APPROVED_BLD2022-0561+Storm_Drainage_Report+2.21.2023_4.06.14_PM+3381050RESUB
bld2022-0561 2/21 /2023
CITY OF EDMONDS
DEVELOPMENT
SERVICES DEPARTMENT
DRAINAGE REPORT
COMPLIES WITH APPLICABLE
CITY STORM CODE
Y.!12 5 ... P ,�.
02/27/2023
SHARMA RESIDENCE
8422 238lh St. SW
m
SITE DEVELOPMENT SERVICES
3011 RAVEN CREST
BELLINGHAM, WA 98226
425-481-9687
April 30, 2022
Revised October 10, 2022
Revised February 17, 2023
TABLE OF CONTENTS
Section Title Section Number
Drainage Summary
Minimum Requirements II
BMP Design III
SWPPP IV
Operation & Maintenance V
Appendix A
Geotechnical Reports
DRAINAGE SUMMARY
SECTION I
DRAINAGE SUMMARY
Consisting of one tax parcel, the site is occupied by one single family dwelling and a detached
garage. The balance of impervious area consists of a concrete driveway and a walkway. The
driveway accesses 2381h St with a single driveway approach located on the east side of the site.
Pervious area is grass and brush with sparse trees. Roadway drainage consists of a ditch system
east of the site and a storm drain system across the frontage and on to the west.
The proposed project will remove the existing house and construct a new single family home in
its place. The driveway access will be in the same location but widened. Drainage
improvements will be in accordance with the city's 2017 addendum to the 2014 DOE
Stormwater Management Manual for Western Washington, hereinafter referred to as the
"Manual".
Site Areas:
Lot size: 11,254 SF (0.258Ac)
Predeveloped:
Roof: 1174 SF (0.027Ac)
Dwy/Sidewalk: 799 SF (0.018Ac)
Total: 1973 SF (0.045Ac)
Lawn: 9277 SF (0.213Ac)
Postdeveloped:
Roof: 3224 SF (0.074Ac)
Dwy: 771 SF (0.018Ac)
Walks: 348 SF (0.008Ac)
Total: 4343 SF (0.100Ac)
Lawn: 6911 SF (0.158c)
A geotechnical engineer evaluated the soils and found that they are loamy sands of increasing
density with depth. The depth at which the density becomes "medium dense to dense" is
about 4 feet, so that is considered the "restrictive" layer. Since the soil logs were done in
August, no perched ground water was encountered; however, iron oxide staining was noted. It
is reasonable to assume a seasonal ground water rise to a height of at least 6 inches above the
restrictive layer. For this report the estimated depth to groundwater will assumed to be at a
depth of 3 to 3.5 feet (approximately one foot above the restrictive layer). Thus, the bottom of
any BMP should be no deeper than 2 to 2.5 feet.
An infiltration test done by the geotechnical engineer was performed at a depth of 4.5 feet
(into the restrictive layer), so it is not representative of the less dense soils which the above
paragraph estimates are present to a depth of about 4 feet. In a memorandum by the
geotechnical engineer dated August 26, 2022, he stated that he reviewed the infiltration design
and overflow and approved them, as designed. Thus, standard BMPs are feasible.
As shown by Figure 2.4.1 on the following page, the project is subject to Minimum
Requirements 1 thru 5 as defined in Volume I, Chapter 2 of the Manual. It is a Category 1
project in the city's drainage code. Current city regulations consider detention as a BMP to be
utilized to manage all areas not otherwise managed by the other BMPs. Furthermore, since this
project will fully infiltrate and will have no connection to the city's storm drain system, flow
control is not required.
All site driveways and walkways will be modeled as permeable pavement. Downspout
infiltration trenches are feasible in the rear portion of the lot. Due to a rising elevation in the
front of the house, no BMPs are feasible there. Fortunately, the back yard is large enough to
infiltrate all roof areas.
With regard to water quality treatment of runoff is not required, as the total PGIS is less than
5000 square feet.
Does the project result m 2.000 square feet_ or greater. of new plus replaced hard surface area.'
OR
Does the land &=bine acm-it:- total 7, .000 square feet or greater.'
Yes
No
Mlmn nm Requirements No 1 throu_h ; apply.- I `.linrmum Requu-ement'.o 2 apphe:
Next Question
Doe: the project add S.400 square feet or more of new plus replaced hard surface.'
OR
Com-ert 0 acres or =re of vegetanon to :att-n or:and caper area:"
OR
Convert 2.5 acre: or more of native ve_etation to pasture'
No
Yes
h flu: a road related project'
bilmmum Requuement..
apply to the new and replaced Yes
hard sur6ces and converted �—
vegetanon area:
.Ul blmunum Requuements
apply to the new hand surfaces
and converted . egetanon area
No
Yes
Doe. the oro-ect add
5.000 scuare feet or No
more of new hard
surface:'.
Yes
Do new hard surface. add `0°v or
more to the e_m.tine hard surfaces
within the project limits"
No
Figure 3.1. Flow Chart for Determining Requirements for Development.
I \o addrnona:
recunemer t:.
SECTION II
MINIMUM REQUIREMENTS
MINIMUM REQUIREMENTS
MR#1: Preparation of Stormwater Site Plans
The elements of a stormwater site plan can be found in the civil plan set.
MR#2: SWPPP
See Section IV of this report that discusses the SWPPP elements, together with the TESC Plan,
which directs the contractor as to how to apply the elements.
MR#3: Source Control of Pollution:
No significant sources of pollution will be created with this project, as it consists of access and
utility construction for residential improvements on a small scale.
