Geotechnical Engineering Study (2).pdfGEOTECHNICAL ENGINEERING STUDY
PROPOSED MULTI -FAMILY RESIDENCES Of 546 PARADISE LANE �I'FE
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EDMONDS, WASHINGTON
ES-5839
INTRODUCTION
General
This geotechnical engineering study was prepared for the three proposed multi -family residential
structures to be constructed at 546 Paradise Lane in Edmonds, Washington. To complete the
scope of services detailed in our proposal PES-5839 dated January 17, 2018 we performed the
following:
• Subsurface exploration and characterization of soil and groundwater conditions by way of
test pits excavated at accessible areas of the site;
• Laboratory testing of soil samples obtained during subsurface exploration;
• An infiltration evaluation based on observed soil conditions and the results of a Small-
scale Pilot Infiltration Test (PIT);
• Geotechnical engineering analyses, and;
• Preparation of this report.
The following documents and resources were reviewed as part of our report preparation:
• Preliminary Drainage and Plot Plan, prepared by Michel Construction, Inc., dated July 3,
2008;
• Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangle,
Washington, prepared by James P. Minard, dated 1983;
• Department of Ecology Stormwater Management Manual for Western Washington,
Volume III Hydrologic Analysis and Flow Control BMPs, dated August 2012;
• Edmonds City Code, Chapter 23.80 (Geologically Hazardous Areas), and;
• Online Web Soil Survey (WSS) resource provided by United States Department of
Agriculture (USDA), Natural Resources Conservation Services.
Proiect Description
Based on the site plan provided to us, the existing single-family residential structure and garage
will be demolished and three new multi -family residential structures will be constructed. We
anticipate grading activities will primarily include cuts to establish the planned building alignments
and roadway improvements. Based on the existing grades, we estimate cuts to establish building
pad and foundation subgrade elevations will be on the order of up to four feet. Site improvements
will also include underground utility installations.
Earth Solutions Nw, LLC
Paradise Heights, LLC ES-5839
March 5, 2018 Page 2
At the time this report was prepared, specific building load values were not available. However,
we anticipate the proposed residential structures will consist of relatively lightly loaded wood
framing supported on conventional foundations. Based on our experience with similar
developments, we estimate wall loads on the order of one to two kips per linear foot and slab -on -
grade loading of 150 pounds per square foot (psf).
If the above design assumptions are incorrect or change, ESNW should be contacted to review
the recommendations in this report. ESNW should review the final design to verify the
geotechnical recommendations provided in this report have been incorporated into the plans.
SITE CONDITIONS
Surface
The subject site is located at 546 Paradise Lane in Edmonds, Washington, as illustrated on the
Vicinity Map (Plate 1). The site consists of one residential tax parcel (Snohomish County parcel
number 27032500302600) totaling approximately 0.91 acres of land area. The property is
currently developed with a single-family residence and associated improvements. The majority
of the site is relatively level with a steep slope descending on the north side of the property
adjacent to Paradise Lane and gentle slope descending to the east for the driveway adjacent to
Paradise Lane. Based on site observation and available topographic data, the steep slope
descending to the north has a slope gradient in excess of 40 percent and elevation change of up
to about 12 feet; the steep slope extends beyond the property. Vegetation within the steep slope
area consists of large trees and ivy groundcover. The subject site is bordered to the west by an
easement for Edmonds Way, to the east and north by Paradise Lane, and to the south by a
residential property. The Test Pit Location Plan (Plate 2) illustrates the approximate limits of the
property.
Subsurface
As part of the subsurface exploration, four test pits were excavated in accessible portions of the
site for purposes of assessing soil and groundwater conditions. The test pits were advanced to
a maximum depth of approximately nine feet below existing grade. Please refer to the test pit
logs provided in Appendix A for a more detailed description of the subsurface conditions.
Soil conditions observed at test pit locations consisted of medium dense poorly graded sand
(Unified Soil Classification System: SP) outwash deposits. Overall soil relative density increased
with depth. Approximately nine feet below ground surface (bgs), dense to very dense silty sand
with gravel glacial till was observed (USCS: SM). The glacial till was observed to be weakly
cemented and exhibited iron oxide staining.
