Use of waste gypsum for trench backfill

Abstract

In the UK, legislation is limiting the disposal of gypsum therefore alternative applications are encouraged. Large quantities of red gypsum arise from the production of titanium dioxide and from waste plasterboard. Both red gypsum and plasterboard gypsum were investigated for use in trench backfill. Red gypsum comprised filter cake lumps with moisture content of about 40% and up to about 100mm in size and plasterboard gypsum consisted dust to thin slab like particles with moisture content of about 20%. The latter was both with paper and without paper. The investigation undertaken aimed to develop gypsum-based trench backfill that required zero compaction, and could be mixed and placed using existing plant without specialist labor. Successful mixes containing waste gypsum, limestone quarry fines and Portland cement that complied with the existing specifications were developed. Tests included determination of unconfined compressive strength, soaked strength, resistance to freeze and thaw and sulfate contents. The sulfate levels in these mixes were within acceptable limits. The most suitable mix was evaluated in a short section of full-scale trial trench and was subjected to 1 million cycles of standard axle load. Deformation was found to occur only in the base course and wearing course layers.