20080521074553.pdfEDmONDs, i ASHINGTO
This technical memorandum provides our supplemental geotechnical peer review of:po.rtions of
the permit package submitted to .the City. of Edmonds.(City) for the proposed referenced development.
within the Earth Subsidence.. anti Landslide Hazard Area (ESLHA) .of North Edmonds. Landau::
Associates reviewed an earlier submittal related to this project and -our review coma—tents are -contained in
our technical memorandum dated March 1$, 2008. The purpose of this supplemental geotechnical peer
review was to review portions :of the resubmittal package to confirmthat the previous review co meats
.have been appropriately addressed, and .to .assess the submittal package for compliance with City
development and building permit requirements_ as contained in Edinonds Community Development Code
(ECDC) Chapters 1.9.10 and 23:$0. This supplemental geotechnical peer review was accomplished in
accordance with Task Order No. 08-08 of Landau Associates' On-Call Geotechnical .Engineering
ServicesAgreement with. the City,
We have received the following supplemental: information forwarded by the City for reviews
Soldier- Pile Wall / Iffininuun :Risk Statement Sral)lileMental Letter, Proposed Single-Farnily
Residence, 156`x' Street Southwest and 75`t' Place West, Edmonds, .WA, prepared for Mr. Ron.
Hilliard by Cornerstone:Geotechnical, slated March.26, 20018:
Seismic Hazard Letter; 156r'' .Street ,Saitthwest caul 75r' Place. West, Parcel No
00500900000101, Edmonds, WA prepared for Mr. Ron ldilliard by Cornerstone
Geotechnical, dater! March 27, 2008-
`PESC Parra Review Letter, Proposed Single-FarnHy Residence, 1551: 75`I' Place: West,
E'drnonds, WA, prepared for Mr. Ron Billiard by Cornerstone Geotechnical; dated April 22,
200&
Civil Engineering Drawings (3 sheets, including T'ESC cl;: Grading Plat, Drainage / Utrlhies
Placa, and Nates and Details), prepared by J. C. McDonnell Engineering, PC, dated Marcia 27,
2007, revised March 25,.2008.
130 2nd Avenue South • .Edmonds, FOIA 98020 (42a) 778.0$07 4#ax (425) 778=6409 www.landaulne.com
• Site Plan, Hilliard Residence, 15515 75`h Place W , Edmonds, WA 98026, prepared by
Cornerstone Architectural Group, dated March 3, 2008.
• Landscape Planting Plan, Sheet No. L1.1, prepared by TerraFonna Design Group, Inc., dated
April 21, 2008.
• Letter Re: Hilliard Residence, plan check no. 20080117, prepared by Cornerstone
Architectural Group, dated April 21, 2008.
• Grading Quantities Report for Ron Hilliard SFR, prepared by J. C. McConnell & Associates,
dated April 21, 2008.
• Architectural and Structural Plan Set (including Sheets A3.1, A2.1, A2.2, A2.3, -43.2, A3.3,
A4.1, A4.2, 44.3, A5.1, A6.1, A6.2, A10.1, SI, S2, S3, S4, S5, S6, and S7) prepared by
Cornerstone Architectural Group, dated March 30, 2008.
• North Edmonds Earth Subsidence and Landslide Hazard Areas Map (with revised location of
property location noted and signed certification as to property location), by Steve Barnes,
Architect, dated December 17, 2007, revised and initialed by SB 4124108.
We have reviewed the resubmitted permit application package and note that a number of our
review comments have been addressed. However, we note some remaining issues that have not been
fully addressed as described in the following sections.
FOUNDATIONS
The following text is copied from our March 18, 2008 technical memorandum review comments:
Cornerstone Geotechnical provided recommendations for foundation support,
including a recommended allowable soil bearing pressure of 1,500 pounds per square
foot (psp. They also recommended that the footing subgrade be over excavated and
backfilled with at least two feet of imported, compacted structural fill; that 1 to 2 ft of
compacted structural fill be placed beneath slab -on -grade floors; and that the footings be
designed to span at least ]Oft In our opinion, these recommendations are appropriate
considering the structure will be supported on shallow foundations above slide debris
that provides variable foundation support. It does not appear that the structural engineer
has incorporated these recommendations into the structural plans. The "Foundations"
note on sheet SI identifies an allowable soil bearing pressure of 2, 000 psf, makes no
mention of over excavation and backfill beneath footings or slabs -on -grade, and provides
for compaction criteria that is not consistent with the geotechnical report.