Geologic Setting
According to the referenced geologic map, the subject site is underlain by advance outwash (Qva)
deposits. Soil conditions observed at the test pit locations were generally consistent with outwash
deposits.
According to the referenced NRCS soil survey, the subject site consists of Everett series soils.
Everett series soils are classified as outwash deposits and are consistent with observations made
in the field.
Earth Solutions NK LLC
Paradise Heights, LLC
March 5, 2018
Groundwater
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No groundwater seepage was observed during our fieldwork on January 7, 2018. Groundwater
seepage rates and elevations fluctuate depending on many factors, including precipitation
duration and intensity, the time of year, and soil conditions. In general, groundwater elevations
and flow rates are higher during the winter, spring and early summer months.
Geological Hazards Assessment
As part of this geotechnical engineering study, the referenced chapter of the Edmonds City Code
(ECC) was reviewed. Per the ECC requirements, the following topics related to development
plans and site conditions are addressed.
Slope Reconnaissance
During our fieldwork, we performed a visual slope reconnaissance of the steep slope area. The
main focus of our reconnaissance was to identify signs of instability or erosion hazards along the
site slopes. The typical instability indicators include features such as head scarps, tension
cracks, hummocky terrain, groundwater seeps along the surface and erosion features such as
gulleys and rills. During the slope reconnaissance, no signs of erosion or slope instability were
observed. The slope is vegetated with large trees and ivy groundcover. In general, based on
the slope reconnaissance, stability of the sloped areas within and adjacent to the property can
be characterized as good.
Erosion Hazard Areas — ECC 23.80.020.A.
With respect to erosion hazard areas, section 23,80.020 of the ECC defines erosion hazards as
"at least those areas identified by the U.S. Department of Agriculture's Natural Resources
Conservation Service as having a "moderate to severe", "severe", or "very severe" rill and inter -
rill erosion hazard. Erosion hazard areas are also those areas impacted by shoreland and/or
stream bank erosion. Within the city of Edmonds, erosion hazard areas include:
Those areas of the city of Edmonds containing soils that may experience severe to
very severe erosion hazard. This group of soils includes, but is not limited to, the
following when they occur on slopes of 15 percent or greater:
a. Alderwood soils (15 to 25 percent slopes);
b. Alderwood/Everett series (25 to 70 percent slopes), and;
c. Everett series (15 to 25 percent slopes).
2. Coastal and stream erosion areas which are subject to the impacts from lateral erosion
related to moving water such as stream channel migration and shoreline retreat;
3. Any area with slopes of 15 percent or greater and impermeable soils interbedded with
granular soils and springs or ground water seepage, and;
4. Areas with significant visible evidence of ground water seepage, and which also
include existing landslide deposits regardless of slope.
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Paradise Heights, LLC
March 5, 2018
ES-5839
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The on -site soils are generally consistent with Everett series soils. Based on the ECC definition,
the steep slope areas within and adjacent to the property classify as erosion hazard areas.
Construction of the proposed engineered retaining wall is expected to be sufficient for adequately
managing and mitigating the erosion potential for this project.
Landslide Hazard Areas — ECC 23.80.020.8.
With respect to landslide hazard areas, section 23.80.020 of the ECC defines landslide hazard
areas as "areas potentially subject to landslides based on a combination of geologic, topographic,
and hydrologic factors. They include areas susceptible because of any combination of soil, slope
(gradient), slope aspect, structure, hydrology, or other factors. Within the city of Edrnonds,
landslide hazard areas specifically include:
1. Areas of ancient or historic failures in Edmonds which include all areas within the earth
subsidence and landslide hazard area as identified in the 1979 report of Robert Lowe
Associates and amended by the 1985 report of GeoEngineers, Inc., and further
discussed in the 2007 report by Landau Associates;
2. Coastal areas mapped as class U (unstable), UOS (unstable old slides) and URS
(unstable recent slides) in the Department of Ecology Washington coastal atlas;
3. Areas designated as quaternary slumps, earthflows, mudflows, or landslides on maps
published by the United States Geological Survey or Washington State Department of
Natural Resources;
4. Any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a 25-
foot horizontal run. Except for rockeries that have been engineered and approved by
the engineer as having been built according to the engineered design, all other
modified slopes (including slopes where there are breaks in slopes) meeting overall
average steepness and height criteria should be considered potential landslide hazard
areas;
5. Any slope with all three of the following characteristics:
a. Slopes steeper than 15 percent;
b. Hillsides intersecting geologic contacts with relatively permeable sediment
overlying a relatively impermeable sediment, and;
c. Springs or ground water seepage.
