Evaluation of High-Capacity Composite Spun Piles

Traits of two broad categories of deep foundations-replacement and displacement piles-are best combined by a newly introduced composite spun pile. This composite spun pile is constructed by deep mixing first and then inserting a precast, prestressed, spun concrete pile into the deep mixed column. The spun pile is then driven into an end-bearing layer to meet the criterion of end resistance. The pile uses a larger peripheral area of soil-cement mix to transfer load to its surrounding ground at the same time it uses higher stiffness of the spun pile to minimize axial compression. The piling operation is relatively quiet, minimizes spoil problems, and facilitates better quality control. A pile design principle for the composite pile is presented. The performance of this new pile system was verified by five instrumented field full-scale pile load tests. Improvement on shaft resistance was observed, as depicted by the correlation between standard penetration test N values of the surrounding soil and the measured skin friction along the pile. The contribution of the measured toe resistance over the total applied load varied from 2.3% to 36.7%, depending on the geotechnical properties of the foundation layers. The field tests clearly showed that composite spun piles had high capacities and could be used as a reliable alternative to traditional driven as well as bored piles in current practice.