Just North of the Tropic of Capricorn, in Botswana, a TechSpan™ bridge has been erected over the main rail route from Gaberone to Francistown in just 4 days. This is the first TechSpan structure in Africa
The design comprises a three-hinged arch with footings on either side of the railway line. The precast concrete elements weigh up to 13 t each, and two mobile cranes were used to position the elements. The span of the bridge is 7,6 m and a rise of 6,3m, to accommodate a single track of the Botswana Railways making provision for future electrification. Each element is 2 m wide and the overall length of the structure is 30 m. The ends are scalloped or tapered from the height of the structure down to the ground. The wing wall is not an arch but acts as a propped cantilever, which is positioned into a concrete slot. All the vertical joints were sealed with geofabric strips, When all the segments were in a position a ring beam was cast to prop the top end of the wing wall elements. A crown beam was placed along the top of the structure to tie in the elements longitudinally.
The 35 precast elements were cast in South Africa and transported by 12 trucks into Botswana. Cranes with a lifting capacity of 50 t were needed to be on-site off-load the trucks and then proceed with the construction.
Once all the elements were in place and ring beams are cast backfilling to provide a metre coverage over the top began. Backfill should not be able to go up more than half a metre higher on one side than on the opposite side because of the nature of the structure. Crusher dust was used for three metres on either side of the structure. Beyond this normal backfill was placed.
Contractor: Ridgeway Construction (LTA subsidiary); precaster – Grinaker Duraset Brakpan
Consulting Engineer: Haas Consult of Germany
Client: Botswana Railways