HomeMy WebLinkAboutRS_Geotechnical_Report_Addendum2_180309_v1
400 NORTH 34TH STREET, SUITE 100
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206-632-8020 FAX 206-695-6777
TDD 1-800-833-6388
www.shannonwilson.com 21-1-22210-002
December 8, 2017
Mr. Ian McKelvey
Brown and Caldwell
701 Pike Street, Suite 1200
Seattle, WA 98101
RE: GEOTECHNICAL REPORT ADDENDUM, BIOGAS AND HEAT SYSTEMS
IMPROVEMENTS PROJECTS, KING COUNTY SOUTH TREATMENT PLANT,
RENTON, WASHINGTON
Dear Mr. McKelvey:
We are pleased to submit this letter to serve as an addendum to our Revised Geotechnical Report
for the South Treatment Plant Biogas and Heat Systems Improvements project dated
January 6, 2017. This addendum summarizes our recent geotechnical evaluation of three
geotechnical issues that were not addressed in our previous report:
1. Infiltration rates to aid in the design of stormwater infiltration facilities
2. Pavement sections for a small area of driveway/parking
3. Evaluation of coal mine hazards
Our services were performed in accordance with Amendment No. Three to the Subcontract
between Brown and Caldwell and Shannon & Wilson, Inc. for Geotechnical Services, dated
June 5, 2017.
STORMWATER INFILTRATION RATES
Shannon & Wilson staff visited the proposed project site from October 9 to 12, 2017, to perform
pilot infiltration tests (PITs) to evaluate suitability of soils for infiltration facilities. Clearcreek
Contractors (Clearcreek), under subcontract to Shannon & Wilson, Inc., excavated three test pits
designated TP-1, TP-2, and TP-3 at locations determined by the project civil engineer, Davido
Consulting Group. The approximate locations of the test pits are shown on the attached
Ian McKelvey
Brown and Caldwell
December 8, 2017
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Figure 1 – Test Pit Locations. Clearcreek used a vacuum truck to pothole and locate subsurface
utilities prior to excavating the pits. Previously unknown utilities were encountered at all three
test pit locations before and during excavation. Photo 1 shows a presumed electrical conduit
running east to west at TP-2 at a depth of approximately 3 feet below ground surface (bgs).
Photo 1: Electrical Conduit Running East to West at TP-2.
Clearcreek used a mini excavator to excavate the test pits. TP-1 was excavated to a depth of
approximately 7 feet bgs, while TP-2 and TP-3 were excavated to a depth of approximately
10 feet bgs. Soils encountered during excavation generally consisted of dense, silty sand and
gravel, followed by soft, gray silt. After excavation, we performed PITs at each of the test pits.
PITs were performed in accordance with the 2017 City of Renton Surface Water Design Manual.
We filled each test pit with water to a depth of approximately 12 inches, then allowed the base of
the PITs to soak for six hours. We then performed constant head and falling head tests.
Ian McKelvey
Brown and Caldwell
December 8, 2017
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We observed relatively little infiltration at all three PITs during the constant and falling head
tests. At TP-1, we observed an infiltration rate of approximately 0.02 inch/hour (without
correction factor), while PIT data at TP-2 and TP-3 indicated negligible infiltration rates. Based
on these observations, in our opinion, water infiltration facilities are unfeasible at the depths
tested in all three PITs.
Following testing, Clearcreek backfilled the test pits with previously excavated soils. We spread
grass seed on the disturbed ground surface at TP-1 and TP-2, as shown in Photo 2. We also
marked the surfaces of each backfilled test pit location at areas where unknown subsurface
utilities were encountered.
Photo 2: Grass Seed Spread on TP-1. White Line Indicates an Unknown Utility Observed.
PAVEMENT SECTION DESIGN
We understand that a relatively small section at the proposed Facility will be paved for vehicle
access and parking. This paved area or areas will likely experience vehicle loading comparable
Ian McKelvey
Brown and Caldwell
December 8, 2017
Page 4 of 7
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to current loading conditions at the South Plant. During our recent site visits, we did not observe
significant damage to the existing roadways, indicating that current pavement design at the South
Plant is adequate for anticipated traffic and loading conditions. We recommend adopting the
existing roadway construction methods when designing for new paved areas around the Facility.
