20080627103045.pdfLANDAU
.ASSOCIATES
TECHNICALMEMORANDUM EwRom,&NTALIQ TECFW 4INATUMRESOURCES
TO: Jeannine Graf, Building Official FCi%jjE®
City of Edmonds Building Department
FROM: Dennis R. Stettler, P.E. f APR 112008
Chad McMullen, P.E. UIL_O' G
DATE: April 8, 2008
RE: GEOTECHNICAL PEER REVIEW
COOPER RETAINING WALLS
7306164n'PLACE SW
EDMONDs, WASHINGTON
INTRODUCTION
This technical memorandum provides our geotechnical peer review of technical documents and
plans submitted to the City of Edmonds (City) for the proposed retaining walls at the Cooper single
family residence. The purpose of this geotechnical peer review was to review portions of the submittal
package and assess its compliance with City development and building permit requirements as contained
in Edmonds Community Development Code (ECDC) Chapter 23.80, and City of Edmonds Retaining
Wall Permit Submittal Requirements (as contained in City Handout B62). This geotechnical peer review
was accomplished in accordance with Task Order No. 08-05 of Landau Associates' On -Call Geotechnical
Engineering Services Agreement with the City.
We have received the following information forwarded by the City for review:
• Landslide .Investigation and Stabilization Recommendations, John Cooper Residence, 7306 -
164`h Place SW, Edmonds, Washington, L & A Job No. 8A010 letter report prepared by Liu &
Associates, Inc. for John Cooper, dated March 4, 2008.
• Design Calculations, Pin Pile Walls, John Cooper Residence, 7306 — 164`" Place SW,
Edmonds, Washington prepared by Liu & Associates, Inc., dated March 8, 2008.
• Slide Repair and Slope Stabilization, Plan Sheet SS -1 of 1 prepared by Liu & Associates, Inc.
for John Cooper Residence, dated March 9-2008.
Two retaining walls are proposed to address a landslide that reportedly occurred behind the
residence on December 3, 2007. Access to the landslide area is limited and the proposed repair scheme
has been developed using materials and construction techniques that can be accomplished with hand or
limited access equipment. The landslide stabilization structure consists of two tiers of pin -pile and
lagging walls. Wall support is supplemented with helical -anchors, which are located at approximately the
mid -height of each wall. The maximum retained height of each of the walls will be approximately 6 feet,
130 2nd Avenue South + Edmonds, WA 98020 + (425) 778-0907 & fax (425) 778-6409. • www.landauinc.com
with an 8 -ft minimum embedded depth of the pin piles. Nearby level ground will exist above the upper
tier and between the bottom of the upper tier and top of the lower tier. The ground profile in front of the
lower tier slopes downward at approximately 35 degrees below horizontal. Backfill behind the walls will
consist of geofoam to within 2 feet of the planned ground surface and structural backfill in the upper 2
feet.
TECHNICAL REVIEW
The proposed concept and approach to the landslide stabilization appears to be appropriate for the
site conditions. The following summarizes our specific technical review comments.
Slope Stability
The landside and the proposed remedial construction occur within a Geologically Hazardous Area
as defined in ECDC 23.80. Consequently, the provisions and requirements of ECDC 23.80 apply to this
project. The geotechnical report for the project addresses some of the ECDC 23.80 requirements. The
report outlines the slope conditions, geologic characteristics, and soils exposed in the landslide scarp area.
The report also addresses the approach to remediate. the recent landslide. However, the geotechnical
report does not address the larger geologic hazard presented by the slope or the overall stability of the
steep slope below the residence. The report should address the report requirements of ECDC 23.80.050,
including, but not limited to, the extent of the geologic hazards area; the relationship of the existing and
proposed construction to the geologic hazard; a hazards assessment of the overall slope; the history of
the site regarding previous landslides, erosion, or grading; the stability of the slope before and after the
proposed retaining wall construction; a discussion of how the proposed retaining walls maintains or
reduces the pre-existing level of risk presented by the geologic hazard; and related requirements outlined
in ECDC 23.80.050.
Earth Pressures
Active and passive earth pressures on the retaining walls are provided in the geotechnical report
and the design plans. The report states that a factor of safety on the wall design for sliding and
overturning of 1.7 is incorporated into the design. The calculations provided in the submittal were
difficult to follow to verify that the recommended factor of safety was provided. We request that the
designer review the calculations to confirm that the factor of safety stated in the report is incorporated
into the design.
The geometry of the passive wedges at the bottom of each wall tier is of concern. The passive
earth pressure provided in the geotechnical report and used for wall design is consistent with level ground
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conditions only. For the lower wall, the ground slopes steeply away from the wall and the available
passive pressure would be substantially reduced. In addition, because of the close proximity of the upper
wall to the lower wall, the passive pressure wedge of the upper wall would intersect with the lower wall,
potentially inducing greater load on the lower wall. The available passive resistance should reflect the
slope geometry within the passive wedge and the influence of the nearby lower retaining wall.
Helical Anchors
There is no stated requirement regarding how long the minimum length of helical anchors should
be. These should be located well behind the active wedge of the retained soil. A minimum installation
length of soil anchors should be provided by the designer. The overall retaining wall design (anchor
length and pin pile depth) should also be assessed with respect to global stability of the wall system.
Site Drainage and Erosion Control
The geotechnical report indicates that a tightline roof drain was extended to the base of the slope
shortly after the slope failure. The report does not comment upon the suitability of the tightline
installation for long-term use, nor does it discuss the suitability of the discharge location and the potential
influence to the subject property or adjacent properties. The report should address the adequacy of the
current tightline installation and if the current system is not adequate, the design documents should
include a permanent roof drain tightline system including size, type, and location of pipe materials, pipe
anchorage, energy dissipation fixtures, and an appropriate location for discharge.
Temporary erosion and sedimentation control measures are required in accordance with the
provisions of ECDC 18.30. Provisions for temporary erosion and sedimentation control are not included
in the retaining wall design plans. Permanent erosion control will consist of re -vegetation of the affected
area. The design plans should include temporary erosion and sediment control measures to be employed
during and after construction, until permanent vegetation is established.
Inspections
City requirements for retaining wall permits (see City Handout B62) outline the owner
requirements for calling for Building Inspections by the City during construction, and Special Inspection
Requirements that call for the Geotechnical Engineer of Record to monitor the construction to verify the
site conditions and construction and to submit Field Reports to the City. The requirements for City
Building Inspections and ,Special Inspections by the Geotechnical Engineer of Record should be included
on the design plans.
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CLOSURE
This technical memorandum has been prepared for use by the City of Edmonds in evaluating the
adequacy of design documents submitted for a permit related to the proposed retaining walls for landslide
stabilization at the Cooper single family residence at 7306 164`' Place SW. The focus.of this review was
the geotechnical aspects of the documents. The purpose of the review was to assess the adequacy of the
application documents for compliance with City requirements contained in ECSC 23.80, City of
Edmonds Retaining Wall Permit Submittal Requirements, 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/CTM/rgm
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