Building construction: Soil type very important ( Part 2)

MOST people have heard that the three most important things to consider with Real Estate are . . . “location, location, location”. While this statement is true, it is not the only thing to consider, especially when it comes to new construction.

One extremely important, but often overlooked, consideration is the type of soil that is at the property. Soil ranges from very clay soil to very sandy soil.

Most land has a percentage of both. Sandy soils do not tend to retain water, drain quickly, and settle quickly. Clay soils retain water, may drain very slowly and may take years to settle.

Clay soils will come in different levels of cohesiveness. This simply means how much does the soil stick together when moisture is added.

If you add water to very clay soil, it will stick together and form mud. You are able to squeeze it to make a ball of mud. If the soil is quite sandy, then it has low cohesiveness. If you add water to it, it will not stick together. It is best to avoid either extreme when building.

One of the worst soils to build on is known as “Black Cotton Soil”. It is called this because it tends to be black in colour and it is good soil for growing crops, such as cotton.

This is rich, cohesive, fertile soil. It is considered a difficult and problematic soil for civil engineers. This is because it swells up during the rainy season and then shrinks down during the dry season.

Both the swelling and shrinking create construction difficulties.

Swelling in black cotton soil during the rainy season, causes the structure to be lifted, also known as “heave”. The lifting action of heave and the dropping action of shrinkage causes structural damage to buildings.

We see this in many Zimbabwean homes, as evidenced by wall cracks. The real danger appears if the foundation becomes severely damaged. The damage can reach to the walls, door frames, window frames, and into the ceiling and roof.

In drastic cases, the building can collapse. When building in an area with heavy clay soils or with black cotton soil, special foundations will be required. This is a major consideration when building.

Depending on the type of foundation required, costs can quickly escalate. Because soil types can vary, you may find it suitable to build on one side of the property, but not the other.

The other extreme is sandy soils. The biggest issue with sandy soils is that they do not really compact, therefore the bearing capacity may be inadequate.

The bearing capacity is how much weight can the soil hold. You may remember that Jesus told a story of a wise man who built his house on a rock, and a foolish man who built his house on the sand. The sand failed to provide enough support, and the house collapsed.

In Zimbabwe, we generally have truly sandy soils in riverbeds and such. Most places where construction is occurring, we have gravel, decomposed rock, or a combination of clay and soil.

With these types of soils, we are able to use a standard type of foundation. The standard foundation is one of the most economical types. When purchasing a property, for construction, the location is important. But remember that the soil under your feet may be more important to a successful construction project than you ever realise.

Jonathan Thompson is a registered Real Estate agent who works in Bulawayo. He has nine years of experience in Real Estate in the USA, and another 11 years experience in Bulawayo. He is the founder and the Managing Director of Thompson Properties (Pvt) Ltd.