biosolids
refereed paper
The performance specification of the Victorian road regulator (VicRoads) for structural fill in road embankments (VicRoads, 2004) was adopted as the objective for developing stabilised clayrich biosolids as a viable product for embankment construction. In particular, the selected target was the quality required to meet the VicRoads “Type B” fill specification that is required for the bulk of the material in a typical road embankment.
Staged investigations Following the recommendation from the 2006 biosolids strategy, a structured program of technical and market development was initiated, as shown in Figure 1. The first phase of investigation determined that it is possible to stabilise the clay-rich biosolids with a pozzolanic material such as cement to enhance the strength of the material and to achieve much improved structural performance. This study involved a laboratory analysis and a plate-loading trial on compacted clay-rich biosolids sampled from several stockpiles, with and without the addition of normal portland cement in proportions from 3% to 8.5% by weight. Typical earthworks construction equipment and methods were used to construct the embankment for the plate load test. Civil engineering industry representatives were invited to witness the construction and a video recording was made of the operation to show that the material behaved similarly to natural clay when handled with typical earthworks equipment.
The conclusions from this study were that: • Clay-rich biosolids stabilised with 3% cement may be suitable to meet VicRoads Type B fill specification; • In the stockpile, the clay-rich biosolids are mostly wet of optimum; • There may be potential for long-term volume change due to degradation of the organic material within the biosolids; • It may be necessary to encapsulate the clay-rich biosolids in an embankment for environmental reasons; • For the cement-stabilised material, negligible creep strain (ongoing settlement) was observed in the plate load test after 10 to 30 days depending on the concentration of cement used. The findings of this initial investigation were discussed with VicRoads, who agreed to produce industry guidelines as “Technical Note 90” (VicRoads, 2007) for the use of the clay-rich biosolids in road projects as Type B fill. This technical note, which was published in October 2007, includes the following qualifications: • Clay-rich biosolids must be cement stabilised; • They must not be placed within one metre of the road subgrade level; • They must be placed above design flood level and not less than one metre above maximum known groundwater level;
• Must be not more than one metre thick; • Settlement monitoring must be undertaken; • The standard specification requirement for zero organics content will be considered for a waiver on a projectby-project basis. The Technical Note 90 suggests that 15mm of settlement (not defined whether long term) could occur under an applied stress of 150kPa (seven metres of embankment). With this positive support from VicRoads, the environmental regulator was consulted to establish conditions that may apply to this beneficial use of the clay-rich biosolids. MWC worked with EPA Victoria to develop EPA Publication 1288, which was published in June 2009 (EPA, 2009). This document provides guidelines that would maximise the sustainable use of clay-rich biosolids as structural fill by documenting good practice. The publication includes guidelines on site selection and management where clay-rich biosolids are proposed. These include: • Recommended buffer distances to sensitive receivers and to waterways; • Not typically to be used where a site is within a one-in-100-year flood plain; • Not less than two metres above the longterm average groundwater table and with a (low permeability) liner below;
Figure 1. Melbourne Water’s Technical and Market Development Program.
water
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