Geotechnical Challenges for Design of a Crude Oil Pipeline Across an Active Normal Fault in an Urban Area

The proposed construction of a crude oil pipeline through a residential area north of Salt Lake City, Utah, with an alignment that crossed the Wasatch fault provides an interesting case history of the numerous uncertainties and competing constraints associated with designing a pipeline fault crossing in an urban environment. Several issues raised during project design needed to be resolved with representatives of the city in which the project was located; the city had obtained technical input from the state geological survey and a local pipeline engineering specialist. The definition of the fault location and design fault displacement required reconciling suggested fault displacement estimates that ranged from 2.4 m to 4.2 m. The desire on the part of the pipeline owner and the city to have the oil pipeline buried relatively deeply (at least 1.5 m of cover) needed to be resolved with the fact that improved pipeline performance for imposed fault displacements typically is achieved with shallower soil cover. Special trench construction measures to increase the pipeline fault displacement capacity, such as reduced burial combined with protective concrete slabs above the pipeline or use of geofoam material as trench backfill, needed to be balanced with potential consequences on normal pipeline operational and maintenance activities, as well as street maintenance by the city. Increases in pipe wall thickness, that would permit an increase in the burial depth of the pipeline, needed to be balanced with concerns regarding potential problems that could be created with the measurement quality of internal inspection devices. The requirement that the pipeline be located beneath city streets, including a 90° corner 125 m from the fault crossing, limited the ability of the pipeline to distribute axial strain developed as a result of the fault displacement and led to optimization of the pipeline bend geometry with respect to available space and impact on existing utility lines. Resolving these issues was facilitated by examining the pipeline response to a variety of postulated design alternatives using finite element analyses. The final design recommendations that satisfied the owner and city provided a reasonable assurance that the pipeline would maintain pressure integrity for a fault displacement of 3.75 m.