Numerical Modeling of Permanent Ground Deformation Hazard to a Natural Gas Pipeline in California

An extensive Permanent Ground Deformation (PGD) study was conducted on a proposed natural gas pipeline to be located north of Sacramento, in Colusa and Butte Counties, California. The majority of the 30-inch (76.2-cm) diameter and 25 mile long buried steel pipe will be cut and cover construction; however several portions of the pipeline will be installed by horizontal directional drilling methods (HDD). The natural gas pipeline will traverse areas of two potential PGD hazards: faulting deformation and lateral spreading. The pipeline crosses the Willows fault, and an HDD portion of the pipeline will underlie the Sacramento River where a potential lateral spreading hazard exists. A detailed analysis was carried out to evaluate the magnitude of PGD and potential impacts to the pipeline associated with each of these potential PGD hazard sources. This paper addresses PDG associated with faulting. Two and three-dimensional numerical modeling of fault deformation using the finite difference computer program FLAC (Fast Lagrangian Analysis of Continua) are presented and discussed. FLAC was used to simulate the fault rupture mechanism considering a 3.3-foot (1-meter) reverse and 1.8-foot (0.55-meter) strike slip displacement across the fault at approximately 1600 feet (488-meter) below the ground surface. The model captured the resulting ground surface deformation patterns. The 3D FLAC results were used in detailed stress-strain evaluations of the pipeline response using another commercially available finite element computer program, ANSYS. Results compared favorably to the allowable limit states that have been established in the American Lifelines Alliance Guidelines for the Design of Buried Steel Pipe.