Cutoff frequencies for impulse response tests of existing foundations

The impulse response test is a nondestructive evaluation technique commonly used for quality control of driven concrete piles and drilled shafts where the pile heads are accessible. When evaluating existing foundations, the presence of a pile cap or other structure makes the pile heads inaccessible and introduces uncertainties in the interpretation of impulse response results. A test section was constructed at the National Geotechnical Experimentation Site (NGES) at Northwestern University to examine the applicability of nondestructive testing methods in evaluating deep foundations under inaccessible-head conditions. This paper focuses on the results of impulse response tests conducted atop the three pile caps at the NGES. Based on field experimentation and numerical simulations, a frequency was determined below which the impulse response test could be used for inaccessible-head conditions. This cutoff frequency primarily depends upon the geometry of the pile cap and pile. A case study is presented that describes impulse response tests obtained on a number of drilled shafts both after the shaft was constructed and after grade beams and walls were built. The results of these tests also follow the trends observed in the NGES tests related to cutoff frequency.