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Concrete In Australia : March 2008
PROJECTS During the course of the works, a number of changes were made to construction joint locations documented on the design drawings to suit the contractor’s formwork and falsework system. In each case, the design implications were assessed promptly and reinforcement detailing revised where required. The stockyard bunds Constructing the temporary ground retention system for Rail Receival Pit RRP3. by means of a network of deep wells around the perimeter of the excavation as well as a grid of drainage points at 1.5m x 1.5m centres in the sprayed concrete between the bored piles). Ultimately, it was agreed that discharge of water from the drains in the bored pile wall would be monitored to provide an indication of the water pressure in the ground behind. In the event that water was found to be jetting from two or more drains in a vertical line (indicating a head of more than 3m behind the wall), the pit would be evacuated. Although the pit was periodically evacuated due to inundation during torrential rain, monitoring of discharge from wall drains did not indicate excessive hydrostatic pressure behind the wall at any stage of construction. The presence of existing conveyor tunnels and other structures in the vicinity of the pit meant that there were a number of constraints imposed on the installation of ground anchors. Limitations of the drilling rig resulted in the need to adjust the level and orientation of some of the ground anchors for the bored pile wall. This generally required additional prestressing strands to maintain equivalent anchorage at steeper angles of inclination. During excavation for the pit, two of the bored piles were found to have voids extending through the full cross-section of the pile and for a height of approximately 0.5m. The cause of these defects was attributed to the rate of withdrawal of the temporary pile liner. The photograph above shows a general view of the ground retention system under construction including repairs to one of the defective piles (at centre of photo). Dump station and tunnel The dump station and tunnel involved more than 4000m3 of cast-insitu concrete. Due to the size of some concrete pours, and in an effort to expedite the schedule, several concrete pours were carried out at night. 46 Concrete in Australia Vol 34 No 1 Description of works Development of the stockyard includes construction of new bunds and the provision of additional stockpiles to increase the yard capacity and improve stockpile effi ciency by increasing the independence of stacking and reclaiming operations. Three new bunds are being constructed, each 1.3 km long: Bund 4A to support the new Stacker ST4, Bund 5A to support the new Stacker ST4 and Bund 6 to support relocated yard machines Stacker-Reclaimer SR6 (formerly SR3 from Bund 4) and Reclaimer RL2 (formerly SR4 from Bund 5). Bunds 4A and 5A are being constructed in the middle of existing stockpile rows. In order to maximise storage capacity, vertical retaining walls will be provided in areas where coal is to be stockpiled and conventional earth bunds with 1V:1.3H batters will be provided in other areas. In addition to a new stacker, each bund supports a conveyor and a roadway. The cross sections for Bunds 4A and 5A are virtually identical and comprise two opposing 6.1m high precast concrete retaining walls supported on a 1.5m high earth embankment. The retaining walls extend approximately 2.4m above road level to provide a physical barrier between personnel on the bund and the bucketwheels of adjacent stacker-reclaimers. A 45° return is provided on top of the wall to reduce coal falling onto the bund surface. It is intended that coal be stockpiled no higher than 3.5m below the top surface of the bund to provide some immunity against overtopping of the wall if the stockpile slumped. This fi gure was determined by considering the likelihood and consequences of slumping which depends on coal type, seasonal effects and other factors. The sides of the embankment are covered with steel fibre- reinforced sprayed concrete and provide a chamfer to the bucketwheel of the stacker-reclaimers on adjacent bunds to maximise recovery of coal. A total of 1100 precast concrete panels were required per bund, each panel being 2.25m long, representing a combined length of 2.5km (1.25 km along each side) and a total concrete volume of 6200m3 . Opposing precast concrete wall panels are tied together with passive anchors at the base (one 32mm-diameter Reidbar) and at mid height (two 36mm-diameter stress bars). As the ties were installed progressively during back fi ll operations, subsequent backfi ll and compaction resulted in partial prestress of the transverse ties. The tied-cantilever confi guration was adopted to prevent sliding of the base and to reduce bending in the stem during construction and in service. The bunds have an overall longitudinal fall of 1:700 from one end of the stockyard to the other. Drainage of the top surface is achieved by provision of 3% cross fall and 1.5% longitudinal fall to gully pits at 36m centres, which in turn discharge