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In civil engineering, the success of any construction project relies heavily on the ground beneath it. For sites where soft clays and silts are present, achieving a stable foundation is a critical challenge. Soil stabilisation offers a practical solution, enhancing the properties of the in-situ soil to support design loads safely and cost-effectively.
What is Soil Stabilisation?
Soil stabilisation involves improving the existing ground to increase strength, reduce compressibility, and lower permeability. By blending soft soils with cementitious binders such as cement or fly ash, civil engineers can transform weak, unreliable soils into a dependable foundation for roads, pavements, and structures.
Two primary in-situ stabilisation methodologies are commonly employed:
1. Deep Soil Mixed Columns (DSMC)
Deep soil mixing is an in-situ treatment where soil is mechanically blended with a binder to form stabilised columns. Using a hollow stem auger and paddle arrangement, the method creates discrete columns of improved material deep below the surface. These columns increase the strength of the soil, reduce settlement, and allow construction loads to transfer safely to competent strata.
2. Mass In-Situ Mixing (ALLU System)
Mass stabilisation follows a similar principle, mixing soft soils with a binder using a paddle system. The key difference is that mass mixing produces a continuous block of stabilised soil rather than individual columns. While this method is limited by achievable depth, it provides a large, homogeneous stabilised area suitable for load transfer platforms and construction tracking.
Managing Groundwater and Binder Mixes
For both deep soil mixing and mass stabilisation, the existing groundwater level has minimal impact on binder design. During installation, water is pumped into the ground to ensure a homogenous mix. If groundwater is higher than usual, the amount of pumped water is adjusted. Binder mix trials, conducted by the stabilisation subcontractor, confirm the most suitable mix for the soil type present, ensuring optimal performance.
Benefits of Soil Stabilisation
- Improved Soil Properties: Both methodologies result in a stronger, less compressible, and less permeable soil. Fully cured after 28 days (workable after 14 days), the improved material behaves like very stiff clay.
- Load Transfer Capability: By combining mass mixing with deep soil columns, a stable, homogeneous load transfer platform can be constructed. This ensures all stabilised elements act as a single unit, preventing drift and directing loads to competent strata. Such platforms also serve as construction working surfaces and can support shallow foundations, pavements, or underground pipework.
- Reduced Material Handling: Utilizing in-situ material eliminates the need for importing or exporting soil, reducing plant requirements, construction costs, and environmental impact.
Applications in Civil Engineering Projects
Soil stabilisation is particularly useful for:
- Foundations on soft clay or silt layers.
- Pavement sub-base construction.
- Load transfer platforms for heavy construction plant.
- Supporting underground utilities.
By improving the existing ground, engineers can safely design structures without the need for extensive excavation or imported fill, saving both time and money while enhancing sustainability.