Trick to building on black cotton soil

When Martin Wang’ombe went to inspect a plot  he wanted to buy in Oloolua in Ngong, he noticed something unusual about his would-be neighbour’s house.

It seemed to be slanting and had a huge crack on the wall. As he would later learn, the house was sinking, sucked in by the black cotton soil on which it stood.

It turned out that the plot was in an area where the soil was more than 10 feet deep.

Normally, black cotton soil swells when wet and shrinks when dry. These changes, known as shrink-swell,  tend to make the foundation sink deeper with time, leading to structural damage to a house built on such soil.

During the construction of the house mentioned earlier, the contractor did make it rest on the bedrock, so the shallow foundation started  sinking  once the soil began swelling  and contracting.

Mr Gathura Njenga, an engineer, says that soil type and depth matters a lot when  buying a plot to build a small house, especially if one doesn’t intend to have a basement.

He explains that this could end up costing a prospective home developer  a tidy sum of money, so it is advisable to consult an expert, preferably an engineer, before buying land.

EXPENSIVE OPTIONS

“It is important to have a proper review by the engineer so that he can advise you, depending on what you want to put up,” he says. “Technically, you can build anywhere but the issue is the cost, particularly if you have black cotton soil;  it’s very weak when it comes to foundations.”

There are two common ways of dealing with the soil: one can excavate all of it or  have a floating foundation. Compared with areas that have red or murram soil, these two options are more expensive .

With excavation, all the top soil is removed and one can  build a basement or backfill with hardcore  or murram. The costs involved in excavation and backfilling or building a basement can be prohibitive, especially if the structure is not a commercial, storeyed building.

Simpler buildings, especially residential ones, are not very heavy and their weight can be accommodated by a floating foundation. This option entails boring several shafts until the bedrock.

Concrete pillars, technically known as piles, are then raised from the bedrock. The piles are then joined with horizontal concrete or steel beams to tie the foundation together near the surface.

However, it does not mean that  black cotton soil is always difficult to build on. Mr Njenga says that sometimes it is very shallow and the cost is not higher than building on  red soil.

“It all depends on what type of construction you are doing and the depth you have to reach before you get to a firm foundation; you have to review each case individually,” he says.

“For example  Embakasi area in Nairobi there has black cotton soil but the bedrock is very near the surface, sometimes less than a metre. The cost only becomes prohibitive beyond two metres.”

But in some cases,  the cost of building on black cotton soil is so high that one  might be forced  to look for an alternative.

Mr Njenga says that areas with murram or red soils are generally good for construction, adding that the type of soil on the land one buys can have a significant effect on  construction costs.