Pipeline Trenching in Permafrost: A Review

BP America Inc., Enbridge Pipelines Inc., and TransCanada PipeLines Limited recently sponsored a comprehensive technical review of the use of wheel and chain trenchers for excavating pipeline ditches for large diameter, long distance oil and gas pipelines in permafrost. The purpose of the review was to identify techniques that could be implemented to improve the productivity of trenchers in permafrost and reduce pipeline construction costs. This paper summarizes the key findings of the study. The study included an analysis of data obtained from previous field trials and construction case histories in permafrost, including the results from proprietary trials that have never been published. The study found that the primary subsurface conditions affecting the productivity of both wheel and chain trenchers in permafrost soil are: 1) the concentration and lithology of cobbles and boulders; 2) the presence and strength of bedrock within the depth of trenching; and 3) the tensile strength of the permafrost soil. With current technology, neither wheel nor chain trenchers can achieve satisfactory rates of production if more than 5 to 10 percent cobbles or boulders are present, or if hard bedrock exists within the depth of trenching. The study evaluated a number of techniques for improving the productivity of both wheel and chain trenchers in permafrost soil which may or may not contain hard inclusions. These methods included pre-blasting along the ditchline using either conventional blasting techniques or shaped charges. In addition, a wide variety of multi-pass trenching techniques were evaluated as part of the study.