Expansive Soils Definition
Expansive Soil: Black Cotton
Expansive soils, also referred as swelling soils, are those soils which have tendency to increase in the volume whenever the moisture content (i.e. water content) in it is increased. Foundation with swelling soil will heave and can cause lifting of a building or structure laid on it whenever the moisture content rises. This can ultimately lead to the failure of foundation and structure laid on it. They swell when water is added to them and shrink when they dry out. These are common in countries like Australia, India, South Africa, USA, Indonesia etc. “Black Cotton Soil” is an example of Expansive soil.
Geographical chart representing abundance of swelling soil
Often, damage from expansive soils can be seen within the first few months or year after a home is constructed. As water from irrigation or rainfall migrates underneath the home’s foundation, the soil around the edge of the foundation expands, pushing up on the edges of the foundation. This condition, called edge-lift, can cause cracking in the drywall and in the foundation itself. Over a period of years, as the moisture further migrates underneath the center of the slab, center-lift can occur, causing additional damage to the home. Each year in the United States, expansive soils cause $2.3 billion in damage to houses, other buildings, roads, pipelines, and other structures. This is more than twice the damage from floods, hurricanes, tornadoes, and earthquakes combined.
Cause of expansion
Soils are composed of a variety of materials, most of which do not expand in the presence of moisture. However, a number of clay minerals are expansive. These include: smectite, bentonite, montmorillonite, beidellite, vermiculite, attapulgite, nontronite, illite and chlorite. There are also some sulfate salts that will expand with changes in temperature. Soil containing large amount of above mentioned expansive minerals, they tend to possess potential of significant expansion or swelling. Whereas, soil containing these minerals in lesser amount are non-expansive in nature.
Design of Foundation
Different builders have tried different techniques to design foundation on swelling soil so that the designed foundation would be good enough to resist the adverse effects of expansion. Under-reamed pile foundations may be used in case of swelling soils. But it is not a full-proof solution as it has often been unsuccessful where there is possibility of rise in water table.
Some of the recommendations for the design of foundation in expansive soil are given below:
- Design of size and depth of footing, length and dimensions of under-reamed piles/piers, thickness of slab, etc. must be carried out only after detail soil investigation on the basis of collected data. Minimum recommended depth is 2m.
- Heavy reinforcement in foundation can prevent uneven heaving and reduce problems regarding cracking. Reinforced concrete bands can be used at the foundation, plinth and lintel levels.
- Area around the building can be paved so that moisture wouldn’t ingress into foundation and cause swelling. Also proper drainage system has to be constructed.
- Plantation of trees 3 m around the building must be avoided.
Builders and engineers have their own opinions and concepts regarding precautions and solution for structures founded on expansive soils. The opinions may not always be on facts and as a result, it can be quite misleading at times. Here are few misconceptions regarding the design of structures on swelling conditions which are not always true:
- The shallow foundations if designed for a low allowable bearing pressure helps to minimize the expansion of soil.
- Soils which aren’t black in color are unlikely to be expansive soil.
- If site is ponded before construction, the chances of swelling after construction are eliminated.
- Chemical stabilization of swelling soil takes care of all the problems related to swelling soil.
- Coarse-grained material placed below the floor slab reduces the uplift pressure by allowing the swelling clay to fill in the voids.
- What is expansive soil? by Jeffrey P. Kerrane
- US Army Corps of Engineers
- Basic and Applied Soil Mechanics, Gopal Ranjan, A.S.R. Rao, New Age International Publishers