Global Companies, LLC engaged S. W. COLE ENGINEERING, INC. to provide geotechnical engineering services for a new above-ground fuel storage tank at their tank farm in South Portland, Maine. The 160-foot diameter, 52-foot tall steel tank holds 6.6 million gallons of No. 2 fuel oil. This was the first new tank proposed for this tank farm in many years, and limited sites were available for its construction. The general area was known to be underlain by a deep deposit of low strength compressible clay. The challenge was to safely found this new tank on a shallow foundation, since it was anticipated that the cost of a pile foundation would make the project economically unfeasible.

Understanding Global Companies LLC's needs relative to the project schedule, their tolerance for risk, and their specific requirements regarding construction standards provided a basis for scoping the project. S. W. COLE ENGINEERING, INC. undertook a subsurface exploration program and worked with other design team members to develop a foundation system. We also provided construction oversight and monitored the performance of the tank foundation utilizing geotechnical instrumentation. Data gathered through this process was evaluated to develop a schedule for safe filling of the tank.

The simple solution to founding the proposed tank on the deep, low strength compressible soils would be end bearing piles and a heavy pile cap. This solution posed low risk and met the required schedule, but the cost of this alternative was prohibitive.

S. W. COLE ENGINEERING, INC. explored a variety of shallow foundation alternatives. Issues that were important in this process included the magnitude of short and long-term settlement and the bearing capacity provided by the site's soils. The utilization of a shallow mat foundation without pile support would result in excessive settlement, and bearing capacity safety factors were unacceptable. A program of site preloading, including wick drains to expedite consolidation of the clay soils would provide a solution to the long-term settlement and improve the bearing capacity, but the time required for site preloading did not fit the client's schedule for bringing the tank on line. A non-conventional approach would be needed to make this project work.

We proposed to provide an unload of the site to lessen the magnitude of total and differential settlement and to provide some stiffening of the foundation materials to improve bearing capacity. We concluded that a lightweight cellular concrete fill would provide stiffening of the foundation materials and would offset the weight of some of the tank and its contents.

One major obstacle remained, that being the overall stability of the soils beneath the tank relative to a deep rotational failure. Computer modeling of the site indicated that a failure could occur if the tank was fully loaded without allowing consolidation and strength gain in the foundation soils. We determined that by carefully staging the loading sequence of the tank to allow for consolidation (settlement) of the underlying compressible soils, the necessary strength gain would occur. The associated risk and schedule of this option were discussed in detail with Global Companies, LLC and the tank designers. Considerations for this alternative included the difficulties associated with excavation and backfilling, the handling of groundwater, and provisions for long-term drainage, since if groundwater became too high during the construction phase, or while the tank is empty, it could float the lightweight fill materials and maybe the tank. Geotechnical instrumentation was installed beneath the tank to monitor the progress of consolidation and assess strength gain to allow safe staging of the tank filling.

How did it work?