CONSIDERING COLLAPSIBLE SOIL HAZARDS FOR SITING AND DESIGN OF NATURAL GAS PIPELINES

Agencies regulating interstate natural gas pipelines in arid and semi-arid regions tend to require that collapsible soils be considered even though hydrocompaction is not one of the hazardous geologic processes specifically mentioned in Federal Energy Regulatory Commission (FERC) Resource Report 6. Dry, low-density sediments with high dry strength consolidate when wetted and produce differential ground-surface settlement up to about 10% of the susceptible deposit thickness. Collapsible sediments typically are deposited as arid to semi-arid Holocene eolian loess and alluvial fans with multiple channels on unstable surfaces, and may include colluvial and some residual soils, and poorly engineered fills. Subsurface wetting by water ponding, irrigation, or leaking utility lines can produce decreased shear strength or dissolution of weak cement, resulting in ground-surface settlement. Estimated maximum potential settlement is based on deposit thickness, whereas projected qualitative strain in buried pipelines is based on the lateral extent of the deposit and the length of pipe that could be affected. Typical natural gas pipeline wall thickness can sustain considerable stress as indicated by Finite Element Method (FEM) analyses. A responsible approach for mitigating collapsible soil hazards for buried pipelines typically is with operations and maintenance (O&M) plans rather than in design. Recognition of potentially collapsible soil deposits allows O&M planning to include periodic ground surface monitoring and a threshold subsidence amount that would trigger a pipeline structural integrity assessment. Some siting and design mitigation can be effective in certain circumstances (e.g., plastic [HDPE] liners for nearby ponds and at canal crossings).