HomeMy WebLinkAboutC_GeoEngineers_Review_Letter_191028.pdf
1101 South Fawcett Avenue, Suite 200
Tacoma, Washington 98402
253.383.4940
October 28, 2019
City of Renton
Community & Economic Development
1055 South Grady Way
Renton, Washington 98057-3232
Attention: Matt Herrera
Subject: Geotechnical Engineering Review Services
Cedar River Apartments
Renton, Washington
File No. 0693-084-00
INTRODUCTION
This report presents comments from our review of geotechnical engineering analyses of the proposed Cedar
River Apartments located at 1915 Maple Valley Highway in Renton, Washington. The site is located south
of Cedar River Park, southwest of Maple Valley Road, and directly north of the Cedar River. Our services are
being provided to the City of Renton Department of Community and Economic Development in accordance
with our agreement dated July 11, 2018 and executed August 2, 2018.
The proposed development will require development in an area currently designated as Channel Migration
Zone (CMZ). We understand that the owner is proposing to have the site reclassified as Disconnected
Migration Area (DMA) based on the presence of an existing concrete wall located at the riverbank. An
analysis of the stability of the wall by Terracon and DCI Engineers has been provided to the City. Our review
of the project is limited to the concrete wall and how it relates to the regulated CMZ.
DOCUMENTS REVIEWED AND REVIEW APPROACH
We reviewed the documents “Bulkhead Wall Stability Addendum” prepared by Terracon Consultants, Inc.,
dated June 10, 2019 and “Bulkhead Wall Stability Addendum” prepared by DCI Engineers, dated June 12,
2019. We provide comments on these documents below.
We also reviewed “Revised Geotechncial Engineering Report, Cedar River Apartments, Renton,
Washington” by Terracon Consultants, Inc. dated October 31, 2018 and preliminary plan sheets prepared
by KPFF Consulting Engineers and Runberg Architecture Group. These were reviewed for background
information only.
City of Renton | October 28, 2019 Page 2
File No. 0693-084-00
“Cedar River Channel Migration Study” prepared by King County Department of Natural Resources, dated
April 2015 was also reviewed to provide context as to standards used to establish the regulated CMZs and
DMAs within the system. This document states that: “A Disconnected Migration Area (DMA) is the area
located landward of an artificial structure that is likely to restrain channel migration and that meets criteria
in Washington Administrative Code 173-26-221(3)(b) and King County (2014).” The document further
states that: “An artificial structure was considered likely to restrain channel migration if its construction,
condition, and configuration are consistent with current relevant design and construction standards and if
the present channel is unlikely to migrate landward of the structure (King County 2014).”
The purpose of our review is to determine if the analyses required to establish if the existing concrete
bulkhead is likely to restrain channel migration have been performed with current relevant design
standards. We have not not evaluated if the structure meets the criteria in the Washington Administrative
Code or other criteria.
GEOTECHNICAL REVIEW
The following presents our review comments with regard to the geotehchnical and structural analysis:
1. Analysis for the bulkhead and its ability to restrain channel migration should be based on relevant
design guides and methods specific to riverine structures. In our opinion United States Army Corps of
Engineers (USACE) design guides provide appropriate design standards. Specific design documents
could include EM 1110-2-1418 “Channel Stability Assessment for Flood Control Projects” and
EM 1110-2-2502 “Retaining and Floodwalls”. Not all elements of these documents will be applicable.
Additionally, there might be other appropriate design guides or design methods that could also be used.
2. Analysis of the bulkhead should include an evaluation of hydraulic conditions. This evaluation should
include, at a minimum, predicted scour at the toe of the bulkhead and the effects of long-term bed
degradation or aggredation and localized bend scour. The report states that scour was not considered
and that scour is being evaluated by others. The hydraulic and scour analysis should be provided for
review and the results should be incorporated in the stability analyses as appropriate. This hydraulic
analysis should also address the length of the bulkhead and if this is sufficient to manage risk of
channel migration at the upstream and downstream ends; specifically, the potential for the river to
flank the bulkhead on the upstream end.
3. We agree with the decision to use a 100-year return period seismic event for the evaluation of the
bulkhead wall for evaluating resiliency and function as a flood or hydraulic control structure. If slope
stability or lateral spread issues will impact inhabited site structures, the appropriate International
Building Code (IBC) seismic loading must be used.
4. The concrete was modeled as a cohesive material. This assumes some tension capacity in the material
and, therefore, must assume that there are no cracks in the concrete. The concrete mass is
unreinforced and, therefore, has likely cracked due to shrinking while curing. The fact that no perched
groundwater was noted on top of the concrete is, in our opinion, evidence that the concrete, while
massive and intact at a local scale, is likely cracked on a larger scale. We believe that a cohesive soil
model can be an appropriate and conservative assumption over short distances such as when
considering potential lateral earth pressures on the bulkhead wall and locally at the toe of the wall.
However, over larger areas, such as were analyzed in the global stability analysis, cracks in the concrete