LPG Conversion, US Virgin Islands

Two ground improvement solutions for one project success

As part of a major project in the US Virgin Islands to convert electric generating stations to Liquid Petroleum Gas (LPG), new storage terminals were built on St. Croix by VTTI, a subdivision of the Vitol Group, for the island’s Water and Power Authority (WAPA). US Wick Drain provided innovative ground improve­ment for the construction of two 20,345 square foot earthen containment mounds, each housing four new bullet tanks for LPG storage.

The ground improvement phase of the proj­ect was scheduled to be completed in six weeks. To accommodate the heavy loads and address seismic concerns, US Wick Drain in­stalled both Bi-Modulus Columns and Controlled Modulus Columns based on the varying nature of the liquefiable soil layers of the two working pads.

The site soils consisted of an approximate 15 to 20 foot deep layer of loose sand overlay­ing a 20 to 30 foot thick layer of stiff to very stiff clay over a hard clay layer, which would serve as the bearing stratum. After excavation of the site to the working platform elevation, the south pad did not have any remaining loose sand, and the working platform was built directly on the upper clay. The north pad still had approximately 13 to 15 feet of the loose sand layer below the working pad that was considered liquefiable and required mitigation.

Due to the varying nature of the liquefiable soil layers between the two working pads, US Wick Drain proposed separate techniques: Bi-Modulus Columns installed on the north pad and, due to the absence of the loose sand, Controlled Modulus Columns on the south pad. The Bi-Modulus Columns consisted of a Vibro Stone Column atop a Controlled Modulus Columns. This technique combines the liquefaction mitigation benefits of a traditional Vibro Stone Column with the load carrying capacity of Controlled Modulus Columns into a singular element.

Using these two ground improvement solutions, US Wick Drain’s design met the project settlement requirements of approximately two inches. Additionally, the design provided the necessary subgrade modulus of 38 pci.