MR#4: Preservation of Natural Drainage Systems and Outfalls:
No concentrated surface water enters or leaves the site. The developed site will result in
virtually the same condition. The geotechnical report allows the use of BMPs if a safe route of
runoff is provided for overflow conditions. Since the city requires overflow go to a storm drain
in the ROW if at all possible, a piped connection from the roof drains to the storm drain in 2381h
St. has been provided. Because the infiltration system is in the back of the house, which is lower
than the street level, a separate, gravity draining pipe from that location to the street was not
possible. Thus, the overflow pipe is connected to the roof drain in front of the house. The
design presented will result in water that backs up in the roof drain to flow out the pipe to the
street before the water level reaches the grate of the catch basin that feed the infiltration trenches.
MR#5: On -site Stormwater Management:
List#1 in Volume I, Section 2.5.5 for cases requiring adherence to MR#1-5 states that dispersion
or infiltration are the first priorities. Examining these in accordance with Appendix A of the
City's 2017 Stormwater Addendum, the infeasibilities are discussed below. Infeasibility will be
discussed for both roof and other hard surfaces (driveway and roads).
BMP Infeasibility
Full Dispersion A minimum vegetated flow path of 100 feet cannot be
achieved for either roofs or other hard surfaces.
Downspout Full Infiltration This is feasible for the back of the house, as a vertical
separation of one foot can be achieved. This separation
cannot be achieved at the front of the house. Therefore, this
will be used at the back of the house. As shown in the plans
and this report, there is adequate area to manage all of the
roof area.
Bioretention or Rain Gardens: Soil logs show depth to seasonal groundwater (iron oxide
staining) of about 3 feet. The restrictive layer ("dense to
medium dense) is at a depth of 3' to 6'. The extremely low
infiltration rate suggests the bottom of the infiltration pit was
in the restrictive layer. No groundwater was encountered but
that was because the tests were done in August. Therefore, it
cannot be demonstrated that bioretention or rain gardens can
provide the required one foot of separation.
Downspout Dispersion: Downspout dispersion flow paths of 50 feet for splash blocks
or 25 feet for trenches cannot be maintained in the front yard.
However, that distance could be available in a rear yard on
this site, and the natural ground slopes are away from the
building; however, as stated above, downspout infiltration
will meet all feasibility requirements, and can serve the entire
site, therefore that BMP is selected.
Perforated Stub -out Connection: Because downspout infiltration is proposed this BMP is not
necessary.
Driveway BMPs: Since dispersion in infeasible as discussed above, permeable
pavement will be used, as it has been determined to be
feasible based on the site investigations discussed in this
report.
BMP DESIGN
SECTION III
SIZE INFILTRATION TRENCHES
Soil type: loamy sand
Roof area: 3224 sf
Req'd T wide trench: 751f/1000sf, or 150sf/1000sf
Total 2' wide trench area required: 150(3224/1000) = 484sf
Total 4' wide trench length required: (484/4)=121LF
Trench length provided: 121LF
MAIM
SECTION IV
SWPPP
See the other sections of this report for existing site conditions and project areas. No
critical areas are on or adjacent to the site. Soils consist of medium dense silty sand
overlying dense to very dense silty sand (glacial till). Although these depths vary on the
site, the geotechnical engineer stated that onsite infiltration is feasible for standard
BMPs, and that the erosion hazard is low. The following is a summary of the SWPP
Elements, how each is addressed, the type and location of BMP used to satisfy the
required element, and justification if an element is found not be applicable.
Element 1: Mark Clearina Limits:
This was addressed on the SWPPP (TESC Plan) with callouts along the project
perimeter defining the clearing limits, as required to perform the necessary
construction. The BMPs used are C101, Preserving Natural Vegetation, and
C104, except the fence is a silt fence (BMP C233). High visibility fence (C103) is
required, as there are trees to be saved.
Element 2: Establish Construction Access:
A Construction entrance is proposed using the existing driveway. The plans call
for quarry spalls in case the existing gravel is inadequate to remove mud and dirt
from the construction equipment tires. A parking area covered with quarry spalls
is called out. BMPs used are C105, Stabilized Construction Entrance, and C107
Parking Area Stabilization. A wheel wash should not be required due to the fact
that virtually no vehicular traffic should have to drive off of protected surfaces.
Element 3: Control Flow Rates:
Flow rates are not controlled, as they are small due to the small size of the work
area and minimum area of disturbance. The most vital objective is to maintain
sheet flows through the silt fence as much as possible. The measures to
address silt runoff issues are discussed below. Thus, BMP C240, Sediment
Trap, and C241, Sediment Pond are not specified.
Element 4: Install Sediment Controls:
Sediment traps or ponds are not suitable for this project, as they will disturb more
space than they will benefit. In addition, it is more effective to remove the silt
before it becomes concentrated in a pond. Because of the short runoff paths,
lack of steep slopes, and low erosion potential of the soils, this can be done with
proper ground cover practices and silt fences (BMP C233) as proposed in the
TESC Plan. Other BMPs that will perform this function are specified below. All
other BMPs relevant to this element are not applicable for reasons discussed
above and because there will be no channels of concentrated runoff.
Element 5: Stabilize Soils:
Specific details governing the covering of disturbed and stockpiled soils. These
are specified in the TESC Plan and in the Notes Sheet. They provide for straw
mulch (C120, C121, C122, C123, C124, & C125) over ground that will not be
disturbed again for a specified time period and over stockpiles. In addition,
traveled areas are specified to be overlain with appropriate gravelly soils to
prevent tracking and the exposure of silt to rainfall. Due to the absence of steep
slopes and the limited site area, BMPs not used consist of Plyacrylamide, surface
Roughening, Gradient Terraces and Dust Control.
Element 6: Protect Slopes:
There are no significant slopes on this project, so no BMPs are proposed.
Element 7: Protect Drain Inlets:
The TESC Plan calls for filtering or sealing inlet grates that could receive runoff
from the site (BMP 220). A detail of the catch basin filter is provided on the
Details sheet.
