Settlement Of A Pipeline On Thawing Permafrost

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Download or read book Ditch Settlement and Imported Fill Methodology Report written by and published by . This book was released on 2006 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "The Mackenzie Gas Project (the project) is proposing to construct: -189 km of NPS 16, NPS 18, NPS 26 and NPS 32 gathering pipelines north of the Inuvik area facility; -a 1,194 km NPS 30 gas pipeline from the Inuvik area facility to a tie-in point in northern Alberta; -a 457 km natural gas liquids (NGL) NPS 10 pipeline to transport liquids from the Inuvik area facility to Norman Wells, where it will tie into the existing Enbridge Norman Wells oil pipeline. From north to south, the pipelines will traverse continuous, discontinuous and sporadic permafrost. Therefore, settlement associated with permafrost thawing is expected along the pipeline. This report presents the method used to estimate the settlement of the native backfill material over the pipeline, i.e., the ditch settlement [and] volume of imported fill that might be required to offset the settlement. This volume was used as the basis for the preliminary designs and corresponding cost estimates. Estimates of ditch settlement, and the imported fill quantities required to offset it, are based on route soil information obtained from borehole and test pit data and terrain mapping from interpreting air photographs. Subsurface soil information from boreholes and test pits was initially used to estimate ditch settlement fill, imported fill or both, at specific locations. These discrete estimates were then grouped and applied to the entire route, using terrain mapping information along the pipeline right-of-way. The following were developed as part of this study: -a computer program that estimates settlement associated with thaw on the right-of-way and local settlement over the ditch line. The program computes settlement on a layer-by-layer basis for each borehole within the engineering borehole database. -a numerical model that estimates the amount of imported fill required to either completely or partially offset local ditch settlement. Imported fill is estimated on the basis of the average soil strain predicted from each borehole's data. Two reclamation scenarios were considered: 1. Leaving some fill on the trench shoulders after thaw. 2. Transferring any stranded fill left on the trench shoulders after the roach has thawed to the ditch line. The following activities will be considered in detailed engineering: -Update the standard moisture content look-up tables and, where practical, fill in some of the existing data gaps using engineering judgement. -Reassess, where practical, the assumptions related to strain development in organic materials that result from mechanical disturbance, with the objective of correlating these strains to actual northern field data. -Refine the use of mean settlement and imported fill values to calculate imported fill demands. -Re-evaluate the current zero allowable settlement criterion to reflect the Norman Wells pipeline experience. -Revisit the approach for the current stranded fill reclamation model, where no re-grading of the roach or backfill conditioning is proposed following initial construction, i.e., adopt the model that transfers stranded fill to the ditch line. -Upgrade the settlement estimates using data from additional boreholes to be either drilled, or other data sources pursued, in data gap areas in the extreme northern and southern portions of the project. -Continue quality control of the engineering borehole database to identify and correct variations, especially with boreholes that are causing unrealistically high or low settlement estimates. -Determine, during detailed design, the optimum time frame (construction, post-construction or both) for placing imported fill"--Executive summary.