6. Any area potentially unstable as a result of rapid stream incision or stream bank
erosion;
7. Any area located on an alluvial fan, presently subject to, or potentially subject to,
inundation by debris flow or deposition of stream -transported sediments, and;
8. Any slopes that have been modified by past development activity that still meet the
slope criteria.
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Paradise Heights, LLC ES-5839
March 5, 2018 Page 5
Based on site observation and the referenced topographic survey, a north -descending steep
slope with gradient in excess of 40 percent is located to north of the subject property; based on
site observations, the steep slope area was likely over steepened by historic excavations made
to construct Paradise Lane. Per the above definition of landslide hazard areas, the steep slope
to the north of the subject property classifies as a potential landslide hazard area based on a
slope gradient of 40 percent or steeper with a vertical relief of 10 feet or more. The overall stability
of the site can be characterized as good and it is our opinion that the potential landslide hazard
for the slope to the north of the site can be considered very low. Additionally, we understand
grading for the proposed development will include construction of engineered retaining walls that
will effectively eliminate the steep slope hazard.
Mapping of Geologically Hazardous Areas — ECC 23.80.030
Review of available geologically hazardous areas mapping did not reveal geologically hazardous
areas within or immediately adjacent to the subject site.
Special Study and Report Requirements — ECC 23.80.050
A. This geotechnical engineering study and geological hazards assessment was completed
by a professional engineer licensed in the state of Washington with experience analyzing
geologic hazards throughout the Puget Sound region.
B. 1. The project area includes the subject site as delineated in the referenced preliminary
plans.
2. No further geologically hazardous areas are located within 200 feet of the property or
will be affected by construction on the property.
C. This geological hazards assessment included a field investigation and an assessment of
geologic hazards. This geotechnical report has been prepared, stamped, and signed by
a qualified professional.
1. It is our opinion the level of analysis completed for this geological hazards assessment
is appropriate for the scale and scope of the project and scale of the geological hazard
areas present.
2. A discussion of all geologically hazardous areas on the site and any geologically
hazardous areas off site potentially impacted by the proposed project is provided in
this report.
3, Based on the results of our study and on -site observations, the proposed project will
not decrease slope stability or pose an unreasonable threat to persons or property
either on or off site. These conclusions are based on the current conditions of the
slope in question and proposed replacement of the slope with an engineered retaining
wall.
4. This geological hazard assessment is provided as adequate information to comply with
requirements of ECC geological hazards.
5. This geotechnical report generally follows the guidelines set forth in the Washington
State Department of Licensing Guidelines for Preparing Engineering Geology Reports
in Washington (2006).
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Paradise Heights, LLC
March 5, 2018
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6. It is our opinion a landslide hazard minimum building setback is not necessary, and
erosion hazard mitigation recommendations are provided in this report.
D. We are not aware of a previous study completed for the subject site.
E. The mitigation recommendations include permanent solutions, such as construction of
engineered retaining walls within the slope area in question and erosion control
recommendations.
F. This geological hazards assessment and geotechnical engineering study should be
reviewed as part of the overall submittal package.
1. Please refer to preliminary plans prepared by Michel Construction, Inc. for the site plan.
a. The height of the slopes and slope gradients are discussed in this report and
displayed within the preliminary plans.
b. Springs, seeps, or other surface expressions of groundwater were not observed on
or within 200 feet of the project area.
c. No surface water runoff features were observed during our site visits.
