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Environmental considerations led to the careful use of a piled embankment on a road widening project in an Australian wetlands area.

The Five Islands Road is a major arterial route crossing the north end of Lake Macquarie in New South Wales (NSW), Australia. It is being widened from two lanes to a four lane dual carriageway to cope with the burgeoning volume of traffic.

The 1.8km stretch of road crosses fragile wetlands that are home to a variety of plant and bird species.

Pedestrian and cycle paths are having to be accommodated in the scheme and are remaining in use throughout the project.

New South Wales Road Traffic Authority selected consultant Coffey Geotechnics to do the environmental and geotechnical consulting on the project. Coffey had an intimate understanding of the scheme from its involvement with the road's construction back in 1968. 'We had retained data on ground conditions and settlement monitoring from this time, which could be reused in the new development, ' says Coffey Geotechnics regional manager for north NSW, Steve Morton.

The Five Islands Road is situated between protected wetlands. So conventional preloads with stabilising berms that would extend into them were not suitable. Other options considered included using small preloads but this would have been too time consuming.

The risk of potential settlement induced effects on the adjacent road embankment and bridge piles, presents another challenge. A potential problem that Coffey had to consider was the disruption of the existing drainage. With the high water table on nearby land, it could also adversely affect the area's natural ecosystem by submerging the ground altogether or by exposing acid sulphate soils (ASS).

The excavation required could lead to the disturbance of ASS, which contain naturally occurring pyrite and can produce acid runoff if exposed. Load induced heaving of the ground adjacent to the embankment could also result in elevation and exposure of ASS above a near surface water table, creating oxidation and acid production.

To reduce settlement effects on existing road infrastructure and nearby wetlands, Coffey designed piled embankments comprising lightweight bottom ash fill sourced from a local power station. The material has a bulk density of 1.4t/mgreater than or equal to. 'The ash material is a good source of lightweight fill with the environmental advantage of reusing a power station byproduct, ' says Morton.

The piled embankments include a basal layer of high strength geosynthetic tensile fabric to distribute the load to the piles, without having a rigid connection to the piles. The embankments provide stability and meant that Coffey did not need to preload the foundations.

'Sections of the new embankment required a retaining wall, ' says Morton. 'The support of this structure was incorporated into the piled design, which avoided the embankment encroaching on the protected wetlands.'

The consultant faced the challenge of differential settlement in the areas between embankments and bridge structures, and between piled embankments and conventional preload areas. It used piles to make the embankment support progressively stiffer in areas closest to the bridge.

The company completed the piling work nearest to the existing bridges first to limit any effects of lateral ground movements on the bridge piles already in place.

For the majority of the site, adequate berms to form the piling platform already existed from the 1968 embankment construction. This meant the piling platform did not need to spread into the wetlands.

To avoid excessive bending moments, shears and deflections arising from fill induced lateral soil movements, the pile heads beneath the embankment were not fixed rigidly to the supporting mattress.

Bridge piles were designed to cope with anticipated settlement and structural action effects and could be installed once the embankments were constructed.

The design and construction of the embankments minimised disturbance of acid sulphate soils.

Where disturbance was unavoidable, soils were treated with lime and incorporated into the project landscaping mounds, pedestrian paths and cycle ways.

Sinclair Knight Merz is the principal design consultant and Thiess the contractor on the project. Work at the site began in 2005 and is due to be completed in June.

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