18109-18111 Lots 1 and 2 - 3rd Peer Review Letter.pdfU "W, HWAGEOSCIENCES INC.
Geotechnical &Pavement Engineering Hydrogeology -Geoenvironmental Services -Inspection& Testing
May 4, 2018
HWA Project No. 2017-007-21 Task 300
City of Edmonds
Public Works Engineering Division
City Hall — 2 d Floor
121 5thAvenue N
Edmonds, Washington 98020
Attention: Jeanie McConnell
Engineering Program Manager
Subject: EXCAVATION AND SHORING PLAN
Peer Review — Proposed Residences — Lots 1 and 2
18109 & 1811184 1h Avenue W
Edmonds, Washington
As requested, HWA GeoSciences Inc. (HWA) has completed a review of the applicant's
responses to our second peer review letter dated March 19, 2018. Responses were provided by
Robert Pride in his letter dated April 9, 2018 and Joseph Lucia in his letters dated April 7 and
April 9, 2018 regarding the two proposed residences on Lots I and 2 at 18109 and
18111 84th 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 91, dated April 7, 2018, prepared by Lucia
Engineering Inc., for the New Residence, 18109 84th Avenue W, Edmonds, WA 98026.
Excavation and Shoring Plans — Lot 92, dated April 7, 2018. prepared by Lucia
Engineering Inc., for the New Residence, 18111 84th Avenue W, Edmonds, WA 98026.
Final Plan Review, Proposed Residences —Lots I and 2, 181091111 84'h Avenue West,
Edmonds, Washington, Permit #BLD 2017-0178, dated April 9, 2018 and prepared by
Robert M. Pride, LLC.
21312 30' Drive SE
Suite 110
Bothell, WA 98021-7010
Fax: 425.774.2714
www.hwageo.com
Federal & State Certified DBE / MWBE
May 4, 2018
HWA Project No. 2017-007-21 Task 300
• Plan Check: BLD2017-0718, ProjectAddress 18111 841h Ave W (Lot 2), City Review &
Reply to City E-mail Message datedApril 4, 2018, dated April 7, 2018, and prepared by
Lucia Engineering Inc. for SCR Architects.
• Plan Check: BLD201 7-0 718, Project Address: 18111 84th Ave W, Plan Disapproval
notice dateddanuary 9, 2017, HWA GeoSciences, Inc. Letter datedMarch 19, 2018,
dated March 23, 2018, prepared by Lucia Engineering Inc. for SCR Architects.
• Geotechnical Recommendations, Proposed Residences —Lots I and 2, 18109 & 18111 —
84th 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 84h Avenue West, Edmonds, Washington, dated
July 30, 2008, prepared by Associated Earth Sciences, Inc. (AESI) for Mr. Steve
Krassner.
• Geotechnical Report Update, Single -Family Residence — Lot, 18109 84 th Avenue W,
Edmonds, Washington (Edmonds Permit No. BLD201 7-0717), dated September 25, 2017,
prepared by Associated Earth Sciences, Inc. (AESI) for Mr. Dino Hass.
REVIEW COMMENTS
Our letter March 19, 2018, requested that additional design information be provided to HWA for
our review of the proposed temporary excavations required to construct the proposed residential
structures. Our key concern is that the adjacent structures could be impacted by the proposed
excavations. The proposed design would allow an unsupported slope steeper than 3/4H:IV to
the neighboring property and could very likely result in damage to structures and utilities on that
property. To this end, we requested that the applicant provide a shoring plan, ground water and
settlement monitoring, and slope stability analyses for the proposed excavation. Based on the
information provided we have the following comments.
HWA Review Comment Regarding Presence of Ground Water
From our letter dated March 19, 2018, we provided the following comment: According to the
AESI report submitted in 2008, the boringsfor the geotechnical investigation were drilled in July
when the groundwater 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 groundwater
level. In our experience, it is typicalfor wet soils observed in borings to represent the presence
of a static ground water table whether or notftee water is encountered. We also reviewed the
material descriptions to see if there are any signs of oxidation that would indicate the ground
water tablefluctuates within these zones. The color variations of light brown and light gray soils
in the boring logs at I Ofeet in both EB- I and EB-3, are indicators of oxidation and show that
groundwater has likely reached as high as I Ofeet below the ground surface.
18109-18111 Lots I and 2 - 3rd Peer Review Letter.doc 2
May 4, 2018
HWA Project No. 2017-007-21 Task 300
Applicant Response
The applicant responded in the letter by Lucia Engineering Inc. dated April 9, 2018 saying:
The borings do not indicate "wet soils". Only Boring I indicates water. Boring 2 shows wet
soils at 10feet, but states "noftee groundwater between 10 and20feet". Boring 3 does not
mention wet soils until 20feet below grade. We do not agree that these borings indicate
water at I Ofeet, as stated in the HWA report. Further, the borings do not indicate changes in
color, as stated in the HWA report. Borings 2 & 3 do not mention any change in color until
20feet below grade, not I Ofeet as the HWA report states.
Following up with this comment the letter states "it would be cost effective tojust wait and
do this work in July, CJuly when the ground water elevation is typically lower compared
with wetter periods of the year.) Would this eliminate your water table argument? .... If the
City 'just wants shoring" then you should state so.
HWA Comment on Applicant Response
First, we maintain our view that the reference to "scattered wet zones" in EB-2 and the
color description of "light brown and light gray" at 10 feet in EB-3 that changes to "light
gray" at 20 feet, provide sufficient evidence that saturated soil conditions are likely
present above the base of the proposed excavation at site. Saturated soil conditions
would reduce the stability of the proposed excavations. Instability of these excavations
pose considerable risk to the integrity of the structures and utilities adjacent to the
properties.
The applicant asks if waiting until July to perform the excavations would eliminate the
concerns regarding for the presence of ground water and saturated soils. We considered
this; however, utilities and a stormwater detention vault and are in close proximity to the
excavations, as shown in Section A -A and B-B of the Plans for Lot 2. Utility backfill is
generally permeable and seepage within the backfill could infiltrate into the slopes
following a rain event. This seepage can cause sloughing of the proposed excavations,
which can happen rapidly, such that there is not adequate time to implement the proposed
shoring plan should instability of the slope occur. Based on these considerations, we
recommend that temporary shoring be required for the proposed excavations. In
additional, the shoring should extend up to an elevation level with the depth adjacent
utilities, including the stormwater detention vault shown on Section B-B Plan
Sheet SH-3.0 for Lot #2.
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.
18109-18111 Lots I and 2 - 3rd Peer Review Letter.doc
May 4, 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 GEOSCIENCES INC.
JoLyn Gillie, P.E.
Geotechnical Engineer, Principal
18109-18111 Lots I and 2 - 3rd Peer Review Letter.doc 4