Impulse Response Evaluation of Foundations using Multiple Geophones

A new method to nondestructively evaluate existing deep foundations is described for the situation where access to the top of a deep foundation is prevented by an intervening structure. The test consists of striking a surface of a structure and simultaneously recording velocities with vertical geophones at several locations on an impacted surface. Arranging the geophones at different distances from both the impact location and the source of surface wave reflections allows one to minimize the interfering effects of surface waves on recognizing the compression wave reflections from an underlying deep foundation. The processed data can be evaluated as either a conventional sonic echo or impulse response test. Results of field tests of the system are described for groups of shafts at the National Geotechnical Experimentation Site at Northwestern University. Results of these tests indicate that there are preferential locations for the geophones that enhance the interpretation of the condition of an underlying drilled shaft. A case study is presented that describes the application of the method to evaluate possible concrete deterioration of existing bridge piers.