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400 North 34th Street Suite 100 PO Box 300303 Seattle, Washington 98103-8636 206 632-8020 Fax 206 695-6777
www.shannonwilson.com
March 26, 2021
Mr. Jeff Hansen, PE
HDR Engineering, Inc.
905 Plum Street SE, Suite 200, Town Square 3
Olympia, WA 98501
RE: KING COUNTY COMPOST FACILITY GEOLOGIC HAZARD REVIEW,
RENTON, WASHINGTON
Dear Mr. Hansen:
HDR Engineering, Inc. requested Shannon & Wilson provide geotechnical
recommendations related to mapped steep slope and coal mine hazard at the proposed King
County Biosolids Compost Facility (Project) at the King County South Treatment Plant
located at 1200 Monster Road Southwest in Renton Washington. This letter, which includes
the requested geotechnical recommendations, was prepared for the exclusive use of HDR
and their representatives for the Project. Our assessment, interpretations, conclusions, and
recommendations are based on:
The limitations of our approved scope, schedule, and budget described in an email from
HDR dated March 2, 2021, and an existing King County Contract (No. E00548E18) for
the King County Wastewater Treatment Division Resource Recovery Work Order No. 5
dated March 24, 2021.
Our understanding of the Project and information provided by HDR on March 2, 2021.
Conditions we observed during our visit site on March 18, 2021, and our understanding
of information that you provided prior to and during that visit.
Our previous experience near the Project site from the King County South Treatment
Plan Biogas and Heat Systems Improvements Project in 2017.
Review of publicly available documents.
PROJECT UNDERSTANDING
Shannon & Wilson understands the Project will consist of constructing a pilot biosolid
compost facility within the approximate footprint of a former pilot biogas facility. Based on
our correspondence with HDR, the existing pilot biogas facility will be removed and the
replacement compost facility will generally consist of a paved surface with cells for the
EXHIBIT 12
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HDR Engineering, Inc.
March 26, 2021
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various stages of compost, contained by precast concrete blocks (e.g., ecology blocks),
stacked one to two blocks high.
SITE DESCRIPTION
The Project site is located adjacent to a steep slope as defined by the City of Renton Critical
Areas Regulation No. 4‐3‐050. Cells for the Screening Area, Curing Area, Compost Bunker,
and Biofilter will be located near the top of the north facing slope. As shown in Figure 1, the
slope is identified to be approximately 15% to more than 40%. Based on the Paving and Site
Grading Plan (HDR, 2021) and our site visit, the slope appears to be inclined at
approximately 40% and is up to approximately 12.5 feet tall. The steeper portion of the
mapped steep slope area is a contouring artifact associated of a concrete structure.
The Project site is also partially within a mapped moderate coal mine hazard. We
understand the site has this designation because of its proximity to the mapped historical
coal mine. The coal mine consists of a series of mine adits extending north from the former
mine entrance near the northeast corner of the Cogeneration Building, as shown in Figure 1.
SUBSURFACE CONDITIONS
Our experience with past projects and mapped geology at this location indicates that the
following geologic units (as encountered from lower to higher elevation) are present at the
Project site: Holocene Fill, Holocene Alluvium, layers of floodplain and organic deposits,
and bedrock consisting of the Renton Formation. Based on the previous explorations
completed by Shannon & Wilson and others nearby, the Project site has encountered loose
to medium dense, silty sand with gravel; soft to stiff silt; and medium stiff, organic silt. The
location of the borings in conjunction with the soil types generally indicate that the soils are
of alluvial or lacustrine origin overlain by recent fill materials in some areas.
FIELD RECONNAISSANCE
We visited the site on March 18, 2021, to review the existing topography and document
observed surficial conditions that could indicate either evidence of slope instability or an
active hazard associated with the former coal mine. The existing slope along the north side
of the site appears to be a constructed cut to provide vehicle access to the digesters located
to the east of the site. The mapped “protected” steep slope, identified in Figure 1, is a
vertical concrete wall. The remainder of the slope is approximately 40% or less and
landscaped. During our site visit, we did not observe any indications (e.g., tension cracks,
scarps, or pistol butted trees) of instability. Our observations of the slope indicate the
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HDR Engineering, Inc.
March 26, 2021
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surficial soils are predominantly silty sand with no visible seepage emitted from the slope
face. Our recent observations are generally consistent with the soils observed and
documented in the historic borings.
During our visit, we also visited the Project area where the mapped coal mine hazard is
located. We did not observe any evidence of subsidence that would suggest an immediate
hazard or that conditions at the site are different than previously identified.
GEOLOGIC HAZARD REVIEW
Coal Mine Hazard
According to the reference documents linked to Renton’s COR Map geographic information
system (GIS) mapping website (City of Renton, 2021a), there was previous coal mining
activity below the Project site. Based on the publicly available information, this mine
operation belonged to the Diamond Coal Company. No other information is available.
Based on the relatively specific mapped location of the adits associated with the Diamond
Coal Company mine, it is our opinion that the mine adits do not extend beneath the
footprint of the proposed pilot biosolid compost facility.
