Underpinning is used to improve the strength of the foundation in an existing structure. This article provides answers to three questions that should serve as an introduction to the mentioned topic for the benefit of those studying to become structural engineers.

Why The Need To Underpin?

Factors that may create the need to strengthen the foundation of an existing building include, but are not limited to, the following:

Natural disasters: These include floods, earthquakes and landslides among several others. Many times, such disasters effect a change in the original position of the foundation of the building. Thus, the structural integrity of the building is considered to have been compromised. Structural engineers use underpinning to stabilise the foundation of the building and the soils beneath.

Building improvements: Certain improvements to a building may create the need for underpinning. The best example of these improvements is the addition of one or more storeys to an existing building. In such situations, underpinning is used to increase the load-bearing capacity of the building's foundation in order to accommodate the extra weight of additional storeys.

Change in soil properties: This is especially common in areas that have expansive clay soil. This type of soil expands and contracts with a respective increase or decrease in its moisture content. This often has an adverse effect on the stability of building foundations that rest on the mentioned soil type, hence the need to underpin.

What Techniques Are Used For Underpinning?

Concrete Underpinning: This technique is perhaps the most commonly used thanks to its relative affordability. For concrete underpinning, structural engineers dig cavities under an existing foundation and gradually pour fresh concrete in these cavities. By the time they're done, the engineers will have 'built' a new foundation under the original foundation.

Resin injection: For this technique, engineers pump a structural resin and a hardening compound into the soil beneath the foundation. The reaction between the compounds makes the resultant mixture expand as it penetrates the ground beneath the foundation. In the process, crevices and voids that may have been present beneath the foundation are filled with the mixture. Resin injection is often preferred over concrete underpinning because it reduces the extent of excavation required for the process. The structural resin and hardener used is often more expensive than concrete, making it a more expensive alternative.

Why Is Underpinning Effective For Foundation Reinforcement?

Concrete underpinning increases the depth or breadth of an existing foundation so that the weight of the building is distributed across a greater area. Resin injection makes the underlying soil stratum more supportive, thereby improving its capability to support the weight of the building.