To ensure that the soil around the coil has high thermal conductivity, one promising innovation for slinky installations is flowable backfill.
With flowable backfill, a concrete-like mixture containing cement, water, sand, and fly ash, rather than water-soaked soil, is first poured in to cover the pipe before the trench is backfilled with the soil removed from the trench.
The backfill flows around the pipe, providing high thermal conductivity and enhanced heat transfer between the pipe and the ground.
Flowable backfill is especially applicable where the soil is not fine enough to provide desirable heat transfer. Examples include clay soils that produce spoils in the form of clods, or locations with rocks or other materials that could create voids around the pipe.
After the slinky has been placed in the slit trench, the procedure is to use a commercial concrete truck to mix and deliver the prepared flowable backfill material.
Pouring should start at the loop head. Hold the free end of the plastic pipe up and clear of the flowable backfill material while pouring.
Pour the flowable backfill material into the trench by moving the truck slowly forward to insure equal depth and distribution. The flowable backfill should cover the top of the pipe by about six inches.
Complete the backfill by placing the spoils back into the trench. Any remaining spoils should be disposed of elsewhere.
Research is ongoing at Oklahoma State University and at South Dakota State University. Objectives include establishing the best mixtures for flowable backfill, which might be in the general range of 5 percent cement, 15 percent water, 25 percent fly ash and 55 percent masonry sand.
The right percentage of each component will produce a mixture with favorable structural and heat transfer characteristics. Comparisons are also being made with conventional backfill.