APPROVED RESUB 1-Storm_Drainage_Report+6.13.2022RES U B
Jun 14 2022
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
Drainage Report BLD2022-0279
Solomon SFR
for
Genet Solomon & Desta Getachew
20204 37th Ave W
Lynnwood, WA 98036
SITE LOCATION:
1400 9" Ave N
Edmonds, WA 98020
Tax Parcel: 00608300000201
Prepared by:
Joseph M. Smeby, P.E.
Job No: 19-0117-S
January, 2022
Revised: June 2022
COMPLIES WITH APPLICABLE
CITY STORMWATER CODE
dr�
07/26/2022
TABLEOF CONTENTS.......................................................................................................................... 1
LISTOF FIGURES.................................................................................................................................. 2
1. INTRODUCTION &SITE CLASSIFICATION.................................................................................... 3
2. STORMWATER SITE PLAN........................................................................................................... 4
3. SWPPP NARRATIVE..................................................................................................................... 4
A. SITE GRADING/EROSION CONTROL RISK ASSESSMENT.............................................................. 5
B. SWPPP MINIMUM ELEMENTS..................................................................................................... 5
4. WATER POLLUTION SOURCE CONTROL...................................................................................... 8
5. PRESERVATION OF NATURAL DRAINAGE SYSTEM...................................................................... 8
6. ON -SITE STORMWATER MANAGEMENT..................................................................................... 8
7. RUN-OFF TREATMENT.............................................................................................................. 11
8. FLOW CONTROL........................................................................................................................ 11
9. WETLANDS PROTECTION.......................................................................................................... 11
10.OPERATIONS AND MAINTENANCE MANUAL.............................................................................. 12
APPENDIX A -
Solomon SFR 19-0117-S
June 2022 Page 1
LIST OF FIGURES
ITEM
PAGE
FIGURE 1
VICINITY MAP .......................................................................................
10
FIGURE 2
EXISTING DRAINAGE BASIN MAP ......................................................11
FIGURE 3
DEVELOPED DRAINAGE BASIN MAP .................................................12
FIGURE 4
SNOHOMISH COUNTY SOILS MAP .....................................................
13
FIGURE 5
UPSTREAM/DOWNSTREAM TRIBUTARY AREA MAP
........................14
Solomon SFR 19-0117-S
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1. INTRODUCTION & SITE CLASSIFICATION
This document is intended to provide engineering information necessary to support the building
permit application submittal to the City of Edmonds for the new SFR on Lot 2 of the recently
approved/recorded Sundstone SP. The current property is approximately 13,448 sf. Access to the
parcel will be from 9th Ave S along the western property line.
The total area of disturbance including clearing and grading does not exceed % acres and the new
total impervious surfaces does exceed 5,000 sf. Therefore, this project is classified as a Category 1
Small Site Project. This classification was completed using figure 2-1 of the 2017 Storm water
Code Supplement to Edmonds Community Development Code Chapter 18.30. Due to the existing
soil conditions, on -site infiltration for all the site runoff is not feasible for this project and a mix of
infiltration and dispersion will be the selected LID BMPs for this project where feasible. Refer to
Section 6 of this report for a detailed feasibility analysis. As a result, only minimum requirements
1-5 will be required to be reviewed and addressed for this project.
This proposed project will create approximately 4,979 sf of new impervious surfaces including the
proposed SFR and driveway. No impervious surfaces will be considered removed or replaced for
this project. All the new (on -site) impervious surfaces will be mitigated using on -site stormwater
management BMPs including permeable pavement, dispersion, perforated stubout, collection and
conveyance to existing City drainage system and amended soils. The portion of the new driveway
(617 sf) within the existing R/W is too low to collect and manage runoff on -site. In addition,
another 4,979 sf of retrofit area will be mitigated on -site along the east side of Lot 2 via a level
spreader trench and sheet flow dispersion area/vegetated flow path. This will meet the
requirements per ECDC 18.30.060.D.5.b.i. The original SP proposal was for a smaller retrofit area
of 3,480 sf, so an additional 1,520 sf of retrofit area will be added to the design to address the
additional impervious surfaces proposed under this building permit.
For the on -site drainage improvements/mitigation proposed on Lot 2, excluding the retrofit areas,
the design is intended to provide a mix of permeable pavement, splash blocks, level spreaders and
a perforated stubout as BMP mitigation for all of the new on -site impervious surfaces. The retrofit
areas will be mitigated using a vegetated sheet flow dispersion area and a level spreader with a
25-foot vegetated flow path.
The site is located at 1400 9th Ave N, Edmonds, WA 98020, and in Section 13, Township 27N,
Range 3E, Willamette Meridian. See Figure 1- Vicinity Map.
Solomon SFR 19-0117-S
June 2022 Page 3
As noted in Section 1 of this report this project is classified as a Category 1 Small Site Project.
Therefore, this drainage design has been prepared to address the requirements contained in the
City of Edmonds Stormwater Supplement and applicable handouts. This document is intended to
provide the supporting information to justify the BMPs used and to detail how the design of the
selected BMPs meet the required standards.
For this project some LID techniques were used to aid in minimizing the project's impacts to the
neighboring properties and downstream system. The new NPGIS runoff will be dispersed where
feasible on -site and the soils within the proposed area of disturbance will be amended to
landscaping conditions as per BMP T5.13 out of the 2014 DOE SWIVIIVIWW. This is done to reduce
the stormwater runoff generated by the proposed impervious surfaces. Future on -site driveway
surfaces (PGIS) will be mitigated using permeable pavement.
3. SWPPP NARRATIVE
Clearing, grading, and temporary erosion and sediment control plans will be prepared as part of
the final engineering drawings. However, since a construction site is dynamic it will be necessary
to reassess the erosion control BMP's during construction and install additional measures when
necessary.
Proposed temporary measures possible for this project will include the following BMPs:
• Installation of stabilized construction entrance.
• Retention of Existing Vegetation
• Straw mulch, hydroseed or other mulching and planting method to stabilized unworked
areas.
• Silt fencing if necessary
Permanent measures to reduce or eliminate erosion or water quality degradation will include the
following BMPs:
• Paving all traffic areas (concrete/asphalt)
• Permanent landscaping in pervious areas.
• Limiting cut and fill slopes to 2:1 maximum and 3:1 maximum where exposed to standing
water.
• Routine maintenance and inspection of the grounds and response to developing
problems.
The listed erosion control BMP's will be engineered for anticipated conditions in compliance with
City and DOE guidelines. With proper installation, maintenance and inspection the proposed
BMP's should result in minimal impact to the surrounding environment. The City retains the
authority by code to require additional measures should the existing BMPs prove insufficient.
Solomon SFR 19-0117-S
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A. SITE GRADING/EROSION CONTROL RISK ASSESSMENT
Area proposed to be cleared/worked: 14,000 sf = 0.32 acres
Average slope for the site (w/in the area of disturbance): 10%
Erosion Hazard of Soil Moderate
Critical Areas downslope No
Site is upstream of an ESA Stream No
Based on the above information and the fact that the site will retain some existing vegetation and
construction site runoff will filter through the soil, and that if site conditions warrant, additional
BMP's can be implemented as corrective measures the Risk Category for this site is Low Risk.
1: Mark Clearing Limits
The first step in the "Construction Sequence" included on the clearing and grading plan
sheets is for the limits of clearing to be flagged and to have construction fencing placed
along the limits prior to any other construction activity.
2: Establish Construction Access
The SWPPP calls for the proposed construction entrance to be installed as the second
step after the staking of clearing limits. A detail is provided on the plans.
3: Control Flow Rates
This project is below the thresholds requiring flow control for the project.
4: Install Sediment Controls
This site and SWPPP proposes to construct/maintain a construction entrance, retained
vegetation and silt fencing. These features are intended to minimize the opportunity for
sediment to leave the site via stormwater or on vehicles. The construction of these
features is one of the first items required in the "Construction Sequence". Mulch will also
be used on the exposed soil as necessary to limit erosion.
5: Stabilize Soils
The "Construction Sequence" calls for the stabilization of soils that remain unworked for
certain lengths of time based on the time of year. Stabilization techniques may include but
not limited to mulching, plastic sheeting or hydroseeding, notes have been added to the
plan regarding protection for the stockpile area if necessary. A stockpile area has been
identified on the SWPPP and is setback a minimum of 25-feet from any down slope
property line.
6: Protect Slopes
All disturbed slopes on site during construction are required to be protected with mulch or
other means as specified in the construction sequence. No concentrated runoff or
significant amounts of sheet flow will be directed to new cut or fill slopes during
construction.
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June 2022 Page 5
7: Protect Drain Inlets
All on -site yard drains (if any) and offsite adjacent/downstream CBs will be provided with
CB inlet protection as shown on the plans.
8: Stabilize Channels and Outlets
No new channels or outlets are proposed for this site.
9: Control Pollutants
No outside chemicals are expected to be necessary for the construction of this project. All
vehicles working on and around the site would need to meet the State requirements for
emissions.
10: Control DeWatering
DeWatering will not be necessary for this project. However, the existing vegetation
retained on site would be available to spread any water from construction for disposal.
11: Maintain BMPs
The construction supervisor will be responsible for maintaining all BMPs during
construction and working with the City to relocate or add BMPs as necessary as site
conditions change.
12: Manage the Project
It will be the responsibility of the Contractor and Developer to manage this project and
coordinate with the City Inspector and Engineer.
Inspection and Monitoring:
Site inspections shall be done by a person who is knowledgeable in the principles and
practices of erosion and sediment control. The person must have skills to first assess the
site conditions and construction activities that could impact the quality of stormwater, and
second assess the effectiveness of erosion and sediment control measures used to
control the quality of stormwater discharges.
Whenever inspection and/or monitoring reveals that the BMPs identified in the
Construction SWPPP are inadequate, due to the actual discharge of or potential to
discharge a significant amount of any pollutant, appropriate BMPs or design changes shall
be implemented as soon as possible.
Maintaining an Updated Construction SWPPP:
The construction SWPPP shall be retained on -site or within reasonable access to the site.
The SWPPP shall be modified whenever there is a change in the design, construction,
operation, or maintenance at the construction site that has, or could have, a significant
effect on the discharge of pollutants to waters of the state.
The SWPPP shall be modified if, during inspections or investigations conducted by the
owner/operator, or the applicable local or state regulatory authority, it is determined that
the SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater
discharges from the site. The SWPPP shall be modified as necessary to include
Solomon SFR 19-0117-S
June 2022 Page 6
additional or modified BMPs designed to correct problems identified. Revisions to the
SWPPP shall be completed within seven days following inspection.
13: Protect Low Impact Development BMPs
The area for the future permeable pavement driveway on -site will be flagged/fenced in the
field with orange construction fencing to prevent compaction or disturbance of this area
during construction. The proposed construction entrance has been located to not impact
this area.
