Direct-push based seismic crosshole testing for geotechnical engineering applications

In areas with an unknown geology, boreholes are usually placed either at the planned location of buildings and infrastructure or following a semiregular pattern. The number of boreholes is typically limited by installation cost, especially the number of boreholes to be used for geophysical testing, such as those used for downhole, crosshole, or tomographic analyses. An alternative approach to conventional drilling is the use of mobile pushing devices, i.e., direct-push procedures. By placing geophysical tools into the pushing rods, geophysical methods become more flexible and adaptive during drilling, and investigation techniques can be implemented more expeditiously. From a geoengineering perspective, the in-situ tests are relatively efficient because they generate near continuous data and are considerably more accurate in comparison to laboratory consolidation tests. In this paper we present a combination of a direct-push system with seismic crosshole measurements as a cost effective alternative to standard investigation techniques. The new methodology was successfully tested at the site for Technical Safety (TTS) in Horstwalde, Germany. A complete crosshole dataset of P-, SV- and SH-waves was acquired between previously installed PVC cased boreholes and the direct-push borehole. Furthermore, the in-situ profiles of paired shear wave velocity profiles (SH and SV) were used to evaluate the stress history of the soils.