Element 8: Stabilize Channels and Outlets:
No channels or outlets are proposed during construction so no BMPs are
proposed.
Element 9: Control Pollutants:
Because of the nature of this work, no significant pollutants will be generated.
Using equipment that is properly maintained prior to bringing it on site is the best
way to control pollutants. No on -site maintenance of equipment is anticipated
due to the small size and short duration of the project. No concrete truck clean -
out shall be performed on site unless the contractor provides an Ecopan or equal
system as approved by the city's inspector
Element 10: Control Dewatering:
Dewatering will not be necessary on this project, due to the shallow excavation
depths and the fact that no ground water was encountered in the soil logs done
for the geotechnical report. Therefore, no BMPs are specified.
Element 11: Maintain BMP's:
This is provided for in the Construction Sequence on the TESC Plan, and in the
Notes sheet. The level of maintenance required depends on the amount of
precipitation and the extent of ground disturbance. A CESCL (BMP C160) will be
identified at or before the pre -construction meeting. The TESC Plan calls for
extra erosion and sedimentation control materials to be stored on site and
available for immediate application as the need arises (BMP C150).
Element 12: Manaae the Proiect:
This requires good communication on the part of the owner and the contractor.
The contractor shall have a contact person (CESCL) in the event that emergency
maintenance is required (BMP C160). The TESC Plan calls for extra erosion and
sedimentation control materials to be stored on site and available for immediate
application as the need arises (BMP C150). At or before the preconstruction
meeting a project schedule shall be provided (C162).
Element 13: Protect Stormwater Manaaement BMPs:
The TESC Plan calls for protection measures. These include prohibiting travel
by wheeled vehicles over infiltration areas, fencing said areas, and limiting
disturbance by all equipment.
Ownership and Financial Responsibility:
This report is required by the City of Edmonds to identify the ownership and financial
responsibility for the project. Both are by Chander Sharma, who is identified as the
owner in the construction documents. Although supervised by the contractor Mr.
Sharma is responsible for the environmental liabilities associated with the project.
OPERATION & MAINTENANCE
SECTION V
OPERATION & MAINTENANCE (PER DOE MANUAL)
The facility -specific maintenance standards contained in this section are
intended to be conditions for determining if maintenance actions are
required as identified through inspection. They are not intended to be
measures of the facility's required condition at all times between
inspections. In other words, exceedence of these conditions at any time
between inspections and/or maintenance does not automatically constitute
a violation of these standards. However, based upon inspection
observations, the inspection and maintenance schedules shall be adjusted
to minimize the length of time that a facility is in a condition that requires
a maintenance action.
CATCH BASIN MAINTENANCE:
Maintenance
Det+ed
Candtions V t m Maintenance is Needed
Results Expected When
Component
Maintenance is
performed
-r-,-il
Trash &
Debris
Trash or debris which is located immediately
in front of the catch basin opening or is
',lc. Trash or 3ebris located
-- na?diately in front of
blocking ink ting capacity of the basin by
cwch basin or on grate
more than 10%.
opening_
Trash or debris On the basin) that exceeds 6C
No trash or debris in the
percent of the sump depth as measured from
catch basin.
the bottom of basin to invert of the lowest
pi pe into or out of the basin, but in no case
less than a minimum of six inches dearanoe
from the debris surface :o the invert & tw
lowest pipe.
Trash or debris in arty inlet or outlet pipe
nlet and ou-Jet pipes `ree
blocking more than 1.3 of its height
of trash or debris.
Dead animals or vegetation that could
No dead animals or
generate odors that could cause complaints
Vegetation present within
or dangerous gases (e.g., methane 1.
:he catch basin.
Sediment
Sediment (in the basin) that exceeds 60
No sediment in the catch
percent of the sump depth as measured from
basin
the bottom of basin to invert of the Iawesz
p4 pe into or out of the basin, but in no case
less than a minirnum of 6 inches clearance
from the sediment surface to the invert of t-
lowest pipe.
Smaure
Damage to
Top slab has holes larger than 2 square
inches or cracks wider than 'A inch
Top slab is tee of holes
and cracks.
Frame and or
Top Slab
l Intent is to make ore no material is running
into basin)_
Frame not sitting flush on top stab. i.e.,
::ran-pe is sitting flush on
separation of more than 14 inch of the `rarre
:he riser rings or top slab
from, the top slab. Frame not securely
and firmly attached.
a7ached
Fractures or
Maintenance person judges that structure is
Basin replaced or repaired
Cracks in
unsound.
:o design standards.
Basin Walls/
Bottom
Grout fillet has separated or cracked wider
wipe is regmuted and
than V2 inch and longer than 1 foot at the
secure at basin wall -
joint of any inlet'outlet pipe or any evidence c-
soil particles entering catch basin through
cracks.
Set•Jement,
If failure of basin has created a safety,
Basin replaced or repaired
Misalignment
function, or design problem_
:o design standards.
Vegetation
Vegetation growing across and blocking more
No vegetation blocking
than 10% of the basin opening.
opening to basin.
Vegetation growing in inletbutlet pipe joints
No vegetation or root
that is more than six inches tall and less thar
growth present.
six inches apart
Contamnajon
See "Detention Ponds" (No- 1).
No pollution present.
and Pollution
CATCH BASIN MAINTENANCE, CONT'D
Maintenance
Defect
Conditions V'1h:-n Maintenance is Needed
Results Expected When
Component
Maintenance is
perfonfned
Ca:ch Basin
Cover Not in
Cover is missing or only partially in place.
Catch basin cover is
Corer
Plaoe
Any open cash basin requires maintenance.
dosed
Locking
Mechanism cannot be opened by one
Mechanism opens with
Mechanism
mairtenanoe person wit proper tools. Bol-s
nroper tools.
