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Concrete In Australia : March 2008
Dalrymple Bay Coal Terminal, south of Mackay in central Queensland, is being expanded from 59Mt/a capacity to 85Mt/a. The project is expected to be completed towards the end of this year. load (wagons fully laden with coal) and wagon derailment. The latter occurs occasionally on the existing pits due to the use of bottom dump wagons. Accumulation of coal on top of the rail beams causes the wagons to ride up mounds of coal resulting in loss of contact between wheel fl ange and rail head. Measures were implemented to minimise the potential for derailment in Rail Receival Pit RRP3 by promoting coal flow. Construction sequence The construction sequence assumed by the designer was critical to the design itself. For example, it was assumed that the perimeter walls at the deepest section of the pit would be constructed in two lifts. The lower lift could be backfilled while the wall was acting as a cantilever, however, the upper section of wall required the completion of the track slab to provide propping action prior to backfi lling operations. It was therefore imperative that the assumed construction sequence be documented on the design drawings. The designer subsequently worked with the contractor to accommodate adjustments in construction sequence, primarily related to the location of construction joints. Temporary ground retention system Due to the critical nature and proximity of adjacent infrastructure and time constraints on the overall project, design of the temporary ground retention system was undertaken by Connell Hatch rather than by the contractor as would traditionally be the case for temporary works. The temporary ground retention system comprised 900mm mm diameter bored piles at 2m centres on three sides of the excavation. Piles were anchored top and bottom with temporary ground anchors to enable subsequent excavation below the pile toe level. Infi ll panels between piles and the exposed rock face beneath the piles were covered with sprayed concrete. Although a geotechnical investigation was conducted prior to the commencement of works on site, an intensive monitoring regime was implemented during construction which included the use of survey controls and inclinometers to verify the geotechnical parameters assumed in design. Field observations confi rmed that assumptions were conservative and no design changes were required. In order to accommodate for geotechnical uncertainty, the number and length of piles, ground anchors, rock nails and shotcrete were priced on a schedule of rates basis with the remainder of works being a lump sum contract. Construction issues Rail Receival Pit RRP3, the fi rst major construction package on the project, was awarded to Golding Contractors. The temporary ground retention system and rail receival pit were constructed throughout 2006, including the wet season, and completed in 2007. The main issues were: Temporary ground retention system Flowing groundwater proved to be an issue during construction with the grout in some ground anchors being washed out and needing to be done again. There was also concern expressed by the contractor that the temporary ground retention system had not been designed for a full head of water (the design assumed dewatering of the pit area Concrete in Australia Vol 34 No 1 45