Environmental Factors Affecting Swelling

If a clay soil is subjected to drying conditions, for example, when evaporation is removing water from the soil near land surface, a suction effect is exerted on the soil that causes water molecules that are not held tightly to clay particles to be drawn out into the large pores of the soil and to move upward to replace the evaporated water. This loss of water from the clay leads to shrinkage, the reversal of swelling process (Kehew, 1995).

2.3. Factors Affecting Swelling

The factors influencing the shrink-swell potential of a soil can be considered in three different groups:

• The soil characteristics that influence the basic nature of the internal force field (Table 2.1)

• The environmental factors that influence the changes that may occur

Plasticity In general, soils that exhibit plastic behavior over wide ranges of moisture content and that have high liquid limits have greater potential for swelling. Plasticity is an indicator of swelling potential.

Table 2.2. Environmental Conditions that Influence Swelling Potential (Nelson and Miller, 1992)

FACTOR

DESCRIPTION

Climate

Amount and variation of precipitation and evapotranspiration greatly influence the moisture availability and depth of seasonal moisture fluctuation. Greatest seasonal heave occurs in semiarid climates that have short wet periods.

Groundwater

Shallow water tables provide a source of moisture and fluctuating water tables contribute to moisture.

Drainage

Poor surface drainage leads to moisture accumulations or

Ponding.

Vegetation

Vegetation (trees, shrubs, grasses, etc.) deplete moisture from the soil through transpiration, and cause the soil to be differentially wetted in areas of varying vegetation.

Permeability

Soils with higher permeabilities, particularly due to fissures and cracks in the field soil mass, allow faster migration of water and promote faster rates of

Readings of swell are taken at 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0, 15.0, and 30.0 minutes and 1, 2, 4, 8, 24, 48, and 72 hours. The specimen is left to swell till the end of primary swell (Figure 2.9, Step 3 – 4). After completion of primary swell a vertical pressure of approximately 5, 10, 20, 40, 80, etc., kPa is applied until the specimen is recompressed to its initial void ratio/height (Figure 2.9, Step 4 – 6). Method A can be modified to place an initial vertical stress, σ1, on the specimen equivalent to the estimated vertical pressure on the in situ soil within 5 minutes of placing the seating pressure and securing the zero deformation reading.

Deformation is read within 5 minutes of placing the vertical pressure (Figure 2.9,

Step 1 – 2). Then the vertical stress is removed, except for the seating pressure.

Deformation is recorded within 5 minutes after removal of σ1 (Figure 2.9, Step 2 –3), the specimen is inundated, and the test is continued as explained in the preceding paragraph. Figure 2.9 Void Ratio – Log Pressure Curve for Method A

This method measures (a) the primary swell, (b) percent heave for vertical confining pressures up to the swell pressure, and (c) the swell pressure.

2.4.2. Method