How To Construct A Slab Culvert Over a Drainage
Engr. Osaz' Enobakhare
Lagos, Nigeria.
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
