MITIGATING RISKS TO SHALLOW TRENCHLESS PIPELINE CROSSINGS USING GEOPHYSICS AND TEST PITTING

ABSTRACT: Shallow linear infrastructure, like roads, rail and existing pipelines, are common obstacles for new pipeline projects. For these relatively short and shallow crossings, pipelines can usually be installed using a range of available boring methods. Although the value of detailed engineering assessment often conducted for larger scale Horizontal Directional Drill (HDD) or Pipe Thruster Installation crossings is not often perceived for these shallow crossings, they are not without risks associated with complex subsurface geology, largely pertaining to intersection of bedrock or coarse granular material. In this case study, we show how a combination of geophysical profiling and test pitting can provide a cost-effective means to characterize shallow subsurface geology with minimal ground disturbance. We demonstrate this approach for a series of 10 road and pipeline crossings spread over a 23-kilometre-long section of a proposed 42-inch natural gas pipeline in a previously glaciated area of Northeast British Columbia. A combination of seismic refraction and electrical resistivity tomography (ERT) profiles were acquired to non-invasively map the subsurface geology, from which select locations were proposed for targeted test pitting to ground truth the geophysical survey results. The results of the geophysics and test pitting surveys were used primarily to evaluate the risks of the 10 proposed track bore and auger bore crossing alignments. At crossing locations where the combined investigation results identified potential risk with the preliminary bore path (e.g., in and out of shallow bedrock), the results were then used to optimize the depth profile of the bore path, and to select the best-suited crossing configuration and tooling.