Water Journal : Water Journal August 2012
refereed paper technical features 68 AUGUST 2012 water biosolids • Unless EPA is satisfied that alternatives will provide an equivalent environmental outcome, design requirements are to include the provision of a marker layer in the fill, a low permeability cap and liner, and appointment of an environmental auditor to oversee the works. EPA also accepted that the stabilised clay-rich biosolids could be used in this application subject to approval of an Environmental Improvement Plan that covers all aspects of the use from the MWC stockpile to the completed road project in operation. This provided a more manageable and streamlined process than the alternative of obtaining an EPA Works Approval for the use. Pilot trials & consequential investigations Initial consultation with the civil engineering industry identified concerns about the following issues: • Potential delays due to industrial action based on health and safety concerns; • Potential delays due to MWC inability to consistently produce the stabilised clay-rich biosolids at optimum moisture content and at a rate to match progress on the road construction site; • Disruption to time-critical construction activities due to the insertion of a material requiring special stabilising methods, including the construction of a high-quality clay encapsulation of the clay-rich biosolids cell in the embankment; • Disruption to time-critical construction activities due to a lack of industry experience working with the material; • Liability for long-term subsidence in the event that the material does not behave as predicted; • Potential delays in obtaining final detailed approvals from VicRoads and EPA. These concerns were clear barriers to entry of the stabilised clay-rich biosolids into the roads market, so a program of responses was initiated. Further consultation with road designers and contractors indicated that the use of "quick lime" in addition to the cement would assist with controlling the variability of the moisture content in the clay-rich biosolids as well as the achievement of a consistent product. An addition of around 3% lime by weight was recommended, based on road industry experience in dealing with the stabilising of marginal clays and soils. A short section of access road on the Eastern Treatment Plant property was constructed as a full-scale production trial to demonstrate the feasibility of manufacturing the stabilised clay-rich biosolids as a consistent product using lime and cement additives. The lime and cement were mixed in sequentially using conventional rotavator equipment and a production rate of close to 1000 cubic metres per day was achieved when constructing the one-metre-high embankment. Subsequent in situ and laboratory testing confirmed that it is practical to achieve a minimum dry density of 95% standard using a nett 2% lime and 2% cement content by weight. Furthermore, both the strength and particle size requirements for VicRoads Type B fill were achieved. However, compressibility testing (long-term subsidence) produced confusing results compared with earlier tests of laboratory-produced samples. Ongoing industry consultation was used to raise the profile of this new product as well as to define development tasks that would address specific industry concerns. The key issues raised were: • Production of a consistent product at a high rate; • Preference to use the material in locations that were less time-critical and where the material formed the total structure such as an overpass embankment. This would require fill depths of, say, four metres and exacerbate the VicRoads concern about long-term subsidence. • The need for a clay liner around the cell of stabilised clay-rich biosolids, which was costly and time consuming, and absorbed much of the available depth in a typical six-metre high overpass abuttment. In response to these issues, MWC committed to another full-scale trial, building a 500-metre-long access road at the Eastern Treatment Plant. This trial was closely planned and supervised and a specialist contractor was engaged for the critical blending of lime and cement, as well as for compaction of the product in the road embankment. Key features of this trial were: • The blending of lime and subsequent stockpiling to establish sufficient material with consistent moisture content (close to optimum) to match the total requirement for the road project; • Loading out from the stockpile over its full depth to further blend the material as it was delivered to the road construction site; • Strict limits on the depth of layers placed on the road for cement stabilisation; • A range of cement concentrations (2%, 4% and 8%) were used to gauge the sensitivity of test properties to cement content; • To gauge the effect of in situ curing of the stabilised clay-rich biosolids in the compacted state, undisturbed samples were recovered from a one-metre-thick test zone in the embankment by coring two 63mm tube samples across the depth from each of the seven boreholes. Duplicate testing of saturated and unsaturated samples Typical example of blending lime with biosolids.
Water Journal September 2012-1
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