Not'Norking
into `came have less tan ':_ inch of tread
Cover Difficult
One maintenance- person cannot remo-.* lid
Cover can be rerrovwd by
to Remove
after applying normal lifting pressure.
one maintenance person.
(Intent is keep cover from sealing off access
to maimenance.I
Ladder
Ladder Rungs
Ladder is unsafe due to rrissing rungs, not
_adder meets desig-
Unsa`e
securely at'ached to basin wall,
standards and alloris
misalignment rust, cracks, or sharp edges.
-'aintenance person safe
acoess.
fvte al Grates
Grate opening
Grate wrth opening w+der tan ? 8 inch.
.:lra-e opening meets
I. If Applicable)
Unsa`e
design standards.
Trash and
Trash and debris tat is blocking more than
3ra*e free & trash and
Debris
20% of graze surface inlet•.ing capacity
debris
Damaged or
Grate missing or broken rremberls) of the
T:----w a-.fis in place and
Missing
grate.
- eets design standards.
INFILTRATION TRENCH MAINTENANCE
Maintenance
Defect or Problem
Conditions When Maintenance is Needed
Results Expected When
Component
Maintenance is Performed
Preventative
Blocking, obstructions
Debris or trash limiting flow to infiltration trench.
Infiltration trench able to receive full
flow prior to and during wet season.
Site
Trash and debris
Trash or debris that could end up in the
No trash or debris that could get into
infiltration trench is evident.
the infiltration trench can be found.
Pipes
Inlet is plugged
The entrance to the pipe is restricted due to
The entrance to the pipe is not
sediment, trash, or debris.
restricted.
Vegetation/roots
Vegetationtroots that reduce free movement of
Water flows freely through pipes
water through pipes
Plugged
Sediment or other material prevents free flow of
Water flows freely through pipes.
water through the pipe.
Broken or joint leaks.
Damage to the pipe or pipe joints allowing water
Pipe does not allow water to exit
to seep out.
other than at the outlet to the trench.
Structure
Flow not reaching
Flows are not getting into the trench as
Water enters and exits trench as
trench
designed
designed.
Cleanout/inspection
The cleanout/inspection access is not available
Cleanout/inspection access is
access does not allow
available.
cleaning or inspection
of trench
Fitter Media
Filter media plugged
Filter media plugged.
Flow through filter media is normal
Inspection
Frequency
Annually and prior to and following significant
Inspect infiltration trench system for
storms.
any defects of deficiencies.
Recommended Frequency.
Component
Condition when Maintenance is Needed
Action Needed
Inspection
Routine
(Standards)
(Procedures)
Maintenance
Surface/Wearing Course
Permeable
A, S
Runoff from adjacent pervious areas deposits soil, mulch or
Clean deposited soil or other materials from permeable pavement or other adjacent surfacing
Pavements, all
sediment on paving
Check if surface elevation of planted area is too high, or slopes towards pavement, and can be regraded (prior 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
A or B
None (routine maintenance)
Clean surface debris from pavement surface using one or a combination of the following methods:
pervious concrete
• Remove sediment, debris, trash, vegetation, and other debris deposited onto pavement (rakes and leaf blowers can be used
for repaving leaves)
• vacuum/sweep permeable paving installation using:
o Walk -behind vacuum (sidewalks)
o High efficiency regenerative air or vacuum 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.
An
Surface is clogged:
Review the overall performance of the facility (note that small clogged areas may not reduce overall performance of facility)
Ponding on surface or water flows off the permeable
• Test the surface infiltration rate using ASTM C1701 as a corrective maintenance indicator. Perform one test per installation,
pavement surface during a rain event (does not infiltrate)
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:
• 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 annuallbiannual routine maintenance standard to clean the pavement surface is conducted using equipment from
the list above, corrective maintenance may not be needed.
.A
Sediment present at the surface of the pavement
• Assess the overall performance of the pavement system during a rain event. If water runs off the pavement andlor there is
ponding then see above.
• Determine source of sediment loading and evaluate whether or not the source can be reducedleliminated. If the source
cannot be addressed, consider increasing frequency of routine cleaning (e.g., twice per year instead of once per year).
Summer
Moss growth inhibits infiltration or poses slip safety hazard
Sidewalks: Use a stiff broom to remove moss in the summer when it is dry
• 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.
A
Major cracks or trip hazards and concrete spalling and
• Fill potholes or small cracks with patching mixes
raveling
Large cracks and settlement may require cutting and replacing the pavement section. Replace in -kind where feasible.
Replacing porous asphalt with conventional asphalt is acceptable if it is a small percentage of the 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
. rrequency: A= Annually; a= atannuauy (tvnce per year); = rerrorm inspections atter major storm events (tin-nour storm event Mtn a to -year or greater recurrence interval).
n Inspection should occur during storm event.
PERMEABLE PAVEMENT MAINTENANCE
GEOTECHNICAL REPORTS
APPENDIX A
NELSON GEOTECHNICAL
ASSOCIATES. INC.
August 27, 2021
ChanderSharma
VIA Email: chander.sharma@fsbwa.com
Geotechnical Engineering Evaluation
Sharma Residence Development
8422 — 238th Street SW
Edmonds, Washington
NGA File No. 1269821
Dear Chander.
17311-135th Ave. N.E. Suite A-500
Woodinville. WA 98072
(425) 486-1669
www.nelsongeotech.com
We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Sharma
Residence Development — 8422 - 238`h Street SW — Edmonds, Washington." This report summarizes
our observations of the existing surface and subsurface conditions within the property and provides
general recommendations for the proposed site development. Our services were completed in general
accordance with the proposal signed by you on July 12, 2021.
The property is rectangular in shape and covers 0.24 acres in area. It is currently occupied by a single-
family residence, detached shed, and concrete driveway within the northeastern corner of the site. The
property is bordered by 238th Street SW to the north, neighboring residential properties to the east and
south, and by a parking lot to the west. Topographically, the site is relatively flat to gently sloping from
northeast to southwest. We understand the plans for development include the construction of a new
residence and the relocation of the existing residence to serve as an accessory structure.
