Some soils will change volume significantly depending upon their moisture content. This poses a special problem for pavement design because this volume change can cause overlying pavements to sink down or heave up unevenly potentially resulting in a cracked or uneven pavement. Jones and Holtz (1973) estimated that shrinking and swelling soils cause about $2.3 billion of damage annually in the U.S. alone.
Shrinking and swelling are generally associated with fine-grained clay soils.
Soil shrinkage is generally confined to the upper portions of a soil. Shrinkage and shrinkage cracks are caused by a reduction in soil moisture content through:
- Evaporation from the soil surface in dry climates.
- Lowering of the groundwater table.
- Desiccation of soil by trees during temporary dry spells in otherwise humid climates.
The following process is illustrated in Figure 1: As moisture content decreases, capillary stress in the void spaces increases due to the increased surface tension. This increased surface tension tends to pull adjacent soil particles closer together resulting in an overall soil volume decrease. As moisture content continues to decrease, capillary stress continues to increase, which continues to reduce overall volume. The point where no further volume reduction occurs but the degree of moisture saturation is still 100 percent is called the shrinkage limit (SL), which is an Atterberg limit, just as plasticity index (PI) is. At this point the capillary menisci just begin to retreat below the soil surface, which can be seen by a change in surface appearance from shiny to dull. The shrinkage limit is not commonly tested because of various difficulties. However, a soil near the shrinkage limit typically have lower void ratios that can be achieved by compaction because of the associated high capillary stress.
When the climate changes and the shrunken soils again have access to water they tend to swell.
Swelling (Expansive) Soils
Swelling soils, also known as expansive soils, are ones that swell in volume when subjected to moisture. These swelling soils typically contain clay minerals that attract and absorb water. When water is introduced to expansive soils, the water molecules are pulled into gaps between the soil plates. As more water is absorbed, the plates are forced further apart, leading to an increase in soil pore pressure (Handy, 1995). If this increased pressure exceeds surcharge pressure (including the weight of the overlying pavement) the soil will expand in volume to a point where these pressures are once again in balance. Swelling pressures can be on the order of 100 – 200 kPa (14.5 – 29 psi) and have been measured as high as 1000 kPa (145 psi). Table 1 gives a general idea of the types of expansion that can be expected.
|Degree of Expansion||Probable Expansion
(as a percent of the total volume change)1
|Colloidal Content (percent less than 1μm)||Plasticity Index||Shrinkage Limit|
|Very High||> 30||> 28||> 35||< 11|
|High||20 – 30||20 – 31||25 – 41||7 – 12|
|Medium||10 – 20||13 – 23||15 – 28||10 – 16|
|Low||< 15||< 15||< 18||> 15|
|# Under a surcharge of 6.9 kPa (1 psi).