The future dispersion and perforated stubout BMPs can be installed near the end of the
SFR construction with soil amendments being provided prior to final site stabilization. This
will prevent the amended soils from being disturbed during construction and provide the
best amended soil/vegetated flow path possible at the end of construction.
Solomon SFR 19-0117-S
June 2022 Page 7
The City of Edmonds Stormwater Supplement and DOE Drainage manual was reviewed to
determine if the proposed land -use for this project required any site -specific source control BMPs
to be constructed. SFR projects are not listed as a proposed use that requires source control.
Therefore, no site/development specific source control BMPs are required for this project.
However, S411 BMPs for Landscaping and Lawn/Vegetation Management does apply to this
project. The BMP except from the DOE SWMMWW is provided in the Appendix along with the
maintenance checklists for the proposed on -site BMPs which will be provided to the property
owner for their ongoing maintenance of the property after construction is completed.
5411 BMPs include amended soils which is provided as part of BMP T5.13 Post Construction Soil
Quality and Depth as noted in this report and shown on the engineering plan set. However, the
entire 5411 section is provided in the Appendix so the future property owner will also have the
recommended Operational BMPs, Use of Pesticides, Vegetation Management, Irrigation, Fertilizer
Management, and guidance for setting up an integrated Pest Management Plan.
5. PRESERVATION OF NATURAL DRAINAGE SYSTEM
The runoff from the on -site basin drains to the west towards the western property line and the
adjacent R/W as sheet flow. It was determined that the runoff from the site improvements will be
infiltrated via permeable pavement, dispersed on -site, the maximum extent feasible, or directly
connected to the existing drainage system in the R/W. The proposed driveway will be constructed
with permeable pavement while the roof will disperse flows over amended soils when the
required flow paths are available, otherwise roof downspouts will be collected and directed to a
perforated stubout connection before discharging to the existing drainage system within 9th Ave
N. This will allow the maximum amount of runoff to be treated and naturally absorb into the on -
site soils while not concentrating any of the flows. By not grading or compacting most of the
pervious surfaces to remain after construction in the area of disturbance a maximum amount of
site runoff from the developed project will be absorbed into the existing on -site soils and remain
as sheet flow as it leaves the site which will match the existing conditions.
6. ON -SITE STORMWATER MANAGEMENT
The on -site soils for this project have been mapped by the web soil survey as Alderwood Urban
Land Complex. This soil type is typically a till soil in urban/developed areas where the existing
ground has been disturbed/graded. Typically, the depth to the top of the till (hard pan) layer is
between 2-3 feet below the existing grade. The soil unit is gravelly ashy sandy loam. Refer to
Figure 4 to view Soils map. This project proposes to construct approximately 4,979 sf of new plus
replaced roof surfaces and/or walks.
Using the City of Edmonds Appendix A — On -Site Stormwater Management BMP Infeasibility
Criteria the following describes the BMP selection process for this project.
Solomon SFR 19-0117-S
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Lawn and Landscaped Areas:
Post -Construction Soil Quality and Depth: This option was selected, and the specifications will be
provided on the future construction engineering plans.
Roofs (3,324 sf to be mitigated):
Full Dispersion: Not selected since the 65:10 required ratio could not be met for this project and
since the 100-foot retained/native vegetative flow path cannot be provided.
Downspout Full Infiltration:
Based on the disturbed nature of the site and the mapped soil type there is not more than 3-feet
of permeable soils available for this project. In addition, the project Geotech has prepared a
report based on their site investigations and found the long-term infiltration rate for the existing
soils to be less than 0.05"/hr which is well below the infeasibility rate for full infiltration.
Bioretention or Rain Gardens:
Based on the disturbed nature of the site and the mapped soil type there is not more than 3-feet
of permeable soils available when considering the depth of the storage area and 18" of
engineered soil mix and based on Geotechnical findings noted above.
Downspout Dispersion Systems:
SELECTED (where feasible) — No more than 700 sf of roof shall be directed to any one dispersion
system. Refer to Preliminary Development Plan.
Perforated Stubout Connections:
SELECTED for any roof downspout that is located such that it cannot meet the necessary minimum
flow path length or setback from property line. A detail has been added to the plans to show how
to adjust the system construction if high groundwater or hard pan it found to be less than 2.5'
deep in the area of the trench. In addition, a note has been added to the plans to ensure the
contractor coordinates with the project Geotech prior to trench construction.
No further Roof BMP evaluation for feasibility is necessary.
Other Hard Surfaces:
Permeable Pavement (1,038 sf to be mitigated):
The driveway will be graded to be at grade or in slight fill. Therefore, due to the shallow nature of
the permeable pavement section this BMP can be used to mitigate PGIS runoff on -site. Based on
Geotechnical findings a low infiltration rate can be used for this project.
Bio-Retention or Rain Gardens:
See discussion for roof areas above.
Sheet -Flow Dispersion:
Runoff from walks and/or patios will be dispersed along the downslope edge onsite.
Solomon SFR 19-0117-S
June 2022 Page 9
Concentrated Flow Dispersion:
Not selected due to a higher LID option being feasible.
Detention Vaults and Pipes:
The total hard surfaces that cannot be mitigated using one of the higher priority options for this
project totals 617 sf. Therefore, since this area for the entire project totals less than 1,000 sf this
option is infeasible, and the driveway runoff within the City's R/W will be collected and conveyed
to the existing city drainage system within 9th Ave N.
The roof will require a minimum of five splash blocks or level spreaders with additional locations
allowed as needed based on the final roof/gutter layout. However, based on the current
layout/site constraints it appears that only three locations will be feasible so a perforated stubout
connection will be provided for any additional downspout that cannot meet the necessary
flowpath and setback requirements. Each splash block will provide a minimum flow path of 50-
feet over amended or natural vegetation/topsoil and level spreaders will provide 25' flow paths.
A total of approximately 1,655 sf of PGIS is proposed for this project with only 1,038 sf of that
being on -site. The on -site driveway runoff, on -site, will be mitigated using permeable pavement.
A typical pavement section has been provided with this design. Driveway runoff within the R/W
will be collected and discharged to the existing City drainage system via a new CB.
Any other walks or patios would be designed to allow runoff to sheet flow off the surface and
spread over amended soils.
All permeable surfaces that are graded or compacted during construction will be amended using
topsoil or compost as provided in the specifications provided on sheet 6 of the plan set.
The retro fit area for the approved short plat proposal consists of pavement for the parking/access
to the condos on the future eastern lot of this short plat. The parking lot, based on site survey by
Acreage Land Surveying, shows that 3,480 sf of the pavement drains to an area that can be
captured with an asphalt berm installed to direct flows draining northwest down the pavement to
a new CB. The runoff entering this CB is then routed to a series of pipes and CBs to a level
spreader that was sized for 10' of trench length for each 700 sf of pavement collected. However,
the maximum trench length of 50' is shown on the plans which slightly exceeds the required
length. The trench and conveyance system is located within an easement on the east side of Lot 2
in an area that drains to the west. A minimum vegetated flow path of 25-feet is also provided
within the easement to ensure that the maintenance and function of the system is ensured after
this project is recorded.
Due to the increased amount of proposed impervious surfaces for this building permit application
over the assumed area as part of the short plat, it is necessary to provide retrofit mitigation to an
additional 1,499 sf. In reviewing the existing condo layout and drainage patterns it was found that
1,520 sf of patio/walk/shuffleboard area west of the existing pool and southwest of the condo
building slopes to the west. The maximum flow path over impervious surfaces in this -retrofit area
was measured to be 30-feet. Therefore, per Checklist 8 of the City of Edmonds Stormwater
Addendum the sheet flow dispersion BMP design requires a vegetated flow path of 10-feet for the
Solomon SFR 19-0117-S
June 2022 Page 10
first 20-feet of contributing area width, plus an additional 10-feet of vegetated flow path for each
additional 20-feet of contributing area or fraction thereof. As noted, the contributing area width
is a maximum of 30-feet so a 15' vegetated flow path is being provided.
7. RUN-OFF TREATMENT
This project proposes to construct less than 5,000 sf of new PGIS and is therefore exempt from
any run-off treatment.
8. FLOW CONTROL
This project proposes to construct less than 5,000 sf of new impervious surfaces and is therefore
exempt from any flow control requirements.
9. WETLANDS PROTECTION
This project will not impact any wetland on -site or downstream of the project site.
Solomon SFR 19-0117-S
June 2022 Page 11
10. OPERATIONS AND MAINTENANCE MANUAL
The Property Owner will be responsible for maintaining the stormwater and landscaping facilities
within this development. Included in this manual are checklists for each feature specific to this
project. Copies should be made of the checklists as necessary during routine inspections and
required maintenance. Specific problems can be recorded along with the appropriate action
taken.
These checklists are a guide for inspections and maintenance. The frequency of the
inspections/maintenance is identified in the left-hand column with the following abbreviations:
A = Annual (March or April preferred)
M = Monthly
S = After Major Storms (Use 1-inch in 24 hours as a guideline)
Routine inspections and maintenance will improve the long-term performance of the stormwater
facilities. If at any time you are unsure if a problem exists or how to address a specific problem,
contact a Professional Engineer.
Refer to Appendix A for a list of each facility to be maintained and the appropriate maintenance
checklist (To be provided with final engineering report).
Solomon SFR 19-0117-S
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APPENDIX A
Solomon SFR 19-0117-S
June 2022 Page 13
Main Office
17311= 135t" Ave NE, A-500
Woodinville, WA 98072
(425) 486-1669 - FAX (425) 481-2510
December 21, 2018
Ms. Lindell Graham
1414 - 911 Avenue North
Edmonds, WA 98020
lindellgghotmail.com
Geotechnical Engineering Evaluation
Sundstone Short Plat
1414 - 9" Avenue North
Edmonds, Washington
NGA File No. 1074518
Dear Ms. Graham:
NELSON GEOTECHNICAL
AssociATEs, INC.
..' : w 0" n
Engineering -Geology Branch
5526 industry Lane, #2
East Wenatchee, WA 98802
(509) 665-7696 - FAX (509) 665-7692
We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Sundstone
Short Plat — 1414 - 9" Avenue North — Edmonds, Washington." This report summarizes our
observations of the existing surface and subsurface conditions within the site, and provides general
recommendations for the proposed site development. Our services were completed in general accordance
with the proposal signed by you on November 18, 2018.
