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BP, ExxonMobil, Phillips to test trenching equipment, rehabilitation methods
Kristen Nelson
BP Exploration (Alaska) Inc., ExxonMobil Production Co. and Phillips Alaska Inc. have applied to the U.S. Army Corps of Engineers to place up to 210,500 cubic yards of fill material into 147.17 acres of waters, including wetlands, for a project to test trenching equipment and rehabilitation methods for burial of a large gas pipeline in permafrost wetlands.
The companies said objectives include determining the production rate that can be achieved with chain trenchers in various types of permafrost and observing and assessing trench backfill subsidence, subsurface thermal changes and the re-vegetation process.
Test trench sites — two in the Deadhorse area and two in the Washington Creek area off the Elliot Highway some 20 miles north of Fairbanks — were selected to test soil types expected to be encountered along a potential gas pipeline route between Prudhoe Bay and the Canadian border. Conditions include: poorly drained tundra with deep organic ice-rich silts and sands; shallow overburden over frozen gravel; deep organic ice-rich silts; and bedrock overlain with residual weathered soils.
Thirteen test trenches excavated Thirteen test trenches would be excavated at each site, with a minimum space of 35 feet between the trenches, using different trenching techniques. Each trench would be six feet wide, eight to 12 feet deep and 1,500 feet long. Explosives would be used at the bedrock site and at the gravel site to test how use of shape charges could affect trenching rates.
All trenching work and access road construction — ice or gravel — would be conducted in winter.
Backfilling of trenches will test thaw settlement and re-vegetation methods. Some trenches would be backfilled only with native soils, some with imported thaw stable gravel under native soils and some sections of 48-inch culvert would be installed to test thaw settlement. Frost depth indicators, thermistor strings and survey markers would be used to monitor the sites after site rehabilitation.
Rehabilitation experiments would include three treatments: trenches in which material is not segregated as it is excavated, but all is placed back in the trench; trenches in which the top 1.5 feet of soil is segregated during excavation and this topsoil is placed as the cap over the backfilled trench; and trenches in which material is not segregated and the backfilled material is capped with one foot of imported thaw stable material.
Seeding and fertilization treatments would then be applied to test various plant cultivation methods.
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