2. a. Vegetative cover across the landslide hazard areas generally consists of non-native
groundcover (i.e. ivy), shrubs, and large trees.
b. Subsurface conditions are described in the Subsurface section of this report.
c. Surface and groundwater conditions are discussed in previous sections of this
report. Roughly two and one-half feet of fill was observed in TP-3. Based on current
conditions and elevation changes present, it is evident that some degree of land
modification has occurred within and adjacent to the potential landslide hazard
areas. The potential landslide hazard areas were likely artificially steepened by
excavations completed during construction of Paradise Lane. Local natural
topography was likely more of a gradual consistent slope.
d. The slopes within and adjacent to the subject site generally exhibit good overall
stability.
e. The slopes within and adjacent to the subject site are not characterized as bluffs
and we do not anticipate a retreat of the slopes to occur.
f. Based on the sand soils present on the site and no indications of groundwater, we
would anticipate the run -out hazard of landslide debris to be confined to a distance
of up to half the height of slope away from the toe. If such a landslide occurred,
impacts of run -out on downslope properties or right-of-way would likely consist of a
few feet of landslide debris covering the ground adjacent to the toe of the slope.
Repair of the slide would likely require cleanup of debris and engineered
reconstruction of the slope and/or construction of an engineered retaining wall at
the toe of the slope; construction of the proposed engineered retaining wall will
effectively mitigate this potential. It should be noted, based on the results of our
study, likelihood of such a landslide event occurring adjacent to the subject site is
very low and the proposed project will not increase the likelihood.
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Paradise Heights, LLC
March 5, 2018
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g. We believe this report meets the criteria of a critical areas report to a degree
adequate with respect to the severity of the steep slope hazard.
h. Recommendations for building siting limitations are provided in this report.
Proposed surface and subsurface drainage includes collecting surface water from
impermeable surfaces and directing them to infiltration systems located along the
eastern side of the site. The proposed drainage design is engineered and located
in a manner which utilizes the native sand soils and minimizes impacts to sloped
areas. The site soils are susceptible to erosion if exposed to surface water runoff
without the implementation of BMP measures. However, typical temporary and
permanent BMP measures will effectively mitigate the erosion potential.
3. This geotechnical engineering study was prepared by a licensed engineer.
a. Geotechnical design parameters are provided within this report.
b. Drainage and subdrainage recommendations are provided within this report.
c. Earthwork recommendations are provided within this report.
d. Recommendations for mitigation of adverse site conditions are provided within
this report, as necessary.
G. It is our opinion the site erosion hazard areas should be considered stable.
H. Based on the results of our study, the site does not contain any seismic hazard areas.
Development Standards (General Requirements) — ECC 23.80.060
Based on the results of our geological hazards assessment, the proposed project will not increase
the threat of the geological hazard to adjacent properties beyond predevelopment conditions, the
proposed project will not adversely impact other critical areas, the proposed project is designed
so that the hazard to the project is eliminated or mitigated to a level equal to or less than
predevelopment conditions, and the project is certified as safe as designed under anticipated
conditions.
Development Standards (Specific Hazards) — ECC 23.80.070
A. 1. We believe that with the construction of the engineered retaining wall adjacent to
Paradise Lane at the toe of the slope hazard area, any structure placed atop the wall
will not be at risk for landslides for the design life of the structure.
2. Based on the results of our study, it is our opinion that a buffer is not necessary for the
landslide hazard areas. The proposed project will include reducing the overall height
of the landslide hazard area as well as construction of engineered retaining walls in
the slope area.
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Paradise Heights, LLC
March 5, 2018
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3. The proposed project will include construction of engineered retaining walls within the
slope hazard area, effectively removing any permanent hazard found on site. The
alteration will include establishing permanent erosion control and drainage measures
at the top of the landslide and erosion hazard areas. Provided the recommendations
in this report and subsequent geotechnical recommendations are incorporated into
the design and construction of the project, the proposed alteration will not increase
surface water discharge or sedimentation to adjacent properties beyond
predevelopment conditions, will not decrease slope stability on adjacent properties,
and such alterations will not adversely impact other critical areas.
