BEHAVIOUR OF PILE FOUNDATIONS UNDER IMPACT/BLAST LOADS

Abstract

The behaviour of pile foundations under blast loads is quite different when compared to that of vibratory loads. The duration of blast load is very short so that it has the nature of an impulse. Simulation of this type of loading is impracticable in laboratory conditions. Blast loading has thus been simulated by impact load, which is also impulsive in nature. In the present investigation an experimental study was carried out on aluminium piles [L/d = 26.67, dia = 15 mm] arranged in a square pattern at a spacing Of three times the pile diameter. The pile groups are driven into homogeneous sand placed at three different relative densities, namely, dense [Dr = 70 %], medium [Dr = 55 °A], and loose [D = 20 %]. Lateral load tests on model pile groups were carried out and the stiffness and ultimate resistance of the soil-pile system is computed. Suitable mass was fixed rigidly to the pile cap to simulate the vertical load (of the structure) on the pile group. The influence of the vertical load on the pile group and relative density of the soil is also observed. From the lateral impact load test the response of the pile foundation is observed for different input impact energies. From this study damping in the soil has been computed which is found to lie in the range of 2.5% to 4.5%. The total damping, including absorption of energy in plastic deformations, would be much more, but the latter is not considered since it is automatically accounted in the non-linear analysis. Further analytical study is carried out both considering damping and without damping. On the basis of results of the analytical study it is concluded that the undamped peak response is more than the damped peak response by 15-20% for the observed damping value of 3.5%.