Pipeline Construction through Sensitive Glaciomarine Clay

During the construction of Coastal GasLink (CGL) through the town of Kitimat, BC, the Project encountered sensitive glaciomarine clay (GMC) that extended up to 40 m in depth along the pipeline right-of-way. While sheet piling is a common solution utilized to stabilize otherwise unstable trench wall conditions, cantilever sheet piles rely on competent soil at depth for toe resistance which provides the lateral support required for safe trench excavation. Due to the significant depth of the GMC, the lack of toe resistance severely limited the capacity of any installed sheets regardless of the steel section utilized. Reinforcing the entire length of cantilever sheet pile wall (SPW) trench with structural cross-bracing was not a suitable alternative for the soft GMC area due to interference with pipe installation sequences and project schedule implications. To meet regulatory pipeline installation requirements and successfully construct through this sensitive area, the Project utilized a combination of shallow trench and soil mounding construction to successfully install the pipeline, minimizing the need for cross trench bracing to short sections only at watercourse crossings, where the pipe had to be deeper. A robust field monitoring program was also implemented to ensure that construction was completed within the predicted bounds of a finite element soil model, which was calibrated with deflection limits that considered the sensitive nature of the GMC.