This document is the Flowable Fill Backfill for Thermoplastic Pipe. It explains advantages/disadvantages of flowable fill and installation considerations.

The use of flowable fill, also known as controlled low strength material (CLSM), controlled density fill (CDF), and slurry fill, as pipe bedding and backfill material has steadily been increasing. The term “flowable fill” encompasses a variety of fill materials that are used as alternates to compacted granular fill. The materials are comprised of mixtures of sand, Portland cement, Class C or Class F fly ash, and water.

Flowable fill is an alternative to conventional soil or stone backfill and has been used for unique applications and installations of pipe for some time. It has the advantage of providing adequate strength quickly, while providing an easy and efficient placement system.

The advantages of the use of Flowable Fill are it allows for narrower trench and less disturbance to the native material. Eliminates the need for backfill compaction. Ensured proper distribution of support around the pipe. Reduces the amount of material excavated on a project. Time, personnel and equipment required to install flowable fill

are typically less than that required for proper placement and compaction of conventional backfill materials, particularly fine-grained soils. Flowable fill may be made on-site using native soil as part of the mix where sands or silty sands exist. Time and equipment required for compressive strength testing is often less than that required to test soil compaction.

Disadvantages are that it’s more costly than granular backfill due to the many components required and specialized delivery. Improper mix components can cause difficult future excavation if taps or extensions are required. Cannot be stockpiled on site like granular backfill. Time saved during the placement of the flowable fill can be wasted waiting on ready-mix delivery. Unless precaution is taken, the potential for pipe flotation is high during the installation process.

Mix design is discussed along with installation considerations of environment, joints, placement of flowable fill, anchoring systems. Tables supporting recommendations is provided.