4. a. As part of this geological hazards assessment, we assessed the current slope
conditions and consider them to be stable in their current condition and
configuration. We do not feel it is necessary to perform full slope stability analysis
within the landslide hazard, as construction of engineered retaining walls will
mitigate the hazard area prior to foundation loading and construction; however, an
ESNW representative should be on -site during wall construction to observe
changes to the steep slope area.
b. It is our opinion that the structures and improvements have been located and
engineered in a manner which sufficiently mitigates impacts to geologically
hazardous areas; ESNW should review any plan changes.
c. Historic development of Paradise Lane likely altered the natural contour of the
slopes. The proposed project will construct an engineered retaining wall within the
slope area, effectively eliminating any slope hazard.
d. The structures and improvements have been located and engineered in a manner
that effectively eliminates the most critical portions of the site by proposing
engineered retaining walls to replace the artificially steepened slopes.
e. The proposed development will not result in greater risk or a need for increased
buffers on neighboring properties.
f. Based on our study, building setback are not necessary.
5. We did not observe vegetation within the subject site which should be retained to
maintain site and slope stability.
6. It is our opinion that this site should not be restricted to seasonal clearing and grading
work. This opinion is based on the relatively small scope of the project and
predominantly sandy soils, which are not as moisture sensitive as glacial till, of the
geological hazard areas.
7. We are not aware of proposed point discharges.
Earth Solutions NW, LLC
Paradise Heights, LLC
March 5, 2018
Analysis of Proposal
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The proposed development will involve demolition of the existing single-family residential
structure and construction of three new multi -family residential structures. Based on the
referenced site plans, the grading will involve minor cuts to establish level building pad areas; we
understand that an engineered retaining wall will be constructed at the base of the slope hazard
area adjacent to Paradise Lane at the north edge of the site. The proposed project will eliminate
the slope hazard area without creating any new geological hazards adjacent to the property.
These determinations are based on the relatively small extent of the previously modified steeply
sloped areas, stable nature of the site soils, and the construction of an engineered wall to retain
the site grades.
DISCUSSION AND RECOMMENDATIONS
General
Based on the results of our study, construction of the proposed residential structures at the
subject site is feasible from a geotechnical standpoint. The primary geotechnical considerations
associated with the proposed development include foundation support, temporary excavations,
retaining walls, infiltration and drainage, and the suitability of the on -site soils for use as structural
fill.
The proposed residential structures can be supported on a conventional foundation system
bearing on competent native soil, recompacted native soil, or structural fill. Competent soils
suitable for support of foundations should be encountered beginning at a depth of roughly two
feet below existing grades across the majority of the site. Slab -on -grade floors should be
supported on dense native soil, re -compacted native soil, or structural fill. Where loose, organic
or other unsuitable materials are encountered at or below the footing subgrade elevation, the
material should be removed and replaced with structural fill, as necessary.
This study has been prepared for the exclusive use of Paradise Heights, LLC and their
representatives. No warranty, expressed or implied, is made. This study has been prepared in
a manner consistent with the level of care and skill ordinarily exercised by other members of the
profession currently practicing under similar conditions in this area.
Site Preparation and Earthwork
Based on the referenced site plans and given the existing topography, we anticipate grading for
the project will involve cuts of up to about four feet to establish building pad and foundation
subgrade alignments. Silt fencing and temporary erosion control measures should be placed
along the perimeter of the site prior to beginning grading activities.
Earth Solutions NW, LLC
Paradise Heights, LLC
March 5, 2018
Temporary Erosion Control
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Temporary construction entrances, consisting of at least six inches of quarry spalls, can be
considered in order to minimize off -site soil tracking and to provide a temporary road surface. Silt
fences should be placed along the margins of the property. Interceptor swales and a temporary
sediment pond may be necessary for control of surface water during construction. Erosion control
measures should conform to the Washington State Department of Ecology (DOE) and City of
Edmonds standards.
In -Situ Soils
From a geotechnical standpoint, the soils encountered at the test pit locations are generally
suitable for use as structural fill. However, successful use of the on -site soils will largely be
dictated by the moisture content of the soils at the time of placement and compaction. The site
soils were generally in a damp condition at the time of the exploration on January 7, 2018.
