![]() ![]() Where h = difference of water level of overhead tank and bottom tank. As the length of the soil sample ‘L’ is fixed throughout the test, the hydraulic gradient ‘i’ remains constant all through the test Constant head ‘h’ is maintained throughout the test. Water flows from the overhead tank consists of three tubes: inlet, outlet and overflow tube. The figure 4.3 shows diagrammatical representation of the test. ![]() (i) Computation from grain (K = CD 10 2) size The coefficient of permeability can be determined by the following methods: Method of Determination of Co-Efficient of Permeability: The permeability of the same soil is more when the flow is parallel to the layer than the permeability when the flow is perpendicular to the layer. Layered soil posses different permeability characteristics. Forces of adsorption and development of diffuse ion-layer around the clay particles create immobilized hydrodynamic layers of water, thereby reducing the effective pore space available for seepage. However the pressure of entrapped air in soil pores obstructs the flow of water.įine particles of clay are surrounded by films of adsorbed water. Thus the more the saturated soil, more will be the permeability. The permeability of soil is observed to vary directly with the cube of the degree of saturation. The effect of structural disturbance on permeability is much pronounced in fine grained soils. The permeability of disturbed sample may be different from that of the undisturbed sample at the same void ratio. The structural arrangement of the soil particles vary, at the same void ratio, depending upon the method of compaction of soil mass. (iv) Structural arrangement of soil particles: A plot of log K (log scale) Vg e (linear scale) is approximately a straight line both coarse grained and fine grained soil. A semi-log relationship also exists between K and e. more the void ratio of soil more will be the permeability. The marked influence of void ratio on permeability of soil as shown in the equation 4.3 has been experimentally verified.įrom the above equation it is clear that K is directly proportional to the void ratio i.e. coefficient of permeability than the fine grained soils. A coarse soil has larger pore sizes and here greater K i.e. This is most significant factor affecting permeability of soil as they decide the void ratio, size and shape of pores in a soil mass. Permeability of soil is directly proportional to the square of the particle size as shown in equation 4.3. The viscosity decreases with increase in temperature and therefore permeability increase with increase in temperature. The value of density of water does not change much with the change in temperature but there is a great variation in viscosity. The coefficient of permeability is directly proportional to density of water and inversely proportional to its viscosity. (iv) Structural arrangement of soil particlesįrom equation 4.3 it is clear that the density and viscosity are the two physical properties of pore fluid (or water) which affects the permeability. The factors which affect the permeability are: Permeability can be obtained from the theoretical equation of Kozeny-Carman for flow through porous medium The seepage velocity is also proportional to the hydraulic gradient. Where K = co-efficient of permeability (cm/s)Ĭ = Hazen’s coefficient = 0.8 to 1.2 (1.0 is commonly used) The empirical relationship between K and D 10 developed by Hazen (1911) for loose, clean sand is The unit of K is similar to that of velocity i.e., either m/s or, cm/s etc. co-efficient of permeability, also known as hydraulic conductivity, may be defined as the rate of flow of water under laminar flow conditions through a unit cross-sectional area of a porous medium under a unit hydraulic gradient and standard temperature conditions (usually 27☌ in India). Putting A = 1 and I = 1 in equation we get As (1+e/e) is always greater than unity, Vg is a always greater than V. ![]()
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