Proposed development of the Mastiff Road Culvert, Midrand, Johannesburg

Upgrading of Mastiff Road culvert near the Midrand industry park, in Commercia Midrand, within the City of Johannesburg Metropolitan Municipality

Option 1 – Precast Portal Culvert Causeway and Road Vertical Realignment
This option consists of precast concrete portal segments placed on a cast in-situ concrete base slab. The portal segments will be held together by placing concrete longitudinally between the cells such that all the cell segments form a monolithic structure. A high strength concrete topping to be placed over the precast units such that it ties in with the finished road level. Vertical reinforcement will be provided between the precast units to tie the structure such that the structure forms a complete unit. Splayed free standing cantilever concrete wing walls will be used at the inlet and outlets to support the fill embankments and allow unrestricted flow. Gabions mattresses will be used for adjacent road embankment protection.

Energy dissipators will be cast onto the outlet apron to assist with reduction of flow velocity. The services that are required to traverse the crossing will be accommodated beneath a proposed walkway. Fixing of services to the face of the structure will also be considered.

Option 2 – Cast In-situ Concrete Culvert with Road Vertical Realignment
This option consists of cast in-situ concrete portal culverts. The base slab, wall, and roof slab of the structure will be cast in different stages to form a monolithic multi-cell structure. Selected material will be backfilled over the structure to provide adequate cover if required. Splayed free standing cantilever concrete wing walls will be used at the inlet and outlets to support the fill embankments and allow unrestricted flow. Gabions mattresses will be used for adjacent road embankment protection.

Energy dissipaters will be cast onto the outlet apron to assist with reduction of flow velocity. The services that are required to traverse the crossing will be accommodated beneath a proposed walkway. Fixing of services to the face of the structure will also be considered. Findings from the detailed hydraulic analysis will make provisions for impact debris fins, stilling basins and any factors relating to reduce the scour.

Option 3 – Additional Cast In-situ Concrete Culvert Widening
This option retains the existing structure whilst a new structure is constructed adjacent to it to increase the hydraulic capacity. Demolition and reconstruction of the inlet and outlet wingwalls and headwalls, as well as the apron slabs, will be required.

This option will also include an engineered soil raft made up of compacted rockfill on suitable in-situ sediment material this may be included subjected to the findings from the geotechnical investigations to support the proposed foundations for the new widened culvert and apron slabs. Construction of additional cast in-situ concrete cells for the upgrade of this structure on a new widened concrete cast in-situ concrete base slab. Splayed free standing cantilever concrete wing walls will be used at the inlet and outlets to support fill embankments and allow unrestricted flow.

Gabion baskets and reno mattresses both upstream and downstream of the new structure to widen the river channel and channel the flow through the proposed new widened culvert at the inlet approaches, provide embankment protection to the existing embankments on the outlet side, and generally minimise the long-term scour potential in the proximity of the structure.

Province(s): Gauteng