Constant Force Shaking of a Group of Four Drilled Shafts

A foundation system comprising a group of four drilled shafts, each 750 mm in diameter by 5.5 m long, constructed in silt/sand/gravel with a rigid pile cap, was subjected to constant force shaking by a servo-hydraulic actuator reacting against a heavy anchor block. Forces of up to 150 kN peak to peak were achieved with resulting displacements of up to 2 mm and accelerations of up to $0.5g\text{.}$ The displacement response of the pile cap was measured over a range of frequencies from 2 to 12 Hz and the resonant frequency and damping ratio were determined for three different levels of excitation. The foundation behaved much like a simple harmonic oscillator with a natural frequency of approximately 10 Hz. This resonant frequency was found to decrease slightly with increasing excitation while the damping ratio decreased slightly. Values for generalized stiffness, mass, and damping for an equivalent simple harmonic oscillator were back-calculated from the test data. The results of this study suggest that it may be possible to predict values for generalized mass and damping a priori by using simple empirical methods.