Soil-Pipe Interaction Analysis: A Forensic Evaluation

A gas leak at a residence caused an explosion. It was hypothesized that the soil shrink/swell potential caused a break in the polyethylene (PE) gas pipe-coupler connection, which subsequently resulted in a leak and an explosion. However, this claim was without any scientific reasoning or engineering principles such as soil-structure interaction. Review of prior information indicated that sometime prior to the explosion a water line was installed below the PE gas pipeline and a BOMAG multipurpose compactor was used to compact the backfill in the water pipe trench. The waterline and PE gas pipeline were perpendicular to each other. Using finite element method (FEM) analysis of the soil-pipe system and the scientific method using site-specific soil, geophysical, and geotechnical data it was shown that the break in the PE pipeline and coupler was due to deformation (displacement) of the soils in the trench area, where the PE gas service line was present and not due to soil movement alone. This deformation is the result of external and applied loads from the BOMAG compactor in the static and dynamic compaction modes. A field re-creation of the construction sequence on a later date corroborated very similar pipe displacements as that predicted by the numerical modeling. This confirmed the results from numerical modeling and confirmed that the construction activities during the installation of water service and/or sewer lines underneath the gas line caused the gas line to sag and pull out at the adjacent pipe-coupler connection.