Blends of recycled materials made for backfilling sewage trenches

The outcome

Excavated trenches for sewerage pipes and infrastructure situated in residential and commercial zones require backfill material to protect them from decay and damage. This project demonstrated effective use of 100% recycled materials for trench backfill materials consisting of recycled glass (RG), recycled plastic (RP) and tyre derived aggregates (TDA) with varying moisture content of up to 10%.

The research project looked at the most significant cause affecting sewage pipelines: the backfill material type. The project developed 2 blends of 100% recycled material as alternatives to conventional sewer backfill material.

The developed blends offer less sensitivity to soil moisture variations caused by rainfall after hot and dry summer months. They were specifically designed for deep excavated trenches located in non-trafficable areas in the outer western suburbs of Melbourne covered by basalt rocks and associated expansive clays.

The need

The damage to sewer infrastructure by expansive soils can bring environmental consequences especially given the reliance on raw materials in traditional trench backfill mixes. They also result in significant repair and rehabilitation costs to water authorities and councils.

The project was aimed at addressing the following 4 needs:

• offering a low-cost recycled material alternative to conventional backfill mixes
• demonstrating performance of recycled materials to existing performance standards and to key stakeholders including the waste authorities and industry
• improving the life of council assets like roads and footpaths in the vicinity of sewer pipelines
• reducing damage to local property impacted by backfill degradation and conserving valuable resources.

Developing the solution

Laboratory testing was conducted to determine suitable blends of recycled glass, rubber and plastics as alternatives to conventional sewer backfill material with sections of conventional material as a benchmark.

The testing breakthrough of this project included developing project specific testing tools and methodologies to simulate real-life site conditions and this included:

• a sand-rain-technique set-up to simulate the real-life backfilling procedure of excavated trenches using the excavator bucket
• a segregation test set-up to simulate the potential settlement and migration of fines as a result of rain events
• modifying the conventional oedometer testing scheme to suit the recycled material of this research for the determination of the compressibility characteristics of the proposed blends.

The project learnings included:

• Forward thinking to obtain large amounts of raw recycled materials of consistent quality and at affordable costs compared to virgin materials.
• Significant time was saved in the backfilling implementation due to the different mixing, backfilling and compaction processes adopted for the recycled material mix. In comparison with raw material mix, the test blend was dropped into trench from a height of greater than 2 metres.
• Installing settlement plates at different depths was recommended in order to monitor the movement of the backfilled area.
• As opposed to the raw material mix, the recycled material blend does not require onsite mixing which is time consuming and logistically complex. In future, the product will become commercially available from current recycling facilities and may be ordered in the same way crushed rock products are ordered for trench backfill.

The partnership

Victoria University led the project and worked in partnership with the University of Melbourne. The research team utilised both universities' knowledge of recycled materials in geotechnical structures.

The technical expertise of Ground Science, as the lead industry partner, and Greater Western Water was drawn upon for quality control, field and laboratory testing and assistance in the design, construction and site management of the full-scale trial sites.

Pathway to commercialisation

Commercialisation of the proposed blends is being pursued. The project team and the industry partners are making attempts to promote the project’s products and obtaining official approvals from relevant authorities. One such approval required is from the water industry’s Technical Approval Group which has representatives from various departments that discuss the various technologies and products.

Another area that will be investigated by the team is producing the blends at an attractive cost. To do this, several recycling and batching industries will be part of the negotiations.

With successful outcomes achieved in the current project, the research team aims to extend the application of blends of 100% recycled materials to backfilling deep excavated trenches located in “trafficable areas”. The “trafficable areas” along sewer infrastructures undergo repeated loadings of the moving vehicles. In addition to static strength testing, a series of pavement tests which are of a dynamic nature is required for proposing the most suitable blend. This project has recently been funded by Sustainability Victoria and has commenced in late 2021.

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