We explored the site subsurface soil conditions with three trackhoe-excavated test pits, one of which
was utilized for infiltration testing. Our explorations indicated that the site was underlain by competent
native glacial till soils at relatively shallow depths, below a layer of undocumented fill and/or topsoil.
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. We recommend that the new structure foundations be founded on medium dense or better
native bearing glacial soils for bearing capacity and settlement considerations. These soils should
generally be encountered approximately 1.5- to 2.0-feet below the existing ground surface based on our
explorations. However, deeper areas of loose soil and/or undocumented fill could also exist within
unexplored areas of the site. If undocumented fill is encountered in unexplored areas of the site, it
should be removed and replaced with structural fill for foundation and pavement support.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
SVGA File No. 1269821
August 27, 2021
Summary — Page 2
We also performed on -site infiltration testing in accordance with the 2019 Department of Ecology (DOE
Stormwater Management Manual for Western Washington
, utilized by the City of Edmonds. Our on -site
testing consisted of one small pilot infiltration tests (PITS) performed in accordance with City of
Edmonds requirements. Subsurface soils generally consisted of silty fine to medium sand with varying
amounts of gravel and cobbles in a medium dense or better condition which we interpreted as native
glacial till soils. Based on our observations, testing in the field, and the silty nature of the native glacial
till deposits that underlie the site, it is our opinion that the on -site native soils encountered at depth are
not conducive for traditional stormwater infiltration systems; however, shallow, low -impact systems
may be feasible, provided they incorporate an overflow component directed to an approved point of
discharge.
In the attached report, we have also provided general recommendations for site grading, slabs -on -
grade, structural fill placement, erosion control, and drainage. We should be retained to review and
comment on final development plans and observe the earthwork phase of construction. We recommend
that Nelson Geotechnical Associates (NGA) be retained to review the geotechnical aspects of the project
plans prior to 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 construction 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
NELSON GEOTECHNICAL ASSOCIATES, INC.
TABLE OF CONTENTS
ION...................0...................................................................................... 1iNTR�D�J�1'
SCOPE11414101414100090000000*00 gooses 00*0 0000........................................................... 1
SITECONDITIONS....................................................................................................... 2
2
Surface Conditions................................................g....................g.................................
2
Subsurface Conditions...........see..e............•.a.....................•.....•..ggee..a.... .........................
Hydrogeologic Conditions .41*40 smosse'066*06 goes as assess **41oosell 00111111114968098
.......,.....assess......,......,...... 3
SENSITIVE AREA EVALUATION..................................................... ............................... 3
Hazard............................................................................................................. 3
Seismic
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Slab -on -Grade ..............................................................................................................
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LIST OF FIGURES,
Figure 1—Vicinity Map
Figure 2 —Site Plan
Figure 3 —Soil Classification Chart
Figure 4 — Exploration Logs
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
8422 — 2381h Street SW
Edmonds, Washington
INTRODUCTION
This report presents the results of our geotechnical engineering investigation and evaluation of the
planned residence development project located at 8422 — 2381" Street SW Street in Edmonds, Washington
as shown on the Vicinity Map in Figure 1. The Snohomish County parcel number for the subject property
is 004633-009-002-00.
The site consists of a rectangular property occupied by a single-family residence. Topography within the
site is generally level. Vegetation within the site includes grass yard areas bordered by shrubs and young
to mature trees. We understand the plans for development include the construction of a new residence
and the relocation of the existing residence to serve as an accessory structure. The existing site layout is
shown on the Site Plan in Figure 2.
SCOPE
The purpose of this study was to explore and characterize the site's surface and subsurface conditions and
to provide geotechnical recommendations for the planned residence development.
Specifically, our scope of services included the following:
1. Reviewing available soil and geologic maps of the area as well as other relevant
geotechnical information, as provided.
2. Exploring the subsurface soil and conditions within the site using trackhoe-excavated test
pits. Excavation services provided by NGA.
3. Providing long-term design infiltration rates based on on -site Small Pilot Infiltration
Testing (PIT) per the 2014 SWMMWW, if feasible.
4. Performing laboratory grain -size sieve analysis on soil samples, as necessary.
5. Providing general recommendations for earthwork and foundation support.
6. Providing recommendations for subsurface utilities and pavement subgrade preparation.
7. Providing our opinion on stormwater infiltration feasibility.
8. Providing recommendations for infiltration/bioretention system installation, if feasible.
9. Providing general recommendations for site drainage and erosion control.
10. Documenting the results of our findings, conclusions, and recommendations in a written
geotechnical report.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
SITE CONDITIONS
NGA File No. 1269821
August 27, 2021
Page 2
Surface Conditions
The subject site consists of a rectangular -shaped parcel covering an area of approximately 0.24 acres. The
site is currently occupied by an existing residence, detached garage structure, and concrete driveway
within the northeast corner of the property. Vegetation within the site consists of grass yard areas
bordered by shrubs and young to mature trees. Topographically, the site is relatively level to gently
sloping from northeast to southwest. The site is bound to the north by 238th Street SW, to the west by a
parking lot, and to the east and south by neighboring residential properties. Surface water was not
observed throughout the site during our site visits on August 4, 2021.
Subsurface Conditions
Geology: The geologic units for this site are shown on the Geologic Map of the Edmonds East and Part of
the Edmonds West Quadrangles. Washington, by Minard, J.P. (USGS, 1983). The site is mapped as glacial
outwash (Qvt). Glacial till is described a dense diamicton, or mixture of silt, clay, sand, and gravel. Our
explorations within the site generally encountered surficial topsoil and/or undocumented fill underlain by
medium dense or better, fine to coarse sand with varying amounts of silt and gravel that we interpreted
as native glacial till deposits throughout the property.
