If the soil contains dispersed structure, the particles are in face to face orientation hence, permeability is very low. The permeability of stratified soil deposits also varies according to the flow direction. If the flow is parallel, permeability is more. If it is perpendicular, permeability is less. Partially saturated soil contain air voids which are formed due to entrapped air or gas released from the percolating fluid or water.
This air will block the flow path thereby reduces the permeability. Fully saturated soil is more permeable than partially saturated soil. Various properties of water or fluid such as unit weight and viscosity also effects the permeability. However, unit weight of water will not affect much since it does not change much with temperature.
But when temperature is increased viscosity decreases rapidly. From equation 1 , permeability increase when viscosity decreases. Temperature also affects the permeability in soils. From equation 1 , permeability is inversely proportional to the viscosity of the fluid. It is known that viscosity varies inversely to the temperature. Hence, Permeability is directly related to temperature. Greater the temperature, higher will be the permeability.
That is the reason, seepage is more in summer seasons than in winter. Adsorbed water is the water layer formed around the soil particle especially in the case of fine-grained soils. If a soil mass has stratified structure then its permeability varies according to the direction of flow. It has been observed that when the flow of water is parallel to the bedding planes the permeability of the soil mass is higher than when water flows perpendicular to the bedding planes.
Including these factors permeability of soil deposit also depends upon the structural defects like cracks or fissures in the soil mass. If volume of voids in the soil increases the flow path becomes wider and voids interconnectivity increases. Hence permeability of soil increases. So in general we can say permeability of a soil mass increases with the increase in its void ratio and decreases when voids ratio decrease.
But it is not true for all types of soils. Clay soils have higher void ratio because of their flocculated structure but still their permeability is very low because the flow path through voids in case of clays is extremely small and poorly connected.
Fine grained soil particles carry charges on their surface and because of that dipolar water molecules are strongly attached to their surface. This attached water is called adsorbed water. As this water is bounded by electrical forces it is not free to move under gravity.
This adsorbed water layer causes an obstruction to the flow of water in the pores by blocking the voids or reducing their effective size and hence reduces the permeability of soils. The saturation of the soil mass also affects its permeability.
If the soil is partially saturated then it may have some voids which contain entrapped air. These entrapped air pockets may block the flow path which may reduce the permeability of the soil. While if the soil is fully saturated there will not be any such blockage.
Hence we can say the permeability of a partially saturated soil is smaller than that of fully saturated soil. Permeability of the soil not only depends on the soil properties but also depends upon the properties of its permeant that is water. Let us consider the viscosity of the water. Start Now. Get Started for Free Download App.
More Permeability Questions Q1. The permeability of coarse grained soil depends:. Constant head permeability test 1. Specific gravity 2. Consolidation test 2. Grain size analysis 3. Pycnometer test 3. Permeability and factors affecting permeability. Upcoming SlideShare. Like this presentation? Why not share! Abdulmannan Orabi views. Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Next SlideShares. Download Now Download to read offline and view in fullscreen.
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