HomeMy WebLinkAboutExh.20_Secondary_Geotechnical_Review
1101 South Fawcett Avenue, Suite 200
Tacoma, Washington 98402
253.383.4940
December 19, 2019
City of Renton
Community and Economic Development
1055 South Grady Way
Renton, Washington 98057-3232
Attention: Matt Herrera
Subject: Geotechnical Engineering Review Services
Canopy Subdivision
Renton, Washington
File No. 0693-084-00
INTRODUCTION
This report presents comments from our review of geotechnical engineering analyses of the proposed
Canopy Subdivision located at 4196 Lincoln Avenue NE in Renton, Washington. Our services are being
provided to the City of Renton Department of Community and Economic Development in accordance with
our agreement dated November 19, 2019 and executed December 3, 2019.
The proposed development will include encroachment into a protected slope set back. Rockeries and
mechanically stabilize earth (MSE) retaining walls will be constructed as part of the development.
We understand that the main road and utilities will be transferred to the City at the completion of the
project. Alleys will remain as property of the future Homeowner’s Association. We understand that a
variance will be required for the setback encroachment and for proposed MSE retaining walls that exceed
6 feet in height.
DOCUMENTS REVIEWED AND REVIEW APPROACH
We reviewed a geotechnical report prepared for the project by Terra Associates dated March 19, 2019 and
revised August 22, 2019. We also reviewed the Canopy site development plans dated September 11, 2019
and a retaining wall exhibit that includes a manufacturer’s brochure for the Flex MSE retaining wall system.
We provide comments on the geotechnical aspects of these documents below.
EXHIBIT 20
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GEOTECHNICAL REVIEW
Plan and Geotechnical Report Review
The following presents our review comments on the report and project plans. Our review only addresses
geotechnical aspects of retaining walls and slopes.
1. The soil parameters selected for the slope stability analysis and reported in the text appear appropriate
for the soils described and interpreted geologic conditions.
2. The report states that a horizontal acceleration of 0.2g was used in the pseudo-static analysis. It’s not
clear what seismic design level (i.e., return period) this is based on or if a reduction was included. A
seismic event with a 2 percent probability of exceedance in 50 years (about 2,475-year return period
in accordance with the IBC) is typically used to analyze slopes and potential failures that will impact
inhabited structures. A seismic event with a 7 percent probability of exceedance in 75 years (about
1,000-year return period in accordance with American Association of State Highway and Transportation
Officials [AASHTO] and Washington State Department of Transportation [WSDOT]) is typically used to
analyze slopes and potential failures that will impact structures such as retaining walls surrounding or
supporting roadways. For a pseudo-static analysis, one-half of the Peak Ground Acceleration (PGA) is
typically used based on the assumption that some permanent slope movement is acceptable.
3. Groundwater, perched or regional, is not included in the stability analysis. The reported critical failure
surfaces do occur at the contact between the upper weathered glacial till or fill and the lower
unweathered glacial till. This contact is where perched groundwater is indicated in the report. In our
opinion the perched groundwater should be included in the stability analyses at the locations indicated
in the logs and as described in the text. If this perched groundwater condition is expected to be transient
and is only expected to appear in response to recent rains, it would be appropriate in our opinion, to
reduce the extent or omit the perched groundwater from the seismic analysis.
4. The analysis of section B-B’, at the protected slope, models the rockery as having “infinite strength”
and the failure surface search feature limits the failure surface to an area above and below the rockery.
Failure surfaces through the rockery were apparently not considered or not reported. In our opinion the
analysis should either consider a failure surface through a rockery (modeled with an appropriate friction
angle) or a separate calculation should be provided indicating that the internal stability of the rockery
or other retaining system is not the critical failure mechanism. Additionally, both analyses might be
prudent given the proximity to an inhabited structure. As noted in the report Section 4.9 “a rockery is
not intended to function as an engineered structure.”
5. We generally concur with the conclusion in Section 3.4.3 that slopes “are not at risk of a deep-seated
failure in their current state.” However, proposed modifications in the Landslide Hazard Area consist of
cutting into the toe of the slope and leaving the above slope unmodified. In our opinion this increases
the risk that shallow surficial sloughs or even erosion could impact adjacent structures. We suggest
that either the standard minimum setbacks be maintained or that additional details be considered to
mitigate risk of run out damage to the adjacent structures. For example, increasing the height of the
retaining wall above the ground surface to provide a catchment for surface sloughs and erosion.
6. Subsurface explorations are limited in the vicinity of the Landslide Hazard Area and Protected Slope.
The geotechnical report states in Section 5 that Terra Associates, Inc. “should also provide geotechnical
service during construction to observe compliance with our design concepts, specifications, and
recommendations. This will allow for design changes if subsurface conditions differ from those
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anticipated prior to the start of construction.” In our opinion, the geotechnical engineer of record should
confirm that the soil and groundwater conditions in the Protected Slope area are consistent with those
assumed in the analysis and should provide appropriate documentation to the City at the completion
of the project.
7. In Section 4.7, lateral earth pressures do not include pressures for the condition with slopes above the
wall. This condition is shown in multiple locations in the plans. It is also not clear what seismic design
level was used to develop the seismic loading. Different seismic loadings might be appropriate based
on the location of the retaining walls and risks as described in comment No. 2, above.
