Use of Surface Pipeline Segments to Mitigate Slide Problems on the Fort Nelson Natural Gas Mainline

Abstract

Skid mounted surface pipelines have been used to cross four major creek valleys on the Westcoast Energy 762 mm Fort Nelson mainline where very large deep-seated slides are progressively failing. The surface pipeline segments range up to 2.2 km long and were used to cross very large slides on the creek approach slopes, or in one case, down valley sliding of the valley floor, which had resulted in major operational problems for the originally buried pipelines. The surface pipeline segments were used for slides where there were no other options for a conventional buried pipeline. During the studies to support the design of the surface pipelines, comprehensive geotechnical investigations included drilling, installation of slope indicators, and electric piezocone penetrometer testing to monitor subsurface pore water pressures and to detect deep seated slide surfaces. Typical peak movement rates of the slides varied to a maximum of several meters per year. The unstable soils were predominantly medium to high plastic silty clay tills and high plastic glaciolacustrine clays with residual internal angles of friction of 7.5 to 8.5°.</jats:p> <jats:p>The pipelines were placed on pressure treated timber skids on graded rights-of-ways (RoW) with comprehensive surface and subsurface water control. Aerial clear span crossings were used across the watercourses with geogrid reinforced abutments on the unstable soil. Geogrids were also used to add stability to structural fills and to reinforce an area of the RoW subject to encroachment from a rapidly moving earth flow up to 6 m deep. The general maintenance and operational guidelines are also discussed. Provincial and Federal regulatory concerns and their abatement are presented and discussed. Over the past five years of operation of the pipelines, one of the slopes moved 1.5 m in a twenty-four hour period and related total movements left a 5 m high head scarp on the RoW. Displacement of soil and areas of thrusting resulted in unsupported pipe spans of 35 m with negligible deleterious impact on the pipeline. This magnitude of movement would have almost certainly caused a failure of the original buried line. All of slopes have continued to move and would have caused a varying magnitude of operational problems had the pipelines still been buried.