Performance of Tanks Supported on DMM Columns in the Gulf Coast: Hydrotest Monitoring and Settlement Analysis

A series of cylindrical steel storage tanks with fixed roofs were constructed for expansion of an industrial facility in the Gulf coast of the USA. Tank diameters ranged from approximately 80 to 210 feet (24.4 to 64.0 m). Soils at the site consist of an upper layer of very loose silty sand and soft clay underlain by medium to stiff clay to depth of about 100 feet (30.5 m). For the case being presented, ground improvement using Deep Mixing Method (DMM), popularly known as Cement Deep Soil Mixing (CDSM) was selected as an efficient and cost effective foundation alternative to reduce settlement and achieve an acceptable factor of safety against bearing capacity failure. The design consisted of 8 feet (2.4 m) diameter DMM columns, spaced at 11 feet (3.4 m) center to center and extending to a depth of about 20 feet (6.1 m). Settlement monitoring during hydrotesting was performed via topographical surveys of the ring wall and settlement profilers installed below the tank bottom plate. Vertical inclinometers monitored lateral movements around selected tanks. The data were used to: (1) assess differential settlement since excessive movement could affect the shell, the roof structure and the bottom plate; (2) estimate tank settlement during operation with the aid of hydrotest settlement and (3) observe bearing capacity performance. Performance of the first hydrotested tank is discussed in this paper.