The level to gently west -sloping site is occupied by a multistory multifamily residential structure on the
eastern portion of the property, and is vacant on the western portion. The property is bordered to the
north by existing residential properties, to the west by 9th Avenue North, to the south by an easement
along Puget Drive (WA-524), and to the east by commercial development. We understand that the 1.18-
acre parcel will undergo a short plat to create at least one additional residential parcel within the vacant
western portion, and that the Sundstone Condominium structure is intended to remain in the east. We
understand that stormwater generated within this site will be directed to on -site infiltration facilities if
feasible. Specific -grading _and stormwater .plans were_ not available when this report was prepared, but
based on conversations with you, we understand that stormwater may be directed to on -site infiltration
facilities, if feasible. In addition to providing recommendations for the development of the new single-
family residence(s), we have been requested to evaluate the infiltration capacity of the site soils. The City
of Edmonds utilizes the 2014 WSDOE Stormwater Management Manual for Western Washington to
determine the design of infiltration facilities. According to this manual, on -site infiltration testing
consisting of the small Pilot Infiltration Test (PIT) is used to determine the long-term design infiltration
rates.
We performed four test pit explorations throughout the property, one of which we utilized for our small-
scale pilot infiltration testing (PIT). Our explorations indicated that the site was underlain by surficial
undocumented fill with competent, native soils at depth.
NELSON GEOTECHN/CAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Summary — Page 2
It is our opinion that the proposed site development is feasible from a geotechnical engineering
standpoint, provided that our recommendations for site development are incorporated into project plans.
In general, the native soils underlying the site should adequately support the planned structures.
Foundations should be advanced through any loose soils down to the competent native bearing material
interpreted to underlie the site, for bearing capacity and settlement considerations. These soils should
generally be encountered approximately two to three feet below the existing ground surface, based on our
explorations. If loose soils or undocumented fill are encountered in unexplored areas of the site, they
should be removed and replaced with structural fill for foundation and pavement support. Final
stormwater plans have also not been developed, but we understand that on -site infiltration is being
considered for this site. Based on our onsite testing, it is our opinion that the onsite soils are not
conducive to traditional methods of stormwater infiltration, however low -impact design systems may be
feasible. The subsurface soils generally consisted of surficial undocumented fill soils underlain by silty
fine to medium sand with varying amounts of gravel and iron -oxide weathering that we interpreted as
native glacial soils at relatively shallow depths. We recommend that any low impact stormwater
infiltration systems be designed in accordance with the 2014 Department of Ecology Stormwater
Management Manual for Western Washington.
In the attached report, we have also provided general recommendations for site grading, slabs -on -grade,
structural fill placement, retaining walls, erosion control, and drainage. We should be retained to review
and comment on final development plans and observe the earthwork phase of construction. We also
recommend that NGA be retained to provide monitoring and consultation services during construction to
confirm that the conditions encountered are consistent with those indicated by the explorations, to provide
recommendations for design changes should the conditions revealed during the work differ from those
anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with
contract plans and specifications.
It has been a pleasure to provide service to you on this project. Please contact us if you have any
questions regarding this report or require further information.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Khaled M. Shawish, PE
Principal Engineer
INTRODUCTION.............................................................................................................1
SCOPE...............................................................................................................................1
SITECONDITIONS......................................................................................................... 2
SurfaceConditions....................................................................................................... 2
SubsurfaceConditions.................................................................................................. 2
3
Hydrogeologic Conditions.
SENSITIVEAREA EVALUATION............................................................................... 3
SeismicHazard............................................................................................................. 3
ErosionHazard............................................................................................................. 4
CONCLUSIONS AND RECOMMENDATIONS.......................................................... 4
General......................................................................................................................... 4
ErosionControl............................................................................................................ 5
_ Site' Preparation- and Grading....................................................................................... 5
Temporary and Permanent Slopes............................................................................... 6
Foundations.................................................................................................................. 7
RetainingWalls.....................................................................:...................................... 8
StructuralFill................................................................................................................ 9
Slab-on-Grade.............................................................................................................. 9
Pavements.......................................:........................................................................... 10
Utilities....................................................................................................................... 10
SiteDrainage.............................................................................................................. 10
CONSTRUCTIONMONITORING.............................................................................12
USEOF THIS REPORT................................................................................................12
LIST OF FIGURES
Figure 1—Vicinity Map
Figure 2 —Site Plan
Figure 3 —Soil Classification Chart
Figure 4 —Test Pit Logs
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
1414 - 9' Avenue North
Edmonds, Washington
INTRODUCTION
This report presents the results of our geotechnical engineering investigation and evaluation of the
planned Sundstone Short Plat Development project in the Edmonds, Washington. The project site is
located at 1414 - 9th Avenue West, as shown on the Vicinity Map in Figure 1. The purpose of this study
is to explore and characterize the site's surface and subsurface conditions and to provide geotechnical
recommendations for the planned site development. For our use in preparing this report, we have been
provided with an undated and untitled preliminary site plan showing the existing and proposed conditions
within the site.
The property is currently occupied by a multi -level multi -family residential structure within the eastern
portion of the site, while the subject western portion of the site is undeveloped. We understand the
proposed developments within the site will include short platting the property and constructing a new
residential structure within the lower western parcel. Topography within the subject portion of the site
slopes gently from east to west. Vegetation generally consists of grass -covered yard areas and scattered
young to mature trees. Final development and grading plans have not been prepared at the time this
report was issued. Final stormwater plans have also -not been developed, however, we understand that
stormwater may be directed to on -site infiltration systems, if feasible. The existing and proposed site
layout is shown on the Site Plan in Figure 2.
SCOPE
The purpose of this study is to explore and characterize the site surface and subsurface conditions, and
provide general recommendations for site development. Specifically, our scope of services includes the
following:
1. Review available soil and geologic maps of the area.
2. Explore the subsurface soil and groundwater conditions within the site with track -
mounted backhoe-excavated test pits. Excavation services were provided by NGA.
3. Perform laboratory grain -size sieve analysis on soil samples, if necessary.
4. Provide recommendations for earthwork, foundation support, and slabs -on -grade.
5. Provide recommendations for temporary and permanent slopes.
6. Provide recommendations for pavement subgrade.
7. Provide recommendations for site drainage and erosion control.
8. Provide ,our opinion on the feasibility of infiltration for the onsite soils.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
-Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 2
9. Provide long-term design infiltration rates based on on -site Pilot Infiltration Testing (PIT)
per the 2014 WSDOE Manual. One test to be performed within each proposed property.
10. Provide recommendations for infiltration system installation.
11. Document the results of our findings, conclusions, and recommendations in a written
geotechnical report.
SITE CONDITIONS
Surface Conditions
The overall property consists of an approximately 1.18-acre parcel with a multi -family residential
structure within the eastern portion of the site and vacant undeveloped land to the west. We understand
the proposed improvements will consist of dividing the property into two parcels and constructing a
residential structure within the western parcel. The proposed western parcel is vegetated with grass lawn
and few scattered young to mature trees. The ground surface slopes gently from east to west. The subject
site is bound to the east by the multi -family structure, to the north by a residential property, to the south
by Puget Drive, and to the west by 9' Avenue North. We did not observe surface water throughout the
site during our visit on December 3, 2018.
Subsurface Conditions
Geology: The site is mapped on the Geologic map of the Edmonds East and part of the Edmonds West
quadrangles, Washington, by James P. Minard (US Geological Survey, 1983). The site is mapped as till
and transitional beds (Qtu and Qtb, respectively). Till deposits generally consists of a compact non -sorted
mixture of clay, silt, sand, pebbles, and boulders. Transitional beds are described as thin bedded clay, silt,
and very fine to fine sand with some layers of peaty sand and gravel lower in the unit. Our explorations
typically encountered undocumented fill underlain by silty fine to medium sand with varying amounts of
gravel consistent with the description of native till deposits at depth.
Explorations: The subsurface conditions within the site were explored on December 3, 2018 by
excavating four test -pits toapproximate -depths in,the range of 4.0 to 5.5-feet,below the existing ground
surface using a mini-trackhoe. The approximate locations of our explorations are shown on the Site Plan
in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic
conditions encountered, obtained samples of the different soil types, and maintained logs of the test pits.
The soils were visually classified in general accordance with the Unified Soil Classification System,
presented in Figure 3. The logs of our test pits are attached to this report and are presented as Figure 4.
We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed
description of the subsurface conditions, the logs of the test pits should be reviewed.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds; Washington
NGA File No. 1074518
December 21, 2018
Page 3
At the surface of each exploration we generally encountered 1.7 to 2.5 feet of dark brown to brown, silty
fine to medium sand with varying amounts of gravel, roots, and organics, which we interpreted as topsoil
and/or undocumented fill soils. Underlying the fill soils and topsoil we generally encountered medium
dense or better orange -brown to gray, silty fine to medium sand with varying amounts of gravel and iron -
oxide staining, which we interpreted as native till deposits at depth. Infiltration Pit 1 and Test Pits 1
through 3 terminated at respective depths of 4.0, 5.0, 5.5, and 4.0 feet below the existing ground surface.
Hydrogeologic Conditions
We did not encounter groundwater within our explorations throughout the site. If groundwater is
encountered during construction we would interpret this as a perched groundwater condition. Perched
water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on
top of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered
soils and undocumented fill. The low permeability soil consists of relatively silty native deposits.
Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched
water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount
of perched groundwater to decrease during drier times of the year and increase during wetter periods.
SENSITIVE AREA EVALUATION
Seismic Hazard
We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project.
Since competent�glacial,soils are inferred to underlie the site at depth, the site-conditions,best fit the IBC
description for Site Class D.
Table 1 below provides seismic design parameters for the site that are in conformance with the 2018
IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return
interval of 2,475 years), and the 2008 USGS seismic hazard maps.
Table 1— 2018 IBC Seismic Design Parameters
Site Class
Spectral Acceleration
Spectral Acceleration
Site Coefficients
Design Spectral
at 0.2 sec. (g)
at 1.0 sec. (g)
Response
SS
Si
Parameters
Fa
Fv
SDs
SDI
D
1.278
0.501
1.000
1.5
0.852
0.501
The spectral response accelerations were obtained from the USGS Earthquake Hazards Program
Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude.
NELSON GEOTECHN/CAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 4
The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of
seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend.
Information published in 2013 by the Washington State Department of Natural. Resources suggests the
SWIFZ last ruptured less than 2,700 years ago, and that the fault zone can produce a M7.5 earthquake. In
our opinion, the possibility of faulting ground rupture caused by this fault zone is considered low.
Hazards associated with seismic activity include liquefaction potential and amplification of ground
motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the
groundwater table. � It is our opinion that the medium dense or better deposits interpreted to underlie the
site have a low potential for liquefaction or amplification of ground motion.
Erosion Hazard
The criteria used for determination of the erosion hazard for affected areas include soil type, slope
gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative
cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil
Survey of Snohomish Coupjy Area, Washington, by the Natural Resources Conservation Service (MRCS)
was reviewed to determine the erosion hazard of the on -site soils. The surface soils for this site were
mapped as Alderwood-Urban land complex, 2 to 8 percent slopes. The erosion hazard for this material is
listed as slight. This site is relatively level to gently sloping and there are no steep slopes on the property.