Explorations: The subsurface conditions within the site were explored on August 4, 2021 by excavating
three test pits extending to depths in the range of 4.5 to 6.0 feet below the existing ground surface, one
of which was used for infiltration testing. 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 explorations. The
soils were visually classified in general accordance with the Unified Soil Classification System, presented in
Figure 3. The logs of our explorations are attached to this report and are presented as Figure 4. We
present a brief summary of the subsurface conditions in the following paragraph. For a detailed
description of the subsurface conditions, the exploration logs should be reviewed.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnicaf Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
Page 3
At the surface of all of our explorations, we encountered approximately 1.5 to 2.0 feet of dark brown, silty
fine to medium sand with varying amounts of gravel, roots, organics, and concrete rubble, which we
interpreted as undocumented fill soils. These soils were dry to moist and typically encountered in a
variable loose to medium dense condition. Underlying the undocumented fill soils in each exploration, we
encountered orange -brown to gray, silty, fine to medium sand with varying amounts of gravel, cobbles,
roots,
and iron oxide staining in an
increasingly dense condition.
We interpreted these
lower soils
to be
native
glacial till deposits. All test
pits were terminated within
native glacial till soils
at depths
in the
range of 4.5 to 6.0 feet below existing grade.
Hydrogeologic Conditions
We did not encounter groundwater within our explorations. If groundwater is encountered during
construction, we would interpret this as perched groundwater. Perched water occurs when surface water
infiltrates through less dense, more permeable soils, such as topsoil and the weathered horizon, and
accumulates on top of a less permeable soil. 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 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 two percent 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 Sl Parameters
Fa FV Sos SM
D 1.262 0.493 1.000 1.547 0.841 0.495
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
00M
The spectral response accelerations were obtained from the ATC Hazards by Location (2008 data) for the
project latitude and longitude.
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 glacial 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
Natural Resources Conservation Service (NRCS) lists this area of Edmonds in the Soil Survey of Snohomish
Count as Alderwood-Urban land complex, 2 to 8 percent slopes. The erosion hazard listed for these soils
is slight. Based on our observations we would interpret this site as having a low to moderate erosion
hazard where surficial soils are exposed. 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 planned development within the site is generally feasible from a geotechnical
standpoint. Our explorations indicated that the site was generally underlain by competent native glacial
till soils at relatively shallow depths. The native glacial soils encountered at depth should provide
adequate support for foundation, slab, and pavement loads. We recommend that all structure
foundations planned within the site be founded on medium dense or better native bearing glacial soils for
bearing capacity and settlement considerations. These soils should generally be encountered
approximately 1.5 to 2.0 feet below the existing ground surface based on our explorations. However,
deeper areas of loose soil and/or undocumented fill could also exist within unexplored areas of the site. If
undocumented fill is encountered in unexplored areas of the site, it should be removed and replaced with
structural fill for foundation and pavement support. We recommend that NGA be retained to review
proposed grading plans once they are developed and allowed to provide alternative foundation support
recommendations as needed.
NELSON GEOTEGHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
We also performed on -site infiltration testing based on the 2019 Department of Ecology Stormwater
Manual for Western Washington. The onsite testing consisted of performing one small-scale PIT within
Infiltration Test Pit One. Based on our observations, results of the onsite testing, and the relatively dense,
silty nature of the native till
deposits that
underlie the site, it is our opinion that the on -site
native soils
encountered at depth are
not conducive
for traditional stormwater infiltration systems.
how impact
systems may be feasible on this site as discussed in the Site Drainage section of this report.
The soils encountered on this site are considered moisture -sensitive and will disturb easily when wet. To
reduce cost overruns and delays, we recommend that construction take place during the drier summer
months. If construction takes place during the rainy months, additional expenses and delays should be
expected. Additional expenses could include the need for placing erosion control and temporary drainage
measures, the need for placing a blanket of rock spalls on exposed subgrades and construction traffic
areas prior to placing structural fill, and the need for importing all-weather material for structural fill.
Erosion Control
The erosion hazard for the on -site soils is considered to be low but the actual hazard 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/or 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 approximately 1.5 to 2.0 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
Page 6
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 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnicai Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
Page 7
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:1V)- Temporary cuts in the competent unweathered glacial
outwash soils at depth should be no steeper than 1.51-1:1V. 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 OSHA/WISHA regulations.
Permanent cut and fill slopes should be no steeper than 2H:1V. However, flatter inclinations may be
required in areas where loose soils are encountered.
vegetative cover maintained until established.
Permanent slopes should be vegetated, and the
Foundation Support
Conventional shallow spread foundations should be placed on medium dense or better native bearing
glacial soils or be supported on structural fill or rock spalis extending to those soils. Medium dense or
better bearing soils should be encountered approximately 1.5 to 2.0 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
overexcavation
may be filled with structural fill, or
the footing may be extended down
to the
competent
native 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 overexcavation 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.
removed from the foundation excavation prior to placing concrete.
All loose or disturbed soil should be
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnica! Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
Page 8
For foundations constructed as outlined above, we recommend an allowable 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 Y2-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.
Structural Fill
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.
NELSON GEOTECHNICAL ASSOCIATES,, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
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. 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.
