What Lies Beneath - Geophysical Approaches For Assessing Trenchless Water Crossings

When tied to drilling results, geophysical surveys of trenchless water crossings provide important information on subsurface geotechnical conditions, including bedrock elevation and the locations of zones of granular material within overburden. Because the terrain can change quite dramatically at water crossings, it is difficult to acquire geophysical data that is continuous between the geotechnical boreholes. The resulting data gaps can decrease confidence in understanding the site geotechnical conditions, which increases uncertainties in the detailed engineering design of the trenchless water crossing (e.g., HDD, or MTBM method). We demonstrate here how some of the technical challenges associated with acquiring continuous geophysical data at water crossings can be overcome. These include the use of suspended ERT cables, and complementary waterborne ERT and seismic refraction surveys. To illustrate the efficacy of these techniques, we present case studies from proposed HDD crossings of three different types of water bodies at sites in British Columbia and Alberta.