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What lies beneath

GROUND IMPROVEMENT

A clay that creates sulphuric acid on contact with air is being treated with lime to create engineering fill in Melbourne, Australia. Frank Albrecht reports.

Australia is home to some of the most dangerous animals and plants in the world, including crocodiles, snakes, box-jellyfish, sharks, red-back spiders and even a giant stinging nettle.

Even if the flora and fauna don't get you, the ground could.

In the Maribyrnong and Yarra Rivers delta near Melbourne in southern Australia, there are extensive deposits of Coode Island Silt (CIS), a marine clay that can produce sulphuric acid on contact with air.

Contractor CDL Consolidated is carrying out large-scale treatment of CIS on a new housing development at Edgewater, 10km from the centre of Melbourne.

Quicklime is being added to neutralise its acid-producing potential and convert the soft soil into engineering fill.

The site lies on the flood plain of the Maribyrnong River and was once home to a munitions factory and firing range built in 1872. It includes a canal from the river to a bunded set of buildings.

The development is a joint venture between the Commonwealth Government and Lend Lease Developments. Its consultants are Coffey Partners International and Egis.

Works include the creation of a 2. 4m to 3. 6m high 25ha platform above the flood plain. An 8ha, 3. 6m deep lake will be dug parallel to the river to provide fill for the platform and a hydraulic balance for floodwater which would otherwise be restricted by the fill platform.

The main lake will be connected to the river with two canals to circulate the lake water.

Decorative settlement ponds at each end will lead drainage water from the development into the lake.

The site is covered with 1. 6m of alluvial sandy clay overlying the CIS, a black, organic, very soft to firm marine clay containing shells and sand lenses.

This overlies a lava flow at one edge of the site and increases to depths of more than 12m closer to the Maribyrnong River.

It was decided to treat the 200,000m 3of CIS spoil from the lake on site and use it for the fill platform rather than disposing of it off site, which would have been expensive because of the its acidproducing potential.

Ground work began in mid2001. The first task was remove soil that had been contaminated by the munitions works. The site was then audited and certified as environmentally safe. CDL then drained the low lying areas and bridged soft areas with mudstone from basement excavation sites in Melbourne, supplied at up to 2,000m 3per day.

Laboratory trials carried out by BFP Consultants assessed the best method for treating the CIS so that it could be used for the platform.

Various additives were tried, with quicklime (CaO) found to be the most effective way of neutralising the acid potential.

Quicklime, with its larger particle size, was adopted in preference to the finer, hydrated lime, which is more dusty. However, much more stringent safety measures have to be applied when using quicklime.

CDL carried out full-scale field trials to evaluate a range of techniques for spreading, mixing and compacting the treated CIS. The fill platform specification requires a minimum density of 95% of standard compaction, and moisture content of +/- 3% of optimum.

The alluvium and CIS are markedly different, so they are separated during excavation and treated separately. The CIS is laid and spread in thin layers.

Eighty tonnes of quicklime are supplied from David Mitchell Estate every day and spread over the CIS by Yarra Valley Fertilisers and then mixed using road stabilising machines by Pavement Technology.

Quicklime dosage is adjusted depending on the moisture content of the CIS which varies from 50% to 100%. The mixer blends the quicklime with the CIS, producing a material which can then be compacted with vibrating sheeps-foot rollers.

The alluvium is then laid out and compacted, with moisture adjusted as required. The fill platform is finished with a layer of topsoil.

BFP Consultants is carrying out compaction monitoring.

Short- and long-term monitoring of settlement is being done by Coffey Partners International.

Because the ground is soft and the lake excavation is close to the river, bank stability is a problem for hydraulic excavators.

After talks with Dick Stain of UK consultant Testconsult, it was decided to use well-point spears to lower the water table locally on the river side of the lake.

Temporary batters steeper than 45° could then be built, to allow the hydraulic excavators to trim the base of the lake from ground level.

The site is subject to strict environmental control, with all imported material assessed for contamination and on-site soil movements tracked and recorded.

Water pumped from the site to the river is monitored daily for pH and turbidity; air particle fallout is measured monthly.

Lake excavation and fill platform construction were 35% complete in mid-April, and more than 3,500t of quicklime had been used. This stage of the housing development is due to finish early next year.

Frank Albrecht works for CDL Consolidated (Vic) Pty.

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