Soil suction

Even after a large supply of water has migrated downward through a soil zone, under conditions where gravity is the dominant force, some water will be retained on and between the soil particles as residual saturation. The relationship between several soil horizons in the unsaturated zone is shown in Figure 3.29. Water in each of these zones is held according to the local conditions of soil suction. It is important to note that water in liquid form cannot be held by soil if the soil suction is >0.7 atm (10 psi). The zones of unsaturated soil are listed below ... 

Capillary fringe The zone immediately above the water table which is essentially saturated this is the height that can support saturated conditions at negative pressure (due to soil suction). 

The term capillary action describes the upward movement of a fluid as a result of surface tension through pore spaces. The fluid can rise until the lifting forces are balanced by gravitational pull (see Figure 3.28). The rise of fluid in a small tube above the water table surface, as previously discussed in Chapter 3, can be described using Equation 3.13. Lifting of fluids above the water table is a true negative pressure compared with atmospheric pressure (also described as soil suction). In soil situa-... 

Soil suction is a function of capillary forces acting between water and soil grains. Other factors being equal, soil suction is greatest in fine textured media, in which the menisci between soil water and soil gas have the smallest radii and thus produce the greatest pressure difference between water and air. Large pores require larger menisci between air and water these create smaller pressure differences. This phenomenon is easily demonstrated with water in... 

If poorly heave compensated, soil suction when bit moves up and percussion when bit moves down... 

FIGURE 3 Relative trends in hydraulic conductivity of two soil types as a function of soil suction. 

At each point where moisture menisci are in contact with soil particles, the forces of surface tension are responsible for the development of capillary or suction pressure (Table 4.1). The air and groundwater interfaces move into the smaller pores. In so doing, the radii of curvature of the interfaces decrease, and the soil suction increases. Hence, the drier the soil, the higher is the soil suction. 

Soil suction is a negative pressure and indicates the height to which a column of water could rise due to such suction. Since this height or pressure may be very large, a logarithmic scale has been adopted to express the relationship between soil suction and moisture content this is referred to as the pF value (Table 4.2). 

Soil suction tends to force soil particles together, and these compressive stresses contribute towards the strength and stability of the soil. There is a particular suction pressure for a particular moisture content in a given soil, the magnitude of which is governed by whether it is becoming wetter or drier. In fact, as clay soil dries out, the soil suction may increase to the order of several thousand kilopascals. However, the strength of a soil attributable to soil suction is only temporary and is destroyed upon saturation. At that point, soil suction is zero. 

The moisture characteristic (moisture content versus soil suction) of a soil provides valuable data concerning the moisture contents corresponding to the field capacity (defined in terms of soil suction, this Is a pF value of about 2.0) and the permanent wilting point (pF of 4.2 and above), as well as the rate at which changes In soil suction take place with variations in moisture content. This enables an assessment to be made of the range of soil suction and moisture content that is likely to occur In the zone affected by seasonal changes In climate. 

Kuriyan S.J. Singh D.N. 2003. Estimation of unsaturated hydraulic conductivity using soil suction measurements obtained by an insertion tensiometer. Canadian GeotechnicalJoumal, 40(2) 476-483. 

There are two major approaches in the analysis of slope stability. The first one is the forward approach in the analysis of slope stability that requires data on shear strength properties and pore pressure conditions. The former are derived from a range of field and laboratory techniques, whereas the latter demand improved techniques capable of instrumenting rapid groundwater and soil suction responses to rainfall without damping the transient peak conditions. Probable worst-case parameter values are assumed, and a conservative value of the factor of safety is derived. 

An interesting and somewhat counterintuitive phenomenon is the reversal of relative hydraulic conductivities of unsaturated fine- and coarse-textured porous media as pore water pressure and water content change (Fig. 3.24). When pores are saturated with water and xJ/ is zero, the coarse-textured, sandy soil has a higher hydraulic conductivity than the fine-textured, clayey soil because the sandy soil has larger pores. As pore water pressure decreases, however, water in the sandy soil more easily drains out, whereas the clayey soil with its larger soil suction loses less water. A point is reached at which the pores of the sandy soil are essentially empty, precluding water flow, whereas the pore spaces of the clayey soil remain partially filled and hence still conduct some water. 

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