PILE GROUPS SUBJECTED TO CYCLIC TORSIONAL LOADS

Abstract

The pile groups support large structures namely offshore platforms, wind turbines, high rise buildings, bridges, railway embankments, and traffic and signal pole foundations which often experience axial, lateral and torsional loads. As a result, they undergo vertical, lateral, and eccentric movements. Therefore, the numerical scheme supported by a set of experimental observations has been considered to capture pile-soil interaction for the pile groups subjected to axial, lateral and torsional loads. The coupling effect of axial load on torsional pile response conversely has been studied with a nonlinear three-dimensional finite-element analysis, while the conventional subgrade reaction method of pile analysis cannot consider this interaction. In addition, the geomaterials experience controlled and uncontrolled displacement at varying amplitudes of loading due to earthquakes, wind loads, machine foundations, vibratory compactors and pile driving. The heave due to cyclic loads is often observed in areas such as coastal regions where there are wave actions or in areas with high seismic activity. Appropriate foundation designs considering this phenomenon are to be implemented to interpret heave displacement due to cyclic torsional loads. In the present research, the progressive twist and displacement in the sand are investigated for a set of pile groups (1,2; 1,3; 2,2) corresponding to the application of the torsional loads. The torque mobilisation, progressive twist, and displacement have been computed for a range of the initial shear modulus ratio for the entire set of pile groups. As a result of twisting, the torsional energy zones are set into the soil and there is the formation of a heave and cavity around the pile group. The torsional energy and twist rigidity parameters were evaluated for a range of shear modulus ratios of the soil. It has been observed that the torsional energy of the pile group (1,2) is significantly higher than pile groups (1,3; 2,2). A classification for torsional energy zones associated with twist rigidity and displacement viii rigidity factor has been suggested to set the limits for twist and displacement of the pile groups relative to a single pile. A relationship of the torsional energy with progressive twist and displacement is obtained. The components of torsional energy associated with progressive twist and displacement were obtained in the range of 0.43-0.50 and 0.50-0.57 respectively. An attempt has been made to find out the solutions for pile groups subjected to combined axial and torsional loads. Therefore, a novel numerical scheme has been presented to capture the nonlinear pile-soil interaction in flow-controlled geomaterial to make allowance for the yield effects. Based upon the numerical scheme, a three-dimensional finite element analysis has been performed on pile groups subjected to combined axial and torsional loads in flow-controlled geomaterial using a computational program. The flow potential for the yield surface of flow-controlled geomaterial is a hyperbolic function of stresses in the meridional stress plane and the smooth elliptic function in the deviatoric stress plane, respectively. The load-displacement relationship of a large diameter pile (LDP) and pile groups (1,2 and 2,2) have been compared with the experimental pile load test and the numerical results reported in the literature. It has been observed that the resultant displacement increases significantly with the torsional load for the LDP and pile groups. Similarly, the twist increases significantly with an increase in axial load. The displacement and twist parameters have been classified which in turn depend on the plastic strain and dilation angle of the geomaterial. In addition, this work also presents the study of the response of cyclic torsional loads on pile foundation groups. A numerical scheme is proposed to capture the behaviour of the symmetrical pile group in the flow-controlled geomaterial under cyclic torsional loading. Based on the numerical scheme, three-dimensional finite element analysis was performed ix to capture the nonlinear response of the pile group using a computational program. The results from the numerical analysis have been compared with the experimental observations. The peak twist and peak shear stress logarithmically decrease and then become asymptotic after a number of cycles of loading for a pile group (2,2) in a flow controlled geomaterial. A set of cyclic degradation parameters of flow-controlled geomaterial is identified for the pile group and presented as a function of dilation. A numerical campaign is proposed to capture the surface displacement of the geomaterial. The constitutive relations from the numerical campaign are placed into the computational program to apprehend the nonlinear response of the geomaterial using a three-dimensional finite element analysis (3D-FE). The outcome of the research has been validated with the experimental results. The surface displacement of the geomaterial tends to increase with a number of cycles of loading and the peak angle of friction. After a number of torsional loading cycles, the surface heave consisting of uplift and lateral flow ranges between 10- 25 mm and 5-10 mm, respectively for varied peak angles of friction. Conversely, the measure of surface heave and lateral flow shall allow the designers to predict the permissible number of cycles at the outset of torsional loading. A set of surface displacement parameters for the geomaterial are identified and presented. Having an understanding of the coupling effect of torsional loads inclusive of the cyclic effects and torsional energy around the pile groups in the flow-controlled geomaterial, an appropriate pile foundation design procedure is placed for the use of practitioners and experts in the industry.