It is our opinion that the erosion hazard for site soils should be low in areas where the site is not
disturbed.
CONCLUSIONS AND RECOMMENDATIONS
General
It is our opinion that the site planned development is feasible from a geotechnical standpoint. Our
explorations indicated that the site is generally underlain by competent native bearing soils at relatively
shallow depths. The native soils encountered at depth should provide adequate support for foundation,
slab, and pavement loads. We recommend that the planned structure be designed utilizing shallow
foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on
the underlying medium dense or better native bearing soil, or structural fill extending to these soils. The
medium dense or better native glacial soils should typically be encountered approximately two to three
feet below the existing surface, based on our explorations. We should note that localized areas of deeper
unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require
additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 5
Based on the results of our infiltration testing and soil explorations throughout the site, it is our opinion
that the onsite native soils are not conducive for traditional methods of stormwater infiltration. This is
further discussed in the Site Drainage section of this report.
Erosion Control
The erosion hazard for the on -site soils is interpreted to be slight for exposed soils, but actual erosion
potential will be dependent on how the site is graded and how water is allowed to concentrate. Best
Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction
should be protected from erosion. Erosion control measures may include diverting surface water away
from the stripped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy
water from leaving the site. Disturbed areas should be planted as soon as practical and the vegetation
should be maintained until it is established. The erosion potential of areas not stripped of vegetation
should be low.
Site Preparation and Grading
After erosion control -measures are implemented, site preparation should consist of stripping the topsoil,
undocumented fill and loose soils from foundation, slab, pavement areas, and other structural areas, to
expose medium dense or better native bearing soils. The stripped soil should be removed from the site or
stockpiled for later use as a landscaping fill. Based on our observations, we anticipate stripping depths of
two to three feet, depending on the specific locations. However, additional stripping may be required if
areas of deeper. undocumented fill and/or loose soil- are -encountered in unexplored areas of the site.
After site stripping, if the exposed subgrade is deemed loose, it should be compacted to a non -yielding
condition and then proof -rolled with a heavy rubber -tired piece of equipment. Areas observed to pump or
weave during the proof -roll test should be reworked to structural fill specifications or over -excavated and
replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the
pavement areas, the loose soils should be removed and replaced with rock spalls or granular structural fill.
If significant surface water flow is encountered during construction, this flow should be diverted around
areas to be developed, and the exposed subgrades should be maintained in a semi -dry condition.
If wet conditions are encountered, alternative site stripping and grading techniques might be necessary.
These could include using large excavators equipped with wide tracks and a smooth bucket to complete
site grading and covering exposed subgrade with a layer of crushed rock for protection. If wet conditions
are encountered or construction is attempted in wet weather, the subgrade should not be compacted as this
could cause further subgrade disturbance. In wet conditions it may be necessary to cover the exposed
subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive soils from
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 6
disturbance by machine or foot traffic during construction. The prepared subgrade should be protected
from construction traffic and surface water should be diverted around areas of prepared subgrade.
The site soils are considered to be moisture -sensitive and will disturb easily when wet. We recommend
that construction take place during the drier summer months if possible. However, if construction takes
place during the wet season, additional expenses and delays should be expected due to the wet conditions.
Additional expenses could include the need for placing a blanket of rock spalls on exposed subgrades,
construction traffic areas, and paved areas prior to placing structural fill. Wet weather grading will also
require additional erosion control and site drainage measures. Some of the on -site soils may be suitable
for use as structural fill, depending on the moisture content of the soil at the time of construction. NGA
should be retained to evaluate the suitability of all on -site and imported structural fill material during
construction.
Temporary and Permanent Slopes
Temporary cut slope stability is a function of many factors, including the type and consistency of soils,
depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open, and the
presence of surface or groundwater. It is exceedingly difficult under these variable conditions to estimate
a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the contractor to
maintain safe slope configurations at all times as indicated in OSHA guidelines for cut slopes.
The following information is provided solely for the benefit of the owner and other design consultants and
should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job
site safety. Job site safety is the sole responsibility of the project contractor.
For planning purposes, we recommend that temporary cuts in the upper undocumented fill soils be no
steeper than 2 Horizontal to 1 Vertical (2H:IV). Temporary cuts in the competent native soils at depth
should be no steeper than 1.511:1V. If significant groundwater seepage or surface water flow were
encountered, we would- expect that flatter inclinations would be necessary. We recommend that cut
slopes be protected from erosion. The slope protection measures may include covering cut slopes with
plastic sheeting and diverting surface runoff away from the top of cut slopes. We do not recommend
vertical slopes for cuts deeper than four feet, if worker access is necessary. We recommend that cut slope
.heights. and inclinations conform to appropriate O.SHA/WISHA regulations.
Permanent cut and fill slopes should be no steeper than 211:1V. However, flatter inclinations may be
required in areas where loose soils are encountered. Permanent slopes should be vegetated and the
vegetative cover maintained until established.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds; Washington
Foundations
NGA File No. 1074518
December 21, 2018
Page 7
Conventional shallow spread foundations should be placed on medium dense or better native bearing
soils, or be supported on structural fill or rock spalls extending to those soils. Medium dense soils should
be encountered approximately two to three feet below ground surface based on our explorations. Where
undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be
over -excavated to expose suitable bearing soil. The over -excavation may be filled with structural fill, or
the footing may be extended down to the competent native bearing soils. If footings are supported on
structural fill, the fill zone should extend outside the edges of the footing a distance equal to one half of
the depth of the over -excavation below the bottom of the footing.
Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost
protection and bearing capacity considerations. Foundations should be designed in accordance with the
2018 IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure.
Water should not be allowed to accumulate in footing trenches.. All loose or disturbed soil should be
removed from the foundation excavation prior to placing concrete.
For foundations constructed as outlined above, we recommend an allowable design bearing pressure of
not more than 2,000 pounds per square foot (psf) be used for the design of footings founded on the
medium dense or better native bearing soils or structural fill extending to the competent native bearing
material. The foundation bearing soil should be evaluated by a representative of NGA. We should be
consulted if higher bearing pressures are needed. Current IBC guidelines should be used when
considering increased allowable bearing pressure for short-term transitory wind or seismic loads.
Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less
than 1-inch total and 1/2-inch differential between adjacent footings or across a distance of about 20 feet,
based on our experience with similar projects.
Lateral loads may be resisted by friction on the base of the footing and passive resistance against the
subsurface portions of the foundation. A coefficient of friction of 0.35 may be used to calculate the base
friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a
triangular equivalent fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic foot
(pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This
level surface should extend a distance equal to at least three times the footing depth. These recommended
values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and
passive resistance, respectively. To achieve this value of passive resistance, the foundations should be
poured "neat" against the native medium dense soils or compacted fill should be used as backfill against
the front of the footing. We recommend that the upper one foot of soil be neglected when calculating the
passive resistance.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
Retaining Walls
NGA File No. 1074518
December 21, 2018
Page 8
Specific grading plans for this project were not available at the time this report was prepared, but
retaining walls may be incorporated into project plans. In general, the lateral pressure acting on
subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount
of lateral wall movement which can occur as backfill is placed, wall drainage conditions, and the
inclination of the backfill. For walls that are free to yield at the top at least one thousandth of the height
of the wall (active condition), soil pressures will be less than if movement is limited by such factors as
wall stiffness or bracing (at -rest condition). We recommend that walls supporting horizontal backfill and
not subjected to hydrostatic forces, be designed using a triangular earth pressure distribution equivalent to
that exerted by a fluid with a density of 40 pcf for yielding (active condition) walls, and 60 pcf for non -
yielding (at -rest condition) walls. A seismic design loading of 8H should also be included in the wall
design, where H represents the total height of the wall.
These recommended lateral earth pressures are for a drained granular backfill and are based on the
assumption of a horizontal ground surface behind the wall for a distance of at least the height of the wall,
and do not account for surcharge loads. Additional lateral earth pressures should be considered for
surcharge loads acting adjacent to walls and within a distance equal to the height of the wall. This would
include the effects of surcharges such as traffic loads, floor slab loads, slopes, or other surface loads. We
could consult with the structural engineer regarding additional loads on retaining walls during final
design, if needed.
The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and
by passive resistance acting on the below -grade portion of the foundation. Recommendations for
frictional and passive resistance to lateral loads are presented in the Foundations subsection of this
report.
All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report.
Care should be taken to prevent the buildup of excess lateral soil pressures due to over -compaction of the
wall backfill. This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting the
backfill with small, hand -operated compactors within a distance behind the wall equal to at least one-half
the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower
compactive energy of the hand -operated equipment. The recommended level of compaction should still
be maintained.
Permanent drainage systems should be installed for retaining walls. Recommendations for these systems
are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to
evaluate the proposed wall drain backfill material and observe installation of the drainage systems.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
Structural Fill
NGA File No. 1074518
December 21, 2018
Page 9
General: Fill placed beneath foundations, pavement, or other settlement -sensitive structures should be
placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and
standards, and is monitored by an experienced geotechnical professional or soils technician. Field
monitoring procedures would include the performance of a representative number of in -place density tests
to document the attainment of the desired degree of relative compaction. The area to receive the fill
should be suitably prepared as described in the Site Preparation and Grading subsection prior to
beginning fill placement.
Materials: Structural fill should consist of a good quality, granular soil, free of organics and other
deleterious material, and be well graded to a maximum size of about three inches. All-weather fill should
contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing
the U.S. 3/4-inch sieve). Some of the more granular on -site soils may be suitable for use as structural fill,
but this will be highly dependent on the moisture content of these soils at the time of construction. We
should be retained to evaluate all proposed structural fill material prior to placement.
Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling
should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be
thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas
and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density.
Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557
Compaction Test procedure. The moisture content of the soils to be compacted should be within about
two percent of optimum so that a readily compactable condition exists. It may be necessary to over -
excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All
compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree
of compaction and should be tested.
Slab -on -Grade
Slabs -on -grade should be supported on subgrade soils prepared as described in the Site Preparation and
Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches
of free -draining gravel with less than three percent by weight of the material passing Sieve #200 for use
as a capillary break. We recommend that the capillary break be hydraulically connected to the footing
drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic
sheeting (6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick
moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor
barrier membrane and to aid in curing the concrete.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
Pavements
NGA File No. 1074518
December 21, 2018
Page 10
Pavement subgrade preparation and structural filling where required, should be completed as
recommended in the Site Preparation and Grading and Structural Fill subsections of this report. The
pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft
or yielding areas that require repair. The pavement section should be underlain by a minimum of six
inches of clean granular pit run. We should be retained to observe the proof -rolling and recommend
repairs prior to placement of the asphalt or hard surfaces.
