The use of permeable materials for backfilling trenches

The use of cementitious permeable backfill materials containing pulverized fuel ash based aggregates for trenches was investigated by means of laboratory tests and a field trial. Single-sized Lytag and Aardelite aggregates were used in this investigation. Laboratory measurements included unconfined compressive strength, 10% values, flowability, permeability and freeze–thaw resistance. These results were compared with those of foamed concrete. It was found that the flowability of Lytag and Aardelite depended in part on the spherical shape of the particles and therefore low water–cement ratio was required to achieve a workable mix which does not need compaction. Aardelite mix showed high potential for backfilling trenches because during pilot testing it performed as well as Lytag and it was much cheaper than the latter aggregate. However, more work is needed to develop a mix design. A field trial was conducted on a Lytag mix and its performance was compared with that of granular type 1 backfill. The trench backfill materials complied with the Highways Authorities and Utilities Committee specification and required no compaction. Field trial showed that Lytag backfill performed better than type 1 granular sub-base in terms of having lesser long-term settlement. A cost estimate of the backfill materials suggested that the Aardelite backfill is similar to granular based sub-base. In the long term it may be cheaper, as it may require little or no maintenance.