How To Construct A Slab Culvert Over a Drainage
Engr. Osaz' Enobakhare
Controlling flood is taking new dimensions. Canal-like ‘V’ drainages are taking root in many cities across the country. The advantage of utilizing less quantities of concrete and reinforcement than the regular drainage makes it cost less to construct and relatively flexible. This type of deigns allows the drain to be supported largely by the earth on its sides and bottom than its own weight. Most constructors of this type of drainage use BRC Wire Mesh as the reinforcement material in the concrete mass hence it is relatively light and not safe enough to support drain covers. Although this types of drainage using wire mesh, if not constructed properly are known to fail quickly under intense water pressure-forces. For this reason building a culvert ‘bridge’ over them to allow passage of vehicles become imperative so that the slab do not by any means bear on the drainage to avoid collapse. But the design of slab-culverts is quite different from the usual drain cover supported fully by the drainage because they would have to stand alone. To get it right, you should engage a structural engineer in designing the culvert which takes into account the expected load (or weight) the culvert will bear par functionality. Culverts designed to largely support heavy vehicles, trucks and trailers like those constructed to serve fuel/filling stations, warehouses, heavy duty parking lots and major streets are quite different from those constructed to support cars and light moving loads for small residential apartments hence the special consideration given to them. Generally in constructing culverts, you have to;
1 Get a structural design. Set-out the area to construct it by its sides on both faces.
1 Excavate the foundation trench to the recommended depth, usually beyond the depth of the drainage. Ensure that trench excavation is properly done so that the formwork can fit in properly. Let the trench be wide enough up to 500mm to allow for clearance (or extra space) between the earth and the formwork as well as the face of the drainage and the formwork.
Install the formwork using timber (wood) or steel with adequate bracing. The formwork should neither lean wholly on the drainage nor the earth so that it can be removed once the concrete sets. Build the slab formwork together at once. Brace the soffit (bottom) of the slab properly so it doesn’t collapse during construction.
Look up the structural design and fix reinforcement accordingly. Usually the foundation is constructed as a solid raft (with main reinforcement and distribution reinforcement spanning from bottom of the trench to the top in all directions) and the slab is constructed like a beam (with top and bottom reinforcement, up and down bound by stirrups) and fixed together, not like typical floor slab ‘mat’ in buildings.
Prepare and place concrete of good mix into the formwork (depending on the strength requirement a mix of 1:1.5:2 is normally recommended). With the aid of a poker vibrator, vibrate the concrete in the formwork properly to avoid pores and void. Then leave to set properly before detaching the formwork. Then you are good to go