REVIEWED RESUB BLD2021-0088+Geotechnical_Addendum_Letter_1+5.21.2021_8.15.37_PM+2211573Irerracon
September 18, 2019
CA Senior Living Holdings, LLC
130 E. Randolph St., Ste 2100
Chicago, IL 60601
Attn: Mr. Michael Duggan — Vice President of Development
P: (312) 248-2091
E: mduggan@ca-ventures.com
Re: Geotechnical Engineering Report — Addendum Letter No. 1
Edmonds Senior Living
21200 72nd Ave. W
Edmonds, Washington
Terracon Project No. 81185173
Dear Mr. Duggan:
This letter serves as an Addendum to the Geotechnical Engineering report dated December 20,
2018 and was developed in general accordance with the Supplement to Agreement for Services
(Change Order No. 1) dated September 9, 2019 and your email authorization dated September
10, 2019. We are providing this letter to provide additional geotechnical engineering
recommendations for the following:
Bearing capacity of shallow foundations for the recessional outwash soil unit
Assumptions of groundwater to be used for stability calculations by the structural
engineer (KPFF)
Temporary slope cuts for soils at the base of the proposed excavation
These recommendations are to be used with the geotechnical engineering report stated above.
BEARING CAPACITY
Our current understanding of the excavation design is based on communication with the shoring
designer during a conference call on August 15, 2019. We understand the excavation side wall
will vary in elevation such that the dense to very dense glacial till soil unit will be several feet
below the planned foundation elevation. To accommodate shallow foundation loading within the
medium dense, recessional outwash soil unit, we provided additional bearing capacity
recommendations. The table presented below supersedes the table presented in the Shallow
Foundations section (i.e. pages 9 and 10) of the geotechnical engineering report:
Terracon Consultants, Inc. 21905 64th Ave. W, Suite 100 Mountlake Terrace, WA 98043
P (425) 771 3304 F (425) 771 3549 www.terracon.com
Geotechnical Engineering Report — Addendum Letter No. 1
Edmonds Senior Living Edmonds, Washington
September 18, 2019 Terracon Project No. 81185173
Description
Net allowable bearing pressure
Structural fill/medium dense native soil
Dense/very dense native soil
Minimum dimensions
Minimum embedment below finished grade
Approximate static total settlement from
foundation loads for condition specified
Estimated static differential settlement from
foundation loads'
Ultimate passive pressure
Compacted structural fill
Spread Footing
3,000 psf
5,000 psf
24 inches
18inches
<1 inch
Irerracon
Wall Footing
3,000 psf
5,000 psf
18inches
18inches
<1 inch
About 2/3 of total settlement
350 pcf (equivalent fluid unit weight)
Ultimate coefficient of sliding friction 0.40
The maximum net allowable bearing pressure is the pressure in excess of the minimum surrounding
overburden pressure at the footing base elevation. An appropriate factor of safety has been applied. These
bearing pressures can be increased by 1/3 for transient loads unless those loads have been factored to
account for transient conditions. Assumes that exterior grades are relatively level adjacent to the structure.
Values provided are for maximum loads noted in Project Description. A maximum footing width of 12 feet
and 8 feet are assumed for spread and wall footings, respectively.
3. For frost protection and to reduce the effects of seasonal moisture variations in the subgrade soils. For
perimeter footing and footings beneath unheated areas. For sloping ground, maintain depth below the
lowest adjacent exterior grade within 5 horizontal feet of the structure.
4. Differential settlements are as measured over a span of 50 feet. We should review the settlement estimates
after the foundation plan has been prepared by the structural engineer.
5. Use of passive earth pressures require the sides of the excavation for the spread footing foundation to be
nearly vertical and the concrete placed neat against these vertical faces or that the footing forms be
removed and compacted structural fill be placed against the vertical footing face.
6. Passive resistance in the upper 2 feet of the soil profile should be neglected.
7. Can be used to compute sliding resistance where foundations are placed on suitable soil/materials. Should
be neglected for foundations subject to net uplift conditions.
Should foundation subgrades within the medium dense, recessional glacial outwash soil unit become
overly disturbed, or deleterious soils such as existing fill are identified, replacement of these soils
using compacted structural fill can be accomplished per the illustration below and the
recommendation provided in the section:
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Geotechnical Engineering Report — Addendum Letter No. 1 Irerracon
Edmonds Senior Living Edmonds, Washington
September 18, 2019 Terracon Project No. 81185173
DESIC
FOOTI
RECOI
EXCP
OVER -EXCAVATION / BACKFILL ZONE
NOTE: EXCAVATIONS ARE SHOWN VERTICAL; HOWEVER, THE
SIDEWALLS SHOULD BE SLOPED AS NECESSARY FOR SAFETY
GROUNDWATER AND UNDERVAULT DRAINAGE
We understand the City of Edmonds has presented the following comment in the Civil Plan
Review for referenced application number BLD20190835 dated August 9, 2019:
• Provide Buoyancy calculations to verify that Double check vault will not be affected by
groundwater. As discussed, assume groundwater at finish grade, since this area is
subject to flooding.
Per email communication with KPFF dated August 19, 2019, the City of Edmonds does not
consider the site a flood area but that groundwater information is to be provided by the
geotechnical engineer (Terracon). In our opinion, an assumed groundwater elevation at finish
grade is overly conservative. Based on our observations at the time of the exploratory program,
the groundwater present at the site appears to be perched on, and within, the glacial till soil unit.
An assumed groundwater elevation at approximately 357 feet would be more appropriate;
however, footing drains at the base of the vault should be adequate to collect the groundwater
seepage and reduce the likelihood for hydrostatic pressures to build-up against the walls of the
vault. Provided the Civil Engineer can design a reliable pumping system to remove the water from
the under -vault collection system, the potential for hydrostatic buildup and hence uplift pressure
on the vault should be negligible. The Geotechnical Engineer observing the earthwork activities
during construction, should be able to confirm, or recommend modifications to, the permanent
drainage system.
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Geotechnical Engineering Report — Addendum Letter No. 1 Irerracon
Edmonds Senior Living Edmonds, Washington
September 18, 2019 Terracon Project No. 81185173
TEMPORARY SLOPE CUTS
Recommendations for temporary slope cuts were requested by Ankrom Moisan Architects, Inc.
(AMA; project architect) via email on August 16, 2019. At the time the geotechnical engineering
report was published, we were not aware open cuts were being considered. As a minimum,
excavations should be performed in accordance with OSHA 29 CFR, Part 1926, Subpart P,
"Excavations" and its appendices, and in accordance with any applicable local, and/or state
regulations. The recessional glacial outwash soil unit appears most consistent with OSHA's
description for Type C soils. Similarly, the glacial till soil unit appears most consistent with Soil
Type B.
Construction site safety is the sole responsibility of the contractor who controls the means,
methods, and sequencing of construction operations. Under no circumstances shall the
information provided herein be interpreted to mean Terracon is assuming responsibility for
construction site safety, or the contractor's activities; such responsibility shall neither be implied
nor inferred.
We annraniata tha nnnnrh Inity to ha of service to you on this project. If you have any questions
concl further service, plea;
Sinc(
Terr:
Zach
Project Engineer
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