Kerbs and Edge Restraint in Nairobi: The Part Nobody Quotes

Paving is only as good as what is holding it in

A cabro field works by interlock: blocks share load through their joints and act as a flexible surface. Take away the restraint at the edges and the blocks migrate outwards under traffic, the joints open, jointing sand escapes, and the interlock stops working.

Once that starts it accelerates, and the failure looks like poor blocks. It is not.

What proper restraint looks like

Kerbs or edging bedded on concrete and haunched, a fillet of concrete supporting the back face. Edging pushed into the sand or bedded without haunching moves in the first rainy season, and then it is no longer restraint.

The haunch is invisible when the job is finished, which is why it is the first thing omitted on a cheap quotation.

Where it matters most

Along a driveway edge with soil or lawn beside it: the paving has nothing to push against. At the head of a slope, where gravity is already working on the field. Anywhere vehicles turn, which loads the edge sideways.

Against a building or a boundary wall the wall itself provides restraint, so no kerb is needed there.

Kerb types

Road kerbs for driveways with vehicle traffic, upstanding to protect a lawn or a bed. Flush edging where paving meets a path or a lawn at the same level. Channels where the edge also has to carry water to a gully.

Precast concrete kerbs and channels are readily available in Kenya. What is not readily available is anyone pricing the installation honestly, and installation frequently equals or exceeds the cost of the product itself.

Why we quote it at survey

Kerb runs depend on length, the number of curves, whether the ground needs excavating and how the haunching is formed. Publishing a per-metre figure would be a number we would revise, so we measure instead.