Constructing on Black cotton soil in Kenya.
Black cotton soil is found abundantly in most regions in Kenya.
Black cotton soil tends to soak water during the rainy season and expand and shrink during the dry season.
This expansion and shrinkage creates up and down movements on the ground floor slab, resulting to cracks.
To prevent this cracking, the black cotton soil is either excavated at a minimum depth of 1m or the building foundation raised and suspended above the ground level.
1.5m minimum depth.
Generally, if the black cotton soil is more than 1.5m in depth, its uneconomical to excavate it. This requires a suspended type of foundation whereby columns are dug till solid rock and protrude slightly above the ground level.
Black cotton soil areas have costly road works as opposed to red soil or murram soil areas. The black cotton soil will need to be excavated and the voids left backfilled with hardcore, or murram, hence increasing the cost.
With the current land price increase in Kenya, the % of cost increase due to black cotton soil is negligible compared to the cost of the land. In earlier times, land buyers could choose not to buy plots with black cotton soils because there were other available better plots. Currently, the land available has become scarce and expensive so buyers have no choice but to think of solutions to construct on top of black cotton soil.
The same applies to marshlands. Land buyers have no choice but to think out for solutions to construct. This is the case with Runda area which has several areas with march land or Karen area which has several patches of black cotton soil dotting the mainly red soil land.
Francis Gichuhi Kamau, Architect.