20080806132813.pdfRECEIVED
Technical Memorandum AUS®� 2Q��
BUILDING
Date: 04 August 2008
Subject: Blackboum / Cannon Landslide Repair
Structural Engineering Peer Review
From: Howard Wells, P.E.
James Bohanek, E.I.T. -cK3
To: Landau Associates
Dennis Stettler, P.E.
130 2nd Avenue South
Edmonds, WA 98020
=10DUCTiON
BFRGER/ABAM
E H•b1 NEER] I NC.
This technical memorandum serves as a structural engineering peer review of the technical
documents and plans submitted to the City of Edmonds for slope restoration and retaining wall
design consisting of a series of anchored micropiles at the Blackbourn / Cannon residences,
located at 1201 and 1205 Viewland Way, Edmonds, Washington. The purpose of the structural
engineering peer review is to evaluate the submitted documents for adequacy of the design to
achieve the desired function. Documents from Merit Engineering, dated 7 December 2007 and
14 January 2008, were reviewed. This review is in accordance with a subconsultant agreement
between Landau Associates and Berger/ABAM Engineers under Task Order No. 08-12 of
Landau Associates' On -Call Geotechnical Engineering Services Agreement with the City of
Edmonds. This Technical Memorandum is intended to serve as a companion document to the
geotechnical peer review performed by Landau Associates.
We understand that a landslide occurred on 3 December 2007 behind the residences at 1201 and
1205 Viewland Way, in Edmonds, Washington. To stabilize the slope, three anchored pin -pile
retaining walls, 4 -feet in height and 18, 20, and 24 -feet in length have been proposed.
Additional pin piles without anchors or a wall facing are proposed to reinforce the slope on
either side of the slide.
The scope of this peer review includes an analysis of the reinforced shotcrete wall panel, the
panel to anchor head connection, the anchor bar capacity, and the pin pile capacity.
BE=RGER/ABAM Engineers Inc. • 700 Northeast Multnomah Street, Suits 900 • Portland, OR 97232
Phone 503/872-4100 • Fax 503/872-4101
PAPO&09-M Technical Memorandum
04 August 7006
Page 2
DESCRIPTION OF RETAINING WALL SYSTEM
The proposed retaining wall system consists of a four foot tall, six inch thick reinforced
shotcrete panel that spans vertically and horizontally between tie -back anchors spaced 2'-0" on
center vertically and T-0" on center horizontally. Three inch diameter schedule 80 steel pin
piles, spaced T-0" on center, provide vertical support at each set of tie -back anchors. The tie-
back anchor is a 718" diameter grade 95 steel all -thread bar, grouted into a drilled hole with a 20
foot long bond length. The anchor is oriented at a 20 degree angle below horizontal. The space
behind the existing slope and the shotcrete facing is to be backfilled with lightweight geofoam
topped with a one foot thickness of topsoil, to create a series of three small terraces.
STRUCTURAL ADEQUACY OF RETAINING WALL SYSTEM
The calculations supplied by Merit Engineering Inc. indicate that each anchor must be able to
resist a tension load of 1.7 kips. This is a very small load and this load can easily be transferred
from the slope through the shotcrete panel to the anchor head and through the proposed anchor
bar to the slope beyond the assumed slide plane. In fact, we have calculated that the system is
capable of transferring a service load in excess of 10 kips per anchor beyond the slide plane.
The vertical component of a 1.7 kip anchor load is a negligible 581 pounds. Each pin pile is
required to support twice this load, or 1.2 kips, again a very small load. If the soil loading that
Merit Engineering has used for their design is correct (we have not verified this), the system
proposed is a conservative and relatively stiff system.
CONCLUSION
The proposed retaining wall appears to be an appropriate system to promote slope stabilization
at this site. The design is conservative with respect to the loads that the designer has calculated.
Please feel free to contact us with any comments or question.
