Pipe Zone Bedding And Backfill: A Flexible Pipe Perspective

Bedding and backfill materials play a critical role in the long-term structural integrity of buried municipal pipelines. Soil strength is resistance to soil slip, and is a function of soil friction angle and any cohesion in the soil. It determines the stability of the soil. Soil stiffness is the modulus of soil elasticity, E', and is a function of soil type and the applied level of compaction. E' is also affected by depth of burial which increases confining pressure. E' determines the ring deflection of flexible rings. The Modified Iowa equation is used to predict ring deflection of buried flexible pipes. Common types of backfill material can range from native soils (which usually have fines, silt and clay) to imported crushed rock, to soil-cement slurry (or flowable fill/CLSM). Whenever backfill material other than native soils is specified on a project, the reason for doing so must be justified. This paper reviews and recommends best practices for the selection of common bedding and backfill materials for flexible pipes and conducts an economic analysis of the various options utilized. Theoretical aspects of pipe-soil interaction as it relates to ring deflection of pipe for various soil types are discussed.