Soil which expand when they gain water and shrink when they loose water (desiccation)

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, Chlorite. Beidellite Vermiculite,

Attapulgite, Nontronite, illite Montmorillonite,There are also some sulfate

salts that will expand with changes in temperature.

CLAY: Particles exhibit a net negative charge

As the water is adsorbed, it pushes the clay particles apart, causing an expansion or swelling of the clay.

The most obvious way in which expansive soils can damage foundations is by uplift as they swell with moisture increases.

Swelling pressure from expensive soil can exceed 30,000psf(F.H CHEN 2000)

More typical value of swell pressure are 2000 to 4000psf

A single pound of montmorillonite can

have a surface area of 800 acres Soil pressure is important if the structure

is to be designed to resist the uplift due to heave

Polygonal pattern of surface cracks in the

dry season. These cracks are

approximately one inch wide at the top. Note sewer manhole

in background.

This crack is at least 32 inches deep. The yardstick was easily

inserted to this depth; narrower, less straight cracks may

extend much deeper.

At the beginning of

the rainy season, the

piers are still supported by friction with

the soil. When it

begins to rain, water enters deep into the soil through the

cracks.

After 5 to 10 large storms,

the soil swells, lifting the house and

piers.

In the dry season, the groundwater table falls and the soil dries and contracts. As tension

cracks grow around the pier, the skin friction is

reduced and the effective stress of the soil increases (due to

drying). When the building load exceeds

the remaining skin friction, or the effective

stress of the soil increases to an all-time

high, adhesion is broken by this

straining, and the pier sinks.

Plastic index (PI)can be used as a reasonable surrogate to indicate expansive soil

PI>20 considered expansive PI>40 considered highly expansive Expansion index considered effect of clay

mineral

Widely used properties for soil Plastic limit(PI) is the moisture

content(MC) at which the soil behavior change from semi solid to plastic

Liquid limit is the moisture content at which the soil change from plastic to liquid

Transition from solid to semi solid behavior

Defined as the water content at which saturated specimen ceases to shrink when dried

Generally below the plastic limit Commonly calculated as a function of

liquid limit and plastic limit index using casagrande chart

The plasticity index is the mathematical difference between the PL and LL.

PI =LL -PL Most soil expansion occur as the moisture

content move from the PL to LL The higher the PL, more the water that

can be absorbed during expansion, and hence the greater the potential movement of soil.

Clay mineral type Amount of clay heave Soil structure and fabric Depth of active zone Potential for change in moisture content

For a given structure, there are five ways to control shrinkage swell

Control heave Replace the expansive soil with non-

expansive soil(select fill) Control change in moisture content Isolate the structure from the soil Chemical soil treatment

Three general types of foundations can be considered in expansive soils. They are

1. Structures that can be kept isolated from the swelling effects of the soils

2. Designing of foundations that will remain undamaged in spite of swelling

3. Elimination of swelling potential of soil.

Provide granular fill around the foundation

Provide air gap

RCC plinth beam

PREWETTING

COMPACTION

CHEMICAL STABILIZATION OF SOIL

Increase the moisture content of soil by ponding.

4 to 5% hydrated lime may be added to the top layer of the soil to make it less plastic and more workable .

Achieve most of the heave before construction.

This technique is time consuming because the seepage of water through highly plastic soil is slow

Heave of expansive soils known to decrease substantially when the soil is compacted to a lower unit weight on high side of optimum moisture content (possibly 3 to 4% above the optimum moisture content)