Bi-Directional Static Load Testing Results of an Augered Cast-in-Place Pile and a Drilled Displacement Pile

Abstract

Augered cast-in-place (“ACIP”) piles and drilled displacement piles (“DDP”) are becoming increasingly common, particularly for providing high bearing capacities with relative ease of installation, reduced overall costs, and potential for shortened construction schedule and reduced construction material quantities. The static analysis methods used for these methods’ designs contain uncertainties, particularly for DDP where the surrounding geomaterials are reformed, resulting in increased unit side-shear resistance. Load testing is often performed to assess the side-shear and end-bearing resistances, and to evaluate the selected construction method and required pile lengths. Bi-directional static load testing (“BDSLT”) can be advantageous for determining mobilized side-shear and end-bearing resistances at lower cost compared to conventional top-loading static load tests. A case study is presented where an ACIP pile and a DDP were constructed in close proximity to each other, and both were bi-directionally load tested. The benefits and limitations of each foundation type are addressed. A comparative study of load test results is presented showing the response of each pile type in similar subsurface conditions. Critical aspects of the test data and their interpretation are reviewed, focusing on fundamental aspects of performing a BDSLT, and methods applicable to converting measured strain to calculated internal force for drilled deep foundations. Higher mobilized side-shear resistances were observed for the DDP, which exhibited 50 to 100% greater resistance compared to the ACIP pile in dense deposits. However, limitation in the DDP construction method (manifested as relatively shallow drilling-tool refusal) was encountered, limiting the DDP maximum achievable total capacity.