Expansive soil is also known as shrink-swell soil, which is a serious challenge in construction due to its ability to expand with moisture and shrink when dry. Expansive soils, primarily composed of clay, can cause serious damage to pipes, roads, and structures if not properly identified and managed. It is important for an engineer, builder or homeowner to know the features of expansive soil to avoid costly damages and ensure structural stability.
This blog will look at the features and problems of expansive soils, as well as methods for identifying them in the field and laboratory and various treatment methods. It will also discuss different foundation construction methods of foundations on expansive soil.
What is Expansive Soil?
Expansive soils are soils rich in clay with large volume changes due to changes in moisture content. They will swell when wet and shrink when dry because of the presence of minerals like montmorillonite and smectite that absorb water into their layered crystal structures. In expansive soil, there are a few other swelling minerals like vermiculite, beidellite, bentonite, nontronite, sulfate salts, chlorite and attapulgite. The level of swelling depends on a few factors, like ground load, stratum thickness, drying intensity and moisture content.
Key Characteristics of Expansive Soil
- High plasticity and moisture sensitivity.
- Ability to undergo substantial volumetric changes.
- Poor drainage, which worsens the swelling behaviour.
Problems with Expansive Soil in Construction
Doing a construction project on expansive soils can lead to a range of serious and costly problems.
- The nature of expansive soil is to absorb water and swell which puts a lot of pressure on the foundation soils resulting in foundation movement and structural damage. These can lead to common problems like cracked concrete, tilted structures, uneven surfaces, bowed walls, and subsidence.
- Expansive soils can result in cracks in foundation walls, which will allow the water to seep into basements and can result in secondary problems like mould growth, water damage, and wood rot.
- In hot and dry conditions, expansive soils lose moisture, become weak, and shrink. This will result in soil movement and foundation settlement, which will cause structural instability.
Solutions and Treatments for Expansive Soil
A few methods can be used to control the difficulties caused by expansive soils and reduce the possibility that they will cause damage. The goal of these treatments is to increase the general stability and reduce volume changes.
1. Moisture Control
Careful moisture control is important to reduce soil swelling and shrinkage. High water penetration can be avoided by properly grading, creating drainage systems and managing irrigation around the structures. Foundations can also be protected from water exposure with waterproofing techniques and moisture barriers.
2. Chemical Stabilisation
Chemical treatments can be done with expansive soils to reduce the swelling by adding materials that react with clay minerals. Common methods are:
- Lime treatment- Clay soils can be more stable and stronger when lime is added to them and their plasticity is decreased.
- Cement treatment- Using cement can reduce the volume changes in the soil and improve the load-bearing capacity and properties of the soil.
- Polymer-based soil stabilisers- Polymers change the behaviour of the soil, decreasing its tendency to swell due to changes in moisture content.
- Chemical injections- Solutions like potassium-based stabilisers can be injected directly into the soil using rigs with multiple injectors.
3. Mechanical Stabilisation
Mechanical methods aim to physically strengthen the soil and reduce movement. These are:
- Soil compaction: It increases soil density through compaction that reduces void spaces and limits moisture-based expansion.
- Aggregated piers: They are installed as compacted stone columns in the soil to provide additional support and stability.
- Soil mixing: The process of mixing expansive soil and non-expansive soil, known as soil mixing, improves the overall performance of the soil.
4. Use of Geosynthetics
Geotextile and geogrids are used to reinforce soil, manage moisture migration and control erosion, thereby lessening the impacts of expansive behaviour.
5. Foundation Design Adaptations
Proper foundation design is vital when building on expansive soils. Solutions are:
- Deep foundation: Piles or piers transfer structural loads to deeper, more stable soil layers unaffected by moisture changes.
- Reinforced slab-on-grade: Using stiffened slabs with crossbeams provides resilience against soil movement.
- Flexible foundation systems: Designs like suspended slabs can accommodate minor soil shifts without damaging the structures.
6. Vegetation Management
Strategic landscaping helps control moisture around structures. Trees should be planted at least 20 feet away from buildings, and drip irrigation should be used to water plants carefully. By making sure that water drains away from foundations, proper yard grading reduces the possibility of soil expansion.
7. Cement/Lime Treatment
Widespread use of cement and lime for soil stabilisation reduces soil plasticity and saturation potential, increasing overall soil strength and reducing the likelihood of volume change.
By using a combination of these methods, the risk linked with expansive soils can be reduced, protecting structures from costly damage.
Methods of Foundation Construction on Expansive Soil
The design of the foundation in soil with shrink-swell behaviour needs specialised approaches to accommodate or reduce soil movements. Here are some of the most effective foundation construction methods:
1. Pier and beam Foundations
This method involves constructing deep piers that extend below the active soil layer into stable strata. Beams can support the structure that is above the ground, which reduces the effect of soil movement on the foundation.
2. Slab-on-Grade Foundations
A slab-on-grade foundation will be effective under reinforced post-tensioning cables or constructed on a moisture-stabilised subgrade. The slab is designed such that it distributes loads evenly, reducing the possibility of cracking by ground movements.
3. Floating Foundations
A floating foundation is also called a compensating foundation that is designed to “float” on the soil. The weight of the structure is balanced against the weight of the displaced soil, minimising differential settlement.
4. Deep Foundations
Pile foundations are ideal for structures on expansive soil. When trying to avoid the active zone completely, piles are drilled or pushed deep into the stable soil or rock strata.
5. Underpinning
Underpinning is the process of supporting a current foundation by extending it to a deeper, more stable soil layer. It is mostly used as a solution for already affected structures by expansive soils.
Case Studies and Real-World Applications
1. Residential Buildings
Homes constructed on expansive soil are prone to foundation cracking and wall deformations. Slab-on-grade foundations with moisture barriers have been found to be effective in reducing these issues.
2. Infrastructure Projects
In highway construction, lime-treated subgrades are frequently used to stabilise expansive soil, preventing cracking and deformation of road surfaces.
3. Commercial Buildings
For high-rise structures, deep pile foundations have the building securely anchored below the active zone of expansive soil, decreasing long-term maintenance costs.
Future Trends in Addressing Expansive Soil Challenges
The construction industry for buildings still looks for new ways to build on expansive soil. Possible research topics are developments in geosynthetics, soil stabilising nanomaterials and smart monitoring systems.
- Geosynthetics: It is used to reduce the need for several chemical treatments by improving soil drainage and strength.
- Nanotechnology: Nano additives can improve soil stabilisation and are environmentally friendly.
- Smart sensors: By tracking the soil movement and water content in real-time, sensors placed in foundations allow for predictive maintenance.
Conclusion
The construction on expansive soil is very difficult, but with the right knowledge and methods it is possible to construct safe and stable structures. Expert builders and engineers can reduce the risk successfully by defining expansive soil properties, taking care of the swelling of soil by stabilisation methods and selecting suitable foundation techniques. The secret to success is a proper understanding of the performance of soil and the application of specific solutions, either with soil stabilisation, efficient drainage or innovative foundation designs. Increasing the use of advanced technologies will better enhance our ability to construct safely and sustainably in areas with expansive soil.
Hemali Patel