Utilities
We recommend that underground utilities be bedded with a minimum six inches of pea gravel prior to
backfilling the trench with on -site or imported material. Trenches within settlement sensitive areas
should be compacted to 95% of the modified proctor as described in the Structural Fill subsection of this
report. Trenches located in non-structural areas should be compacted to a minimum 90% of the
maximum dry density. The trench backfill compaction should be tested.
Site Drainage
Infiltration: We conducted a Small PIT within Infiltration Pit 1, located within the northwest portion of
the site as shown on the attached Schematic Site Plan in Figure 2. The test was conducted within a pit
that measured 4.0-feet long by 3.0-feet wide by 4.0-feet deep. The pit was filled with 12-inches of water
at the beginning of the day and we began the soaking period of the PIT for approximately 6 hours. At this
time, the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 075 water
flow meter for the pre-soak period.
After the 6-hour soaking period was completed, the water level was maintained at approximately 12-
inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 0.02
gallons per minute (1.20 gallons per hour). This equated to an approximate infiltration rate of 0.182
inches per hour. The water was shut off after the steady-state period and monitored at least every 15
minutes for one hour. After 60 minutes, the water level within the pit dropped approximately 0.125
inches, resulting in a measured infiltration rate of 0.125 inch per hour.
In accordance with the Table 3.5 of the Department of Ecology 2014 SWMMVIWW, correction factors of
0.8, 0:5,and°0.9 for CFv, CFt, CFm, respectively were applied to the field measured infiltration rate of
0.125 inches per hour, obtained from the falling -head portion of the testing in Infiltration Pit 1. A total
correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head
portion of the test to determine the long-term design infiltration rate.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds; Washington
NGA File No. 1074518
December 21, 2018
Page 11
Using the above correction factor, we calculated a long-term design infiltration rate of approximately
0.045 inches per hour. In our opinion, a long-term design infiltration rate of 0.045 inches per hour could
be utilized to design low -impact infiltration systems, such as rain gardens, bioswales, or permeable
pavements within the native, silty fine to medium sand with gravel found on this site at depth. We
recommend these systems be sized and designed in accordance with the 2014 Department of Ecology
Stormwater Management Manual for Western Washington and in conjunction with the provided long-
term design infiltration rate of 0.045 inches per hour. Additionally, due to the very low infiltration rate we
recommend incorporating an overflow component into any infiltration systems within the site, which
should be directed towards an approved point of discharge.
We recommend that any proposed infiltration systems be placed as to not negatively impact any proposed
or existing nearby structures and also meet all required setbacks from existing property lines, structures,
and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be
located within proposed fill areas within the site associated with site grading or retaining wall backfill as
such condition could lead to failures of the placed fills and/or retaining structures. We should be retained
to evaluate the infiltration system design and installation during construction.
Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an
approved stormwater collection system. Water should not be allowed to stand in any areas where
footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from
the residences. We suggest that the finished ground be sloped at a minimum gradient of three percent, for
a distance of at least 10 feet away from the residences. Surface water should be collected by permanent
catch basins and drain lines, and be discharged into an approved discharge system.
Subsurface Drainage: If groundwater is encountered during construction, we recommend that the
contractor slope the bottom of the excavation and collect the water into ditches and small sump pits where
the water can be pumped out and routed into a permanent storm drain.
We recommend the use of footing drains around the structures. Footing drains should be installed at least
one foot below planned finished floor elevation. The drains should consist of a minimum 4-inch-
diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material wrapped in a filter
fabric. We recommend that the free -draining material consist of an 18-inch-wide zone of clean (less than
three -percent fines), granular material placed along the back of walls. Pea gravel is an acceptable drain
material. The free -draining material should extend up the wall to one foot below the finished surface.
The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper
to minimize surface water or fines migration into the footing drain. Footing drains should discharge into
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds; Washington
NGA File No. 1074518
December 21, 2018
Page 12
tightlines leading to an approved collection and discharge point with convenient cleanouts to prolong the
useful life of the drains. Roof drains should not be connected to wall or footing drains.
CONSTRUCTION MONITORING
We should be retained to provide construction monitoring services during the earthwork phase of the
project to evaluate subgrade conditions, temporary cut conditions, fill compaction, and drainage system
installation.
USE OF THIS REPORT
NGA has prepared this report for Ms. Lindell Graham and her agents, for use in the planning and design
of the development on this site only. The scope of our work does not include services related to
construction safety precautions and our recommendations are not intended to direct the contractors'
methods, techniques, sequences, or procedures, except as specifically described in our report for
consideration in design. There are possible variations in subsurface conditions between the explorations
and also with time. Our.report, conclusions, and interpretations should not be construed as a warranty of
subsurface conditions. A contingency for unanticipated conditions should be included in the budget and
schedule.
We. recommend that NGA be retained to provide monitoring and consultation services during
construction to confirm that the conditions encountered are consistent with those indicated by the
explorations, to provide recommendations for design changes should the conditions revealed during the
work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation
activities comply with contract plans and specifications. We should be contacted a minimum of one week
prior to construction activities and could attend pre -construction meetings if requested.
Within the limitations of scope, schedule, and budget, our services have been performed in accordance
with generally accepted geotechnical engineering practices in effect in this area at the time this report was
prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are
a means to identify and reduce the inherent risks to the owner.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 13
It has been a pleasure to provide service to you on this project. If you have any questions or require
further information, please call.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Alex B. Rinaldi, GIT
Staff Geologist H
Maher A. Shebl, PhD, PE, M.ASCE
Senior Engineer
Four Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
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Woodinville Office
East Wenatchee Ofte
17311-135th Ave. NE, A-500
5526 Industry Lane, #2
Woodinville, WA 98072
East Wenatchee, WA 98802
(425) 486-1669 / Fax: 481-2510 wvw.nelsongeotech.com
(509) 665-7696 / Fax: 665-7692
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NELSON GEOTECHNICAL
Graham Short Plat N �, ASSOCIATES, INC.
Site Plan GEOT,ECHNICAL-ENGINEERS-&.•GE.OLOGISTS
Woodlnvllle Office East Wenatchee Oftice
17311-135th Ave. NE, A-500 5526 Industry Lane, #2
Woodinville, WA 98072 East Wenatchee, WA 98802
(425) 486-16691 Fax: 481-2510 vAyw.nelsongeotech.com (509) 665-76961 Fax: 665-7692
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No. I Date I Revision I By I CK
1 1 12/5/18 1 Original I DPN I ABR
UNIFIED IL CLASSIFICATION SYSTEM
GROUP
MAJOR DIVISIONS
GROUP NAME
SYMBOL
CLEAN
GW
WELL -GRADED, FINE TO COARSE GRAVEL
COARSE-
GRAVEL
GRAVEL
GP
POORLY -GRADED GRAVEL
GRAINED
MORE THAN 50 %
GRAVEL
GM
SILTY GRAVEL
OF COARSE FRACTION
SOILS
RETAINED ON
NO.4 SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
SW
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
SP
POORLY GRADED SAND
MORE THAN 50 %
MORE THAN 50
RETAINED ON
RETAIN
R
OF COARSE FRACTION
SAND
SM
SILTY SAND
NO. 200 SIEVE
PASSES NO.4 SIEVE
SC
CLAYEY SAND
WITH FINES
FINE -
SILT AND CLAY
ML
SILT
INORGANIC
CL
CLAY
GRAINED
LIQUID LIMIT
LESS THAN 50 %
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SOILS
SILT AND CLAY
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
INORGANIC
MORE THAN 50 %
PASSES
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
LIQUID LIMIT
NO. 200 SIEVE
50 % OR MORE
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES:
1) Field classification is based on visual SOIL MOISTURE MODIFIERS:
examination of soil in general
Dry -Absence of moisture, dusty, dry to
accordance with ASTM D 2488-93.
the touch
2) Soil classification using laboratory tests
Moist -Damp, but no visible water.
is based on ASTM D 2488-93.
Wet - Visible free water or saturated,
3) Descriptions of soil density or usually soil is obtained from
consistency are based on below water table
interpretation of blowcount data,
visual appearance of soils, and/or
test data.
Project Number
NELSON GEOTECHNICAL
No.
Date
Revision
By
CK
1074518
Graham Short Plat
A ASSOCIATES, INC.
1
12/5/18
Original
DPN
ABR
SoII CIaSSillCatl"orb"'"Ch art
GEOTECHNICAL -ENGINEERS &.GEOL.OGISTS
I g u re 3
WoodlnAlle Office East Wenatchee OfficeF
17311-135th Ave. NE, A-500 5526 Industry Lane, #2
Woodinville, WA 98072 East Wenatchee, WA 98802
(425) 486-16691 Fax: 481-2510 www.nelsongeotech.com (509) 665-7696 / Fax: 665-7692
Table V-A.S: Maintenance Standards - Catch Basins
Maintenance
Defect
Conditions When Maintenance is Needed
Results Expected When Maintenance is per -
Component
formed
Trash or debris which is located immediately in front of the catch basin opening or is blocking inletting capacity of the basin by more than 10%.
No Trash or debris located immediately in front of
Trash or debris (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the
catch basin or on grate opening.
basin, but in no case less than a minimum of six inches clearance from the debris surface to the invert of the lowest pipe.
No trash or debris in the catch basin.
Trash & Debris
Trash or debris in an inlet or outlet pipe blocking more than 1/3 of its height. Y p�p 9 9
Inlet and outlet pipes free of trash ordebris.
Dead animals orvegetation that could generate odors that could cause complaints ordangerous gases (e.g., methane).
No dead animals orvegetation present within the
catch basin.
Sediment (in the basin) that exceeds 60 percent of the sump depth as measured from the bottom of basin to invert of the lowest pipe into or out of the
Sediment
basin, but in no case less than a minimum of 6 inches clearance from the sediment surface to the invert of the lowest pipe.
No sediment in the catch basin
General
Structure Damage to
To slab has holes lar er than 2 square inches or cracks wider than 1/4 inch. Intent is to make sure no material is running into basin
P 9 q ( 9 )�
Top slab is free of holes and cracks.
Frame and/or Top Slab
Frame not sitting flush on top slab, i.e., separation of more than 3/4 inch of the frame from the top slab. Frame not securely attached
Frame is sitting flush on the riser rings or top slab
and firmly attached.
Fractures or Cracks in
Maintenance person judges that structure is unsound.
Basin replaced or repaired to design standards.
Basin Walls/ Bottom
Grout fillet has separated or cracked wider than 1/2 inch and longer than 1 foot at the joint of any inlet/outlet pipe or any evidence of soil particles entering
Pipe is regrouted and secure at basin wall.
catch basin through cracks.
