Field Test on Group Piles under Machine Induced Coupled Vibration

Abstract

Dynamic response characteristics of reinforced concrete group piles with embedded pile-cap condition are investigated in the field under varying levels of coupled harmonic excitations. The piles are constructed by bored cast-in-situ method. The site is located at Indian Institute of Technology Kharagpur, West Bengal, India. The soil properties are determined by laboratory tests on both disturbed and undisturbed soil samples collected from various boreholes at the site. Two in-situ tests, namely, standard penetration tests to determine N-value and cross hole seismic tests for determining the shear-wave velocity of soil are conducted at various depths of soil layer. Forced coupled vibration tests are conducted on pile groups using Lazan type mechanical oscillator with four different eccentric moments. Both the horizontal and rocking motions of pile groups are measured simultaneously for different operating frequencies of oscillator. Two resonant peaks are observed at two different frequencies. It is also observed that as the eccentric moment increases, the resonant amplitude increases, but the natural frequency decreases for both horizontal and rocking responses. The test results on piles are then compared with the numerical approach with two different soil-pile models – (i) A linear visco-elastic medium composed of outer infinite region and an inner weaker layer with reduced shear modulus, and (ii) a boundary zone with parabolic variation of shear modulus and with a non-reflective interface. From the comparison between the test results and numerical results using the first model it is found that the prediction of nonlinear response of the pile foundation is in good agreement with the test results. For the second model, the numerical results are not found satisfactory when compared to the test results. The nonlinear parameters like separation length, shear modulus ratio, weak zone damping factor and thickness ratio are also predicted for the group piles under coupled vibration.