HomeMy WebLinkAboutRS_Geotechnical_Report_Addendum1_180309_v1SHANNON &WILSON, INC.
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
March 16, 2017
Mr. Ian McKelvey
Brown and Caldwell
701 Pike Street, Suite 1200
Seattle, WA 98101
RE: ADDENDUM TO GEOTECHNICAL REPORT, KING COUNTY SOUTH
TREATMENT PLANT BIOGAS AND HEAT SYSTEMS IMPROVEMENTS,
RENTON, WASHINGTON
Dear Mr. McKelvey:
This letter is an addendum to our geotechnical report dated January 6, 2017, for the King County
South Treatment Plant Biogas and Heat Systems Improvements. We understand that a cut -and -
cover tunnel is proposed to run under the biogas facility, and that lateral earth pressure
equivalent fluid densities are needed for design of the tunnel. According to drawings provided
by Roberts Engineering, tunnel finished grade will be about 13 feet below the main building
floor slab. We understand that backfilling around the tunnel will commence simultaneously on
both sides of the tunnel following tunnel roof construction. The main building floor slab/tunnel
roof will therefore provide lateral support, preventing lateral deflection of the tunnel walls, and
allow for passive and at -rest lateral earth pressures to develop rather than active earth pressures.
We recommend that passive soil resistance on tunnel walls be designed based on an equivalent
fluid density of 300 pounds per cubic foot (pcf), consistent with recommendations made in our
geotechnical report. We recommend that the tunnel walls also be designed based on an at -rest
equivalent fluid density of 53 pcf using a triangular pressure distribution. The total at -rest earth
pressure should be analyzed for seismic loading conditions using a dynamic load increment
equal to 12H, where H is the height of excavation. The tunnel mat foundation bearing on
compacted imported or existing fill may be designed using a coefficient of base friction of 0.40.
These values are based on the assumption that tunnel construction is above groundwater and that
all backfill material placed behind tunnel walls will be structural fill systematically compacted in
6- to 10 -inch -thick loose lifts.
400 NORTH 34TH STREET, SUITE 100
P.O. BOX 300303
SEATTLE, WASHINGTON 98103-8636
206-632-8020 FAX: 206-695-6777
www.shannonwilson.com
21-1-22210-001
Mr. Ian McKelvey
Brown and Caldwell
March 16, 2017
Page 2 of 2
SHANNON 6WILSON, INC.
Please call if you have any questions regarding this letter. We appreciate the opportunity to be of
service.
Sincerely,
SHANNON & WILSON, INC.
Martin W. Page, PE, LEG
Vice President
Geotechnical Engineer
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