REVIEWED RESUB1 BLD2021-0742+Geotechnical_Report+2.4.2022_10.39.04_AM+2661979COBALT
G E 0 S C I E N C E S
December 6, 2021
Massimo Ramella-Pezza
mmcedar(&icloud.com
RE: Geotechnical Addendum
Proposed Residential Development
702 Cedar Street
Edmonds, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss soldier pile shoring wall design information. A pile wall will extend along the south
property line and a short portion of the east property line. A total of 10 piles are proposed.
Cantilever Soldier Pile Walls
A soldier pile wall with pressure treated timber (wood) lagging would be suitable to support the
proposed excavations and to support adjacent properties and right-of-way where required.
Soldier piles typically consist of steel W or H-beams inserted into oversized drilled shafts, which
are backfilled with structural concrete, lean mix {Controlled Density Fill (CDF)}, or a combination
of lean mix to the base of the excavation and structural concrete below the excavation to anchor
the soldier piles.
Due to the potential for local caving during drilling operations for the soldier pile holes due to soft
soil conditions and shallow groundwater, consideration should be given to using slurry or drilling
fluid to reduce the risk of caving of the pile holes during installation. If water is present within
the pile hole at the time of soldier pile concrete placement, the concrete should be placed starting
at the bottom of the hole with a tremie pipe and the column of concrete should be raised slowly to
displace the water.
We recommend that soldier piles have a maximum spacing of eight feet on center. To account for
arching effects, lateral loading on the lagging can be reduced by 50 percent. Unlagged excavation
heights should not exceed three feet. No portion of the excavation should remain unsupported
overnight. Lagging sections may be up to 6 feet in height depending on stability.
Cantilever soldier pile walls for this site may be designed based on an active lateral earth pressure
Of 35 pcf for level backslope conditions, provided the wall is unrestrained (not fixed; permitted to
move at least 0.2 percent of the wall height). The pressure will act on the soldier pile width below
the base of the excavation as well. A lateral uniform seismic pressure of 7H is recommended for
seismic conditions (active). Any road or building surcharges should be included into the design.
In front of the soldier piles, resistive pressure can be estimated using an allowable passive earth
pressure of 275 pcf acting over 2 times the soldier pile diameter, neglecting the upper 2 feet below
the base of the excavation. A factor of safety of 1.5 has been incorporated into the passive
pressure value for temporary conditions. A lateral pressure reduction of 50 percent may be used
for design of the lagging for a pile spacing of three diameters. Lagging should be backfilled with
5/8 inch clean angular rock to minimize void spaces.
www.cobaltgeo.com (2o6) 331-1097
December 6, 2021
Page 2 of 2
Geotechnical Addendum
The shoring system and any nearby existing structures, including roadways, should be monitored
for movement during construction. A system of survey points should be established prior to
commencing with the excavation activities. Readings should be taken periodically (weekly) until
the permanent wall is in place and these readings should be compared to the original baseline
measurements. Permanent walls should have a pile coating or be upsized.
Sincerely,
Cobalt Geosciences, LLC
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Phil Haberman, PE, LG, LEG
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www.cobaltgeo.com (2o6) 331-1097