Plant 129 Reservoir R-1 – Structural Analysis, Covina CA

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Suburban Water Systems (SWS) needed to perform a structural analysis and evaluation of the existing concrete reservoir (R-1) at Plant 129 in West Covina, CA.  The plant, located off South Lark Ellen Ave, is comprised of two separate above-ground reservoirs, a newly constructed pump station (2005), and associated piping.  Reservoir R-1 is a 120’ diameter, 2.5 MG concrete structure constructed circa 1956.  Reservoir R-2 is a newer 170’ diameter, 5.0 MG steel reservoir whose construction date  isunknown (assumed circa 1985).  When initially constructed around 1956, Reservoir R-1 was designed and constructed below grade.  The sequence of construction would have consisted of excavating below the existing ground surface at that time, sloping down the excavations around the perimeter at a 1:1 gradient, constructing the ring foundation, forming and pouring the outer side of the tank wall, and then shotcreting against the outside of the concrete walls. Following construction of the perimeter wall, backfilling was performed with onsite soils. However, with the construction of Reservoir R-2 and more importantly with the construction of the new pump station in 2005, the existing grades around R-1 were lowered exposing a side of the reservoir for approximately half of its circumference.  The grades were lowered exposing as much as twenty feet of the concrete wall that was previously below grade.

Although there are no visible signs of structural damage or failure, SWS requested PacRim to perform a structural analysis of the existing concrete reservoir to determine if the removal of soil around the perimeter of the reservoir will have any long term negative impacts to the structural integrity.

PacRim made an engineering judgment and determination of the as-built material properties for the purpose of structural analysis. Assessing the material structural parameters requires determining a statistical or empirical “aging” reduction factor to apply to the material strength of the 60 year old reservoir.  Based on current guidelines for assessing similar existing structures and materials, the reduced structural material strength was used in verifying the adequacy of structural member capacities based on computed demands of the structural analysis.

PacRim analyzed the existing structure, including service and dynamic loading conditions.  Dynamic loading includes convective and impulsive inertial forces created during a seismic event, and was determined using methodology outlined in ANSI/AWWA D-110 standard and site geotechnical information.   PacRim identified potential deficiencies related to the reservoir’s global stability due to overturning and sliding, structural member and foundation capacity and approximate remaining service life.

Upon the completion of the structural evaluation study in Task B, PacRim provided retrofit alternatives to provide stabilization of the reservoir and increase its overall lifespan.    Service life of the structure can be increased by renewing regions of the structure under maximum distress.