Updated Geo Letter 4.29.20COBALT
G E 0 S C I E N C E S
March 25, 2020
Troy Grasseth
G2 Contracting
G2contractingolive.com
RE: Retaining Wall & Excavation Considerations
Proposed Single -Family Residence
15712 — 72nd Avenue West
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 free-standing retaining wall construction east of the proposed residence at the referenced
site. This letter follows our receipt and review of a third -party review letter by Landau Associates
dated February 19, 202o as well as a recent site visit to observe the current site topography. Their
letter discusses a previous 20 feet tall cut and not the 12 to 14 feet tall cut as described to us
during our previous site visit.
Observations and Recommendations
We recently visited the site to observe the current topography. Based on our review, it appears
that the proposed 10 feet tall concrete retaining wall east of the residence will be situated on
buttress fill. We estimate that a 1.5H:1V (horizontal to vertical) temporary slope can be created
safely in the buttress fill to allow for construction of the upper retaining wall, provided the wall is
relocated to the west.
Figure 1 shows the current topography and proposed wall location (original plans and
recommended location), along with location of buttress fill. We added a 1.5H:1V temporary slope
angle to illustrate a possible excavation to allow for basement wall construction and upper
concrete wall construction.
The originally proposed wall location would require steep temporary excavations in buttress fill
that may not be stable. We recommend moving the retaining wall about 8 feet to the west of this
location. This location will allow for wall construction while maintaining a 1.5H:1V slope in
buttress fill up to 72nd Avenue West.
Since the wall will be situated in fill, we recommend supporting the concrete wall on small
diameter pipe piles driven to refusal in underlying glacial till. The following recommendations
may be used in foundation design considerations for the concrete wall located in the yard area.
The proposed concrete wall may be supported by small diameter driven steel pipe piles extending
to refusal in the underlying dense to very dense native soils.
Pin piles used for foundation support may consist of two, three, or four -inch diameter, Schedule
40 or 8o, galvanized steel pipes. Allowable axial compression capacities of 3, 6, and 10 tons may
be used for these piles, respectively.
www.cobaltgeo.com (2o6) 331-1097
March 25, 2020
Page 2 of 3
Geotechnical Recommendations
The required pile length in order to develop the recommended pile capacity is expected to vary
depending on the depth of loose/soft soils below the foundation elements. For estimating
purposes, a pile length of about 15 to 25 feet may be anticipated.
Three and four -inch diameter piles are typically installed using small (approximately 650 to 1,100
pound) hammers mounted to an excavator. Refusal criteria is the minimum amount of time (in
seconds) required to achieve one inch of penetration, and it varies with the size of hammer used
for pile driving. Penetration resistance required to achieve the capacities will be determined from
the hammer size.
The following is a summary of typical driving refusal criteria for different hammer sizes that are
commonly used for three and four -inch diameter piles.
Hammer Weight
3" Pile Refusal Criteria
4" Pile Refusal Criteria
Hammer
(lb) / Blows per
(seconds per inch)
(seconds per inch of
minute
penetration)
Hydraulic TB
650 /
225
550 - 1100
12
20
Hydraulic TB
850 /
325
550 - 1100
10
16
Hydraulic TB
1,100 /
425
550 - 1100
6
10
Alternative hammer sizes may be used; however, the contractor should provide adequate
information to verify equivalence and refusal criteria. Pile splices may be made with compression
fitted sleeve pipe couplers (mechanical couplers).
Two inch pin piles are driven with a 140 pound pneumatic hammer. Refusal criteria is 3 cycles of
30 seconds per inch of embedment.
A total of 3 percent of the pin piles (one pile minimum) should be load tested to verify the design
capacities. All load tests shall be performed in accordance with the procedure outlined in ASTM
D1143. The maximum test load shall be 2 times the design load (i.e. 2 x 10 tons = 20 tons).
A representative of the geotechnical engineer shall provide full time observation of pile
installation and testing to verify the driving refusal criteria.
We can provide additional recommendations upon request.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site. The contractor and owner are responsible for project safety, risk to
adjacent properties, and excavation stability.
Sincerely,
www.cobaltgeo.com (2o6) 331-1097
March 25, 2020
Page 3 of 3
Geotechnical Recommendations
Cobalt Geosciences, LLC
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Section Showing Current and Originally Proposed Wall Location
Proposed Wall at 72nd Ave. W.
285 Proposed Location
i
275' Proposed SFR /
265 / Fill Presumed
/ Former Cut
..........,. . ...............
........................
:'
255' /
1.5H:1V Slope" Native Glacial
Till
1„_30,
Section Showing Current Layout with Proposed Wall Location West of Original Location
Proposed Wall 72nd Ave. W.
285 Location, —8' West of
revious Location
275' Proposed Finish /
Yard Grade /
i
265' / Fill
255' /
i
1.5H:iV Slope Glacial Till
Cobalt Geosciences, LLC
Proposed Residence P.O. Box 82243
1 12 end Avenue West Site Plan & Kenmore, WA 98028
COBALT57 % (2o6) 331-1097
Section
G E 0 S C I E N C k S Edmonds, Washington www.cobaltgeo.com
Figure i cobaltgeoogmail.com