Settlement/ Mis-
If failure of basin has created a safety, function, or design problem.
Basin replaced or repaired to design standards.
alignment
Vegetation growing across and blocking more than 10% of the basin opening.
No vegetation blocking opening to basin.
Vegetation
Vegetation growing in inlet/outlet pipe joints that is more than six inches tall and less than six inches apart.
No vegetation or root growth present.
Contamination and Pol-
See Table V-A.1: Maintenance Standards - Detention Ponds
No pollution present.
lution
Cover Not in Place
Cover is missing or only partially in place. Any open catch basin requires maintenance.
Cover/grate is in place, meets design standards,
and is secured
Catch Basin
Locking Mechanism
Mechanism cannot be opened by one maintenance person with propertools. Bolts into frame have less than 1/2 inch of thread.
Mechanism opens with proper tools.
Cover
Not Working
Cover Difficult to
One maintenance person cannot remove lid after applying normal lifting pressure.
Cover can be removed by one maintenance per -
Remove
(Intent is keep coverfrom sealing off access to maintenance.)
son.
Ladder
Ladder Rungs Unsafe
Ladder is unsafe due to missing rungs, not securely attached to basin wall, misalignment, rust, cracks, or sharp edges.
Ladder meets design standards and allows main-
tenance person safe access.
Grate opening Unsafe
Grate with opening wider than 7/8 inch.
Grate opening meets design standards.
Metal Grates
Trash and Debris
Trash and debris that is blocking more than 20% of grate surface inletting capacity.
Grate free of trash and debris.
(If Applicable)
Grate is in place, meets the design standards, and
Damaged or Missing.
Grate missing or broken member(s) of the grate.
is installed and aligned with the flow path.
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1009
Table V-A.21: Maintenance Standards - Bioretention Facilities (continued)
Recommended Frequency a
Maintenance Com- Condition when Maintenance is Needed (Stand- Action Needed (Procedures)
ponent Inspection Routine Main- ards)
tenance
vegetation man -
protocols)
agement
Note that the inspection and routine maintenance frequencies listed above are recommended by Ecology. They do not supersede or replace the municipal stormwater permit requirements for inspection frequency required of municipal stormwater per-
mittees for "stormwater treatment and flow control BMPs/facilities".
a Frequency: A = Annually; B = Biannually (twice per year); M = Monthly; W = At least one visit should occur during the wet season (for debris/clog related maintenance, this inspection/maintenance visit should occur in the early fall, after deciduous
trees have lost their leaves); S = Perform inspections after major storm events (24-hour storm event with a 10-year or greater recurrence interval).
IPM - Integrated Pest Management
ISA - International Society of Arboriculture
Table V-A.22: Maintenance Standards - Permeable Pavement
Recommended Frequency a
Component Condition when Maintenance is Needed Action Needed (Procedures)
Inspection I Routine Maintenance (Standards)
Surface/Wearing Course
• Clean deposited soil or other materials from permeable pavement or other adjacent surfacing
Permeable Pave- A S Runoff from adjacent pervious areas deposits • Check if surface elevation of planted area is too high, or slopes towards pavement, and can be regraded (prior
ments, all soil, mulch or sediment on paving to regrading, protect permeable pavement by covering with temporary plastic and secure covering in place)
• Mulch and/or plant all exposed soils that may erode to pavement surface
Porous asphalt or per-
vious concrete
Ab
AorB
None (routine maintenance)
Surface is clogged: Ponding on surface or water
flows off the permeable pavement surface dur-
ing a rain event (does not infiltrate)
Clean surface debris from pavement surface using one or a combination of the following methods:
• Remove sediment, debris, trash, vegetation, and other debris deposited onto pavement (rakes and leaf
blowers can be used for removing leaves)
• Vacuum/sweep permeable paving installation using:
o Walk -behind vacuum (sidewalks)
o High efficiency regenerative airorvacuum sweeper (roadways, parking lots)
o ShopVac or brush brooms (small areas)
• Hand held pressure washer or power washer with rotating brushes Follow equipment manufacturer guidelines
for when equipment is most effective for cleaning permeable pavement. Dry weather is more effective for
some equipment.
• Review the overall performance of the facility (note that small clogged areas may not reduce overall per-
formance of facility)
• Test the surface infiltration rate using ASTM C 1701 as a corrective maintenance indicator. Perform one test
per installation, up to 2,500 square feet. Perform an additional test for each additional 2,500 square feet up to
15,000 square feet total. Above 15,000 square feet, add one test for every 10,000 square feet.
• If the results indicate an infiltration rate of 10 inches per hour or less, then perform corrective maintenance to
restore permeability. To clean clogged pavement surfaces, use one or combination of the following methods:
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1025
Table V-A.22: Maintenance Standards - Permeable Pavement (continued)
Recommended Frequency a
Condition when Maintenance is Needed
Component
(Standards)
Action Needed (Procedures)
Inspection
Routine Maintenance
• Combined pressure wash and vacuum system calibrated to not dislodge wearing course aggregate.
• Hand held pressure washer or power washer with rotating brushes
• Pure vacuum sweepers
Note: If the annual/biannual routine maintenance standard to clean the pavement surface is conducted using
equipment from the list above, corrective maintenance may not be needed.
• Assess the overall performance of the pavement system during a rain event. If water runs off the pavement
and/or there is ponding then see above.
A
Sediment present at the surface of the pave -
ment
• Determine source of sediment loading and evaluate whether or not the source can be reduced/eliminated. If
the source cannot be addressed, consider increasing frequency of routine cleaning (e.g., twice per year
instead of once per year).
• Sidewalks: Use a stiff broom to remove moss in the summer when it is dry
Summer
Moss growth inhibits infiltration or poses slip
safety hazard
• Parking lots and roadways: Pressure wash, vacuum sweep, or use a combination of the two for cleaning
moss from pavement surface. May require stiff broom or power brush in areas of heavy moss.
• Fill potholes or small cracks with patching mixes
• Large cracks and settlement may require cutting and replacing the pavement section. Replace in -kind where
A
Major cracks or trip hazards and concrete
feasible. Replacing porous asphalt with conventional asphalt is acceptable if it is a small percentage of the
spalling and raveling
total facility area and does not impact the overall facility function.
• Take appropriate precautions during pavement repair and replacement efforts to prevent clogging of adjacent
porous materials
Clean pavement surface using one or a combination of the following methods:
• Remove sediment, debris, trash, vegetation, and other debris deposited onto pavement (rakes and leaf
blowers can be used for removing leaves)
• Vacuum/sweep permeable paving installation using:
A or B
None (routine maintenance)
o Walk -behind vacuum (sidewalks)
o High efficiency regenerative airorvacuum sweeper (roadways, parking lots)
Interlocking concrete
o ShopVac or brush brooms (small areas)
paver blocks and
Note: Vacuum settings may have to be adjusted to prevent excess uptake of aggregate from paver openings
aggregate pavers
orjoints. Vacuum surface openings in dry weather to remove dry, encrusted sediment.
• Review the overall performance of the facility (note that small clogged areas may not reduce overall per-
formance of facility)
Surface is clogged: Ponding on surface or water
• Test the surface infiltration rate using ASTM C1701 as a corrective maintenance indicator. Perform one test
Ab
flows off the permeable pavement surface dur-
per installation, up to 2,500 square feet. Perform an additional test for each additional 2,500 square feet up to
ing a rain event (does not infiltrate)
15,000 square feet total. Above 15,000 square feet, add one test for every 10,000 square feet.
• If the results indicate an infiltration rate of 10 inches per hour or less, then perform corrective maintenance to
restore permeability.
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1026
Table V-A.22: Maintenance Standards - Permeable Pavement (continued)
Recommended Frequency a
Condition when Maintenance is Needed
Component
(Standards)
Action Needed (Procedures)
Inspection
Routine Maintenance
• Clogging is usually an issue in the upper 2 to 3 centimeters of aggregate. Remove the upper layer of encrusted
sediment, and fines, and/or vegetation from openings and joints between the pavers by mechanical means
and/or suction equipment (e.g., pure vacuum sweeper).
• Replace aggregate in paver cells, joints, or openings per manufacturer's recommendations
• Assess the overall performance of the pavement system during a rain event. If water runs off the pavement
and/or there is ponding, then see above.
A
Sediment present at the surface of the pave -
ment
• Determine source of sediment loading and evaluate whether or not the source can be reduced/eliminated. If
the source cannot be addressed, consider increasing frequency of routine cleaning (e.g., twice per year
instead of once per year).
• Sidewalks: Use a stiff broom to remove moss in the summer when it is dry
Summer
Moss growth inhibits infiltration or poses slip
safety hazard
• Parking lots and roadways: Vacuum sweep or stiff broom/power brush for cleaning moss from pavement sur-
face
A
Paver block missing or damaged
Remove individual damaged paver blocks by hand and replace or repair per manufacturer's recommendations
A
Loss of aggregate material between paver
Refill per manufacturer's recommendations for interlocking paver sections
blocks
A
Settlement of surface
May require resetting
• Remove sediment, debris, trash, vegetation, and other debris deposited onto pavement (rakes and leaf
A or B
None (routine maintenance)
blowers can be used for removing leaves)
• Follow equipment manufacturer guidelines for cleaning surface.
Aggregate is clogged: Ponding on surface or
• Use vacuum truck to remove and replace top course aggregate
Ab
water flows off the permeable pavement surface
during a rain event (does not infiltrate)
Replace aggregate in paving grid per manufacturer's recommendations
• Remove pins, pry up grid segments, and replace gravel
Open -celled paving
A
Paving grid missing or damaged
• Replace grid segments where three or more adjacent rings are broken or damaged
grid with gravel
• Follow manufacturer guidelines for repairing surface.
A
Settlement of surface
May require resetting
A
Loss of aggregate material in paving grid
Replenish aggregate material by spreading gravel with a rake (gravel level should be maintained at the same level as
the plastic rings or no more than 1/4 inch above the top of rings). See manufacturer's recommendations.
• Manually remove weeds
A
Weeds present
• Presence of weeds may indicate that too many fines are present (refer to Actions Needed under "Aggregate is
clogged" to address this issue)
• Remove sediment, debris, trash, vegetation, and other debris deposited onto pavement (rakes and leaf
Open -celled paving
A or B
None (routine maintenance)
blowers can be used for removing leaves)
grid with grass
• Follow equipment manufacturer guidelines for cleaning surface.
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1027
Table V-A.22: Maintenance Standards - Permeable Pavement (continued)
Recommended Frequency a
Condition when Maintenance is Needed
Component
(Standards)
Action Needed (Procedures)
Inspection
Routine Maintenance
Aggregate is clogged: Ponding on surface or
Ab
water flows off the permeable pavement surface
Rehabilitate per manufacturer's recommendations.
during a rain event (does not infiltrate)
• Remove pins, pry up grid segments, and replace grass
A
Paving grid missing or damaged
• Replace grid segments where three or more adjacent rings are broken or damaged
• Follow manufacturer guidelines for repairing surface.