HAW.JRB
TO Ann Bullis, Building Official
City of Edmonds Development Services, Building DivisionRECEIVED:
FROM: Dennis R. Stettler, RE. �
Chad McMullen, P.E. t AUG ® 6 2008
DATE: August o, 2008 BUILDING
RE: GEOTECHNICAL PEER REVIEW
BLACKBouRN/CANNQN LANDSLIDE REPAIR, BLD2008-0538
1201/1205 VIEWLAND WAY
EDMONDS, WASHINGTON
INTRODUCTION
This technical memorandum provides our :geotechnical peer review of technical documents and
plans,subrnitted'to the City of Edmonds (City) for the proposed retaining 'wails ,and.l.andslide repair at the.
Blackbourn/Cannon residences, located at 1201 and 1205 Viewland Way in Edmonds, Washington. The
purpose of this geotechnical peer review was to review the submittal package and assess its compliance
with City development and building .pe. rmit requirements; as contained in the Edmonds Community
Development Code (ECDC) Chapter .23.80, and with City. Retaining Wall .Permit Submittal
Requirements, as contained in City Handout 1362. This geotechnical peer review was accomplished in
accordance with Task Order No. 08-12 of Landau Associates' On -Call Geotechnical Engineering
Services Agreement with the City. The Task Order also provided for structural engineering review of the
retaining walls. Landau Associates retained Berger/A.BAM Engineers as a subconsultant to accomplish
the structural engineering review and the results of their review are submitted .in a separate technical
memorandum.
We have received'the.following information forwarded by the City for our review:
a Site Visit — Slope Reconnaissance, 1201 Viewland Way, Edmonds, Washington, Merit Job No.
OL0141601, letter report prepared by Merit. Engineering, Inc. for Ruth Biackbourn, dated
December 7, 2007.
* Partial T opograpliy Survey for Ruth Blackboun:, topographic survey plot prepared by Loveli-
Sauerland & Associates, Inc.,.latest revision date January 8, 2008.
® Slope Restoration, 1201 Viewland Wciy, Eclnzonds, Mzshiaigton, Merit Job No. OL014:1601,
letter report prepared by Merit Engineering, Tnc. for Ruth. Blackbourn, .dated January lel,
2008.
1.30:2nd Avenue South e. Edmonds, WA 98020 a (425) 77.5-0907 . tax (425) 778-6409 9 www.landauine.com
• Plan Sheets 1 through 3 (Site Plan, Wall Detail, Erosion Control, with 6 pages of supporting
calculations and materials) prepared by Merit Engineering, Inc. for Ruth Blackboum, dated
January 14, 2008.
• Geotechnical Engineering Study, Proposed Lockyer Residence, Lot 2, Harbor View Estates,
Edmonds, WA, Project No. T-418, letter report prepared by Terra Associates, Inc. for Group
Four, Inc., dated January 29, 1987.
* A two-page excerpt from an undated memorandum from the Edmonds Engineering Division
to the Edmonds Building Division, regarding comments for a building permit application at
1205 Viewland Way. The memorandum excerpt lists ten specific design and construction
requirements for permit approval and appends page 8 of the January 29, 1987 Terra
Associates, Inc. letter report, referenced above.
We understand that a landslide occurred behind the residences at 1201 and 1205 Viewland Way
on December 3, 2007. The landslide debris flowed down-slope to the north and onto property at 1312
12th Avenue North. The landslide zone was about 18 feet (ft) wide, about 2 to 3 ft deep, and occurred on
a steep slope that ranges from about 35 degrees to over 40 degrees, according to the site topography
prepared by Lovell-Sauerland & Associates. The December 7, 2007 letter by Merit Engineering and the
topographic map by Lovell-Sauerland & Associates refer to plastic drainage pipes leading from the
northwestern corner of the property at 1205 Viewland Way to near the top of the landslide.
Three short retaining walls are proposed to address the landslide repair. Access to the landslide
area appears to be limited and the proposed repair scheme has been developed using materials and
construction techniques that could be accomplished with hand or limited -access equipment. The landslide
stabilization structures consist of three tiers of pin pile -supported vertical concrete walls. Wall support is
supplemented with two rows of soil anchors at each wall. Pin piles and soil anchors will be spaced at 3 -ft
intervals along the length of the walls. The retained height of each wall will be approximately 4 ft.
