Long-term performance of aboveground storage tanks (AST) depends on well-designed foundations and thorough subsurface explorations. At many sites, the optimal tank locations have already been developed, leaving the remaining AST locations with problematic soil conditions, including uncontrolled fill, undocumented fill, dredge spoils and natural low-strength deposits, which may require remediation. In contrast to typical building foundations, the large area load of AST foundations results in loads transmitted to considerable depths nearly twice the tank diameter. Tank diameters can exceed 300 feet, requiring understanding of the subsurface conditions to a depth of 600 feet. Exploration to these depths is expensive and often does not benefit the design. Therefore, engineers frequently test the upper 100 to 200 feet and rely on soil stratigraphy maps from the U.S. Geological Survey or other entities to understand deeper strata and select a foundation option in accordance with API acceptance criteria.
Foundation options for ASTs vary. The economical option of ground improvement (GI) to provide efficient, long-term performance for ASTs has gained wide acceptance in the past 15 years. GI foundations are more flexible than a rigid pile foundation, offering many advantages that reduce schedule and often result in substantial cost savings. GI allows the designer to match overall sensitivity of the tank and piping to withstand some movement to the type of foundation. Current GI options available for ASTs include wick drains with surcharging, stone columns (aggregate piers), rigid inclusions (controlled modulus columns, controlled stiffness columns or displacement piles) and soil mixing.
Surcharging is the preconstruction temporary loading of a site with a mound of soil to reduce the settlement the tank will experience. The approach is economical but time consuming, which may preclude it based on schedule. When supplemented with wick drains, the surcharge time is greatly reduced but may still be excessive.
Stone columns are made of dense crushed aggregate capable of carrying much higher loads than the soil that surrounds them but, if the soil is too soft, settlements can still be excessive. Once constructed on grid spacing, the columns work in a composite action with the untreated soil to reduce settlements and increase bearing capacity.
If stone columns cannot sufficiently reduce settlement or increase bearing capacity, rigid inclusions provide enhanced reinforcement of weak soils. These inclusions use displacement construction techniques, which provide some densification in sandy soil profiles. A special mandrel is advanced to design depth, filled with a high strength grout and then extracted, leaving a small diameter (less than 18 inches) grout cylinder in the soil. These inclusions are placed on designed centers to control settlements and increase capacities.
Soil mixing amends the soil with binders, greatly increasing soil strength to accommodate nearly any tank load. Constructing a grid of columns or treating 100 percent of the tank footprint (mass treatment) allows the highest tank capacities with the least amount of settlement.
If soil is prone to liquefaction in seismically active areas, the tank design and foundation support can be as challenging as dealing with soft soils. Often stone columns or aggregate piers mitigate granular soil sites by densification. In liquefiable soils with sufficient clay or silt fines to resist densification, the arrangement of soil mix columns can form closed cells and control liquefaction induced settlement. Tanks around the world have performed well during earthquakes thanks to GI.
Numerous GI options exist for tank foundations and depend on required performance and other local factors. The speed of construction and the predictable performance characteristics of GI solutions may make them the right choice over time consuming and cost-prohibitive structural solutions.
For more information, visit www.haywardbaker.com, or contact Dennis Boehm at dwboehm@haywardbaker.com or call (281) 668-1870.
