Impulse Response Evaluation of Drilled Shafts

A test section was constructed at the National Geotechnical Experimentation Site at Northwestern University to assess the applicability of nondestructive testing methods to evaluate deep foundations under inaccessible-head conditions. Tests were performed in both the accessible and the inaccessible conditions to evaluate the effects of intervening structure. This paper focuses on the results of impulse response tests performed on the National Geotechnical Experimentation Site drilled shafts before the pile caps were constructed, i.e., in an accessible-head condition. Based on field experimentation and numerical simulations, the use of impulse response tests to identify lengths of accessible-head shafts is limited primarily by the L/D ratio of the shaft, the ratio of the shear-wave velocity of the soil to the propagation velocity of the concrete, and soil stratigraphy. The length of a drilled shaft can be found to within #+5% based solely on errors in assumed propagation velocities. Site-specific construction procedures may be important when interpreting the results of impulse response tests because the soil immediately adjacent to a shaft has a large effect on the resolution of the signals in a mobility plot.