Construction of Drilled Shafts in Variable Ground—A Case Study

In designing drilled shafts with expanded bases, often referred to as belled piles, it is necessary to determine whether these expanded bases can be constructed to the desired design sizes at the depth where these piles are to be founded. In fairly homogeneous cohesive ground, this does not pose a problem and often drilled shaft test piles can confirm the adequacy of the construction of these piles. However, where the ground is variable and non-homogeneous and contains sand deposits, this is not readily determined and the dimensions of piles may have to be varied to meet the design criteria. This situation poses a construction problem, increases the project costs, and may lead to contractual problems if piles have to be redesigned on site during the installation process. This paper describes a case history of fifty one (51) drilled shaft belled piles in which the design shape of a number of the piles had to be changed on account of erratic subsoil conditions. A variety of pile shapes — belled piles with long bottom shafts, short belled piles, straight shaft piles and double belled piles were utilized during installation. These pile shapes were not predetermined but rather dictated by existing conditions encountered during installation. The paper addresses the site conditions, design requirements and changes that had to be made on site to achieve an acceptable product. The need to recognize certain aspects of the subsurface site conditions prior to full scale installation is of importance to the successful installation of these piles.