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This case study describes the events that led to the partial failure of the Olifants River Bridge in South Africa. It gives an overview of how the Routine Road Maintenance (RRM) section of the South African National Roads Agency SOC Ltd (Northern Region) went about having it repaired and open to traffic with the assistance of the project team.
The project referred to in this case study was carried out for the South African National Roads Agency SOC Limited. The contents of the paper reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official view or policies of the South African National Roads Agency SOC Limited. Their permission to publish the paper is gratefully acknowledged.
On the 6th of January 2011 the Olifants River Bridge built in 1977 by the Transvaal Provincial Administration (TPA), was damaged by flood waters. This bridge located on the R555, a major road link between Emalahleni (Witbank) and Middelburg in the Mpumalanga Province, was taken over from the provincial authority by the South African National Roads Agency (SOC) Limited (SANRAL) in 1999. In order to carry out the emergency repairs it was required to close the road to traffic until the work was complete. Approximately 12,000 vehicles per day, on average, had to be diverted to other routes including the N4 toll freeway until beneficial occupation took place on the 17th of October 2011.
Due to the unusually high rainfall during that summer the sluice gates of the Witbank dam, which is located a few kilometers upstream of the bridge, had to be opened. This was done to prevent the possible catastrophic failure of the dam wall. It was estimated that the resultant downstream flow exceeded the 1:50 year peak flow of 1,415 m³/s, for which the bridge had originally been designed by the Provincial Administration. The 64.0 meter long bridge consisting of four spans and with a 15.5 meter wide deck, was not overtopped during the event. The flow was however enough to cause the major failure of the West abutment wall; the East abutment wall showed only minor signs of distress.
The main reason for the partial failure of the bridge was the scouring away of the poor founding material, with a low bearing capacity, from under the spread footing of the West (Witbank side) abutment wall. The weathered material (Silty Sand/Clayey Sandy Silt/Sandy Clayey Silt) had an allowable bearing capacity of 80 to 190 kPa. The lack of footing support, due to scouring, combined with the hydraulic forces caused the downstream side of the abutment wall to deflect and fail at the construction joint, leaving the deck of that span only partially supported. The fill on the approach to the bridge was also partially washed away from behind the abutment wall. It was estimated that the flow velocity of the water, at the time of failure, was in the region of 4.82 m/s.
The East (Middelburg side) abutment wall showed only minor signs of distress at the construction joint. The spread footing of that abutment wall was founded on a somewhat better material, a baked Mudstone with an allowable bearing capacity of 500 kPa.
The proposed design and repair solution was determined following a detailed geotechnical investigation, which involved six boreholes on the founding material to bedrock. The construction work started on the 15th of March 2011.
The construction sequence was as follows:
The project cost was as follows:
Site investigation, design and construction supervision: ZAR 1,193,393-04 (including VAT)
Construction: ZAR 11,966,993-00 (including VAT)
Total cost: ZAR 13,160, 386-04 (including VAT)
The West abutment of the Olifants River Bridge was damaged beyond repair, it had to be demolished and reconstructed 2.5 meters deeper. Besides that it included piles and a pile cap instead of the original spread footing due to the poor founding conditions.
The East abutment was minimally damaged and required only 2 rows of soil nails and 2 reinforced concrete plinths/anchor beams.
The emergency repairs afforded SANRAL the opportunity of replacing the expansion joints and improving pedestrian safety with the introduction of concrete barriers.
This case study was produced by Nuno A. de N. C. Gomes MSc (Eng) (Water), IEng, MICE, PrCPM, South African National Roads Agency Ltd: Operations and Maintenance Manager (Northern Region) and Frans Kromhout MEng (Struct), PrEng, MSAICE AECOM SA (Pty) Ltd: Bridge Engineering Lead, Africa
The repaired bridge meets the recommended hydrological/hydraulic requirements as described in the SANRAL Drainage Manual, for a class 2 road.
South African National Roads Agency SOC Limited (SANRAL) Drainage Manual, Sixth Edition (2013) – Editor: Edwin Kruger, Sub-editors: Nuno Gomes, Fannie van Vuuren and Marco van Dijk.
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