Based on the conditions encountered during our fieldwork, the site soils generally have a low
sensitivity to moisture. During periods of dry weather, the on -site soils should generally be
suitable for use as structural fill, provided the moisture content is at or near the optimum level at
the time of placement. If the on -site soils cannot be successfully compacted, the use of an
imported soil may be necessary. Imported soil intended for use as structural fill should consist of
a well -graded granular soil with a moisture content that is at or near the optimum level. During
wet weather conditions, imported soil intended for use as structural fill should consist of a well -
graded granular soil with a fines content of 5 percent or less defined as the percent passing the
Number 200 sieve, based on the minus three-quarter inch fraction.
Structural Fill
Structural fill is defined as compacted soil placed in foundation, slab -on -grade, and roadway
areas. Fills placed to construct permanent slopes and throughout retaining wall and utility trench
backfill areas are also considered structural fill. Soils placed in structural areas should be placed
in loose lifts of 12 inches or less and compacted to a relative compaction of 95 percent, based
on the laboratory maximum dry density as determined by the Modified Proctor Method (ASTM D-
1557). For soil placed in utility trenches underlying structural areas, compaction requirements
are dictated by the local city, county, or utility district, and in general are specified as 95 percent
relative compaction.
Earth Solutions NW, LLC
H aradise Heights, LLC
March 5, 2018
Excavations and Slopes
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The native soils encountered in the upper approximately nine feet of the test pit locations consist
of poorly graded sands in a dense condition. Temporary slopes within this layer should maintain
a gradient of no steeper than 1 H:1 V. If groundwater is present within acute the minimum gradient
of a slope should be increased to 1.5HA V. The presence of perched groundwater may cause
caving of the temporary slopes due to hydrostatic pressure. ESNW should observe site
excavations to confirm the soil type and allowable slope inclination are appropriate for the soil
exposed by the excavation. If the recommended temporary slope inclination cannot be achieved,
temporary shoring may be necessary to support excavations.
Permanent slopes should maintain a gradient of 2H:1V, or flatter, and should be planted with
vegetation to enhance stability and to minimize erosion. A representative of ESNW should
observe temporary and permanent slopes to confirm the slope inclinations are suitable for the
exposed soil conditions, and to provide additional excavation and slope recommendations, as
necessary.
Foundations
The proposed residential structures can be supported on a conventional foundation system
bearing on competent native soil, recompacted native soil, or structural fill. Competent soils
suitable for support of foundations should be encountered beginning at a depth of roughly two
feet below existing grades across the majority of the site. Where loose, organic or other
unsuitable materials are encountered at or below the footing subgrade elevation, the material
should be removed and replaced with structural fill, as necessary.
Provided the structures will be supported as described above, the following parameters can be
used for design of the new foundations:
a Allowable soil bearing capacity 2,500 psf
Passive earth pressure 300 pcf (equivalent fluid)
• Coefficient of friction 0.40
A one-third increase in the allowable soil bearing capacity can be assumed for short-term wind
and seismic loading conditions.
With structural loading as expected, total settlement in the range of one inch is anticipated, with
differential settlement of about one-half inch. The majority of the settlements should occur during
construction, as dead loads are applied.
Earth Solutions NW, LLC
Paradise Heights, LLC
March 5, 2018
Seismic Considerations
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The 2015 IBC recognizes ASCE for seismic site class definitions. If the project will be permitted
under the 2015 IBC, in accordance with Table 20.3-1 of ASCE, Minimum Design Loads for
Buildings and Other Structures, Site Class D, should be used for design.
In our opinion, the site is not susceptible to liquefaction. The soil relative density and the absence
of an established shallow groundwater table are the primary bases for this opinion.
Slab -On -Grade Floors
Slab -on -grade floors should be supported on a firm and unyielding subgrade consisting of
competent native soil or at least 12 inches of structural fill. Unstable or yielding areas of the
subgrade should be recompacted or overexcavated and replaced with suitable structural fill prior
to construction of the slab. A capillary break consisting of a minimum of four inches of free -
draining crushed rock or gravel should be placed below the slab. The free -draining material
should have a fines content of 5 percent or less defined as the percent passing the Number 200
sieve, based on the minus three-quarters inch fraction. In areas where slab moisture is
undesirable, installation of a vapor barrier below the slab should be considered. If used, the
vapor barrier should consist of a material specifically designed to function as a vapor barrier and
should be installed in accordance with the manufacturer's specifications.
