Suction potential

This entiy suction or suction potential is a measure of the resultant forces tending to draw water from the interior of the bed to the surface. For a pore formed by regularly packed nonporous spheres, the suction potential is given by... 

In fact, an unsaturated soil has no pressure potential, only a matrix potential (expressible as a negative pressure). The negative pressure causes water to move toward the soil with a higher suction potential, in contrast to the saturated flow where water moves from a high pressure potential to a low pressure potential. For soils with the same properties but with different saturation, the less saturated soil has more excessive suction force, causing water to move towards it. 

A negative pressure, known as a suction potential, will exist in the liquid in the capillary. Immediately below the meniscus, the suction potential will be equivalent to the height of the liquid column hs and, if water is used, this will have the value ... 

If equilibrium conditions exist, the suction potential hi at any other level in the liquid, a distance z below the meniscus, will be given by ... 

The theoretical suction potential of a pore or waist containing water is given by ... 

For an idealised bed of uniform rhombohedrally packed spheres of radius r, for example, the waists are of radius 0.155r, from Table 16.2, and the maximum theoretical suction potential of which such a waist is capable is ... 

The maximum suction potential that can be developed by a waist is known as the entry suction potential. This is the controlling potential required to open a saturated... 

When a bed is composed of granular material with particles of mixed sizes, the suction potential cannot be calculated and it must be measured by methods such as those given... 

Even though the effective suction potential at a waist or pore within the bed may be in excess of its entry or limiting suction potential, this will not necessarily collapse or open. Such a waist can only collapse if it adjoins an opened pore, and the pore in question can only open upon the collapse of an adjoining waist. 

Effect of particle size. Reducing the particle size in the bed will reduce the size of the pores and the waists, and will increase the entry suction potential of the waists. This increase means that the percentage variation in suction potentials with depth is reduced, and the moisture distribution is more uniform with small particles. 

Oliver, T. R. and Newitt, D. M. Trans. Inst. Chem. Eng. 27 (1949) 9. The mechanism of drying of solids. Part II. The measurement of suction potentials and moisture distribution in drying granular solids. 

Capillary forces offer a coherent explanation for the drying periods of many materials. If a tapered capillary is filled with water and exposed to a current of air, the meniscus at the smaller end remains stationary while the tube empties from the wider end. A similar situation exists in a wet particulate bed and the phenomenon is explained by the concept of suction potential. A negative pressure exists below the meniscus of a curved liquid surface which is proportional to the surface tension, X, and inversely proportional to the radius of curvature, r. (The meniscus is assumed to be a part of a hemisphere.) This negative pressure or suction potential may be expressed as the height of liquid, expressed by Eq. (25),... 

The suction potential, hj, acting at a depth x below the meniscus is given by Eq. (26). 

Physio- mechanical processes Thermal expansion, diffusion, and convection in fractured rocks and buffer materials Fluid flow in fractured rocks and buffer materials Deformation of fractured rocks and buffer materials Constitutive laws for rock fractures, fractured rock masses, and buffer materials Swelling pressure and suction potential of the buffer materials... 

The osmotic suction potential for clay was measured against the moisture content by Newitt and Coleman [2] and Ohtani et al. [3] in comparison with the capillary suction potential for granular materials. Figure 35.3 [2] shows the relation between the osmotic suction potential and the moisture... 

FIGURE 35.3 Relation between osmotic suction potential and moisture content for china clay. 

The drainage layer consists of appropriate, almost single-size, granular materials, and it is usually 150 or 200 mm in thickness. The depth of the roadside trench below formation level depends on the capillary suction potential of the soil material. 

The osmotic suction potential for clay was measured against the moisture content by Newitt and Coleman [2] and Ohtani et al. [3] in comparison with the capillary suction potential for granular materials. Figure 31.3 [2] shows the relation between the osmotic suction potential and the moisture content for china clay. The potential was observed to be proportional to the exponential of the moisture content. The correlation was influenced significantly by the additives due to electrochemical interactions. The capillary suction potential for the bed of glass spheres of 40 i,m in diameter was much lower than the osmotic suction potential for clay, and it was between 0 and 2500 mm H2O depending upon the moisture content [3]. 

To understand the mechanism further, a microscopic investigation on the moisture content in clay is required. The osmotic suction potential was introduced as the driving force of moisture movement, described in Section 31.3, and was successively applied to the prediction of moisture movement in wet clay [15,16]. The theoretical analysis on the two-dimensional moisture transfer of cylindrical clay was performed taking into account the effects of both osmotic suction and strain-stress caused by the shrinkage [17,18]. However, only the transient mass-transfer equation was analyzed, assuming a constant drying rate on the external surface of the... 

Ohtani, S., Suzuki, M., and Maeda, S., 1963, On the Suction Potential of Granular Material, Kagaku... 

Due to low water contents, high specific surface areas and pore structures (see the previous section), the water-rock interactions within the Tournemire shales must be characterized by strong short range (nanometre-scale) water (or solute) molecules-mineral interactions. Therefore, the physico-chemical characteristics of the water and its solutes will be different from that of free water which is conventionally considered to take part in water-rock interactions (Horseman et al. 1996). The difference arises from factors such as the very low mobility of water in thin films, the high suction potentials developed owing to water-mineral surface electrostatic interactions and the membrane filtration of anions. The 9% porosity given above must be considered as a maximum value since waters bound chemically to mineral surfaces are included in the estimates in reality free waters are of most importance to the present study. 

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