18109-18111 Lots 1 and 2 - 2nd Peer Review Letter.pdf"LM,g HWAGEOSCIENCES INC.
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Geolechnical & Pai,ement Engineering - HydrogeoloD, - Geoenvironniental Services - Inspection & Testing
March 19, 2018
HWA Project No. 2017-007-21 Task 3 00
City of Edmonds
Public Works Engineering Division
City Hall — 2 rid Floor
1215 1h Avenue N
Edmonds, Washington 98020
Attention: Jeanie McConnell
Engineering Program Manager
Subject: ExCAVATION AND SHORING PLAN
Peer Review — Proposed Residences — Lots I and 2
18109 & 18111 841h Avenue W
Edmonds, Washington
As requested, HWA GeoSciences Inc. (HWA) has completed a review of the applicant's
responses to our peer review letter dated December 29, 2017. Responses were provided by
Robert Pride in his letter dated January 22, 2018 and Joseph Lucia in his letter dated
January 30, 2017 regarding the two proposed residences on Lots I and 2 at 18109 and 18111
84 th Avenue West in Edmonds, Washington. The subject of this review concerns the proposed
temporary unsupported slope cuts in the underlying dense to very dense glacial advance outwash
soils. This letter provides our comments based on review of the applicant's responses.
DOCUMENT REVIEW
The following documents were referenced in our evaluation of the applicant's responses to our
peer review comments:
• Excavation and Shoring Plans — Lot #1, dated November 2, 2017, prepared by Lucia
Engineering Inc., for the New Residence, 18109 84th Avenue W, Edmonds, WA 98026.
• Excavation and Shoring Plans — Lot #2, dated November 2, 2017, prepared by Lucia
Engineering Inc., for the New Residence, 18111 84th Avenue W, Edmonds, WA 98026.
• Geotechnical Recommendations, Proposed Residences — Lots I and 2, 18109 & 18111 —
84'h Avenue West, Edmonds, Washington 98026, dated November 10, 2017 and prepared
by Robert M. Pride, LLC.
• Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report,
Edmonds Three -Lot Subdivision, 18111 84)� Avenue West, Edmonds,
Washington, dated July 30, 2008, prepared by Associated Earth 21312 301 Drive SE
Sciences, Inc. (AESI) for Mr. Steve Krassner. Suite I 10
Bothell, WA 98021-70 10
Fax: 425.774.2714
www.hwageo.com
Federal & State Certified DBE / N"VBE
March 19,2018
HWA Project No. 2017-007-21 Task 3 00
• Geotechnical Report Update, Single -Family Residence — Lot, 18109 84'hAvenue W,
Edmonds, Washington (Edmonds PermitNo. BLD2017-0717), dated September 25, 2017,
prepared by Associated Earth Sciences, Inc. (AESI) for Mr. Dino Hass.
• Geotechnical Plan Review, Proposed Residences — Lots I and 2, 181091111 84'h Avenue
West, Edmonds, Washington, dated November 23, 2017 and prepared by Robert M. Pride,
LLC. I
PROJECT SUMMARY
Construction for the proposed residential structures will require deep excavations into the
existing slope on the north side of the properties. The plans currently show these excavations
will be unsupported, the tallest of which is 19 feet and is located within 19 feet of an adjacent
residential structure. The project geotechnical engineer has indicated that cuts can consist of an
unsupported vertical cut up to 9 feet tall with the remaining cut sloped at I H: IV to intersect the
existing ground slope. The use of an unsupported cut for the proposed excavations is currently
under review as discussed in the following section.
REVIEW COMMENTS
HWA Review from December 2017:
In our first review letter for this project, we indicated that shoring for the proposed excavations
should be designed assuming the advance outwash soils will be wet at the time of excavation.
Waiting until the excavation is open to identify that the advance outwash materials will not stand
vertical represents a significant risk ofslopefailure and damage to the adjacent residences. We
also stated that the applicant should revise the excavation and shoring plans to address slope
stability concernsfor the proposed temporary excavations.
Responses from Applicant — Joseph Lucia Letter dated January 30, 2018:
Based on our review of the responses, the applicant has focused on addressing the
presence/absence of ground water within the soil borings. We have reviewed the borings and
their comments and provide the following conclusions.
HWA Comments to Mr. Lucia Responses:
According to the AESI report submitted in 2008, the borings for the geotechnical investigation
were drilled in July when the ground water elevation is typically lower compared with wetter
periods of the year. In addition, no wells were installed within any of the borings to observe the
static ground water level. In our experience, it is typical for wet soils observed in borings to
represent the presence of a static ground water table whether or not free water is encountered.
We also reviewed the material descriptions to see if there are any signs of oxidation that would
indicate the ground water table fluctuates within these zones. The color variations of light brown
and light gray soils in the boring logs at 10 feet in both EB- I and EB-3, are indicators of
18109-18111 Ldts I and 2 - 2nd Peer Review Letter.doc 2
March 19, 2018
HWA Project No. 2017-007-21 Task 300
oxidation and show that ground water has likely reached as high as 10 feet below the ground
surface.
Given that the ground water table is expected to fluctuate throughout the year, the applicant
should provide design for a shored excavation assuming the ground water elevation is at its
highest level during the year. The applicant should also install a monitoring well to observe and
monitor ground water during the wet season to estimate the depth to seasonal high ground water,
which should be provided for our review of the applicant's design.
Responses from Applicant — Robert Pride Letter dated January 22, 2018:
The geotechnical engineer's letter regarding the stability of the proposed excavated slope
states: Evaluation of the slope stabilityfor these temporary slope cuts shows they will have a
factor ofsafety in excess of 1. 5for static and 1.2for seismic conditions, and willprovide
construction safety during installation of the adjacentfoundations and retaining walls.
HWA Comment on Robert Pride's Letter
The applicant's geotechnical engineer states that the proposed excavation slopes will have
adequate factors of safety against slope failure; however, no documentation of the slope
geometry and soil parameters used in these analyses are provided. The geotechnical engineer
should submit documentation showing the design assumptions and results of the analyses for
review. Where the sloped, benched, or shored excavation will be accessed by workers it should
be designed in accordance with WAC 296-155 Part N.
CONDITIONS AND LIMITATIONS
We have prepared this review letter report for the City of Edmonds, for use in permitting
considerations for the proposed project. It should be noted that HWA performed no subsurface
explorations within the scope of our review and has relied solely on available information
provided by others and existing within the public domain. Accordingly, our evaluations of site
conditions and appropriateness of designs submitted is limited by any limitations in the
information available to us, and we cannot confirm that the actual geotechnical site conditions
are as presented.
18109-18111 Lots I and 2 - 2nd Peer Review Letter.doc 3
March 19, 2018
HWA Project No. 2017-007-21 Task 3 00
We appreciate the opportunity to be of service. If you have any questions regarding this report,
please do not hesitate to contact either of the undersigned at (425) 774-0106.
Sincerely,
HWA GmSCIENCES INC.
4�:A�
Zakeyo Ngorna, P.E.
Geotechnical Engineer
JoLyn Gillie, P.E.
Geotechnical Engineer, Principal
18109-18111 Lots I and 2 - 2nd Peer Review Letter.doc 4