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Reservoirs Seismic Evaluation Study

Structural, Water


Clackamas County, WA

Oak Lodge Water District


Structural, Water



RH2 evaluated four reservoirs for structural deficiencies and seismic vulnerability. Two of the reservoirs were 5.0 MG partially buried, pre-stressed concrete reservoirs; the other two reservoirs were 2.8 MG welded steel reservoirs. Construction records were utilized to obtain available structural and geotechnical information, and visual inspections and non-destructive tests were performed to check the dimensions and condition of the structures, foundations, and coatings. The structural evaluations were performed, checking each reservoir for conformance with current standards for welded steel and pre-stressed reservoirs. Coupons were cut from the 1966 reservoir and tested to determine the material properties of the steel used at each course, allowing our team to use much more accurate material properties in the structural analysis. This approach saved the District approximately $200,000 in costly weld repairs.

The evaluations found that the steel reservoirs require improvements to their anchorage and foundations to eliminate uplift and reduce soil bearing pressures. The pre-stressed concrete reservoirs could be brought up to code by constructing interior shear curbs to supplement the existing seismic wall cables. RH2 also evaluated the non structural and mechanical elements at the reservoir sites and provided recommendations for improvements.

Our team’s comprehensive analyses determined that it would be significantly less costly to retrofit the existing tanks, opposed to demolishing and replacing them. Evaluations found that both tanks required the addition of anchors to address uplift, and one required a foundation expansion to reduce soil bearing pressures. The retrofit solutions included an expanded foundation, as well as the addition of anchor bolts and chairs at both reservoirs.

RH2 designed improvements for the two 2.8 MG steel reservoirs at the View Acres site based on the phase 1 analysis results. The structural design work includes foundation expansions and the addition of anchor bolts and chairs at both reservoirs. The mechanical design included the addition of a new 16-inch inlet at each reservoir. The existing configuration uses a single inlet and outlet that has been creating problems with their residual levels. The new inlets will help to improve mixing and turnover in each reservoir. Other items include a seismic shutoff valve and expansion joints as well as miscellaneous access and safety improvements.

The steel reservoir improvements at the View Acres Reservoir site were completed in 2013. The analysis for improvements for the two 5.0 MG pre-stressed concrete reservoirs at the Valley View site is complete, and construction will occur in the future.

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