Gibson040405April 4, 2005
Mr. Jim Gibson
City of Yelm
Public Works Department
P.O. Box 479
Yelm, WA 98597
RE: TAHOMA TERRA
Sanitary Sewer Design Concept
SCA #04104
Dear Jim:
The sanitary sewer design for Tahoma Terra Master Planned Community will consist of a typical gravity sewer collection system i.e. minimum 8” diameter PVC collection mains, 48” diameter
concrete manholes spaced approximately 400’ apart along the main line. Each lot will have a 4” diameter gravity sewer service line stubbed to it. Resultantly there will be no onsite
S.T. E. P. tanks for the single-family lots unless otherwise specifically shown on our civil drawings where gravity flow cannot be achieved.
Domestic sewerage will flow by gravity to three 50,000 gallon fiberglass underground septic tanks installed near the intersection of Longmire Road extended to the new Tahoma Boulevard,
i.e. the community park area. The septic tanks will be designed using City of Lacey design criteria (Chapter 7) of one 50,000 gallon tank per 80 ERU’s. It is anticipated the system
will serve the 216 proposed lots in Divisions 1 and 2, plus the commercial building along Tahoma Boulevard.
The proposed Fairway Townhomes will be served by their own S.T.E.P. system and effluent force main discharging into the new S.T.E.P. force main in Longmire Road.
Septic effluent from the three 50,000 gallon S.T.E.P. tanks will be pumped by a new duplex effluent pump system located on the outlet side of the tanks and discharging to a 4” diameter
force main in Longmire Road. Pump controls will be installed in a locked room in the community park building.
The tanks, pumps and controls will be sized and operated to control or restrict flow to the sewage treatment plant to flow volumes within down stream infrastructure capacities.
The new 4” diameter force main on Longmire Road will also be sized to service future development between Tahoma Terra and Mossman Avenue to the east.
In conjunction with the road improvements of Killian Road and Tahoma Boulevard, a new sewer effluent force main will be designed and installed along Tahoma Boulevard that will eventually
collect sewerage from Tahoma Terra Commercial and Residential Divisions 3 through 9 to the west.
The piping and controls for Divisions 1 and 2 system will be designed to switch force main flows between Killian Road and Longmire Road outlets to the Sewage Treatment Plant as the City
sees fit in the future.
This system design is intended to reduce the number of individual S.T.E.P. tanks and reduce overall long term City maintenance costs of individual pumps. The new system will require
the City to provide somewhat different maintenance i.e. gravity and force main sewer flushing and larger community effluent pump operation and maintenance tasks.
We are in the process of preparing final engineering drawings for all road and utility improvements. The sewer collection and pumping design drawings will be included in this process.
We are using the City of Lacey’s Development Guidelines, specifically, Chapter 7 for design specification. This information was provided to you in our recent Master Plan Submittal.
Should you have any questions please call me.
Yours truly,
SCA CONSULTING GROUP
Steven L. Chamberlain, P.E.
President
CC: Doug Bloom
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