Cottonwoods Connection Pipeline Crossing of the Wasatch Fault

Culinary water suppliers in Utah’s Salt Lake Valley collaborated on a new intersystem connection pipeline which crosses the Salt Lake City Segment of the Wasatch Fault. With owners selecting a design hazard level of 2% probability of exceedance in 50 years (corresponding to a return period of 2,475 years), the pipeline was designed to accommodate a 10.0-ft vertical displacement along the main fault, with smaller antithetic fault displacements on the other side of the graben. One feature of the project was that the crossing occurred under major roadways; hence, potential settlement of backfill was a concern. The higher density and stiffness of trench backfill otherwise needed to reduce settlement counteracted desirable flexible behavior, leading to excessive strain in the modeled pipeline. Evaluation of potential source materials at differing densities led to the development of backfill material and placement criteria that balanced these competing interests.