Backfill will be placed behind each wall after construction, with an approximately level ground surface
between the top of the wall and the existing slope surface behind it. Backfill behind the walls will consist
of geofoam, to within 1 foot of the planned ground surface, with topsoil in the upper foot. In addition to
the retaining walls proposed within the area affected by the December Yd slide, additional pin piles are
proposed on either side of the slide area in order to reduce the likelihood of future sliding at these
locations. These pin piles will be spaced at 3 -ft intervals across the slope.
TECHNICAL REVIEW
The proposed design concept for slope stabilization, both within and outside of the slide area,
appears to be appropriate for the site conditions. In our opinion, the proposed construction would Iikely
result in an increase in the overall stability of the slope at the subject property, as compared to existing
816!08 L EdmdataProjects10741145XFileRoomlRlBlackboum_ReWall Review Final TM.doc LANDAU ASSOCIATES
2
conditions. However, certain aspects of the design are lacking or do not address requirements of ECDC
Chapter 23.80. The following summarizes our specific technical review comments.
Overall 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 area. The report also addresses the approach to remediate the recent Iandslide. 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. For retaining walls within designated geologically critical
areas (the steep slope is within a geologically hazardous area), the City requires that the Geotechnical
Engineer of Record submit a stamped and signed Ietter acknowledging that the criteria of ECDC
23.80.070 have been reviewed and all applicable design considerations for code compliance have been
incorporated into the plans.
The residential structures, and their related site improvements, appear to be at the crest of the
slope with virtually no buffer zone. The proposed slope repair addresses repair of the slope at the location
of the recent landslide, but an opinion has not been provided regarding the overall slope stability and
potential impact to the residences above or their associated site improvements. The December 7, 2007
field reconnaissance letter implies that disturbance from the landslide extended beneath the concrete pad
walkway at the top of the slope. It is not clear if repair or slope reinforcement is also needed in this area
to prevent further progression of the landslide damage.
The geotechnical report should address the requirements of ECDC 23.80.050, including, but not
limited to: the extent of the geologic hazards area; a hazards assessment of the overall slope; the history
of the site (including neighboring properties) regarding previous landslides; and related requirements
outlined in ECDC 23.80.060. The report should also address the requirements of ECDC 23.80.070,
including, but not limited to: identifying preservation requirements for vegetation affected by
construction; seasonal limitations to construction (or application for exemption from seasonal
limitations); and surface water point discharge limitations.
Pin Piles
Three-inch diameter pin piles are proposed to support the retaining walls. Pin piles are also
proposed on either side of each wall tier in order to provide slope reinforcement outside of the existing
slide area. Typically, a minimum embedment length into competent soil is necessary to provide wall
815108 IlEdmtlatalProjectsL07411434FileRoomlRSiackboum_Revvall ReAew_Final rM.doc LANDAU ASSOCIATES
3
support and slope reinforcement. A minimum embedment length for the pin piles is not indicated on the
documents submitted to us for review. Also, the project plans state that pin piles are to be driven "to
refusal." For pin piles, refusal is usually defined as some minimum rate of pile penetration for a given
soil and pile/hammer combination. Refusal criteria are not indicated on the documents submitted to us
for review. A minimum embedment length for the pin piles should be provided by the designer. Refusal
criteria should be established by the designer.
Soil Anchors
The proposed soil anchors are closely spaced. The designer may want to consider using different
installation angles for the top and bottom anchor in order to provide greater separation and limit the
potential for anchor interference during construction.
Slide Debris Removal
The December 7, 2007 field reconnaissance letter indicates that the owner of the down-slope
residence expressed interest inremoving the slide debris which occupies a portion of their property. At
the time of the reconnaissance, the Merit Engineering representative advised the parties present that the
soil mass likely provided a stabilizing effect to the slope and that removal of the mass should be
postponed to a later date. We are not aware of whether this landslide debris has now been removed and
the proposed design does not indicate whether portions of the slide mass may be removed following
construction. If the landslide mass has not already been removed by the down-slope property owner, the
designer should indicate whether portions of the slide mass may be removed and, if so, should provide
guidance regarding phasing with construction, impacts to surrounding properties, recommended grading,
and recommended erosion and sediment control.
