Shrinkage is one of the major causes for volume change associated with variations of water content in soil.

The change from points a to b is called normal shrinkage, as the volume changes linearly with a decrease in water content. This linear shrinkage is due to surface tension forces in the capillary moisture. When the soil color changes, a small amount of volume change, termed residual shrinkage or curvilinear shrinkage, occurs between points b and d.

• frequently estimated from the liquid limit and plasticity index as given by Casagrande

• Types of Shrinkage • Volumetric shrinkage: The volumetric shrinkage of soil is the

decrease in volume, expressed as a percentage of the soil mass, when the water content is reduced from a given percentage to the shrinkage limit. The relationship between volumetric shrinkage and shrinkage limit is shown in Equation

Linear shrinkage: Linear shrinkage is defined as the one-dimensional decrease in soil mass expressed as a percentage of the original dimension, when the water content is reduced from a given value to the shrinkage limit (ASTM D427).

Factors affecting shrinkage: a. fluctuations in moisture content b. clay content, c. Drying process, d. soil particle orientation, e. unit weight of soil, and f. grain size distribution.

Swelling characteristics and mechanism

When soil loses its water content, the volume decreases or shrinks. However, if water is added to the dry soil, the volume of soil increases and this phenomenon is called swelling or expansion

Soil swelling is a spontaneous process that occurs when there is a decrease in free energy or an increase in system entropy in the presence of moisture.

The rate of swelling is directly proportional to the available free energy, with the following consequences:

(a) the rate of swelling decreases with time, and (b) the swelling pressure, as a measure of free energy, also decreases.

Characteristics of swell–shrinkage processes.

Identification Swelling US Bureau of reclamation method for identification of swelling soils

An empirical equation relating the expansion and plasticity indexes is given by Equation, (Fang, 1997):

A term called the expansion index, IE, has been proposed by Fernando et al. (1975) in assessing swelling on the basis of dielectric dispersion. The classification of expansive soils based on magnitude of dielectric dispersion is presented in Table

Table 4.2 Classification of expansive soils based on magnitude of dielectric dispersion

Infiltration, percolation, and retention

Infiltration : the passage of water through the ground surface into the subsurface soil layer

Infiltration normally begins at a high rate and then decreases to a minimum.

Infiltration capacity: Infiltration capacity is the maximum rate at which a given soil condition can absorb rain as it falls. This value decreases exponentially in time from a maximum initial value to a constant rate. It also decreases exponentially with time as the soil becomes saturated and soil particles swell

Infiltration through soil layer:

Rainfall or surface drainage water infiltrated into the subsurface soil layer is an important parameter for stability evaluations of soil foundation systems or earth slope stability problems. The thickness of this saturated zone can be estimated by Equation

Modified after Beattie and Chau (1976)

Percolation: Percolation is the water movement within the soil mass. Percolation and infiltration are closely related but infiltration cannot continue unless percolation provides sufficient space such as voids in the soil layer for infiltration water. Retention is also a part of the water movement process and is closely related to channel precipitation and overland flow. Measurement of percolation: The instruments suitable for measuring suction include tensiometer, pressure membrane apparatus, and vapor adsorption method. Retention: Retention is defined as a capability of a soil to retain water in the soil mass. At the beginning, almost all of the rainfall is collected on the ground as surface retention. After a sufficiently long time, surface retention approaches a steady state.

Capillarity Phenomena

•Caused by fluid surface tension •Differences of forces of attraction between molecules of diff. material at interface •At the interface of two different materials, soil for instance, at water surface, mineral grains and air

Rate of capillary rise

• Rate of rise is critical than max. rise for practical purposes

Rate of capillary rise

After Terzaghi (1942)

Estimation of capillary rise

Factors affecting capillary rise

Pressure membrane aparatus (ASTM-D3152) – fine grained soils Porous Plate device (ASTM-D2325) –Course grained soils Tensiometer-in field conditions

•Time-soil particle size: a curve linear relation •Moisture content and types of pore fluid-for unsaturated of partially saturated soil, contact angle between water menisci and soil will be greater than zero and vice versa •Temperature-inversely related