Most pipeline design assumes the ground stays where it is. Geohazards are the cases where it does not. Unlike internal pressure, which the pipe is explicitly designed for, ground movement imposes displacement-controlled loading — the soil moves a certain distance and the pipe must accommodate that displacement or fail trying. It is the mechanism behind a disproportionate share of transmission-line incidents in mountainous and mined terrain, and because ground movement is progressive, the damage accumulates silently between inspections. Routing is the primary control: a hazard avoided by a half-mile reroute costs nothing to manage for the next fifty years.
Karst and sinkholes
Karst is terrain developed on soluble bedrock — limestone, dolomite, gypsum, and salt — where groundwater has dissolved the rock into a network of voids, conduits, and caves. The USGS karst map compilation puts about 18% of the United States over soluble rock with karst or the potential to develop it, concentrated in the Appalachians, the Ozarks, central Texas, Florida, and parts of the Midwest.
What karst does to a pipeline:
- Cover-collapse sinkholes open abruptly beneath or beside the line, removing support over a span of several feet to several tens of feet and leaving the pipe as an unsupported beam — the classic karst failure mechanism.
- Raveling and soil piping gradually wash trench backfill down into bedrock voids, creating a void beneath the pipe that is invisible from the surface until it collapses.
- Pinnacled rock makes for extremely difficult, expensive trenching with abrupt hard/soft transitions that concentrate bending at the contact.
- Drilling fluid loss in trenchless crossings — a bore that intercepts a conduit loses total returns instantly, with the added environmental exposure that karst conduits discharge to springs and drinking-water supplies, as covered in the frac-out article.
Mitigation is avoidance first — routing around mapped sinkhole clusters and closed depressions — then investigation (borings, and geophysics such as microgravity or electrical resistivity that can detect voids between borings), then engineering: grouting voids, spanning with reinforced bedding or a structural slab, and increasing wall thickness through the reach so the pipe can carry an unsupported span. Drainage control matters as much as structure, because concentrating stormwater runoff on karst ground is an efficient way to trigger the collapse you were trying to avoid.
Landslides and slope instability
Slope movement is the geohazard that most often governs in the Appalachians and the mountainous West. The relevant distinction is between mechanisms, because they load the pipe differently:
- Deep-seated rotational or translational slides move a large soil mass along a failure surface below the pipe, dragging the line laterally or axially over tens of feet — the most damaging case.
- Shallow debris flows and slumps, common on steep, cleared right-of-way slopes after heavy rain, which can strip cover and expose the pipe.
- Soil creep — slow, continuous downslope movement of the near-surface soil, measured in inches per year, that accumulates axial strain in a line running down the fall line.
- Right-of-way-induced instability, where clearing, grading, and trenching themselves reduce slope stability and create the hazard the route was supposed to avoid.
The pipe response is governed by strain, not stress. A line crossing a slide perpendicular to movement sees bending and ovalization; a line running parallel to movement sees axial tension or compression, and compression at a girth weld is what produces wrinkling and buckling. Modern practice for known slide crossings uses strain-based design — tension-governed limits well beyond first yield, compressive strain limits set by local buckling — rather than the allowable-stress approach used for pressure design.
Mitigation includes routing perpendicular to rather than along an unstable slope, crossing the slide at depth with a trenchless method beneath the failure plane, slope drainage and dewatering (water is almost always the trigger), regrading and buttressing, using heavy-wall pipe and high-toughness girth welds through the reach, and installing strain gauges, inclinometers, or fiber-optic monitoring on slides that cannot be avoided. Geotechnical characterization for these decisions follows the same investigation logic described in the geotechnical investigation article.
Subsidence
Subsidence is the slow, broad settlement of the ground surface, and it has three common causes along pipeline corridors:
- Underground mining — active longwall mining produces a moving subsidence trough with a predictable geometry and can generate several feet of vertical movement plus severe horizontal strain at the trough edges; abandoned room-and-pillar workings can collapse decades later with no warning.
- Groundwater and hydrocarbon withdrawal — regional aquifer depletion in areas such as the Central Valley and parts of Texas and Arizona produces feet of settlement across broad areas, with earth fissures at the margins that are locally abrupt.
- Organic soil oxidation and consolidation — drained peat and organic soils settle continuously, a chronic issue in delta and reclaimed wetland terrain.
Uniform settlement across a wide area does a buried line little harm. Differential settlement does, and the damaging zones are the edges of a subsidence trough and the transitions where the line passes from settling ground onto stable ground or onto a rigid structure such as a piled facility foundation. Those transitions are where curvature and axial strain concentrate. The settlement analysis calculator evaluates the trough geometry and the resulting profile the pipe must accommodate.
Fault crossings
An active fault imposes a permanent ground displacement across a narrow zone — potentially several feet in a single event. Two additional seismic effects act more broadly: liquefaction of saturated loose sands, which removes the soil’s ability to support or restrain the pipe and can drive lateral spreading toward a free face, and transient ground strain from seismic wave passage, which is usually the least damaging of the three for a well-designed buried line.
Fault-crossing design is a specialty of its own, and the principles are consistent: cross the fault perpendicular where possible and oriented so the pipe goes into tension rather than compression under the expected slip; use heavy-wall, high-toughness pipe with matched or overmatched girth welds through the crossing; reduce soil restraint so the pipe can move and distribute strain over a longer length — shallow burial, loose granular or lightweight backfill, geofoam, or a lined trench; and avoid anchors, valves, bends, and tie-ins inside the fault zone, since every restraint concentrates strain. ASCE and ALA guidelines for the seismic design of buried pipelines give the analysis framework.
Screening geohazards before survey
Every one of these hazards has a mapped signature, and none of them require a field program to identify at the corridor stage. The SubTerra geohazard data layers render karst and soluble-rock terrain with mapped sinkholes and closed depressions, USGS landslide inventories and susceptibility mapping, historic and active mine workings, areas of documented subsidence and earth fissures, and USGS Quaternary fault and fold traces with slip-rate attributes — overlaid on the same base a route is drawn on.
Paired with high-resolution lidar-derived terrain, which reveals landslide scarps, hummocky ground, and sinkhole topography far better than aerial imagery, and with soils and surficial geology layers, this is enough to do the triage that matters: which alternatives cross an active slide complex, which reach sits over abandoned workings, which crossing is in karst and therefore needs geophysics in the investigation scope, and which segments will carry heavy wall. It is the same routing-stage discipline the flood zone and contaminated sites articles describe, applied to the ground itself rather than to what is on it.