Recycled Aggregate Mixtures for Backfilling Sewer Trenches in Nontrafficable Areas

Abstract

Sewer infrastructures that are backfilled with available onsite expansive soils may be exposed to damages due to seasonal ground movements. Therefore, for deep excavated trenches located in nontrafficable areas, the suitability of utilizing mixtures of recycled materials as alternative backfilling materials with lower sensitivity to moisture variations was studied. Extensive environmental and geotechnical testing programs were carried out on the performance of four proposed blends comprising different proportions of recycled glass, plastic, and tire aggregates in order to determine an optimum mix design. First, potential environmental hazards of the individual recycled materials were assessed. The investigation was advanced by the development of an application-specific geotechnical testing program utilizing geomechanics theories and methods associated with granular materials. The program included the determination of the maximum dry density achieved using a sand-rain technique (SRT) to simulate the real-life backfilling procedure of excavated trenches, and the determination of the compressibility of the blends using a modified oedometer test. In addition, the potential for the migration of finer particles within the recycled material blends and the possible long-term settlement of the backfilled area were studied. The environmental test results showed that the blends could safely be used as fill materials. The SRT test results revealed that the dry density increased as the moisture content and the height of drop increased and the tire content of the mixture decreased. At moisture contents ≥ 15%, the maximum dry densities obtained through the SRT and the standard Proctor compaction were close, achieving relative densities greater than 80%. Furthermore, by simulating precipitation, inconsiderable segregation of particles was observed in the proposed blends. Based on the results of the developed experimental program, two blends were recommended as the optimum mixtures.