We recommend that the structural plans be revised to:
• Indicate the design allowable bearingpressure of 1,500psf.
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• Incorporate the geotechnical engineer's recommendations for over excavation and backfill
with imported, compacted structural fill beneath foundations and slab -on -grade floors, and
include the geotechnical engineer's recommended criteria for placement and compaction of
structural fill.
We also recommend that the structural engineer confirm that the foundations are
designed to span at least ]Oft.
In our review of Sheet S1, we see that the bearing pressure has been changed to 1,500 ps£
However, it appears that the other issues identified above regarding the geotechnical engineer's
recommendations for overexcavation and backfill beneath footings and slabs have not been incorporated
into the revised plans. We recommend that these issues be addressed in the structural plans.
SOLDIER PILL EXCAVATION SUPPORT AND TEMPORARY EXCAVATIONS
Cornerstone Geotechnical provides information in their March 26, 2008 letter that indicates that
changes to the plans have eliminated the need for a soldier pile retaining wall for shoring. The depth of
the temporary cut envisioned by Cornerstone Geotechnical would be 4 ft or less, and they indicate that
deeper cuts would require shoring. In our opinion, this recommendation is appropriate. The soil
conditions in the temporary cuts should be monitored in the field at the time of construction to confirm
the appropriateness of the temporary cut recommendations.
In our review of the Site Plan and Sheet S7, detail 7 on the structural plans, it appears that the
retaining wall has been designed with a shear key extending to a depth of 5.5 ft below the finished grade.
The finished grade in this area appears to be near Elev. 227 ft. This would indicate that the bottom of the
retaining wall would be at about elevation 221.5 ft, which is deeper than the elevation of 1.5 ft below the
floor elevation of 227 ft that was anticipated in the March 26, 2008 letter by Cornerstone Geotechnical,
and would require a cut near the toe of the existing slope in excess of the 4 ft maximum depth. In
accordance with the March 26, 2008 letter by Cornerstone Geotechnical, it appears that shoring is still
required, or further adjustment is needed to the design to reduce the depth of cut for the retaining. wall
foundation.
Sheets A4.1 and A4.3 show a "maximum slope cut" line near the toe of the steep slope on the
property at an inclination of I horizontal to 1 vertical. This maximum slope cut line was not shown on
the previous submittal. This Iine is contrary to the recommendations of the geotechnical report. The
January 22, 2007 geotechnical report by Cornerstone Geotechnical states: "For planning purposes, we
recommend temporary cuts in the fill soils and landslide debris be no steeper than 1.5 horizontal to 1
vertical." The Cornerstone Geotechnical letter of March 26, 2008 also recommends that if the depth of
the cut in this area exceeds 4 vertical feet, shoring will be required. The "maximum slope cut" line shown
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on the drawings is not consistent with the geotechnical recommendations for either the inclination or the
maximum depth.
OTHER COMMENTS
Although not a geotechnical comment, we note that the elevations presented on Sheet A3.1
reference the first floor elevation at both 0'0" and 10'0". The elevations should be consistent.
This technical memorandum has been prepared for use by the City of Edmonds in evaluating the
adequacy of permit submittal documents related to the proposed Hilliard single family residence at 15515
75t" place West. The focus of this review was the geotechnical aspects of the application. The purpose of
the review was to assess the adequacy of the application documents for compliance with City
requirements contained in ECDC 23.80 and ECDC 19.10 and conformance with conventionally accepted
geotechnical engineering practices. This geotechnical peer review by Landau Associates does not lessen
the requirements for the applicant's geotechnical consultant and other design professionals to prepare an
appropriate design for the site conditions.
We appreciate this opportunity to be of service to the City. Please contact us if you have any
questions or if we may be of further service.
DRS/rgm
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