Geotechnical Characterization of the Weber River Crossing, Utah, for HDD Feasibility for the 2003 Kern River Pipeline Expansion

The 2003 Kern River Pipeline Expansion will loop 635 miles of 36-inch high-pressure natural gas pipeline between southwest Wyoming and southern California and 82 miles of 42-inch pipeline in southern California. The Weber River near Coalville in Summit County, Utah, will be crossed using horizontal directional drilling (HDD) techniques. The original Kern River Pipeline was constructed across the Weber River using conventional open-cut methods in 1991, but environmental factors and the close proximity to other pipelines made HDD a reasonable option to consider. The Weber River floodplain is approximately 3500 feet wide at the pipeline crossing, and contains Interstate Highway 80, US Highway 189, and a paved frontage road. Geometry of the slopes above the floodplain results in an overall HDD horizontal length of 4750 feet with an elevation difference of 88 feet. The geologic setting of the Weber River crossing consists of Cretaceous marine and non-marine sedimentary rocks and Quaternary alluvial and colluvial deposits. The rocks are chiefly sandstone, siltstone, and claystone, with local limestone and coal. Sedimentary beds dip to the west at 10 to 30 degrees, and fractures dip at 60 to 80 degrees. The siltstone and claystone are soft to moderately soft, whereas the sandstone and limestone are moderately hard to hard. The alluvial deposits consist of silty sand to sandy gravel, and the colluvial deposits consist of silty clay to silty sand. A north-trending, west-dipping normal fault shown on published maps nearly coincides with Interstate 80. Geotechnical conditions at the proposed HDD crossing were evaluated by drilling six borings to depths ranging from 100 to 150 feet, and collecting ground penetrating radar (GPR) data at 11 profile lines covering 2170 feet of floodplain. The GPR profile lines were interrupted by channels of the Weber River, canals, and Interstate 80. A 25 MHz antenna was used to obtain radar penetration to effective depths of 50 feet or more. The contact between alluvial deposits and bedrock was detected at depths ranging from 20 to 50 feet. Local relief of the alluvial/bedrock contact was found to be on the order of 10 feet. DrillPath 2 and DrillMud programs were used for a preliminary evaluation of the HDD feasibility. A maximum HDD installation depth of approximately 105 feet was used, and fracture pressures were evaluated for a pilot hole and four stages of back-reaming. Locations of frac-out concern were identified. Final HDD design was performed by others.