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Smart Water Fund Research Grant

Geotechnical Characteristics of Biosolids and their Suitability as Stabilized Fill

Smart Water Fund
The Smart Water Fund is an initiative of the Victorian Government and the Victorian water industry aimed at encouraging innovative solutions to water conservation, water management and biosolids management. This project involves expertise in the areas of civil engineering and geotechnical engineering. By the application of advanced soil mechanics concepts, the geotechnical strength properties of aged biosolids (with and without additives) obtained from a major sewerage treatment plant in Victoria were assessed for use in stabilized fill applications. Biosolids samples obtained from three stockpiles at the Biosolids Stockpile Area, Western Treatment Plant were tested to investigate the geotechnical characteristics of biosolids and the suitability of biosolids as stabilised fill material.

Project Outcomes

Laboratory Testing

Extensive geotechnical laboratory tests were subsequently conducted at Swinburne University of Technology. The tests included triaxial shear strength, consolidation tests with oedometers, consolidation tests with Rowe cells, permeability tests, compaction tests, California bearing ratio (CBR) tests, grain size distribution, atterberg limits, moisture content and other geotechnical tests to assess the geotechnical characteristics of stabilised biosolids as a fill material.

The biosolids samples are classified as organic fined-grained soils of medium to high plasticity with a group symbol of ?H?as per Australian standard for the geotechnical site investigation (AS 1276, 1993). The biosolids samples contain approximately 5% gravel size, 50% sand size, 40% silt size and 5% clay sized particles.

The shear strength test results from the laboratory triaxial tests indicate that the biosolids stabilised with the required proportions of additives possess sufficient shear strength to make them suitable for use as fill material. The CBR values of biosolids stabilised with a minimum of 5% lime, 3% cement, 3% bauxsol and 30% crushed brick satisfies the VicRoads specification for Type B fill material which requires a minimum CBR of 2%.

Finite Element Modelling

Finite element analysis was conducted to analyse the behaviour of an embankment using biosolids stabilised with lime, cement, bauxsol or crushed brick, as well as untreated biosolids. The residual settlement of the biosolids stabilised with various additives including lime (5%), cement (3%, 5%), bauxsol (3%, 5%) and crushed brick (30%, 40%, 50%) were found to be within VicRoad? residual settlement requirement of a maximum of 50 mm over a period of 20 years after 6 months of preloading.

The results of the finite element analysis agree well with the laboratory results and indicate that biosolids, when stabilised with additives to the required percentages, can be used as stabilised fill in embankments. To negate the effect of long term decomposition of organic matter in biosolids, the maximum allowable total thickness of the stabilised biosolids within a road embankment should be limited to 0.5 m. Use of an impermeable geomembrane separator or 0.5 m impermeable clay layer is recommended to encapsulate the biosolids and prevent any seepage or leaching of biosolids into the fill material. The cost of encapsulating the stabilised biosolids with a geomembrane or clay liner is minimal as the geomembrane or clay liner has been included solely for separation purposes.

Technical Note

A technical note has been developed to provide standard best practice for the usage of biosolids as stabilised fill in embankments. Due to the nature of the material, occupational health and safety measures, suitable transportation, controlled storage, site management and environmental management controls have been outlined in this technical note. Biosolids should be stabilised with one of the following additives to the specified minimum mix proportions:

  • lime (5%), cement (3%), bauxsol (3%), 20 mm crushed brick (30%);

In addition, a brief has been prepared for the construction of future instrumented trial embankments to confirm the expected settlement of the stabilised biosolids when used as embankment fill and to compare this with the laboratory testing and finite element modelling results. An estimate of the cost of the stabilised trial embankments has also been included and was found to be only marginally higher than that of a traditional embankment without biosolids.

The project was completed in September 2009.

Contact

Name
Assoc Prof. Arul Arulrajah
Department
Centre for Sustainable Infrastructure
Telephone
+61 3 9214 5741
E-Mail
aarulrajah@swin.edu.au

Funding sources

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