Field Performance Evaluation of Recycled Aggregate Blends Used for Backfilling Deep Excavated Trenches

Abstract

Utilizing natural expansive clays that are available on-site as sewer trench backfill can cause destructive deformations due to volume changes, which are caused by seasonal climatic changes. Such deformations result in manhole structures protruding from the surface, which cause damage to the surrounding infrastructure and generate potential trip hazards. In this study, mixtures of recycled materials with minor sensitivity to moisture variations and superior compactibility were investigated using geomechanics theories associated with granular materials as an alternative backfill material. Blends of recycled glass (RG), plastic (RP), and tire-derived aggregates (TDA) were mixed on-site, wetted to the required moisture content (MC), and used to backfill excavated trenches around two manhole structures and extended to approximately 11 m along the trench. A benchmark trial was constructed by backfilling with natural soils available on-site according to the normal procedure. The full-scale trial sites were instrumented using settlement plates and MC sensors at various locations and depths for performance monitoring. The results of approximately 17 months of field monitoring showed that settlements over both areas that were backfilled with recycled blends were <20% of those over areas backfilled with site-won soils. Approximately 82% of the settlements in the recycled blends occurred during construction. In contrast, trenches that were backfilled with site-won soils continued to exhibit deformation due to consolidation and swell–shrink cycles. The outcome of this study could contribute to the United Nations’ Sustainable Development Goals, in particular, Goal 12, by improving the industry’s confidence in the reuse of wastes in geotechnical applications. This study investigated the application of mixtures of RG, RP, and recycled tire when backfilling deep excavated trenches in nontrafficable areas. In deep excavated trenches, quality control of the traditional compaction methods that use the excavator’s arm is challenging due to safety restrictions that limit the presence of the testing crew in the trench. The blends that were developed in this study are designed to exhibit self-compacting properties, lower the settlement potential of excavated clay and gravel fill materials, and have less sensitivity to moisture variations. Therefore, they are useful in applications, such as backfilling deep (>1.5 m) excavated trenches. The outcomes of this study could promote sustainable construction approaches through the utilization of recycled materials in earthworks and construction projects. In addition, this would mitigate the challenges associated with waste management, because the substitution of virgin materials with recycled materials or wastes could divert waste from landfills. Furthermore, by improving the strength and deformation properties of backfilled trenches, less damage to the surrounding structures at the surface, such as residential houses, sidewalks and fence lines, is expected.