Retaining Walls
Engineered retaining walls are proposed to eliminate the slope within the slope hazard area. The
following parameters can be used for retaining wall design:
• Active earth pressure (yielding condition)
• At -rest earth pressure (restrained condition)
• Traffic surcharge (passenger vehicles)
Passive earth pressure
• Coefficient of friction
o Seismic surcharge
*Where H equals retained height
35 pcf
55 pcf
70 psf (rectangular distribution)
300 pcf
0.40
6H*
Where sloping or other surcharge conditions will be present, supplement recommendations and
design earth pressure values should be provided by ESNW. Drainage should be provided behind
retaining walls such that hydrostatic pressures do not develop. If drainage is not provided,
hydrostatic pressures should be included in the wall design.
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Paradise Heights, LLC ES-5839
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Retaining walls should be backfilled with free -draining material that extends along the height of
the wall, and a distance of at least 18 inches behind the wall. The upper one foot of the wall
backfill can consist of a less permeable soil, if desired. A perforated drain pipe should be placed
along the base of the wall, and should be connected to an approved discharge location. A typical
retaining wall drainage detail is provided as Plate 3.
r%
urainagC
Groundwater seepage was not observed during our fieldwork on January 7, 2018. Groundwater
seepage is possible in site excavations, particularly in the winter, spring and early summer
months. Temporary measures to control groundwater seepage and surface water runoff during
construction will likely involve passive elements such as interceptor trenches and sumps, as
necessary. Surface water should not be allowed to runoff over sloped areas and should not be
allowed to pond near the top of sloped areas or retaining structures.
Surface grades must be designed to direct water away from buildings. The grade adjacent to
buildings should be sloped away from the buildings at a gradient of at least 2 percent for a
horizontal distance of four feet or more as setbacks allow. In our opinion, perimeter footing drains
should be installed at or below the invert of the building footings. A typical footing drain detail is
provided on Plate 4 of this report.
Infiltration Evaluation
We understand drywells, trenches or other methods will be utilized for on -site infiltration. For
design, the long-term infiltration rate was evaluated using a Small-scale Pilot Infiltration Test (PIT)
completed at a depth of three feet in TP-3. Based on consistent nature of the upper outwash
soils, it is our opinion one PIT is sufficient; once infiltration depths and locations are determined,
we should reevaluate the need for additional testing. Table 3.3.1 of the referenced DOE Manual
provides criteria for estimating the long-term infiltration rate based on measured rates and
applicable correction factors. The following long term infiltration rate was calculated using the
following equation and correction factors:
K design rate = K measured rate X CFT
• Measured infiltration rate Kmeasured = 120 inlhr
• Site variability; number of locations tested CFI = 0.33
Test method CFt = 0.50
• Degree of influent control to prevent siltation Urn = 0.90
• Total correction factor CFT = 0.15
Calculated Long Term Infiltration Rate* TP-3 = 18 inlhr
*Per Stormwater Management Manua! for Western Washington, August, 2012. Chapter 3.3.4
Earth Solutions NK LLC
Paradise Heights, LLC
March 5, 2018
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We recommend the proposed infiltration systems be located with a setback of at least 25 feet
from the top of slope hazard area (and proposed retaining wall upon completion) to the north of
the subject site. The geotechnical engineer should observe the excavations for the proposed
infiltration systems to confirm soil conditions at the time of construction. We recommend an
emergency overflow provision be included in the infiltration system design. If an overflow is not
incorporated, we recommend reducing the design infiltration rate by half.
Low Impact Development
The following table provides our evaluation and recommendations regarding low impact
development BMPs for the proposed project:
BMP
Viable?