8. The plans indicate many tall retaining walls close to structures. In our opinion, if the collapse of a
retaining wall can significantly damage an inhabited structure, the wall must be designed to the same
seismic design standards as the adjacent structure. Alternatively, analysis can be provided indicating
that significant movement of the wall in the design seismic event will not impact the adjacent structure
or that the structure would not be damaged if it is impacted. In our opinion, any retaining wall and
structure that are closer horizontally than the vertical exposed height of the retaining wall should be
evaluated with these criteria.
9. The plans indicate a tiered retaining wall system located at Tract C. A global slope stability analysis
should be performed for this condition.
10. In Alley 3, a storm drain and a sanitary sewer are shown behind the proposed MSE retaining wall.
Utilities can be placed close to these walls or even incorporated into the reinforced section. However,
replacing or connecting to these utilities can require demolishing and reconstructing the wall, in whole
or in part. Wall details, once developed, should confirm that utility installation or replacement (including
space for trenching) will not encroach on the reinforced soil section of the wall. Alternatively, if utilities
must encroach on the reinforced zone, there could be a clear written understanding of how costs will
be shared between whomever is responsible for maintaining or replacing the utilities and whomever is
responsible for maintaining the retaining wall.
11. Sheet C3.32 is not sealed by an engineer and does not contain all the information required for
construction. Specifically, soil reinforcement schedule appears to be missing.
12. There are multiple locations, specifically Alley 3 and at the turn around on Alley 1, where a traffic barrier
or guardrail will likely be required above a retaining wall. In these cases, the walls must be designed to
withstand the design impact load from the guardrail and detailed to accommodate the guardrail.
Review of Proposed Flex MSE Vegetated Wall System
In our opinion, MSE walls and retaining systems, also referred to as structural earth walls (SEWs), can be a
very robust systems provided they are properly designed and constructed. The strength of the system
comes from the geosynthetic placed horizontally within the fill. The facing is secondary and prevents surface
sloughing and erosion. Planted faces are often used with MSE wall systems and can be successfully
maintained if appropriate plants are selected. The proposed Flex MSE wall consists of both a geosynthetic
soil reinforcing (geogrid) and a geosynthetic facing (Flex MSE Bag). Although we do not have direct
experience with this specific product, it is a proven concept.
There are some details, however, that should be considered regarding durability of these systems.
The facing, while not critical to the overall stability of the system, is important because it prevents localized
erosion of the face. Facings that only consist of geosynthetics, can be susceptible to damage, especially in
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From: Lyle J. Stone <lstone@geoengineers.com>
Sent: Wednesday, April 8, 2020 11:29 AM
To: Matthew Herrera
Subject: RE: Canopy Subdivision -- Third Party Geotechnical Review
Hello Matt,
We reviewed the comment response letter and reviewed the sections of the report that were indicated
as revised. We did not go into the details of the calculation package or revised report to confirm that
nothing else had changed. It appears everything that was addressed in the Geotech report, but not all
the edits made it to the plan set. There are two remaining issues where it’s not clear that they have
been or will be formally addressed in the final plan set.
1) In the response to comment #4, Terra states that rockeries at the toe of slopes will be replaced with
Ready-Rock walls. The Ready-Rock walls they are proposing are appropriate, in our opinion. But, this is
not yet fully reflected on the plans, only in the area where there is an encroachment on the critical
slope. This original comment was intended for all areas where walls are close to permanent inhabited
structures.
The plans still indicate that there are rockeries right behind the structures in plats 41 – 47. The rockeries
are as tall as 8 feet and have a slope above the rockery. This is not as steep a slope as other areas, but
there is some slope. The condition where there is a rockery with a slope above is not covered in the
typical cross sections. It appears a rockery failure would impact a structure in at least one
location. Furthermore, maintenance, repairs, or replacement of the rockery will be difficult or totally
impractical should it be required in the future.
2) In the response to comment #5 Terra states “a chain-link fence could be added”. In our opinion this
should be addressed more proactively. One of the purposes of that setback at the toe of a steep slope
critical area is to mitigate runout or erosion potential. If the setback is eliminated, there should be
another method to manage the risk of runout or erosion. A chain-link fence can stop some debris, but
it’s not designed for that application. Terra also states that it will be addressed in the field. In general, a
field fit can be an appropriate approach. However, in this case if “additional measures” as stated, are
required it could effectively make the walls taller. It’s not always possible to make those adjustments
once the wall is constructed and it’s clear they are needed. In our opinion it is preferable to design the
walls with the measures included and then omit them if site grades and conditions warrant the change.
Lyle J. Stone, PE, GE
Associate Geotechnical Engineer | GeoEngineers, Inc.
Direct: 253.722.2443
Mobile: 253.820.4874
From: Matthew Herrera <MHerrera@Rentonwa.gov>
Sent: Tuesday, April 7, 2020 11:34 AM
To: Lyle J. Stone <lstone@geoengineers.com>
Subject: RE: Canopy Subdivision -- Third Party Geotechnical Review
[EXTERNAL]
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