A
Settlement of surface
May require resetting
• Restore growing medium, reseed or plant, aerate, and/or amend vegetated area as needed
A
Poor grass coverage in paving grid
• Traffic loading may be inhibiting grass growth; reconsider traffic loading if feasible
As needed
None (routine maintenance)
Use a mulch mower to mow grass
• Sprinkle a thin layer of compost on top of grass surface (1/2" top dressing) and sweep it in
A
None (routine maintenance)
• Do not use fertilizer
• Manually remove weeds
A
Weeds present
• Mow, torch, or inoculate and replace with preferred vegetation
Inlets/Outlets/Pipes
Inlet/outlet
A
Pipe is damaged
Repair/replace
pipe
A
Pipe is clogged
Remove roots or debris
Clean orifice at least bian-
Plant roots, sediment or debris reducing capa-
• Jet clean or rota cut debris/roots from underdrain s
rotary ( )
Underdrain pipe
Clean pipe as needed
nually (may need more fre-
quail cleaning during wet
city of underdrain (may cause prolonged draw-
• If underdrains are equipped with a flow restrictor (e.g., orifice) to attenuate flows, the orifice must be cleaned
season)
down period)
regularly
Clean orifice at least bian-
• Jet clean or rotary cut debris/roots from under-drain(s)
Raised subsurface
overflow pipe
Clean as needed
pipe
nually (may need more fre-
quent cleaning during wet
Plant roots, sediment or debris reducing capa-
city of underdrain
If underdrains are equipped with a flow restrictor (e.g., orifice) to attenuate flows, the orifice must be cleaned
season)
regularly
Outlet structure
A, S
Sediment, vegetation, or debris reducing capa-
• Clear the blockage
cityof outlet structure
Identify the source of the blockage and take actions to prevent future blockages
Overflow
g
Native soil is exposed or other signs of erosion
Repair erosion and stabilize surface
damage are present at discharge point
Aggregate Storage Reservoir
Water remains in the storage aggregate longer
If immediate cause of extended ponding is not identified, schedule investigation of subsurface materials or other
Observation port
A, S
than anticipated by design after the end of a
potential causes of system failure.
storm
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1028
Table V-A.22: Maintenance Standards - Permeable Pavement (continued)
Recommended Frequency a
Condition when Maintenance is Needed
Component
(Standards)
Action Needed (Procedures)
Inspection
Routine Maintenance
Vegetation
Vegetation related fallout clogs or will potentially
• Sweep leaf litter and sediment to prevent surface clogging and ponding
Adjacent large
As needed
clog voids
• Prevent large root systems from damaging subsurface structural components
shrubs or trees
Once in May and Once in
Y
Vegetation growing beyond facility edge onto
9 9 9 Y Y 9
Edging and trimming of planted areas to control roundcovers and shrubs from overreaching the sidewalks, paths and
9 9 9 p 9 9
September
sidewalks, paths, and street edge
street edge improves appearance and reduces clogging of permeable pavements by leaf litter, mulch and soil.
Leaves, needles, and
In fall (October to December)
after leaf drop (1-3 times,
Accumulation of organic debris and leaf litter
Use leaf blower orvacuum to blow or remove leaves, evergreen needles, and debris (i.e., flowers, blossoms) off of
organic debris
depending on canopy cover)
and away from permeable pavement
Note that the inspection and routine maintenance frequencies listed above are recommended by Ecology. They do not supersede or replace the municipal stormwater permit requirements for inspection frequency required of municipal stormwater per-
mittees for "stormwater treatment and flow control BMPs/facilities".
a Frequency: A= Annually; B= Biannually (twice per year); S = Perform inspections after major storm events (24-hour storm event with a 10-year or greater recurrence interval).
b Inspection should occur during storm event.
Table V-A.23: Maintenance Standards - Vegetated Roofs
Activity
Objective
Schedule
Notes
Structural and Drainage Components
Clear inlet pipes: Remove soil substrate, vegetation or other debris.
Maintain free drain-
age of inlet pipes.
Twice annually.
Inspect drain pipe: Check for cracks settling and proper alignment,
Maintain free drain -
and correct and re -compact soils or fill material surrounding pipe, if
age of inlet pipes.
Twice annually.
necessary.
Inspect fire ventilation points for proper operation
Fire and safety.
Twice annually.
Maintain egress and ingress: Clear routes of obstructions and main-
Fire and safety.
Twice annually.
tained to design standards.
Roof garden design should provide drainage rates that do not allow pooling of waterfor periods that promote insect larvae
Insects: (see note)
development. If standing water is present for extended periods correct drainage problem. Chemical sprays should not be
used.
Prevent release of contaminants: Identify activities (mechanical
During construction of
systems maintenance, pet access, etc.) that can potentially release
Water quality pro-
roof and then as determ-
Any cause of pollutant release should be corrected as soon as identified and the pollutant removed.
pollutants to the roof garden and establish agreements to prevent
tection.
ined by inspection.
release.
Vegetation and Growth Medium
Promote selected
Invasive or nuisance plants: Remove manually and without herb-
plant growth and sur-
Twice annually.
At a minimum, schedule weeding with inspections to coincide with important horticultural cycles (e.g., priorto majorweed
icide applications.
vival, maintain aes-
varieties dispersing seeds).
thetics.
2019 Stormwater Management Manual for Western Washington
Volume V - AppendixA - Page 1029
LOG OF EXPLORATION
DEPTH (FEET) USC SOIL DESCRIPTION
INFILTRATION PIT
ONE
0.0 - 0.8 GRASS UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS,
AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
0.8 - 2.0 ORANGE -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, WOOD DEBRIS, AND ROOTS
(LOOSE TO MEDIUM DENSE, MOIST) (FILL
2.0 - 4.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS' COMPLETED AT 4.0 FEET ON 12/3/2018
TEST PIT ONE
0.0 - 2.5 DARK BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ROOTS
(LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
2.5 - 5.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE IRON -OXIDE STAINING
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 4.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 5.0 FEET ON 12/3/2018
TEST PIT TWO
0.0 -1.7 DARK BROWN TO BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH GRAVEL AND
ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
1.7 - 5.5 SM ORANGE -BROWN TO GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE IRON -OXIDE
STAINING, AND ROOTS (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 5.5 FEET ON 12/3/2018
TEST PIT THREE
0.0 -1.0 DARK BROWN TO ORANGE -BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH
GRAVEL AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL/FILL)
1.0 - 4.0 SM LIGHT BROWN TO GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE IRON -OXIDE
STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 4.0 FEET ON 12/3/2018
ABR:KMS NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1074518
FIGURE 4
Applicable Operational BMPs:
Eliminate unpermitted wastewater discharges to s/allowed
er, ground
\ater,r surface water.
unpermitted discharges to a sanitary. seweby the
wer authority, or to other approved treatm
• Obtain ap ropriate state and local permits for ese discharges.
Recommended dditional Operational BM : At commercial and
industrial facilities, onduct a survey of was water discharge connections
to storm drains and t urface water as fol ws:
• Conduct a field surv4 of building /particularly older buildings, and
other industrial areas to ocate st drains from buildings and paved
surfaces. Note where thes 'oi the public storm drain(s).
• During non-stormwater co ' ions inspect each storm drain for non-
stormwater discharges. R cord e locations of all non-stormwater
discharges. Include all ermitted 'scharges.
If useful, prepare a p of each areahow on the map the known
location of storm s ers, sanitary sewers, and permitted and
unpermitted disc ges. Aerial photos m be useful. Check records
such as piping hematics to identify kno side sewer connections
and show thes on the map. Consider using s oke, dye, or chemical
analysis test to detect connections between tw conveyance systems
(e.g., proc s water and stormwater). If desirable, onduct TV
inspectio s of the storm drains and record the foota e on videotape.
Comp e the observed locations of connections with the information
on; e map and revise the map accordingly. Note susp t connections
that are inconsistent with the field survey.
Identify all connections to storm sewers or to surface watand take
the actions specified above as applicable BMPs.
S411 BMPs for Landscaping and Lawn/ Vegetation Management
Description of Pollutant Sources: Landscaping can include grading, soil
transfer, vegetation removal, pesticide and fertilizer applications, and
watering. Stormwater contaminants include toxic organic compounds,
heavy metals, oils, total suspended solids, coliform bacteria, fertilizers,
and pesticides.
Lawn and vegetation management can include control of objectionable
weeds, insects, mold, bacteria, and other pests with pesticides. Examples
include weed control on golf course lawns, access roads, and utility
corridors and during landscaping; sap stain and insect control on lumber
and logs; rooftop moss removal; killing nuisance rodents; fungicide
application to patio decks, and residential lawn/plant care. It is possible to
Volume IV - Source Control BMPs — December 2014
2-21
release toxic pesticides such as pentachlorophenol, carbamates, and
organometallics to the environment by leaching and dripping from treated
parts, container leaks, product misuse, and outside storage of pesticide
contaminated materials and equipment. Poor management of the
vegetation and poor application of pesticides or fertilizers can cause
appreciable stormwater contamination.
Pollutant Control Approach: Control of fertilizer and pesticide
applications, soil erosion, and site debris to prevent contamination of
stormwater.
Develop and implement an Integrated Pest Management Plan (IPM) and
use pesticides only as a last resort. Carefully apply pesticides/ herbicides,
in accordance with label instructions. Maintain appropriate vegetation,
with proper fertilizer application where practicable, to control erosion and
the discharge of stormwater pollutants. Where practicable grow plant
species appropriate for the site, or adjust the soil properties of the subject
site to grow desired plant species.
Applicable Operational BNIPs for Landscaping:
• Install engineered soil/landscape systems to improve the infiltration
and regulation of stormwater in landscaped areas.
Do not dispose of collected vegetation into waterways or storm sewer
systems.
Recommended Additional Operational BMPs for Landscaping:
• Conduct mulch -mowing whenever practicable
• Dispose of grass clippings, leaves, sticks, or other collected vegetation,
by composting, if feasible.
• Use mulch or other erosion control measures on soils exposed for
more than one week during the dry season or two days during the rainy
season.
• Store and maintain appropriate oil and chemical spill cleanup materials
in readily accessible locations when using oil or other chemicals.
Ensure that employees are familiar with proper spill cleanup
procedures.
• Till fertilizers into the soil rather than dumping or broadcasting onto
the surface. Determine the proper fertilizer application rate for the
types of soil and vegetation encountered.
• Till a topsoil mix or composted organic material into the soil to create
a well -mixed transition layer that encourages deeper root systems and
drought -resistant plants.
