The client requested an in-depth study of installation requirements for underground storage tanks (USTs) when installed in poor in situ soils.

Current installation requirements for USTs pertaining to trench width and minimum soil shear strength had been in place but were not quantified by analysis. The client requested a study of UST installations in poor soils as well as a seismic study concerning liquifaction of granular soils. The tank and soil were modeled using the Finite Element Analysis (FEA) program CANDE.

The minimum soil cohesion limit from the client’s installation requirements was 750 psf. The study showed that soils with 500 psf cohesion had adequate factors of safety, provided that the side width of the trench was at least 2 feet.

Soil liquefaction during a seismic event was studied as well. If the soil were to liquefy, there would be an increase in pore pressure and the possibility that the tank would float out. Using conservative assumptions, the probability of liquefaction of soils supporting USTs was estimated as one in approximately 122,000 tank installations.

Alternate installation methods in poor soils were also provided. These methods included using Geogrid, FRP rebar, CMU walls or a Gunite liner for the walls of the trench.