The WRC is often plotted on a semilog plot of suction against moisture content as shown in Fig. 3.9. The curve can be approximated with three distinct line segments having different slopes, with degree of saturation ranging from zero to one, as illustrated in the figure. These segments illustrate four different stages of moisture regimes in the soil with increasing matric suction from zero value at the saturated moisture content. The soil is fully saturated in the first moisture regime where the moisture loss is due to change in the shape of the meniscus. The thickness of this regime depends on the surface properties of the soils that can influence the receding contact angle. The contact angle reaches the receding angle at the end of this regime. As the suction increases, the soil enters the quasi-saturated regime where the mositure is lost by keeping a constant receding angle. The air enters the largest pore of the soil in this regime. Once the air-entry value is reached, the retention curve enters a regime dominated by capillary pore water retention mechanisms. the soil looses the water at faster rate with increase in the suction in this regim. The amount of water retained here is a function of the particle and pore size properties. Additional incerase in suction would bring into residual state where the loss of water is very less for the increase in the suction. The amount of water retained at this stage is a function of the surface area of the soil particles, the surface charge density of the soil mineral, and the type and valency of any adsorbed exchangeable cations . Within the adsorbed film segment, water is retained in the form of thin films on the particle surfaces under the influences of short-range electrical forces, van der Waals attraction, and exchangeable cation hydration. Water molecules are strongly held on the particle surfaces due to electrostatic forces at this stage. In clays greater amount of pore water is required to satisfy the relatively large surface hydration energies associated with the high suction regime. Thus the residual regime extends to very large suction values. On the other hand, In sands very little water is adsorbed under initial surface hydration mechanisms. However, the capillary effects dominate over the majority of the unsaturated water content range in sands.

Dependency of WRC on soil chracteristics

Fig. 3.10. Representative water retention curves for sand, silt and clay

Fig. 3.10 illustrates the general SWCCs of representative samples of sand, silt, and clay. It can be seen from the figure that sandy soil quickly desaturates after the air-entry pressures. The water retention capacity beyond residual state is also very poor because of the poor surface charge characteristics in sandy soils. In such soils, the overall water retention behavior is influenced by the pore size distribution. On the other hand, clays exhibit high water retention characteristics with high air-entry pressures and mild slope from air-entry suction to residual suction indicating high renetion capacity. The negatively charged particle surfaces and high surface areas in clays can substantially influence the retention behavior. The retention behavior in silty soils is moderate and falls between these two extereme behaviors.