NUMERICAL SIMULATION OF UNDER-REAMED PILE SUBJECTED TO VERTICAL AND HORIZONTAL SINUSOIDAL LOADING

Abstract

Under- reamed piles are used especially in expansive soils where large ground movements are associated with varying seasonal moisture content, these pile types stand effective by virtue of their geometry. These piles have enlarged bases, known as the bulbs, in the pile stem at particular locations. This contributes to extra uplift and bearing capacity and also better anchorage at greater depths. Under-ream piles also find applications in machine foundations, foundation for off-shore structures and transmission towers. In these cases it is likely that these piles are subjected to dynamic forces, thus it is necessary to study the response of these structures under dynamic excitations. A study of the Dynamic Soil Structure Interaction is important to examine the critical inter dependent behavior of the pile-soil system. The study consists of two parts; in the first part a series of under-reamed piles, having different embedment and geometries, have been tested in-situ for different vertical and horizontal excitations. For this a motor-oscillator assembly was mounted directly on the pile cap and excited at different levels. The soil at the site consisted of silty-sand up to a depth of 4 metres, followed by a clay layer of about 2 metres and then silty-sand again till depth of 9 metres. For both horizontal and vertical vibration loading it was noted that the natural frequency of the pile-soil system decreased with an increase in force level, indicating a decrease in stiffness of pile-soil system. It was also observed that the resonant frequencies were similar for the different pile geometries (varying length and number of bulbs) when excited to same force level, indicating that only a part of the pile was excited. The effect of embedment in case of horizontal vibration loading was pronounced as we observed a definite increase in the resonant frequency, with an increase in embedment, thus indicating an increase in the pile-soil stiffness for a smaller free standing height. In the second part of the study, a 3-dimensional mathematical model to simulate the experimental behavior of a under reamed pile-soil system subjected to horizontal and vertical loading has been developed using the Finite Element package, ANSYS. Various input data required in this analysis has been obtained from an experimental test programme. The pile-soil system model consists of a layered soil profile restrained normally at the boundaries, and the piles (along with pile cap) of different geometries varying in terms of length, number of bulbs and embedment depth have been considered. Initially a linear model, i.e. linear material properties and linear contact behavior at the pilesoil interface is assumed and afterwards both material non-linearity in soil and contact nonlinearity at the pile-soil interface has been considered. Self-weight analysis and modal analysis are performed to check the stability and modal frequencies. A forced impact loading analysis has been done to check the modal analysis results after which the system is subjected to monotonic horizontal and vertical static loading to study the pile-soil system response behavior to same and load-deflection curves are obtained for all three types of models. Non-Linear (material and contact non-linearity) model showed the softest behavior. Sinusoidal loadings are applied on all three models and the dynamic response of all the pile-system geometries are studied. Natural frequency and displacement amplitudes induced in the pile-soil system are analytically obtained and Non-Linear Model harmonic loading responses of different pile - soil system geometries are compared with the experimental results and thus the model is validated through the experimental results.