The Dorstfontein project in Kriel, South Africa, is DRA Turnkey division’s largest project to date and includes 10.5kms of overland conveyor traversing a 160m long bridge over the Klein Olifants River.
The requirement for an overland conveyor which necessitated a river crossing became apparent due to a change of scope after what was thought to be final conceptual plant design acceptance. The change of scope resulted in a full tender enquiry procedure being implemented by Total Coal and it was DRA’s proposal to launch prefabricated sections across the Klein Olifants River that secured the project under stiff competition from two other very keen tenderers.
Although the launch concept was a project team effort, the structural integrity was supplied by DRA’s civil and structural division, Holley & Associates. Their brief was to design the bridge in its installed form, but they also had the difficult task of ensuring that the structure was structurally sound throughout all aspects of a launch that would take the platform through many different structural loading conditions. Preparation was key to a successful operation and, although up front planning and innovative design was essential, ground level input was also critical. A full launch-pad area was set up on the south side of the river where the incremental launch was to take place.
Bridge sections weighing 14.4t each were supplied as 18m long fabricated latticed girder sections measuring 1.5m in depth and 5.2m in width. Each section was delivered to site as an abnormal load and bolted together to the correct torque specifications. The requisite number of sections needed to be in position in order for the launch to begin. Cantilever calculations were completed to ensure that sufficient mass was on the launch pad to prevent the cantilever section ending up in the river.
Two 32t hydraulic jacks were positioned on either side of the bolted girder sections which incremently jacked the composite unit onto the permanent concrete pillars. Temporary steel support structures were connected on either side of the support pillars to reduce the cantilever distance for the erection period. A temporary nose piece, designed to catch the temporary supports and allow for a calculated deflection of 430mm, was bolted to the front section. The nose piece and the temporary supports were removed once the bridge was locked into final position and the four masts and stays installed. At that stage the maximum unsupported span was 57m.
Although the full launch of the bridge sections brings the ‘bridge launch’ to a close, only half of the overland conveyor construction process is now complete. 22t of suspension steelwork will now be installed and the bridge platform dressed with the idler support frames and completely covered with a sheeted housing. This complies with all of the required environmental specifications when launching a bridge across a river.