Pavements
The pavement subgrade should be prepared as recommended in the Site Preparation and Grading and
Structural Fill subsections of this report, including proof -rolling the subgrade with a loaded dump truck
and repairing areas observed to pump or weave during the proof -roll test. Also, all fill placed within the
pavement areas, including utility trench backfill, should be compacted to 95 percent of the Maximum Dry
Density (Modified Proctor). We should be retained to observe the proof -roll test. Any areas observed to
pump or weave under the wheels of the loaded dump truck should be over -excavated and replaced with
crushed rock.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Shama Residence Development
Edmonds, Washington
NGA Fife No. 1269821
August 27, 2021
Page 10
We have reviewed the Asphalt Institute Information Series No. 91 Publication "Full -Depth Asphalt for
Parking Lots,, Service Stations, and Driveways." Based on
this
publication, and
an assumed
medium dense
to dense native pavement subgrade, we recommend
that
in light traffic
and parking
lot areas, the
pavement section consist of five inches of crushed rock base -course, overlain by 2.5 inches of Class B Hot
Mix Asphalt (HMA). Alternatively, the pavement section could consist of 3.S inches of ATB, overlain by
2.5 inches of Class B HMA. For heavy truck access areas, the pavement section should consist of eight
inches of crushed rock overlain by four inches of HMA. Alternatively, the crushed rock layer could be
substituted by five inches of ATB.
Pavements should
be sloped
to provide rapid
drainage of surface
water. Water
allowed to
pond on or
adjacent to the
pavements
could saturate
the subgrade and
contribute to
premature
pavement
deterioration. In addition, the pavement subgrade should be graded to provide positive drainage within
the crushed aggregate base section.
The pavement sections provided in this report represent minimum recommended thicknesses. Therefore,
preventive maintenance should be planned and provided for through an on -going pavement management
program. Preventive maintenance activities are intended to slow the rate of pavement deterioration, and
to preserve the pavement investment. Preventive maintenance consists of both localized maintenance
(e.g., crack and joint sealing and patching) and globs! maintenance (e.g., surface sealing). Preventive
maintenance is usually the first priority when implementing a planned pavement maintenance program.
Prior to implementing
any maintenance, additional engineering
observation
is recommended to
determine the type and
extent of acost-effective program. Even
with periodic
maintenance, some
movements and related cracking may still occur, and repairs may be required.
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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2021
Page 11
Site Drainage
infiltration: We performed one small PIT within Infiltration Test Pit One. The infiltration pit measured
approximately 4.0-feet long by 3.0-feet wide by 4.5-feet deep. The soils within the pit consisted on gray,
silty, fine to medium sand with gravel and cobbles that we interpreted to be native glacial till soils.
Infiltration Test Pit One was filled with approximately 12 inches of water and this level was maintained for
6 hours for the presoak portion of the test. At this time, the water flow rate into the pit was monitored
with a Great Plains Industries (GPI) TM 050 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 0.021
gallons per minute (1.26 gallons per hour). This equated to an approximate infiltration rate of 0.17 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 inches per hour.
In accordance with the Table V-5.1 of the Department of Ecology 2019 Stormwater Management Manual
for Western Washington, correction factors of 0.8, 0.5, and 0.9 for site variability and number of locations
tested (CFv), testing method (CFt), and degree of influent control to prevent siltation and bio-buildup
(CFm), respectively were applied to the field measured minimum infiltration rate of 0.17 inches per hour,
calculated from data collected from all days. 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. Using this correction factor, we calculated a long-term design infiltration
rate of 0.062 inches per hour. This does not meet the minimum long-term design infiltration rate provided
by the 2019 SWMMWW of 0.30 incher per hour. Based on the results of our infiltration testing and the
dense silty nature of the onsite native glacial till soils at depth it is our opinion that the native glacial till
soils at depth within the site are not conductive to traditional stormwater infiltration systems.
However, a low -impact stormwater infiltration system may be feasible, depending on final site layout.
The project civil engineer should determine possible methods of low -impact stormwater infiltration in
conjunction with our calculated long-term design infiltration rate and the Department of Ecology 2019
Stormwater Management Manual for Western Washington.
NELSON GEOTECHINICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1269821
August 27, 2921
Page 12
We did not observe any groundwater during our time on site. It is our opinion that proposed infiltration
systems should be able to maintain a minimum separation from the base of the infiltration systems to any
impermeable surfaces and/or groundwater table. We recommend that the base of the on -site infiltration
systems be terminated in the native deposits. We also recommend that all on -site limited infiltration
systems include an overflow component directed into an approved point of discharge, likely within 238 th
Street SW.
We recommend that any 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, if necessary.
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 downward 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 generally recommend
the use of footing drains around the structures. Footing drains may be installed at least one foot below
planned finished floor elevation. The drains should
consist of a minimum 4-inch-d
iameter,
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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnicaf Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
NGA File No. 1259821
August 27, 2021
Page 13
The free -draining material should extend up the wail 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 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 recommend that we be retained to provide construction monitoring services to evaluate conditions
encountered in the field with respect to anticipated conditions, to provide recommendations for design
changes should the conditions differ from anticipated, and to evaluate whether construction activities
comply with contract plans and specifications.
USE OF THIS REPORT
NGA has prepared this report for Chander Sharma and associated agents, for use in the planning and
design of the development on these sites 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 we
be
retained to review the
project
plans after
they
have been
developed to determine that recommendations in the report were incorporated into project plans.
We recommend that NGA be retained to review final plans prior to 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 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
opinions are a means
to identify and reduce the inherent risks
to
the owner.
observations, findings, and
50 •
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sharma Residence Development
Edmonds, Washington
It has been a pleasure to provide service to you on this project.
further information, please call.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Sarah L. Dunn
Staff Geologist
0221
Khaled M. Shawish, PE
Principal
SLD:KMS:dy
Attachments: Four Figures
NGA File No. 1269821
August 27, 2021
Page 14
If you have any questions or require
NELSON GEOTECHNICAL ASSOCIATES, INC.
VICINITY MAP N
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Project Number Sharma Residence nELSOn GEOTECHnICAL No. Date Revision sy CK
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1269821 Development ASSOCIATES, InC 1 ennzl Original DPN Dao
Figure 1 Vicinity Map dwiheWAWon W �anift,Wg900
Site Plan
N89e48'58"E
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SITE BENCH.
