20180803134628.pdfNELSON GEOTECHNICAL
AssocrlTEs, ING.
GEorEcHNtcAL Er*ctNrrns & Geouoatsrs
Engineering-Geology Branch
5526 Industry Lane,#2
East Wenatchee, WA 98802
(s09) 66s-7696 . FAX (509) 665-?ó92
Main Office
I 731 I - 135'h Ave NE, A-500
Woodinville, WA 98072
(425) 486.16ó9 . FAX (425) 48 r-25 r0
development be directed to flow into this system.
N G A
July 31, 2018
Mr. James Weis
50 Mill Plain Road
Danbury, CT 0681I
iweis@techair.com
Updated Geotechnical Plan Review Letter
Weis Residence I)evelopment
16630 - 74th Place West
Edmonds, Washington
NGA Job No. 9887818
Dear Mr. Weis:
This updated letter presents the results ofour recent geotechnical engineering review ofthe plans for the
Weis Residence project located at 16630 - 74ù Place West in Edmonds, Washington. We previously
prepared a geotechnical evaluation for the proìect dated April 24,2017. As a part of this evaluation, we
provided geotechnical recommendations for the proposed development.
For our use in preparing this updated plan review and comment response letter, we have been provided
with the following document:
¡ Plan Sheets Cl and C2 titled "Weis SFR," dated June 4, 2018.
PLAN REVIEW
We have reviewed the geotechnical aspects of the provided plans and found the plans to be in general
compliance with our recommendations as presented in our previous geotechnical report prepared for this
project. We were informed that previous plans for the residence included rockery retaining walls and
dispersing stormwater onto the steep slope below. All rockeries along the downslope side of the
residence and stormwater dispersion have been removed from the curent plans. Stormwater drainage
from the residence is to be tightlined through a 6-inch HDPE pipe down to the very lower western
portion of the property and into an existing stonnwater system. The HDPE pipe is to be collared and
anchored to the slope at 2O-foot intervals. In our opinion, the proposed stormwater HDPE pipe system is
feasible and should perform adequately. We recommend that all stormwater associated with RË'S.UB
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Updated Geotechnical Plan Review Letter
Weis Residence Developrnent
Edmonds, Washington
NGA File No. 9887818
July 31,2018
Page2
In addition to the recornmendations provided in our previous report, rve recommend that the lower
\ryestern and western l5 feet of the northern and southern residence foundations be embedded a minimum
of four feet into the native glacial soils encountered within the site, to provide additional protection
against any shallow failures on the steep slope below. All other foundations can utilize standard
foundation depths.
It appears that some minor gfade adjustments may take place within the sloping area below and to the
northwest of the proposed access driveway. We recommend that any fill placed within this area be
placed as structural fill as discussed in our previous report. Any fill placed on sloping ground should be
placed on graded level benches as discussed in our previous report and adequately compacted and
vegetated. The slope fill should have a maximum finished gradient of 2H:lV. No water should be
allowed to flow over the slope.
It also appears that a proposed retaining wall is to be constructed within the upper southeastem corner of
the property. This retaining wall may be as high as seven feet and situated in very close proximity to the
eastern and southern property lines. Due to the close proximity of the proposed wall to the property lines
and neighboring roadway, it may be difficult to maintain temporary excavation slope gradients as
described in our previous report to allow construction of the wall. If safe excavation slopes cannot be
maintained, soldier pile wall shoring systems may need to be incorporated into the overall site design.
We are available to provide recommendations for any proposed shoring systems should they be needed.
COMMENT RESPONSE
The site meets the criteria for landslide hazatd areas and erosion hazard areas, as outlined in the
Edmonds Municipal code (ECDC 23.80.020). Pursuant to ECDC 23.80.070.4, the planned project meets
the requirements of design standards for development within Erosion and Landslide Hazard Areas.
a The proposed deyelopment shall not decrease the faclor ol safety þr landslide occurrences
below the limits of 1.5 for static condifions and L2 for dynamic conditions.
The site slope was analyzed for stability along Cross Section A-A' through the planned area of
modification using the computer program Slope/lV, by Geo-Slope International (version 8.11.1.7283).
SlopeiW is a two-dimensional, limit equilibrium slope stability program that generates random potential
failure surfaces or specific failure surfaces and determines their corresponding factors of safety with
respect to failure. By generating a large number of random surfaces, a critical failure surface with the
minimum factor of safety can be identified,
,VEISON G EOT E CH N I C AL A SSOC'A TES,'ÍVC.
