Pile Load Tests in Permafrost Using Spiral Legs to Support Hot Ice No 1 Drilling Platform

Two full-size piles with a spiral design on the lower part of the piles, augered and backfilled with slurry were tested in natural permafrost in Prudhoe Bay, Alaska to assess their performance as foundations for the Hot Ice No 1 Drilling Platform of Anadarko Petroleum Corporation. The platform was used to drill gas hydrates 40 miles southwest of Prudhoe Bay, Alaska in winters 2002 – 2004. The purpose of this paper is to present the pile displacement results and lessons learned during the testing procedure. The design requirements included a 667-kN ultimate pile capacity for a design life of 2 years, allowable settlement of 15 mm in 2 years and 25 mm in 40 years. The foundation system needed to assist in keeping the permafrost from degrading and to be removable so that the tundra would be left behind undamaged. The function of the hollow spirals was to aid in removing the piles by circulating a heated liquid down the pile, through the helixes and back to the surface, and to possibly increase the pile capacity. Three levels of compressive loads were applied using a load cell and a mechanical actuator. The pile displacement during the test was measured using a potentiometer and three types of secondary systems. Thermistors were used to monitor the ground temperatures during the freeze back period and testing. The displacement rate for primary and secondary creep in permafrost was obtained using the potentiometer and confirmed with the secondary monitoring systems. Dial gages proved to be the best secondary measurement system used. The test results indicated that the required factor of safety of ⩾ 1.5 was obtained for the pile capacity in regards of shear failure for the governing ground temperatures (from –4 to 0°C). However, the creep settlement rate for the temperature range exceeded the tight allowable rate. Recommendations for future testing programs are given.