notes1-3 - Soil Mechanics-I(CENG-2202 3 Chapter 3...

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Soil Mechanics-I(CENG-2202) Chapter 3 : Permeability - 36 - Department of Civil Engineering, Faculty of Technology Addis Ababa University 3. ONE-DIMENSIONAL FLOW OF WATER THROUGH SOILS From the discussions in the previous chapter, we have seen that water changes the soil states in fine-grained soils; the greater the water content the weaker it is. Soils are porous materials due to the presence of interconnected void spaces between the solid grains. Hence, particle sizes and the structural arrangement of the particles influence the rate of flow. Water can cause instability and many geotechnical structures such as roads, bridges, dams and excavations have failed due to instability induced by flow of water. It is therefore necessary to estimate the quantity of underground seepage under various hydraulic conditions, for investigating problems involving the pumping of water for underground construction, and for making stability analyses of earth dams and earth-retaining structures that are subject to seepage forces. The key physical property that governs flow of water in soils is permeability. Prior to discussing permeability in detail, we should first note the following key terms: Groundwater is water under the influence of gravity that fills the soil particles. Head is the mechanical energy per unit weight. Hydraulic conductivity (otherwise referred to as the coefficient of permeability ) is a proportionality constant to determine the flow velocity of water through soils. GROUNDWATER If we dig a hole into a soil mass that has all the voids filled with water, we will observe water filling the hole upto a certain level. This water level is called groundwater level or groundwater table and exists under a hydrostatic condition. A hydrostatic condition occurs when there is no flow; i.e. the flow is zero. The top of the groundwater level is under atmospheric pressure. We will denote the ground water level by the symbol T . HEAD From basic fluid mechanics, we know that, according to Bernoulli’s equation, the total head at a point in water under motion can be given by the sum of the pressure, velocity, and elevation heads, or
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Soil Mechanics-I(CENG-2202) Chapter 3 : Permeability - 37 - Department of Civil Engineering, Faculty of Technology Addis Ababa University Z g v u h w + + = 2 2 γ where h = total head u = water pressure v = velocity Z = elevation head or vertical distance of a given point above or below a datum plane For flow of water through soil, the seepage velocity is very small and can be neglected. The total head at any point can thus be adequately represented by Z u h w + = The figure below shows the relationship among pressure, elevation, and total heads for the flow of water through soil. Open stand pipes known as piezometers are installed at the two points. The levels to which water rises in the piezometer tubes situated at the two points are known as the piezometric level of their respective point. The pressure head at a point is the
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This note was uploaded on 11/29/2011 for the course 209 5303 taught by Professor Andre during the Spring '10 term at Iowa Lakes.

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notes1-3 - Soil Mechanics-I(CENG-2202 3 Chapter 3...

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