Expansive soil can have detrimental effects on structures and their foundations. To prevent these effects, several measures can be adopted during construction, such as constructing deep piers, soil replacement, keeping moisture at a constant level, chemical soil stabilization, and using void forms. It is recommended to consider one or a combination of these preventive methods while planning construction over expansive soil. The soil investigation report should be consulted to determine the depth of expansive soil and the most effective and economical treatment technique.
When expansive soil exerts heave pressure on the foundations, basements, stem walls, and piers of buildings, it can cause extensive cracking and expensive repair work. Cracks often develop initially at the wall-floor junction and between the wall and the roof slab. The expansion of soil can also damage utilities buried in the soil, such as sewage lines, water pipes, gas lines, and telephone lines. However, damages due to soil expansion are usually slow and commonly visible only after several years of construction.
Expansive soil can push up the floor slab and cause extensive crack development, as well as push the footing wall outward, which eventually leads to the formation of cracks on the end of the wall. The magnitude of heave pressure exerted by expansive soil is based on the mineral composition of the soil. The volume of expansive soil can increase up to 20%, and it can impose a force of around 1430 KN/m2.
How to Avoid the Effects of Expansive Soil on Buildings?
1. Drilled Piers
Deep piers are a viable solution for mitigating the adverse impacts of expansive soil on both the foundation and the overall structure. One common type of deep pier, known as a drilled pier as shown in Figure-1, is specifically designed to prevent bearing capacity failure and minimize differential settlement of the building to an acceptable level. To ensure the safety and stability of these drilled piers, they need to be thoroughly examined for their resistance against uplift pressure.
Figure-1: Drilled Pier Foundation
When dealing with expansive soil problems, it may be necessary to address the issue of uplift pressure on drilled piers. If the uplift pressure is significant and the drilled pier is not designed to resist it, an alternative solution is to construct a belled drilled pier. The belled shape of the pier can help counteract the uplift pressure, providing a more stable foundation.
Another approach commonly used in India is the use of under-reamed piles, as shown in Figure-2 and Figure-3. These piles feature an enlarged bulb-like shape at the bottom, known as the under-ream, which helps to increase the bearing capacity of the pile and resist the uplift pressure caused by expansive soils. This design is often employed in areas where expansive soil conditions are prevalent.
The under-reamed pile, as illustrated in Figure-4, is reported to be a popular solution in India for addressing expansive soil problems. By incorporating the under-ream at the base of the pile, this design can help improve the stability and performance of the foundation, mitigating the effects of uplift pressure on the structure. This design has been widely utilized in India to address the challenges posed by expansive soils.
2. Replace Expansive Soil
If the thickness of expansive soil is not significant, replacing it with a compacted, non-permeable soil can potentially resolve the problem. However, it’s important to note that the surrounding soils can have a significant impact on the foundation of a building over time. The lateral pressure exerted by the soils around the structure may have detrimental effects on the soil beneath the foundation.
Therefore, it’s essential to conduct a survey of the properties of the surrounding soil before deciding whether replacing the expansive soil is a viable solution or not. The results of this survey can play a decisive role in determining whether the replacement of the expansive soil is a suitable choice for resolving the issue at hand. Taking the time to carefully evaluate the condition of the surrounding soil can help prevent future problems and ensure the long-term stability and safety of the structure.
3. Keep Moisture at a Constant Level
To prevent further swelling of expansive soil, it is crucial to prevent moisture from reaching it. One effective way to achieve this is by using a bentonite mat. The purpose of the bentonite mat is to maintain a constant level of moisture in the soil. To achieve this, it is recommended to place the bentonite between two layers of fabric, with one layer being permeable and the other non-permeable.
As water enters the bentonite, the particles within it become saturated. This saturation results in the creation of a solid mat that effectively stops any further movement of water into the untreated soil. By utilizing this technique, it is possible to control moisture levels in expansive soil and prevent any further damage caused by soil swelling.
