Black Cotton Soil exhibits considerable swelling and shrinkage with moisture content variation. This soil expands while soaked in water and shrinks in dry weather. This situation poses a problem for the foundation. Foundation on Black Cotton Soil should be either under reamed pile foundation or raft foundation that distributes load coming from the superstructure over a large area.
Challenges Faced in Black Cotton Soil
- Low Bearing CapacityBlack Cotton Soil possesses low bearing capacity. It is tricky to design a foundation that transfers load over a larger area. One other possible solution is to transfer the load to the firm strata available at greater depth.
- Swell/Shrink Behavior of the Black Cotton SoilDuring rainy season black cotton soil absorbs a considerable amount of water and expands in volume. This imparts additional stress on the foundation.
While during hot and dry weather this soil shrinks in volume and foundation can not transfer loads to the beneath soil.
Solutions For Foundations in Black Cotton Soil
- Adopt Raft FoundationRaft foundation or Mat foundation distributes load of superstructure over a larger area. This helps in minimizing the allowable stresses at the footing level.
- Provide Under-Reamed Pile FoundationPile foundation like under-reamed piles with one or more bulbs help transfer the load to a greater depth where firm-bearing strata is available.
- Replacement of Black Cotton SoilIt becomes sometimes necessary to replace the whole foundation soil with soil of desirable properties. This solution is adopted where important structures are going to be built. Black Cotton Soil is fully replaced by non-cohesive and non-swelling soil.
- Use of Soil Stabilization TechniquesUsing chemicals like lime or cement to stabilize the soil and controlling its behavior is a common practice adopted by engineers.