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Number and approximate
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location of infiltration test pit
TP-1
Scale: 1 inch = 30 feet
Number and approximate
location of test pit
Reference: Site
Plan based on a plan dated February 5, 2021 titled "Lot
Survey for Chandler Sharma," prepared by Owyhee Consulting.
Project Number
nELSOn GEOTECHnICAL
"°'
Date
Revision
By
CK
Sharma Residence
`
1269821
Development
ASSOCIATES, inc
1
611721
Original
OPN
DJO
Figure 2
Site Plan
L'
Wn ENv!=ana W . ..M-
17311-139nN. NF,A W 105 PNarce St
xww.n0lwnpeNVF.wm
Ww41nmfl WP 900R WenffiFea, WA 90001
(425)4861809 / Fm:4e1-2510 (5091 �5-]fi�IFm:fiOS]092
UNIFIED SOIL 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. 4SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
Sw
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
SP
POORLY GRADED SAND
MORE THAN 50 %
RETAINED ON
MORE THAN 50 %
NO. 200 SIEVE
OF COARSE FRACTION
SAND
SM
SILTY SAND
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
IIELSOII GEOTECHnICOL
No.
Date
IRevision
ft
CK
Sharma Residence
r
J
8/1]pI
Original
OPN
OJO
1269821
Development
pp
ASSOCIATES, III4
Figure 3
Soil Classification Chart
� Am NE.A36G ,��.oKs.
WenffiM1ee, WA 95001
WaadmNle. WA B80]3
wwx.nNamB�ra��em (4]5)<661668/Fav: 0813510 15W)6P5-]695 /Faz: 66S]BB2
LOG OF EXPLORATION
DEPTH (FEET)
USCS
SOIL DESCRIPTION
INFILTRATION
TEST PIT ONE
0.0-2.0
DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, COBBLES,
CONCRETE RUBBLE, AND METAL DEBRIS (LOOSE TO MEDIUM DENSE, MOIST) F( ILL)
2.0-3.2
SM
ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES,
ROOTS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST)
3.2-4.5
SM
GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND IRON -OXIDE STAINING
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 4.5 FEET ON 08/04/2021
TEST PIT ONE
0.0-1.5
DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, AND IRON -
OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) F( ILL)
1.5 - 3.0
SM
ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES,
ROOTS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST)
3.0-6.0
SM
GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE IRON -OXIDE
STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 3.5 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT MET REFUSAL AT 6.0 FEET ON 0=4/2021
TEST PIT TWO
0.0- 1.7
DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, AND IRON -
OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) F( ILL)
1.7 - 3.0
SM
ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES,
ROOTS, ORGANICS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST)
3.0-5.0
SM
GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE IRON -OXIDE
STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 4.5 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 5.0 FEET ON 08/0412021
DPN:DJO NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1269821
FIGURE 4
INFILTRATION MEMORANDUM
DATE:
TO:
CC:
FROM
I:
NELSON GEOTECHNICAL
ASSOCIATES. INC.
MEMORANDUM
August 26, 2022
Shekhar Sharma — seattlens@gmail.com
17311-135ei Ave. N.E. Suite A-500
Woodinville, WA 98072
(425) 486-1669
www.nelsongeotech.com
Site Development Services — Dave Dougherty — davesds49@gmail.com
Khaled M. Shawish, PE
Thor Christensen, PE
Geotechnical Infiltration Plan Review Memorandum
Sharma Residence
8422 — 238`" Street SW
Edmonds, Washington
NGA File No. 1269821
INTRODUCTION
)22
This memo presents our geotechnical engineering review of the proposed infiltration system for the
proposed residence located at 8422 — 238' Street SW in Edmonds, Washington. We previously prepared
a geotechnical engineering evaluation for the project dated August 27, 2021.
We have been provided with a grading and drainage plan prepared by Site Development Services dated
April 28, 2022. We have been informed that City of Edmonds requires that a geotechnical engineer review
and approve the use of the proposed stormwater infiltration trenches and overflow.
The provided plan shows that roof runoff from the proposed residence will be directed to a catch basin a
short distance south of the residence. From the catch basin, stormwater will be directed to a series of
three infiltration trenches. The trenches are to have a width of 4 feet and a total length of 121 feet. The
infiltration trenches will be excavated about 2 feet below existing grade, and washed rock will be placed
over the bottom of the trenches. We understand that the 484 square feet of the infiltration trenches was
based on the 2014 DOE Stormwater Management Manual which lists 75 linear feet of 2-foot-wide trench
per 1,000 square feet of roof area for loamy sand. We understand that infiltration testing is not required
provided the infiltration system includes an overflow.
Geotechnical Infiltration System Plan Review Memorandum
8422 — 238t" Street SW
Edmonds, Washington
NGA File No. 1269821
August 26, 2022
Page 2
During heavy rain events, if water is directed to the infiltration system at a rate that exceeds the rate of
infiltration, the water will back up and could eventually flow out of the catch basin and onto the ground
surface. Because the terrain south of the residence declines gently toward the southwest, any water that
emerges from the catch basin would flow in that direction, away from the proposed residence.
Our previous test pit explorations encountered silty sand that was loose to medium dense to depths of 3
to 3.2 feet, and then became medium dense to dense. The silty sand can reasonably be considered loamy
sand. In our previous study we completed infiltration testing at a depth of 4.5 feet. That testing found
that the capacity of the site soil at that depth was very low. However, that finding was for soil at a depth
of 4.S feet and those soils are much denser and thus have much lower permeability than the loose to
medium dense at the proposed infiltration depth of 2 feet.
In our opinion, the proposed infiltration system, including the catch basin overflow, is suitable for the
proposed development.
We trust this memorandum should satisfy your needs at this time. Please contact us if you have any
questions or require additional services.
...
NELSON GEOTECHNICAL ASSOCIATES, INC.