During our 2016 subsurface investigation program for the Biogas and Heat System
Improvements (Shannon & Wilson, 2017a and 2017b), we advanced two borings to depths of
about 91.5 to 121.5 feet below ground surface (bgs) at the location of the proposed Heat and
Energy Recovery building, north of the Project site, as shown in Figure 1 (Shannon &
Wilson, 2017a and 2017b). Subsurface information obtained from the explorations indicated
that the Project site is underlain by sandstone bedrock consisting of the Renton Formation at
depths ranging from approximately 80 to 111 feet bgs. This formation is known for
containing coal beds, some of which that were mined. The soils above the bedrock consist
of alluvial sediments that were not associated with coal mine operations.
Previous subsurface explorations, including borings and test pits by others, also did not
locate coal mining evidence at the facility (Shannon & Wilson, 2017a and 2017b). In our
opinion, there is no subsurface or surface evidence of past mining activity at the proposed
pilot biosolid compost facility and there is low probability that construction of the proposed
Project is at risk of coal mine hazards such as subsidence. Also, given our understanding
that the proposed Project will largely consist of paving and stacked ecology blocks, we do
not anticipate coal mine hazards to be a design issue that require mitigation.
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HDR Engineering, Inc.
March 26, 2021
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Steep Slope Hazards
The proposed footprint of the Project site is at the crest of a Regulated Slope as identified by
Renton’s COR Map GIS mapping website (City of Renton, 2021a). Based on the Paving and
Site Grading Plan (HDR, 2021), the slope is between the approximate elevations of 114 to
127 feet. In our opinion, the existing north‐facing slope should be classified as a sensitive
slope under the Renton Municipal Code 4‐3‐050G based on the inclination (City of Renton,
2021b). Also, consistent with the Renton Municipal Code 4‐3‐050G, it is our opinion that a
setback buffer is not required. We anticipate the setback distance from the top of the slope
based on the 60% Design Drawings is sufficient to avoid excessive surcharge on the slope.
RECOMMENDATIONS AND CONCLUSIONS
In our opinion, the development of the King County facility at this location, where the
roadway is below the above‐mentioned critical slope, is likely a cut‐slope created to provide
access to other parts of the facility. The proposed setback buffer and development of the
biosolids compost facility at the top of the slope, in our opinion, will not impact the cut
slope stability. We believe the mapped coal mine hazards do not present a design issue or
hazard that could impact the Project.
For purposes of our recommendations and conclusions, it was necessary for us to assume
that the results of the past explorations are representative of conditions throughout the
Project site. However, subsurface conditions should be expected to vary. We may need to
revise our recommendations during construction if different conditions are encountered.
Our recommendations and conclusions are based on limited past explorations and site visit
on March 18, 2021. Additional explorations may be required to develop final design
recommendations for this Project.
The subsurface conditions are interpreted from materials observed in historic explorations
completed for King County projects at the South Wastewater Treatment Plant. Variations
between the interpretation shown and actual conditions will exist.
We have prepared our recommendations for the construction of the pilot compost facility
and considering the Project configuration as described in Drawings SP220‐G‐00002, SP220‐
C‐00002, and SP220‐C‐10002 prepared by HDR. When the owner or designer develops
additional information about final ecology block wall configurations or other factors, the
recommendations presented herein may need to be revised. Shannon & Wilson should be
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HDR Engineering, Inc.
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made aware of the revised or additional information so that we can evaluate our
recommendations for applicability.
We recommend that during construction of the pilot biosolids compost facility a
representative from Shannon & Wilson complete a weekly site visit to observe the
conditions of the slope and Project site. This will allow Shannon & Wilson to compare the
conditions exposed during construction to our observations on March 18, 2021, and the
interpreted conditions from the past subsurface explorations completed within the King
County facility. These visits will ensure our conclusions are consistent with the conditions
encountered during construction.
LIMITATIONS
Our scope of services did not include:
Evaluating the presence or absence of wetlands, cultural resources, or hazardous
materials at or around the site.
Performing subsurface explorations at the site.
Performing geotechnical engineering analyses for slope stability.
Our recommendations and conclusions rely on part upon explorations performed by others.
We did not review samples and cannot confirm that the explorations performed by others
are representative of the site conditions. We also relied upon the City of Renton mapping
data in our evaluation of coal mine hazards. If conditions different from those described
herein are encountered during construction, we should review our description of the
subsurface conditions and reconsider our conclusions and recommendations.
This letter should not be used without our approval if any of the following occur:
Conditions change due to natural forces or human activity under, at, or adjacent to the
site.
Assumptions stated in this letter have changed.
Project details change or new information becomes available such that our assessment,
conclusions, and recommendations may be affected.
If the site ownership or land use has changed.
A substantial lapse of time has passed since the issuance of this letter.