If as-builts are unavailable, or if construction methods of existing roadways cannot be
determined before new paved sections are designed, we recommend referring to Table 4.3 of
Chapter 4 of the 2016 King County Road Design and Construction Standards. In our opinion,
the native soil subgrades have resilient modulus values of 10,000 pounds per square inch or
greater. For commercial access roads, King County recommends 6 inches of hot mix asphalt and
6 inches of crushed surfacing base course. This is based on a design equivalent single axle load
of less than 2,500,000 trips.
COAL MINE HAZARD EVALUATION
The City of Renton has designated the proposed Facility site as located within a critical area
consisting of a moderate coal mine hazard. The site has this designation because it is within
approximately 200 feet of a mapped historical coal mine consisting of a series of mine adits
extending north from the former mine entrance near the northeast corner of the Cogeneration
Building, as shown on the City of Renton map below:
Ian McKelvey
Brown and Caldwell
December 8, 2017
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According to the reference documents linked to the City’s sensitive areas website, County Map
K29 (King County, 1997 1), this mine operation belonged to the Diamond Coal Company. No
other information is available. Based on the relatively specific mapped location of the adits
1 County, King, 19970224, Coal Mine Hazards: King County, King County, WA.
http://www5.kingcounty.gov/gisdataportal/Default.aspx
Ian McKelvey
Brown and Caldwell
December 8, 2017
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associated with the Diamond Coal Company mine, it is our opinion that the mine adits do not
extend beneath the footprint of the proposed Heat and Energy Recovery building or the proposed
thermal oxidizer.
During our 2016 subsurface investigation, summarized in our January 6, 2017 report, we
advanced two borings to depths of about 91.5 to 121.5 feet bgs at the location of the proposed
Heat and Energy Recovery building. Boring results indicated that the project site is underlain by
sandstone bedrock consisting of the Renton Formation at depths ranging from 80 to 111 feet bgs.
This formation is known for containing coal beds, some used for mining purposes. The soils
above the bedrock consist of alluvial sediments that were not associated with coal mines.
Previous subsurface explorations, including borings and test pits by others, also did not locate
coal mining evidence at the Heating and Energy Recovery building site. Our recent TP-3 and
test pits previously performed by others nearby the proposed thermal oxidizer site did not expose
evidence of mining activity. In our opinion, there is no subsurface or surface evidence of past
mining activity at the proposed Facility site and there is low probability that construction of the
proposed Facility is at risk of coal mine hazards such as subsidence. Coal mine hazards are not a
design issue that require mitigation for the proposed project.
CLOSURE AND LIMITATIONS
This letter was prepared for the exclusive use of Brown and Caldwell and King County to aid in
design of the King County South Treatment Plant Biogas and Heat Systems Improvements. This
letter should be relied on for factual data only, and not as a warranty of subsurface conditions,
such as those interpreted from our observations. The conclusions and recommendations
contained in this letter are based on site conditions observed during our site visit.
Within the limitations of the scope, schedule, and budget, the conclusions and recommendations
presented in this letter were prepared in accordance with generally accepted professional
geotechnical engineering principles and practices in the area at the time this letter was
prepared. We make no other warranty, either express or implied.
The scope of our services for this project did not include any environmental assessment or
evaluation regarding the presence or absence of wetlands or hazardous or toxic materials in the
soil, surface water, groundwater, or air on, below, or around the site. Shannon & Wilson, Inc.
1
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DESIGNED/DRAWN:
PROJECT ENGINEER:
PROJECT ACCEPTANCE:
DESIGN APPROVAL:
CONTRACT NO:
DRAWING NO:
PROJECT FILE NO:SCALE:
FACILITY NUMBER:
DEPARTMENT OF NATURAL RESOURCES & PARKS
WASTEWATER TREATMENT DIVISION
DATE:
SHT NO / TOTAL REV
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SOUTH TREATMENT PLANT
BIOGAS AND HEAT SYSTEMS IMPROVEMENTS
MAR 2017
15-xxxxxxx
NO REVISION DESCRIPTION BY APVD DATE
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PRELIMINARY ISSUE DRAWING
30% DESIGN
APRIL 2017 S. Hildreth
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Page 1 of 2 1/2017
SHANNON & WILSON, INC.
Geotechnical and Environmental Consultants
Dated:
Attachment to and part of Report 21-1-22210-002
Date: December 8, 2017, 2017
To: Mr. Ian McKelvey
Brown and Caldwell
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 which 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.
Page 2 of 2 1/2017
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