Predicting Residual Stress State Around Bored Cast-In-Situ Piles Utilizing Cavity Contraction and Expansion Solutions

Deep foundations are conventionally being adopted in construction of various structures ranging from bridges to skyscrapers. Bored cast-in-situ piles and drilled shafts have become a common choice of deep foundations, especially in urban areas, owing to the relatively minimal construction associated noise and vibration as opposed to driven pile. However, one of the major shortcomings of bored piles is the lower skin and tip resistances due to the installation effects (stress relief). In the current practice, empirical or semi-empirical correlations are generally adopted for estimating the unit skin friction of bored pile/drilled shafts which is not necessarily taking into account the actual soil state around pile subsequent to the installation. Since the soil state (properties and stresses) in the vicinity of pile will be significantly altered due to the installation processes, studying the evolution of soil state during the various construction stages as well as the loading stage will aid to estimate the residual confining stress and thus the unit skin resistance of pile. This can be made possible using cavity expansion and contraction solutions as the stress relaxation during the excavation of hole is analogous to a cylindrical cavity contraction (unloading) problem and subsequent concrete placement and axial loading resemble a cavity re-loading (i.e. expansion) problem. This paper presents a semi-analytical cylindrical cavity contraction and expansion solution procedure to predict the residual horizontal stress and consequently the skin friction of bored pile. The proposed approach was validated using two well-documented field test data of bored piles/drilled shafts in sand.