• Use manual and/or mechanical methods of vegetation removal rather
than applying herbicides, where practical.
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Applicable Operational BMPs for the Use of Pesticides:
• Develop and implement an IPM (See section on IPM in Applicable
Operational BMPs_ or Vegetation Mana eg ment) and use pesticides
only as a last resort.
• Implement a pesticide -use plan and include at a minimum: a list of
selected pesticides and their specific uses; brands, formulations,
application methods and quantities to be used; equipment use and
maintenance procedures; safety, storage, and disposal methods; and
monitoring, record keeping, and public notice procedures. All
procedures shall conform to the requirements of Chapter 17.21.RCW
and Chapter 16-228 WAC (Appendix IV-D R.7).
• Choose the least toxic pesticide available that is capable of reducing
the infestation to acceptable levels. The pesticide should readily
degrade in the environment and/or have properties that strongly bind it
to the soil. Conduct any pest control activity at the life stage when the
pest is most vulnerable. For example, if it is necessary to use a
Bacillus thuringiens application to control tent caterpillars, apply it to
the material before the caterpillars cocoon or it will be ineffective. Any
method used should be site -specific and not used wholesale over a
wide area.
• Apply the pesticide according to label directions. Do not apply
pesticides in quantities that exceed manufacturer's instructions.
• Mix the pesticides and clean the application equipment in an area
where accidental spills will not enter surface or ground waters, and
will not contaminate the soil.
• Store pesticides in enclosed areas or in covered impervious
containment. Do not discharge pesticide contaminated stormwater or
spills/leaks of pesticides to storm sewers. Do not hose down the paved
areas to a storm sewer or conveyance ditch. Store and maintain
appropriate spill cleanup materials in a location known to all near the
storage area.
• Clean up any spilled pesticides. Keep pesticide contaminated waste
materials in designated covered and contained areas.
• The pesticide application equipment must be capable of immediate
shutoff in the event of an emergency.
• Spraying pesticides within 100 feet of open waters including wetlands,
ponds, and rivers, streams, creeks, sloughs and any drainage ditch or
channel that leads to open water may have additional regulatory
requirements beyond just following the pesticide product label.
Additional requirements may include:
• Obtaining a discharge permit from Ecology.
• Obtaining a permit from the local jurisdiction.
• Using an aquatic labeled pesticide.
Volume IV - Source Control BMPs — December 2014
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• Flag all sensitive areas including wells, creeks, and wetlands prior to
spraying.
• Post notices and delineate the spray area prior to the application, as
required by the local jurisdiction or by Ecology.
• Conduct spray applications during weather conditions as specified in
the label direction and applicable local and state regulations. Do not
apply during rain or immediately before expected rain.
Recommended Additional Operational BMPs for the use of pesticides:
• Consider alternatives to the use of pesticides such as covering or
harvesting weeds, substitute vegetative growth, and manual weed
control/moss removal.
• Consider the use of soil amendments, such as compost, that are known
to control some common diseases in plants, such as Pythium root rot,
ashy stem blight, and parasitic nematodes. The following are three
possible mechanisms for disease control by compost addition (USEPA
Publication 530-F-9-044):
1. Successful competition for nutrients by antibiotic production;
2. Successful predation against pathogens by beneficial
microorganism; and
3. Activation of disease -resistant genes in plants by composts.
Installing an amended soil/landscape system can preserve both the plant
system and the soil system more effectively. This type of approach
provides a soil/landscape system with adequate depth, permeability, and
organic matter to sustain itself and continue working as an effective
stormwater infiltration system and a sustainable nutrient cycle.
• Once a pesticide is applied, evaluate its effectiveness for possible
improvement. Records should be kept showing the effectiveness of the
pesticides considered.
Develop an annual evaluation procedure including a review of the
effectiveness of pesticide applications, impact on buffers and sensitive
areas (including potable wells), public concerns, and recent
toxicological information on pesticides used/proposed for use. If
individual or public potable wells are located in the proximity of
commercial pesticide applications, contact the regional Ecology
hydrogeologist to determine if additional pesticide application control
measures are necessary.
• Rinseate from equipment cleaning and/or triple -rinsing of pesticide
containers should be used as product or recycled into product.
For more information, contact the Washington -State University (WSU)
Extension Home Assist Program, (253) 445-4556, or Bio-Integral
Resource Center (BIRC), P. O. Box 7414, Berkeley, CA. 94707, or EPA to
Volume IV - Source Control BMPs — December 2014
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obtain a publication entitled "Suspended, Canceled, and Restricted
Pesticides" which lists all restricted pesticides and the specific uses that
are allowed.
Applicable Operational BMPs for Vegetation Management:
Use at least an eight -inch "topsoil" layer with at least 8 percent organic
matter to provide a sufficient vegetation -growing medium. Amending
existing landscapes and turf systems by increasing the percent organic
matter and depth of topsoil can substantially improve the permeability
of the soil, the disease and drought resistance of the vegetation, and
reduce fertilizer demand. This reduces the demand for fertilizers,
herbicides, and pesticides. Organic matter is the least water-soluble
form of nutrients that can be added to the soil. Composted organic
matter generally releases only between 2 and 10 percent of its total
nitrogen annually, and this release corresponds closely to the plant
growth cycle. Return natural plant debris and mulch to the soil, to
continue recycling nutrients indefinitely.
Select the appropriate turfgrass mixture for the climate and soil type.
Certain tall fescues and rye grasses resist insect attack because the
symbiotic endophytic fungi found naturally in their tissues repel or kill
common leaf and stem -eating lawn insects. However, they do not,
repel root -feeding lawn pests such as Crane Fly larvae, and are toxic to
ruminants such as cattle and sheep. The fungus causes no known
adverse effects to the host plant or to humans. Endophytic grasses are
commercially available; use them in areas such as parks or golf
courses where grazing does not occur. Local agricultural or gardening
resources such as Washington State University Extension office can
offer advice on which types of grass are best suited to the area and soil
type.
Use the following seeding and planting BMPs, or equivalent BMPs to
obtain information on grass mixtures, temporary and permanent
seeding procedures, maintenance of a recently planted area, and
fertilizer application rates: Temporary and Permanent Seeding,
Mulching, Plastic Covering, and Sodding as described in Volume Il.
Adjusting the soil properties of the subject site can assist in selection
of desired plant species. For example, design a constructed wetland to
resist the invasion of reed canary grass by layering specific strata of
organic matters (e.g., composted forest product residuals) and creating
a mildly acidic pH and carbon -rich soil medium. Consult a soil
restoration specialist for site -specific conditions.
• Aerate lawns regularly in areas of heavy use where the soil tends to
become compacted. Conduct aeration while the grasses in the lawn are
growing most vigorously. Remove layers of thatch greater than 3/4-inch
deep.
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Mowing is a stress -creating activity for turfgrass. Grass decreases its
productivity when mown too short and there is less growth of roots
and rhizomes. The turf becomes less tolerant of environmental
stresses, more disease prone and more reliant on outside means such as
pesticides, fertilizers, and irrigation to remain healthy. Set the mowing
height at the highest acceptable level and mow at times and intervals
designed to minimize stress on the turf. Generally mowing only 1/3 of
the grass blade height will prevent stressing the turf.
Irrigation:
The depth from which a plant normally extracts water depends on the
rooting depth of the plant. Appropriately irrigated lawn grasses
normally root in the top 6 to 12 inches of soil; lawns irrigated on a
daily basis often root only in the top 1 inch of soil. Improper irrigation
can encourage pest problems, leach nutrients, and make a lawn
completely dependent on artificial watering. The amount of water
applied depends on the normal rooting depth of the turfgrass species
used, the available water holding capacity of the soil, and the
efficiency of the irrigation system. Consult with the local water utility,
Conservation District, or Cooperative Extension office to help
determine optimum irrigation practices.
Fertilizer Management:
Turfgrass is most responsive to nitrogen fertilization, followed by
potassium and phosphorus. Fertilization needs vary by site depending
on plant, soil, and climatic conditions. Evaluation of soil nutrient
levels through regular testing ensures the best possible efficiency and
economy of fertilization. For details on soils testing, contact the local
Conservation District, a soils testing professional, or a Washington
State University Extension office.
Apply fertilizers in amounts appropriate for the target vegetation and
at the time of year that minimizes losses to surface and ground waters.
Do not fertilize when the soil is dry. Alternatively, do not apply
fertilizers within three days prior to predicted rainfall. The longer the
period between fertilizer application and either rainfall or irrigation,
the less fertilizer runoff occurs.
Use slow release fertilizers such as methylene urea, IDBU, or resin
coated fertilizers when appropriate, generally in the spring. Use of
slow release fertilizers is especially important in areas with sandy or
gravelly soils.
• Time the fertilizer application to periods of maximum plant uptake.
Ecology generally recommends application in the fall and spring,
although Washington State University turf specialists recommend four
fertilizer applications per year.
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• Properly trained persons should apply all fertilizers. Apply no fertilizer
at commercial and industrial facilities, to grass swales, filter strips, or
buffer areas that drain to sensitive water bodies unless approved by the
local jurisdiction.
Integrated Pest Management
An IPM program might consist of the following steps:
Step 1: Correctly identify problem pests and understand their life cycle
Step 2: Establish tolerance thresholds for pests.
Step 3 : Monitor to detect and prevent pest problems.
Step 4: Modify the maintenance program to promote healthy plants and
discourage pests.
Step 5: Use cultural, physical, mechanical or biological controls first if
pests exceed the tolerance thresholds.
Step 6: Evaluate and record the effectiveness of the control and modify
maintenance practices to support lawn or landscape recovery and prevent
recurrence.
For an elaboration of these steps, refer to Appendix IV-F.
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n of Pollutant Sources: Operators typically conduct
loloading of liquid and solid materials at industrial
coal facilities at shipping and receiving, outside sto ge, fueling
araterials transferred can include products, ra materials,
inte roducts, waste materials, fuels, scrap als, etc. Leaks and
spels, ' s, powders, organics, heavy meta , salts, acids, alkalis,
ettransfe ay cause stormwater cont ination. Spills from
hyline breaks e a common proble t loading docks.
Pollutant Control Appr ch: Cover an contain the loading/unloading
area where necessary to pre nt run-o of stormwater and runoff of
contaminated stormwater.
Applicable Operational BMP X.
At All Loading/ Unloadin Areas:
• A significant amo of debris can acc ulate at outside, uncovered
loading/unload' g areas. Sweep these Sur es frequently to remove
loose mater' that could contaminate storm ter. Sweep areas
tempora . y covered after removal of the contai rs, logs, or other
mate ' 1 covering the ground.
• ace drip pans, or other appropriate temporary contai ent device, at
locations where leaks or spills may occur such as hose co Vections,
hose reels and filler nozzles. Always use drip pans when ing and
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