PIPELINES IN SEISMICALLY ACTIVE AND LANDSLIDE AREAS



CONSTRUCTING PIPELINES IN SEISMICALLY ACTIVE AND LANDSLIDE AREAS

For seismic loads, examination of papers and design guidelines show that for a welded steel pipeline, ground shaking and strain do not cause any substantive harm. Problems for buried pipelines exist either where a fault with a surface expression is crossed or where the ground in which the pipeline is laid loses strength and a land slide or slip leaves the pipe without support or imposes large shear forces when moving. The fault movement can be designed in with long straight lengths of pipe crossing the fault at an angle of 60 degrees and the pipeline trench back-filled with cohessionless backfill (e.g. gravel). Identification of potential land slide areas will need to form part of the route survey in seismically active areas and susceptible areas avoided wherever possible.

Pipelines have been built in seismically active areas both as buried and surface run pipelines. In general there is little advantage to having the pipeline above ground as general shaking is not a particular problem for buried pipelines, but pipelines have been known to fall off supports or be excessively strained at individual supports, producing a point load, not associated with buried pipelines.

In the event of mass ground failure, both types of construction are equally vulnerable and the pipeline would be fortunate to survive. Where other considerations apply, supports have been specifically designed to move only under large forces which would be close to the yield strength of the pipeline. Project examples are rare as particular fault areas are normally avoided wherever possible and where ground failure is predicted from soil analysis, re-routing normally undertaken.

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