Building Comments 1.pdf
technical requirements of ECDC 19.10 and ECDC 23.80 that address slope stability and hazard reduction
do apply to the proposed project.
PR
LANEVIEW
We reviewed the plans for the park development to assess their consistency with the geotechnical
recommendations contained in theHWA Geosciences, Inc. (HWA) report and with the provisions of
ECDC 19.10 and ECDC 23.80. We identified a number of inconsistencies between the plans and the
geotechnical report that should be addressed:
·
The plans show a retaining wall andup to 10 ft of fill placedabove existing gradeson and
near the base of the slope on the park property. The park development plan considered in the
geotechnical report (see Figure 6 of the HWA report) apparently did not consider the effect of
the proposed retaining wall and fill on the slope stability.The HWA report states on page 6
“Because placement of fill on the downslope side of the road would add to the weight of a
potential landslide, reducing stability of the slope, we recommend that filling be avoided to
theextent possible.”The geotechnical report needs to address the appropriateness of the
proposed retaining wall/fill configuration. The effect of this proposed grading on overall
slope stability needs to be addressed and it must be demonstrated that the stability is
improved over existing conditions or the grading needs to be altered.
·
Theplans indicate that the retaining wall planned on the park property would be supported on
2-ft diameter augercast piles installed to a depth of 10 ft.The bottom of thesepiles would
still be within landslide debris.No geotechnical recommendations are provided specifically
for this wall to assess the appropriateness of this design.The HWA geotechnical report
recommends thatelevatedwalkways and other structuresshouldbe supported on pin piles
installed to a depth of 20 to 25 ft (which in this area is thought to be through the existing
landslide debris).The proposed wall support and geotechnical recommendations need to be
consistent.
·
Theplannedretaining wall is composed of a modular block wall thatextends to heights of up
to6 to7 ft above the final grade according to Plan Sheet S-11. No geogrid or geotextile
reinforcementof the backfill behind the wall is indicated on the plans. The HWA
geotechnical report states that “Block walls higher than twice the block depth should include
geogrid reinforcing which extends back from the face of the wall a horizontal distance equal
to at least 0.7 times the free-standing wall height.”If the wall remains at its planned height,
geogrid reinforcing needs to be provided or the type of wall changed to be consistent with the
geotechnical recommendations.
·
Retaining wall details appropriately show drainage material and a drainage pipe behind the
retaining wall. However, it is not clear from the plans that the drainage pipe behind the wall
will be connected to the site drainage system.Provide a connection from the drainage pipe
behind the retaining wall to the site drainage system.
·
The geotechnical report recommends a deep trench subdrain (8 to 10 ft deep) near the toe of
the eastern slope on the park property.A slope drain is shown on the plans (Sheet L-4) with a
note to see the Civil Plans for the invert elevations. That information was not apparent on the
plan sheets that we were provided for review.The slope drain appears to be located near the
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toe of the new slope instead of the toe of the existing slope.No typical detail for the deep
trench subdrain is provided on the plan sheets that we were provided for review.Confirm
that the slope drain is at an appropriate location and depth consistent with the geotechnical
report recommendations, and provide a typical detail consistent with the recommendations of
the geotechnical report. In addition, consider constructability issues with respect to
excavating an8 to 10 ft deep trench near the toe of a potentially unstable slope. It may be
appropriate to require temporary shoring or specifically limit the horizontal extent of trench
that can be open at any time in order to reduce the potential for initiating a slope failure
during construction.
·
City requirements for retaining wall permits (see City Handout B62)and the provisions for
permits as contained in the City’s ESLHA documents outline the 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.Therequirements for
City Building Inspections and Special Inspections by the Geotechnical Engineer of Record
should be included on the design plans.
·
The Drawing Index calls out plan sheets L-14 through L-16 related to Irrigation. These plan
sheets were not provided to us for review. However, the City has identified that site
improvements that introduce water into the ground within the ESLHA (including watering or
irrigations systems) are prohibited.The plans should be revised to reflect this requirement.
GRR
EOTECHNICALEPORTEVIEW
The geotechnical report does not reference or address the specific requirements of ECDC 19.10
or ECDC 23.80. There are a number of requirements for grading and retaining wall permits for projects
within the ESLHA that are not addressed by the HWA geotechnical report.The geotechnical report
should be revised or supplemented to address the specific City requirements for geotechnical reportsas
contained in the City’s “Geotechnical Report Guidelines” documentfor projects in the North Edmonds
ESLHA.
The report references the 1979 Geotechnical Report by Roger LoweAssociates, but does not
reference any of the more recent comprehensive landslide hazard area studies of the area, including the
2007 Landau Associates report as required by ECDC 19.10.HWA describes the landslide history and
references theRoger Lowe Associates report to conclude that large-scale ground movement occurred
between two and three thousand years ago. The HWA report further states that “No large-scale earth
movements have been recorded for this area in modern times.” The implication of the discussion in the
report seems to be that no large-scale movements have occurred in historical times, and that is not
consistent with the available historical information. Significant landslides occurred in this area in 1947
and 1955-56 that would be considered by most people to be large-scale.We recommend the HWA report
be updated to address the landslide hazard discussion and background as required by ECDC 19.10.
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TheHWA report addressesslope stability at the park and the influence of placing fill along the
toe of the slope in onlya general way. It does not appear that stability analyses were conducted for either
the existing conditions or the stability after the planned fill and retaining wall is constructed.Placement
of the proposed fill and retaining wall may increase the stability of the slope above (east of) this location,
but since the wall and fill will be placed on about 20 ft of landslide debris, the stability downslope (to the
west)could be adversely affected.Slope stability analyses of the existing and proposed slope
configuration needs to be conducted.
The HWA report also provides the results from monitoring of an inclinometer on the adjacent
property that documents recent slope displacement and notes that another inclinometer was installed in
boring BH-1 that was drilled on the park property as a part of this project. The HWA report states that “A
definitive stabilization design will require additional measurements in the two slope indicator casings
over the course of the coming winter to determine the location of the slide plane and performing stability
analyses to develop stabilization measures. That additional analysis is not within the present scope of
work; however, we recommend that this additional analysis be performed and the results incorporated
into the park design.” To our knowledge, this work was not done.We recommend that additional
monitoring of the slope inclinometers be conducted and slope stability analyses be conducted to assess
current conditions and the expected slope stability conditions during and following construction of the
proposed park retaining wall, grading, and drainage improvements.
Thesite of the park and the proposed constructionare 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, but
limited information is provided for other issues.Thegeotechnicalreport should address all of the report
requirements of ECDC 23.80.050, including, but not limited tothe extent of the geologic hazards area;
the relationship of the existing and proposed construction to the geologic hazard;a hazardsassessment
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.
C
LOSURE
This technical memorandum has been prepared for use by the City of Edmonds in evaluating the
nd
adequacy of construction documents submitted for the City’s 162 Street Park. The focus of this review
was the geotechnical aspects of the documents. The purpose of the review was to assess the adequacy of
the documents for compliance with City requirements contained inECDC 19.10,ECDC 23.80, City of
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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/rgm
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