There are a variety of different bedding/backfill materials available to suit the various circumstances of cable installation.
Geotherm specialise in the design of bedding/backfill materials and recommends the use of a flowable backfill/bedding material, specifically designed to meet thermal property requirements. This is sometimes known as Fluidise Thermal Backfill or Thermally Stable Backfill. These materials can achieve reliable Thermal Resistivity values when supplied and installed as specified.
Flowable thermal backfill is a concrete type product and can be supplied by most concrete companies once Geotherm has provided a mix design. It is typically quite fluid, having a relatively high slump, allowing it to readily flow around the cable/conduits. The backfill can be designed with variable slumps if required (on steep hills etc).
Like concrete, flowable backfill is delivered to site via concrete agitator trucks, and poured directly into the trench. Typically, no vibration is required due to its “flowability”.
Most flowable backfills are typically weak, less than 5mpa to allow easy access to the cable in the future, yet maintain the mechanical strength to protect the cable. It hardens quickly so that the ground surface may be reinstated the following day and provides support for underground and surface facilities (road pavement).
The thermal resistivity of the flowable backfill will depend on the mix constituents. The mix ingredients will depend on those available from the local quarries/batching plants to the area. It would not be very cost effective to import materials from far and wide. When selecting mix constituents for a flowable backfill design, it is important to consider the availability of those materials over the life of the project.
Various sand and stabilised sand (granular materials) can also be used as bedding/backfill. These materials are required to be compacted around the cable in layers, and must be installed at the specified moisture content and density as both test factors greatly affect the thermal resistivity readings. These granular type backfills require strict quality control to ensure the installation criteria are met, the thermal properties can change dramatically if they are not.