Development of Membrane-confined Stem Grouted Piles and their Behaviour under Uplift Loading

Under-reamed piles are extensively used in India and South Asia in situations where large uplift forces are encountered. Such situations include expansive soils (swelling generates uplift force) and structures like chimneys, marine structures, transmission towers, windmills, etc. However, major issue with the construction of these piles is the collapse of under-reaming zone, particularly in sand and soft fine grained soil in saturated condition. The collapse of soil may results in uncertainty in the final size of the bulb. This issue can be eliminated by adopting pressure grouting technique to form a spherical bulb identical to under-reamed bulb. This technique can be used at any depth irrespective of the soil condition. In addition to forming a bulb, the grouting improves the stress state around the bulb significantly, especially in cohesionless soils, as the process is analogous to a spherical cavity expansion problem. This improvement in stress state is expected to increase the pile resistance against both uplift and compression loading. No studies were undertaken in the past on feasibility and uplift response of stem grouted piles. The goal of the proposed project is to investigate the efficacy of stem grouting in forming a spherical grout bulb and its influence on the uplift behaviour of pile. Both small-scale experimental study and finite element modelling will be carried out to accomplish the objectives. An appropriate grout delivery and confinement system are required for successful stem grouting, without which grout may find its own flow path instead of forming a solid grout bulb around the pile. One such grouting system was developed by McVay et al. (2009) and Thiyyakkandi (2013) for side grouting of precast jetted pile. The system had a U-shaped grout distribution network and a membrane wrapping around the pile to confine the groutflow. This system will be further fine-tuned to use efficiently for the stem grouting of bored pile. A test chamber (2 m diameter x 3 m depth) and a reaction system for uplift loading have to be constructed for the experimental study. The test chamber will be instrumented to monitor soil stresses and deformation during the grouting and load testing. The experimental investigation will include installation, stem grouting and uplift loading of the pile to assess uplift resistance and the mechanism of failure. Detailed parametric study will be performed by numerical modelling of the installation stages of pile and uplift loading. As stem grouting resembles a spherical cavity expansion problem, an attempt will also be made to develop an appropriate  model for predicting the uplift capacity of pile with the aid of cavity expansion and contraction solutions.