Cement Bound Sand (CBS) is a standard backfill material which is frequently used to reinstate excavations associated with laying underground high voltage transmission cables. However, this backfill is unsuitable when higher ratings are required as it cannot adequately dissipate the additional heat generated by the conductor. As we seek to increase the amount of undergrounding and capacity of our transmission infrastructure, this is likely to become of increasing importance.

 Liquid Soil (LS) has been identified as a potential alternative to CBS as it claims to have a relatively low Thermal Resistivity (TR), which would allow the heat to dissipate quickly through the material and enable higher rated cables.

LS is formed by using excavated native soil which is then processed together with additives, which allows it to become temporarily flowable.  This means that once prepared it can be poured into the trench and around the Gas Insulated Lines (GIL) or cable. The material adheres closely to the cable or GIL and sets hard without the risk of air gaps forming, thus removing the need for the backfill material to be compacted.

 However, LS has not yet been used in the UK.  In order to deploy this backfill on the High Voltage (HV) transmission system we need to understand the material properties of the LS and in particular how it behaves under certain conditions in terms of moisture retention and the consequent impact on TR.

The project will consist of a literature review to identify any alternative flowable type backfills for potential use in an HV environment and an investigation into the specific properties and behaviours of this type of backfill to assess the suitability of flowable type backfill materials for use on the UK transmission system. Tests will also be undertaken to gain an understanding of the possibility of any potentially harmful ingredients from the backfill material leaking (leaching) into the surrounding environment, and an assessment of the propensity for long term shrinkage of the LS.

Objectives

1. To identify any flowable type backfill materials available with potential for use on the UK transmission system.

2. To gain an understanding of the basic properties of flowable type backfills, specifically LS, when formed from a variety of indigenous materials, to assess the suitability of this type of backfill material for deployment on the UK transmission system.