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REFERENCES
City of Renton, 2021a, City of Renton COR Maps, Interactive online GIS Mapping
Application, City of Renton, available:
https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps
City of Renton, 2021b, Renton Municipal Code: A Codification of the General Ordinances of
the City of Renton, Washington, available:
https://www.codepublishing.com/WA/Renton/
HDR Engineering, Inc., 2021, King County South Wastewater Treatment Plant Loop Pilot
Compost Facility, Drawings SP220-G-00002, SP220-C-00002, SP220-C-10002.
Drawings prepared by HDR Engineering, Inc. for King County Department of
Natural Resources and Parks, Wastewater Treatment Division, King County,
Washington.
Shannon & Wilson, 2017a, Geotechnical Report Addendum, Biogas and Heat Systems
Improvement Projects, King County South Treatment Plant, Renton, Washington:
Report prepared by Shannon & Wilson, Inc. for Brown and Caldwell, Seattle,
Washington.
Shannon & Wilson, 2017b, Draft Geotechnical Report, King County South Treatment Plant,
Biogas and Heat Systems Improvements, Renton, Washington: Report prepared by
Shannon & Wilson, Inc. for Brown and Caldwell, Seattle, Washington.
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Attachment to and part of Report: 106905-001
Date: March 26, 2021
To: Mr. Jeff Hansen, PE
HDR Engineering, Inc.
Important Information About Your
Geotechnical/Environmental Report
CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS.
Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil
engineer may not be adequate for a construction contractor or even another civil engineer. Unless indicated
otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated.
No one other than you should apply this report for its intended purpose without first conferring with the
consultant. No party should apply this report for any purpose other than that originally contemplated without
first conferring with the consultant.
THE CONSULTANT’S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS.
A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set
of project-specific factors. Depending on the project, these may include the general nature of the structure and
property involved; its size and configuration; its historical use and practice; the location of the structure on the
site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the
additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, ask
the consultant to evaluate how any factors that change subsequent to the date of the report may affect the
recommendations. Unless your consultant indicates otherwise, your report should not be used (1) when the
nature of the proposed project is changed (for example, if an office building will be erected instead of a parking
garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered
on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the
location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for
application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are
not consulted after factors that were considered in the development of the report have changed.
SUBSURFACE CONDITIONS CAN CHANGE.
Subsurface conditions may be affected as a result of natural processes or human activity. Because a
geotechnical/environmental report is based on conditions that existed at the time of subsurface exploration,
construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the
consultant to advise if additional tests are desirable before construction starts; for example, groundwater
conditions commonly vary seasonally.
Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater
fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a
geotechnical/environmental report. The consultant should be kept apprised of any such events and should be
consulted to determine if additional tests are necessary.
MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS.
Site exploration and testing identifies actual surface and subsurface conditions only at those points where
samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an
opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or
abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in
your report. While nothing can be done to prevent such situations, you and your consultant can work together to
help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be
particularly beneficial in this respect.
RECEIVED
Clark Close 04/19/2021
PLANNING DIVISION
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A REPORT’S CONCLUSIONS ARE PRELIMINARY.
The conclusions contained in your consultant’s report are preliminary, because they must be based on the
assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions
throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should
retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who
prepared the report is fully familiar with the background information needed to determine whether or not the
report’s recommendations based on those conclusions are valid and whether or not the contractor is abiding by
applicable recommendations. The consultant who developed your report cannot assume responsibility or
liability for the adequacy of the report’s recommendations if another party is retained to observe construction.
THE CONSULTANT’S REPORT IS SUBJECT TO MISINTERPRETATION.
Costly problems can occur when other design professionals develop their plans based on misinterpretation of a
geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work
with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and
environmental findings, and to review the adequacy of their plans and specifications relative to these issues.
BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE
REPORT.
Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site
personnel), field test results, and laboratory and/or office evaluation of field samples and data. Only final boring
logs and data are customarily included in geotechnical/environmental reports. These final logs should not, under
any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may
commit errors or omissions in the transfer process.
To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready
access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If
access is provided only to the report prepared for you, you should advise contractors of the report’s limitations,
assuming that a contractor was not one of the specific persons for whom the report was prepared, and that
developing construction cost estimates was not one of the specific purposes for which it was prepared. While a
contractor may gain important knowledge from a report prepared for another party, the contractor should
discuss the report with your consultant and perform the additional or alternative work believed necessary to
obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken
impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates
them from attendant liability. Providing the best available information to contractors helps prevent costly
construction problems and the adversarial attitudes that aggravate them to a disproportionate scale.
READ RESPONSIBILITY CLAUSES CLOSELY.
Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact
than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against
consultants. To help prevent this problem, consultants have developed a number of clauses for use in their
contracts, reports, and other documents. These responsibility clauses are not exculpatory clauses designed to
transfer the consultant’s liabilities to other parties; rather, they are definitive clauses that identify where the
consultant’s responsibilities begin and end. Their use helps all parties involved recognize their individual
responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report,
and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to
your questions.
The preceding paragraphs are based on information provided by the ASFE/Association of Engineering Firms
Practicing in the Geosciences, Silver Spring, Maryland
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