Guidelines for Seismic Design & Assessment of Pipelines

SECT SECTIO ION N 5: Mitig ation Optio ns

5-4

well-suited for this purpose since they can experience large compression strain under near-constant compression compression load. Cellular concrete is a mixture of sand, cement, and water to which a foaming agent or polystyrene beads are added to create small air  pockets. The use of crushable material as a mitigation measure is typically only a  practical consideration for fault crossings in rock or rock-like materials where very large lateral soil-spring forces can be generated and excavating a trapezoidal trench that can be filled with loose granular material material is difficult or impractical. impractical. Controlled-strength materials improve pipeline response by allowing the pipe to bend in a more gradual manner to accommodate the imposed ground displacement. The distance from the pipeline that is to be filled with controlled-strength material is largely governed by the amount of compressive irrecoverable strain that can be accommodated before the material begins to exhibit much higher compressive strength. This strain level is is commonly referred to as the lock-up strain. The lock-up strain for cellular concrete can vary from 15% to 35%. The lock-up strain for EPS or XPS geofoam can vary from 25% to more than 50%.

5.1.2

Unanchored Length

The capacity of a buried pipeline to withstand ground displacement components can be improved by maximizing the distance from the deformation zone (fault rupture, landslide, lateral spread, etc.) to points of virtual anchorage, typically side bends, overbends, and sagbends. Sharp bends, tees, branch fittings, valves, etc. also also will have a tendency to anchor the pipeline against axial movement and should be avoided within or near a zone of ground displacement. Good design practice is to provide a straight segment of  pipeline as long as practical through and beyond the ground displacement zone to maximize the length of pipeline available av ailable to distribute strain.

5.1.3

Isolating Pipelines from Ground Displacement

Soil loads on buried pipelines are the result of relative movement between the pipe and the surrounding soil. This relative movement can be minimized or eliminated eliminated by  providing space around the pipeline that is greater than the relative movement associated with poor pipeline response. Isolating the pipeline from ground displacement displacement is generally  practical only when the ground groun d displacements d isplacements are relatively small, the length of o f pipeline requiring isolation is relatively short, or the pipeline diameter is relatively small. Mitigation concepts using culverts allow the pipeline to respond in a manner similar to an on-grade condition. The term culvert refers to to any buried structure built partially or completely around the pipeline to provide an unobstructed space for the pipeline to deform in in a direction transverse transverse to its axial alignment. Three conceptual culvert configurations are illustrated illustrated in Figure 5.5. Culvert concepts are essentially specialized casings and the same problems that can arise for cased pipelines generally apply to culverts. In addition, caution is needed to assure assure that axial loads from from thermal changes or internal pressure do not lead to buckling of the pipe within the culvert. Axial buckling can typically be prevented by incorporating bends or expansion loops in the pipeline. PR-268-9823