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EMERGENCY REPLACEMENT OF THE SISHEN-SALDANHA RAIL BRIDGE B31 NEAR VREDENDAL

CLIENT: TRANSNET FREIGHT RAIL CONSULTANT: MOWANA ENGINEERS CONTRACTOR: CAPE CONCRETE AND NAMAQUA ENGINEERING

The Sishen-Saldanha railway line is an 861 km long (535 mi) heavy-haul railway line in South Africa connecting the mines near Sishen in the Northern Cape with the Port of Saldanha Bay in the Western Cape, carrying in excess of 40 million tonnes of iron ore annually.   Daily, up to 7 trains travel the railway line, each with a reported payload of 41 400 ton and total train length of 3.78 km (10 electric and diesel-electric locomotives and 342 wagons), making these the longest production trains in the world.

On 28 November 2018 severe impact damage was caused to the steel composite deck of Bridge B31 on the iron ore railway when an excavator, transported on a low bed vehicle, collided with the bridge.   The incident occurred on the R362 northbound between Vredendal and Lutzville where the road passes under the iron ore rail line.   The load exceeded the normal allowable vertical clearance permitted on public roads.   Fortunately, no one was injured during the accident and all trains on the railway were halted within time.

Mowana Engineers conducted an emergency inspection to assess the extent of the damage and to determine the bridge’s structural integrity.   The existing bridge was a single span, simply supported structure that was completed in 1977.   The deck consisted of two steel plate girders compositely connected to a reinforced concrete top slab.   The emergency inspection revealed that the deck was transversely displaced, off its spherical pot bearings, by the force of the collision.   Severe transverse bending and buckling of the bottom flanges and webs of both steel girder beams were evident, along with buckled and sheared-off lateral bracing connections.   Extensive damage was observed on the southern beam where the web was torn apart and ripped from the flanges.   The spherical pot bearings were damaged beyond repair with severe horizontal and vertical misalignment of the track.

Solutions that included the utilisation of the existing bridge proved to be unsafe or would restrict traffic on the provincial road passing underneath.   The recommendation by Mowana Engineers that the existing bridge be demolished and replaced with a temporary bridge that can withstand the rail traffic loads until the permanent bridge deck can be constructed was accepted by the client.  This approach would ensure the least amount of downtime on the iron ore line for which Transnet’s contractual obligations towards their clients have a penalty clause of roughly R20 million per day for any standing time.

Mowana Engineers’ involvement included the design and implementation of the temporary works and the monitoring of the emergency remedial measures.   Modular emergency steel beam girders from Transnet, transported from Gauteng, were utilised to construct the temporary bridge with wooden sleepers as cross members to support the track.   This resulted in an open deck that does not allow for ballast, making it much shallower than the bridge that was replaced and necessitating modifications to the existing abutments to align the temporary bridge vertically with the existing rail line.   Mowana Engineers together with Cape Concrete designed, assembled shutters, fixed reinforcement, cast, cured and transported two 8 ton reinforced concrete support plinths to the site 300 km away, all within 48 hours from confirmation of the design loads and geometric constraints.   Central to this feat was the use of steam curing where elevated temperatures together with a humid environment are used for early strength gain of the concrete.

The temporary concrete support plinths had to be securely fixed to the existing abutment using threaded stress bars and a high-strength epoxy grouting compound.  Sourcing all the stress bar components in such a short time proved to be a challenge, with a temporary shortage of stock in the Western Cape resulting in the proprietary nuts and anchor plates having to be flown in from Johannesburg.   Namaqua Engineering who provided many of the construction equipment and personnel on site, assisted by flying the epoxy and bedding grout in via helicopter to the construction site, thus enabling the tight construction programme to be adhered to.

The temporary bridge, assembled next to the bridge crossing, was lifted into position using a 440 ton crane, 7 days after the occurrence of the collision.   The reinstatement of the rail track, overhead electrification and signalling infrastructure followed immediately thereafter with the railway line re-opening on 7 December 2018, only 9 days after the collision forced the closure of the line (two days earlier than initially estimated).   A speed restriction of 30 km/h will be enforced until the temporary bridge is replaced.

This project underlines Mowana Engineers’ commitment to providing engineering solutions that require initiative, creativity and excellence.  The shared vision by all stakeholders facilitated delivery of the solution in record time.   Transnet’s spokesperson Molatwane Likhethe stated that the fastest emergency rail bridge replacement prior to this incident took 2 weeks to complete.    Kumba Iron Ore CEO, Themba Mkhwanazi, commended all involved on their swift response, to ensure that the railway line was reopened earlier than planned. The CEO of Africa Rainbow Minerals Ferrous placed this achievement in an economic context by stating that iron-ore exports are vital to generate foreign exchange (forex) for South Africa. By reopening the iron-ore line the South African economy gained an estimated R900-million in forex that would else have been lost.

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