Site Drainage and Erosion Control
The December 7, 2007 field reconnaissance Ietter indicates that several plastic pipes, including
some broken pipes, were observed near the top of the slope and appeared to originate from the property to
the .east (1205 Viewland Way). Broken pipe sections were noted in slide debris. Small water channels
incising the exposed slide surface and seeps at the slide toe were observed. The December 7th letter
indicates that the Merit Engineering representative recommended that the up-slope homeowners involved
connect broken pipes to a temporary drain system and divert other runoff sources away from the slide
area. Terra Associates' 1987 report, which includes design and construction recommendations for
residential construction at 1205 Viewland Way, recommends that roof, footing, and driveway drains be
directed away from the slope and discharged directly to a storm system. The City Engineering Division,
815108 1YEdmdeta\Projects1D7451ABYFileRoomVRTlackboum—ReWall Review.Final rM.doc LANDAu AssoclATEs
4
at the time of permit application, required that the stormwater be discharged at the southwestern corner of
the property, as noted in the two-page memorandum excerpt. From the documents we received for this
review, it is not possible to discern whether roof and footing drains at 1205 Viewland Way were routed to
the southwestern corner, to some other suitable discharge location (such as a tightline conveyance beyond
the slope), or discharged above or on the slope. The origins of the water conveyed in the pipes exposed
by the landslide are not identified in the December 7, 2007 reconnaissance Ietter. The reconnaissance
letter does not discuss the condition of any drain systems at 1201 Viewland Way nor does it discuss the
condition of a landscape irrigation system located at the top of the slope.
The proposed design does not comment upon the suitability of the drainage system, its suitability
for long-term use, or. the suitability of existing or potential discharge locations. Original permit
documents, as well as current requirements outlined in ECDC 23.80, prohibit stormwater discharge to the
slope. The report should address the adequacy of existing drainage installations and the irrigation
system. If drainage installations are inadequate, the design documents should include a permanent
drainage system that includes size, type, and location of collectors (such as catch basins or collection
troughs), pipe materials, pipe anchorage, energy dissipation fixtures, and an appropriate long-term
discharge location.
Temporary erosion and sediment control measures for the proposed design consist of silt fence
around the perimeter of the work zone. Permanent sediment and erosion control, such as vegetation plans
and routing of surface runoff, are not indicated on the plans. The design plans should include temporary
and permanent erosion and sediment control measures, to be employed during and after construction in
accordance with Chapter 18.30 ECDC.
Inspections
City requirements for retaining wall permits (see City Handout B62) outline the owner
requirements for calling to schedule Building Inspections by the City during construction. Additionally,
Special Inspection Requirements call for the Geotechnical Engineer of Record to monitor the
construction, to verify the site conditions, 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 in the design plans.
CLOSURE
This technical memorandum has been prepared for use by the City in evaluating the adequacy of
design documents submitted for a permit related to the proposed retaining walls for landslide repairs and
slope stabilization at 1201 and 1205 Viewland Way in Edmonds, Washington. The focus of this review
815108 5kEdmdetalProjects1074114o5FileRoom1RM31ackboum RewalI Re%iew_Final TM.doc LANDAU AssoclATE$
5
was the geotechnical aspects of the documents. A structural engineering review by Berger/ABAM
Engineers is submitted under separate cover. The purpose of this geotechnical review was to assess the
adequacy of the application documents for compliance with City requirements contained in ECDC 23.80,
City Retaining Wall Permit Submittal Requirements, and conformance with conventionally accepted
engineering practices. This peer review 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/rwt
8008 %lEdmdata%ProjoMW74Al b%FileRoomAR%Blackhoum_ReWell Review—Final TM.doc LANDAU ASSOCIATES
6