4. Stabilize Expansive Soil with Chemicals
Expansive soil can be stabilized and its heaving can be prevented through the use of certain materials such as fly ash, lime, or cement. The process of stabilization involves chemical methods which work to cement the clay particles in place, effectively preventing them from expanding in volume.
Chemical stabilization techniques aim to prevent the expansive soil from undergoing changes in volume that may cause damage to structures built upon it. This can be achieved through the addition of certain materials, such as fly ash, lime, or cement, which work to bind the clay particles together, preventing them from expanding and contracting.
The use of fly ash, lime, or cement in the stabilization process allows for the creation of a stable base upon which structures can be built without fear of damage caused by expansive soil. By chemically bonding the clay particles in place, the soil is prevented from undergoing changes in volume, ensuring that the structures built upon it remain stable and secure. Overall, the process of chemical stabilization is a crucial technique in the construction industry, ensuring the safety and longevity of buildings and other structures built on expansive soil.
5. Use Degradable Void Forms Under Grade Beam, Slab, and Piers
Degradable void forms are a type of sacrificial element that are specifically designed to prevent foundations from being affected by the heave pressure caused by expansive soil. These forms are typically constructed using degradable materials such as carton, and they can be placed in various locations, such as under slabs, grade beams, and around piers, to prevent the heave pressure from impacting these elements.
Although degradable void forms are able to withstand both operational and fresh concrete pressure, they eventually lose their strength and integrity, and begin to degrade when they become wet. As a result, a space is left between the foundation and the soil, which is able to accommodate the maximum amount of soil expansion without affecting the foundation.
To ensure that the void forms are effective, it is important to seal the surface of the form where the concrete will be placed with a wax coating. This helps to prevent the migration of moisture from the concrete into the void forms. Additionally, it is crucial to keep the void forms dry during storage and placement, and to ensure that the concrete has achieved adequate strength to support its own weight before the void forms are removed.
By combining degradable void forms with one of the above solutions, it is possible to effectively manage the pressure caused by expansive soil and protect the foundation from damage.
FAQs
How is construction over expansive soil carried out?
Expansive soil can cause significant damage to structures and their foundations, making it necessary to consider preventive measures during construction planning. There are various methods that can be employed to avoid the detrimental effects of expansive soil on structures. These include construction of deep piers, soil replacement, and maintaining moisture at a constant level.
Chemical soil stabilization is another option that can be utilized to prevent damage to structures caused by expansive soil. Additionally, the use of void forms can be considered to mitigate the impact of expansive soil on the foundation of a structure.
Depending on the specific circumstances of a construction project, one or a combination of these preventive measures can be implemented to ensure that the structure remains stable and secure. By taking these measures, the negative effects of expansive soil can be minimized, and the longevity of the structure can be enhanced.
How is structural damage caused by expansive soil?
Expansive soil can cause significant damage to a building’s substructure and superstructure. This type of soil exerts heave pressure on foundations, basements, stem walls, and piers, which can result in cracking and damage that may require costly repair work.
One of the major issues with soil expansion is that the damages can be slow and gradual, and may only become visible several years after construction. The expansion of soil can cause the floor slab to lift, resulting in the development of extensive cracks. Additionally, the pressure from expansive soil can push the footing wall outward, leading to the formation of cracks on the wall’s end.
In summary, the effects of expansive soil on a building’s structure can be severe and expensive to repair. These damages may take time to manifest and become visible, but they can cause significant structural issues that require immediate attention to ensure the safety and stability of the building.
Up to what extent can the volume of expansive soil increase?
The magnitude of heave pressure in expansive soil is influenced by its mineral composition. Expansive soils have the ability to increase in volume by up to 20%, which can result in a significant force of approximately 1430 KN/m2. This force is exerted on the surrounding structures or foundations, potentially causing detrimental effects. The mineral composition of the soil plays a crucial role in determining the magnitude of heave pressure, as different minerals have varying degrees of swelling potential. Understanding the mineral composition of expansive soil is essential in designing and constructing structures in areas with such soils to mitigate potential damage caused by soil heave.