Tricks of building a house on black cotton soil while saving money

It is an immutable fact that only houses built on solid foundations transform into strong, long-lasting structures.

However, as many Kenyans living in parts of Kiambu, Nairobi, Kajiado and Machakos counties have found out, getting that steady foundation often comes at a steep cost. This is because land in some areas of those counties is notorious for being covered with black cotton soil.

“Building on black cotton soil is a tricky affair,” admits Mr Daniel Mburu, the CEO of Fingerprint Capital, a building and construction firm that specialises in putting up budget homes for Kenyans. Mr Mburu notes that while you can easily build on red soil which requires minimum excavation while setting up the foundation, black cotton soil often requires excavation.

“Black cotton soil,” he says, “Tends to soak water during the rainy season and expands abnormally. Conversely, when the soil loses water during the dry season, it tends to shrink in great proportions and often shows cracks on the surface. These extreme changes, known as shrink-swell, can bring catastrophic effects to a structure whose foundation is built on it.”

Mr Mburu’s colleague at Fingerprint Capital, quantity surveyor Bede Mwangi, says when building on land with black cotton soil, the soil is usually excavated, and this means additional costs to the home builder. “It is important to contact a quantity surveyor before building on black cotton soil. Doing so will enable the professional to calculate for you the entire building cost, including the cost of excavation. Then you can decide whether your budget can support building on the land,” he advises.

Speaking from their offices at Membley’s Milestone Business Centre, Mr Mwangi further advises that when buying land, one should not shun black cotton soil since with expertise, one can still build on it.

“There were times when buyers of land used to shun areas with black cotton soil. However, the price of land has become so usurious that the cost of excavating it is just but a small fraction,” he says.

But before building, the contractor and his team of professionals, including the quantity surveyor, have to determine the depth of the black cotton soil.

“We specialise in putting up homes in Kiambu and Kajiado counties where the black cotton soil is abundant. We usually do a site visit to the aspiring home owner’s land where we excavate a pit. The depth of the pit until we hit the bedrock or murram gives us the depth and extent of the black cotton soil,” says Mr Mburu.

The bedrock or murram, he says, are stable enough for load-bearing purposes.

It is important, says Mr Mburu, to insist that your contractor does this test. This is because some unscrupulous contractors might only excavate a portion of the soil in a bid to cut corners and save costs.

This could later on sink one’s structure leading to irredeemable losses.

According to the contractor, if the depth of the black cotton soil is 1.5m and below, then it would be economical to excavate it. With excavation, the entire top soil is removed using building machinery.

This soil is then used to rehabilitate quarries. “Some contractors who are used to cutting corners do not make the effort to transport the entire black cotton soil to a quarry. They instead dump the soil by the roadside, leading to eyesores and destruction of the roads. If you are environmentally conscious, you should pick a contractor with enough integrity to dump the soil in the appropriate place,” he says.

Once the soil has been excavated, a kind of foundation known as strip foundation is applied. Strip foundation is akin to the normal foundation applied to houses built on red soil and as Mr Mburu assures, it is strong enough to guarantee the structural integrity of the entire building.

What if the black cotton soil extends to a depth of more than 1.5m below the ground? Unless the home builder has an intention of creating a basement, Mr Mburu sees that it would be uneconomical to go ahead with excavation.

“In such cases, we normally apply a suspended type of foundation known as the pad foundation. With pad foundation, the only excavation done will be for the load-bearing components of the house, which are the columns and grand beams. This columns and beams will then support the entire structure above the black cotton soil near the surface,” the seasoned builder explains.

His advice to those building on black cotton soil is that they should get professionals to assess how the type of construction relates with the depth that has to be reached  for a solid foundation. “As you can see, each case is treated individually. If you get unqualified fundis (contractors) to do the excavation, you might end up with an uninhabitable structure,” he said.

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