Lawns and Landscaped Areas
T5.13: Post -construction soil quality Yes
and de&_(Volume V_,_Chapter 5) 1
Roofs
T5.30: Full dispersion (Volume V,
Chapter 5)
T5.10A: Downspout full infiltration
systems (Volume III, Chapter 3)
Bioretention (Volume V, Chapter 7)
T5.10R Downspout dispersion
systems (Volume III, Chapter 3)
f T5.10C: Perforated stub -out
l connections (Volume III, Chapter 3)
Other Hard Surfaces
i T5.30: Full dispersion (Volume V,
Chapter 5)
T5.15: Permeable pavement (Volume
V, Chapter 5)
Yes*
Yes
Yes
Yes*
Yes
Yes*
Yes
Bioretention (Volume V, Chapter 7) 1 Yes
Limitations or
Infeasibility Criteria
Considered infeasible on slopes of 33 percent or
greater
I
T5.30: Dispersion is not recommended near the
slope hazard area/proposed retaining wall of the
site.
T5.10A: A design infiltration of 18 inches per
hour should be used for preliminary design.
Must be setback at least 25 feet from proposed
retaining wall.
A design infiltration of 18 inches per hour should
be used for design. Must be setback at least 25
feet from proposed retaining wall.
Dispersion is not recommended near the slope
hazard are retaining wall of the site.
No limitations.
Dispersion is not recommended near the slope
hazard area/proposed retaining wall of the site.
Near -surface soils generally have a higher fines
content and a design infiltration rate of 4.5 inch
per hour should be used for design of permeable
pavement. Must be setback at least 25 feet from
proposed retaining wall.
A design infiltration of 18 inches per hour should
be used for design. Must be setback at least 25
feet from proposed retaining wall.
T5.12: Sheet flow dispersion Dispersion is not recommended near the slope
T5.11: Concentrated flow dispersion Yes*
(Volume V, Chapter 5) hazard area/proposed retaining wall of the site.
*lf dispersion is utilized all downspout areas should maintain at least a 25-foot buffer from the proposed
retaining wall area. Dispersion is recommended to lead south or east boundaries of the site property
where site topography is relatively level.
Earth Solutions NW, LLC
Paradise Heights, LLC
March 5, 2018
Utilitv Trench Suuaort and Backfill
ES-5839
Page 15
In our opinion, the soils observed at the test pit locations are generally suitable for support of
utilities. In general, the soils observed at the test pit locations may not be suitable for use as
structural backfill in the utility trench excavations, unless the soil is at or near the optimum
moisture content at the time of placement and compaction. Moisture conditioning of the soils
may be necessary at some locations prior to use as structural fill. Utility trench backfill should be
placed and compacted to the specifications of structural fill provided in this report, or to the
applicable requirements of the City of Edmonds.
Pavement Sections
The performance of site pavements is largely related to the condition of the underlying subgrade.
To ensure adequate pavement performance, the subgrade should be in a firm and unyielding
condition when subjected to proofrolling with a loaded dump truck. Structural fill in pavement
areas should be compacted to the specifications detailed in the Site Preparation and Earthwork
section of this report. It is possible that soft, wet, or otherwise unsuitable subgrade areas may
still exist after base grading activities. Areas of unsuitable or yielding subgrade conditions may
require remedial measures such as overexcavation and replacement with structural fill or thicker
crushed rock sections prior to pavement.
For relatively lightly loaded pavements subjected to automobiles and occasional truck traffic, the
following sections can be considered for preliminary design:
• Two inches of hot mix asphalt (HMA) placed over four inches of crushed rock base (CRB),
or;
• Two inches of HMA placed over three inches of asphalt treated base (ATB).
The HMA, CRB and ATB materials should conform to WSDOT specifications.
LIMITATIONS
The recommendations and conclusions provided in this geotechnical engineering study are
professional opinions consistent with the level of care and skill that is typical of other members in
the profession currently practicing under similar conditions in this area. A warranty is not
expressed or implied. Variations in the soil and groundwater conditions observed at the test
locations may exist, and may not become evident until construction. ESNW should reevaluate
the conclusions in this geotechnical engineering study if variations are encountered.
Additional Services
ESNW should have an opportunity to review the final design with respect to the geotechnical
recommendations provided in this report. ESNW should also be retained to provide testing and
consultation services during construction.
Earth Solutions NW, LLC