The soil tests and tricks for your construction project

How the type of soil affects construction

One of the world’s most famous buildings, the Leaning Tower of Pisa in Tuscany, Italy, is one of the earliest examples of why soil is so important to construction. Building started in the 12th century and its soft soil, which became unstable during the rainy season, and the lack of drainage, all contributed to the building leaning. Ironically, researchers have since determined that the interaction of the soil with the stiffness of the tower is also part of the reason why the tower is still standing, despite its five degree tilt and some earthquakes in the area over the centuries.

Jump to modern-day South Africa, and soil and construction took on a new turn when needing to build on dolomitic ground known to form sinkholes, such as a 16km stretch of the Gautrain from Centurion to Pretoria. In the same area, a R600-million head office for a mining group was built on a bedrock of dolomite, which is a translucent mineral consisting of a carbonate of calcium and magnesium, often also containing iron. Undeterred, the developers overcame the challenge by drilling over 100 boreholes to create a three-dimensional model of the bedrock. They blasted the dolomite pointed pieces of rock, compacted the upper layers of soil, and laid a 2.25m reinforced concrete raft foundation. This needed 13 200 m3 of concrete in nine main pours of about 14000 m3 each, with up to 18 trucks on continuous rotation.

These are just some extreme examples of how soil can affect construction.

Soil is the upper layer of earth. It consists of minerals, soil organic matter, living organisms, gas, and water. Its minerals are divided into three classes, what is commonly referred to as the three main types of soil – sand, silt and clay – based on the percentages of particles they contain. But there are combinations of these types.  For example, there is loamy sand and sandy clay, and this mix affects the look and feel of the soil and its possible impact on what is being built on it. There are also different layers within soils called solid horizons.

South Africa has 73 soil forms divided into 14 groups, such as organic or oxidic. This classification helps scientists organise their properties, how they were formed, their distribution, and environmental significance. The World Reference Base (WRB), a system developed by the International Union of Soil Sciences, has 32 soil groups.

With this kind of complexity, considering how soil affects construction is not necessarily simple. It is not always just a question of looking at the ground, touching it, and making a quick assessment. It can require evaluating its geotechnical properties, which means applying technology to engineering factors caused by geological factors.  Geotechnical properties of soil include particle size distribution, compressibility, strength, and optimum moisture content.

You don’t need to be a civil engineer or quantity surveyor to realise that soil needs to be stable to be a good foundation to build on. Soil with a lot of clay absorbs water easily and so can shift depending on how much water it holds. This sort of soil requires specially engineered foundations.

Silty soil can retain water for a long time and so can shift and expand. Sandy soil allows water to drain well but if not compacted, can be washed away. Rocky soil can bear heavy loads, and is resistant to water damage, but can cause problems because of its uneven surfaces.

A good soil for infrastructure needs to have a neutral pH, so that it doesn’t corrode the building materials. It also needs to be stable during wet and dry periods so that the soil doesn’t expand and crack roads or foundations, and to be strong under pressure so that the weight of the building does not cause it to sink into the ground.

Soil found to be unsuitable for construction should first be enhanced with stabilisers such as raft foundations, cement or concrete pylons to reach solid bedrock. If you are not sure about whether the soil you want to build on it suitable for your structure, it is best to consult an engineer or geotechnologist to find the best construction methods.

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