Soil Mixing Supports a Deepwater Bulkhead in Soft Soils

An oil well platform fabricator looked to develop property along the mouth of a Mississippi bayou. They required a deepwater port, 610 m (2000 ft) long, with stable ground for loading and unloading. The marsh ground conditions posed a serious challenge to this development, which included a 10.7 m (35 ft) deepwater access. A sheetpile bulkhead and dead-man system was initially designed to stabilize the future cut. However, earth lateral pressure from a soft, clayey silt/sandy clay behind the sheetpile was excessive and jeopardized the stability of the bulkhead. The engineers elected to specify a soil mixing solution to treat a 14.63-m (48-ft) wide block of soft soils behind the sheetpile to effectively reduce the lateral stresses imposed on the wall. Using a cellular arrangement of 2.14-m (7-ft) diameter soil mix columns, the contractor effectively proposed an in situ gravity wall structure that exceeded the ground improvement requirements. This paper describes the design of the earth support structure, the proposed ground improvement by soil mixing, the construction of the soil mixing, the quality control aspects of the work, and the performance of the structure in service. Testing of the soil mix product will be described, including pre-construction laboratory work and production quality control tests.