Impulse Response Evaluation of Drilled Shafts at the Nges Test Section at Northwestern University

Abstract

The impulse response test is a non-destructive 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 often times makes the pile heads inaccessible and introduces uncertainties in the interpretation of the results. To evaluate the effects of intervening structures on impulse response results in a controlled environment, a test section was constructed at the National Geotechnical Experimentation Site at Northwestern University. The test section consisted of 5 drilled shafts constructed in 3 groups. The drilled shafts were constructed with diameters ranging from 2 to 3 ft and lengths varying from 40 to 90 ft. Pile caps vary in thickness from 2 to 5 ft. Two of the shafts were constructed with defects: a reduced cross-section and a thin, soil filled joint. Impulse response data were obtained in both the accessible and inaccessible head conditions. The ability to determine shaft integrity for inaccessible shafts depends on the thickness of the pile caps and the length-to-diameter ratio of the shafts. The toe of the shaft could be identified for inaccessible shafts with length-to-diameter ratios as high as 23 for cap thickness-to-pile diameter ratios up to 2.5.