Presentation On Expansive Soil Given By Farid And Chatta 4y2t3t

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

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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,

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Attapulgite, Nontronite, illite

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Montmorillonite,

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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.

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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 ed 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

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

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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

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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

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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.

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

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For a given structure, there are five ways to control shrinkage swell Control heave Replace the expansive soil with nonexpansive 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. Deg of foundations that will remain undamaged in spite of swelling 3. Elimination of swelling potential of soil.

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Provide granular fill around the foundation

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Provide air gap

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RCC plinth beam

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PREWETTING

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COMPACTION

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CHEMICAL STABILIZATION OF SOIL

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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

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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)