04-0180 STEP tank installation 121605
City of Yelm
Community Development Department
105 Yelm Avenue West
P.O. Box 479
Yelm, WA 98597
December 16, 2005
Doug Eklund P.E.
Jerome W. Morrissette and Assoc.
1700 Cooper Point Road SW B-2
Olympia, WA 98502
RE: Ridgeline Junior High School – SPR-04-0180 -YL
Step Sewer Tank Installation in High Ground Water
Dear Mr. Eklund
Thank you for your letter dated December 15, 2005. Today, I met with Tim Peterson, Director of Public Works and Jon Yanasak, Sewer Department Manager to discuss the options that you
presented in your letter.
The information presented in your letter is based on the ground water level not exceeding the elevation of 347.4 (the mid point of the tank) and or monitoring the level of the ground
water with a peizometer after the tank is installed. We agree that with the ground water level at mid tank and the tank being empty the safety factor is approximately 1.7. However
the City can not assume that the ground water level will stay at this elevation. The indications are that the winter ground water level changes frequently and often based on the amount
of precipitation each year. The past 2 years we have had fairly dry winters which would suggest that the water table is also unseasonable low. If the ground water elevation was to
rise 1.8’ to 2.0’ above the mid tank elevation the safety factor would drop to approximately 1.2. The 1.2 safety factor is the minimum Buoyancy Safety Factor the Xerxes recommends.
We have consulted the tank manufacture and they have stated, “It is the tank owner’s responsibility to determine the suitability and applicability of installation as stated in our Limited
Warranty.” Xerxes also stated that if the step tank was to be displaced by the ground water table then the warranty may be null and void. When Ridgeline Junior High School has completed
construction they will be completing a bill of sale and dedication of the tank to the City of Yelm for ownership and maintenance.
The City also looked at the idea of using the peizometer to monitor ground water and possibly coordinate maintenance around the high ground water season. This option is not feasible
due to the possibility of the tank requiring maintenance during a high
ground water season. The school district does not have the option of closing down the school because the STEP sewer system can not be maintained. The school will expect the City of
Yelm to maintain sewer service to the site at all times.
After reviewing the information you submitted, the Installation Manual and Operating Guidelines for Xerxes tanks and soil test logs dated October 2004 it is the City requirement that
an anchoring system is required for the 50,000 gallon step sewer tank being installed at Ridgeline Junior High School. This system shall be designed per the Xerxes Installation Manual,
be reviewed by the tank manufacture before installation and approved by the City of Yelm.
If you have any additional questions or comments please do not hesitate to call or contact me at (360) 458-8438 or jimg@ci.yelm.wa.us.
Community Development Department
Respectfully,
Jim Gibson P.E.
Development Review Engineer
Erling Birkland, Yelm Community Schools
Tim Peterson, City of Yelm
Jon Yanasak, City of Yelm