3.2 Blends of recycled materials as sustainable alternatives for backfilling sewer trenches

In this project, the suitability of four blends of recycled materials comprising different proportions of recycled glass, plastic, and tyre aggregates as alternative backfilling materials for deep excavated trenches was investigated. This paper presents results of an extensive testing program carried out for selecting two most appropriate blends for backfilling of trenches located in non-trafficable areas. These blends will be used for construction and instrumentation of trial sites for deformation monitoring over 12 months. Physical properties such as particle size distribution, maximum and minimum density, compaction properties, and field capacity of the blends were determined. Further, an application-specific geotechnical testing methodology was developed. This included determination of the dry density achieved using a proposed sand-raining technique (SRT) to simulate the real-life trench backfilling procedure and determination of the compressibility of the blends using a modified oedometer test. The SRT results showed that the obtained dry density (DD) increased as moisture content (MC) and height of drop increased. Based on the relative density achieved through the SRT compaction and the compressibility properties, Blends 2 and 4 showed the most advantageous characteristics. This paper presents the developed testing procedures and discussions on the results leading to the selection of the two most suitable blends for the proposed application. The paper also makes discussions on the stress-strain conditions expected on site and potential downfalls of the proposed application. The outcomes of this research aim to promote sustainable geotechnical design and construction by improving the industry’s confidence in utilising recycled materials.