Updated Geotechnical Plan Review Letter
Weis Residence Development
Edmonds, Vy'ashington
NGA File No. 9887818
July 31,2018
Page 3
The slope stability analyses were performed using information from the original geotechnical report and
the addendum explorations. Soil properties were assigned to the core of the slope to reasonably reflect
their engineering characteristics. Based on limited explorations, mapped geology, and local experience,
we assigrred the subsurface soils to have a unit weight of 127 pounds per cubic foot, an effective
cohesion of300 pounds per square foot, and an internal friction angle of30o.
Stability analyses were performed within the area to be developed along the cross section, which does not
include additional stabilization associated with revegetation. Stability analyses were performed using the
Morgenstem-Price methodology for non-seismic and seismic conditions for the proposed development,
based on provided plans. Peak ground acceleration of 0.2g was used in seismic analyses.
The City of Edmonds requires design factors of safety of permanent slopes to be greater than 1.5 for
static conditions and 1.2 for dynamic conditions. Vy'e have analyzed slope stability of the proposed
development within the steep slope area utilizing the design parameters discussed in this letter. These
analyses indicated that the proposed development achieved appropriate minimum factors of safety for
both static and dynamic loading conditions for significant failures within affected areas in the vicinity of
the steep northeast-facing slope, with a minimum factor of safety of 1.2 in dynamic conditions and 2.0 in
static conditions.
. Slruclures and improvemenls shall be clustered to avoid geologically hazardous areas and other
critical areas
Proposed structures are located within the upper portion of the site adjacent to 74th Place West to avoid
disturbing slopes within the central portion of the property and affecting the wetland areas in the western
portion of the lot.
. Struclures and improvemenls shall minimize aheralions lo lhe nalural conlour of the slope, and
foundalions shall be riered u,here possible lo conform to existing topography
Foundations for the proposed residence will daylight onto the slope and step up with topography to
reduce disturbance of existing contours.
a Structures and improvements shall be locaÍed lo presente lhe mosl crilical portion of the sile and
ils nalural landforms and vegetation
The eastern portion of the site with the gentlest slopes will be occupied by the proposed development in
order to preserve the critical areas within the site and reduce impact on geological and other critical areas
in the vicinity.
,VEISON GEOTECHNICAT ASSO CIATES, I NC.
Updated Geotechnical Plan Review Letter
Weis Residence Developrnent
Edmonds, Washington
NGA File No. 9887818
July 31,2018
Page 4
The proposed developmenl shall nol resuh in greater risk or a need þr increased buffers on
n e i gh bori n g prop er ti es
Increased buffers unnecessary for adjacent property due to proposed development as part ofthis project.
The use of relaining walls lhal allow the mainlenance of existing naÍural slope area is preferred
over graded artificial slopes
Minor grading is shown in the plans to stabilize the planned access driveway to the property from 741h
Place West. In the southeastem portion of the property, a 3- to 7- foot tall retaining wall is proposed to
provide access to the garage. Permanent retaining walls are utilized to support a daylight basement.
o Development shall be designed to minimize impentious lot coverage
Stormwater drainage from the residence is to be tightlined through a 6-inch HDPE pipe down to the very
lower western portion of the property and into an existing stormwater system. All new impervious
surfaces are to be connected to this system to prevent additional dispersion within the landslide and
erosion hazard areas.
MINIMUM RISK STATEMENT
It is our opinion that construction will not increase the threat of erosion or landsliding to adjacent
properties beyond predevelopment conditions, will not adversely impact other critical areas, is designed
so that the hazard to the proìect is eliminated or mitigated to a level equal or less than predevelopment
conditions, and is safe as designed under anticipated conditions.
CLOSURE
We recommend that NGA be retained to provide monitoring and consultation services during
construction to confirm that the conditions encountered are consistent with those indicated by the
explorations, to provide recommendations for design changes should the conditions revealed during the
work differ from those anticipated, and to evaluate whether or not earthwork activities comply with
contract plans and specifìcations.
o-o-o
o
o
NEISON GEOTECH NICA¿ ASSO CIATES, INC.
Updated Geotechnical Plan Review Letter
Weis Residence Development
Edmonds, Washington
NGA File No. 9887818
July 31,2018
Page 5
We appreciate the opportunity to provide service to you on this proìect. Please aontact us if you have
any questions regarding this letter or require further information.
Sincerely,
wETSOw GEOTECH Nt CAL ASSOCTA rES, rwC.
C'^eM
Carston T. Curd, GIT
Staff Geologist I
Khaled M. Shawish, PE
Principal
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9887818 Updated Plan Review
Weis Residence Development
Dynamic Slope Stability Analysis
NELSoN GeorecaN¡cel-
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