ENGINEERING GEOLOGY FOR DESIGN OF THE PORCUPINE RIDGE PIPELINE AT THE WASATCH FAULT CROSSING, DAVIS COUNTY, UTAH

A proposed 0.3-m-diameter (NPS12) pipeline will transport crude oil from eastern Utah to an existing refinery in North Salt Lake City. The pipeline will descend the western slope of the Wasatch Range in Davis County and enter Centerville in a residential area where the pipeline will be located under city streets. The Weber segment of the Wasatch fault crosses the pipeline alignment at the intersection of Island View Drive and 800 East Street where it is confined to a single, relatively narrow, north-trending zone dipping west at about 60°. The fault scarp ranges from 20 m high 500 m north of the pipeline crossing to 14 m high 400 m south of the crossing. Corresponding vertical displacements are 14 and 11 m, respectively, of the Provo-age (14-ka) Lake Bonneville geomorphic surface result in a range of displacement-per-earthquake event of 1.83 m to 2.33 m, with an average of 2.08 m, assuming they were produced in six earthquake events. For comparison, mean net vertical tectonic displacements range from 1.6 m to 2.0 m for the last three surface-faulting events at the well-documented Kaysville trench site approximately 14 km north of the pipeline crossing. A vertical displacement of 2.08 m produces 2.4 m along the dipping fault plane.

A finite element model developed by D.G. Honegger Consulting to evaluate fault-displacement-induced stresses and strains in the pipeline consisted of 1,440 model elements. Strains were calculated in each element for fault displacements up to 5 m in 120 increments (4.17 cm). The maximum tension strain and compression strain at any element in the model were plotted for each displacement increment, along with diagrams showing the deformed shape of the pipeline and the absolute value of maximum strain at a fault displacement of 5 m. Displacement capacity of the pipeline was taken to be conventional strain limits at the threshold of loss of pressure integrity (4% in tension and 6% in compression). A variety of conditions were analyzed with burial depths ranging from 0.9 to 1.5 m, geotextile wrap on the pipe, and geofoam blocks in lieu of backfill at the fault. A 90° bend in a street intersection 125 m from the fault crossing tended to concentrate strain. The selected design has a displacement capacity of 3.75 m which corresponds to a factor of safety of more than 1.5 over the expected 2.4-m in-plane displacement.