Case History: Effects of Permanent Casing on the Axial Resistance of Drilled Shafts

Permanent casing is commonly used for drilled shafts that extend through very soft soils to reach an underlying stable stratum. However, steel casing will generate a reduction in skin friction resulting from the use of permanent casing compared to concrete placed directly against the soil. The side resistance reduction factors due to the steel casing relative to concrete shaft can vary depending on type, length, and installation method of casing. A large load test program was performed for the Sir Solomon Hochoy Highway Extension Project in Trinidad and Tobago. A total of eight drilled shafts were constructed and tested. The drilled shaft used a vibratory installed steel casing as a construction aid in soft to stiff clay and loose to medium dense sand stratum. A program of dynamic load tests was conducted and analyzed using a combination of CAPWAP and SIMBAT methods to measure the axial response of the drilled shafts. The load test results demonstrated that the case zone of the shaft experiences a reduction in side resistance as compared to an uncased shaft section. This paper presents the site conditions, construction methods, load test results, and comparison between predicted and measured side resistance of the test shafts. The load test program was structured to help optimize side resistance reduction factors resulting from the use of permanent steel casing.