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Module 1: Flow Behaviour of Unsaturated Soils

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Steady-State Flow of Water in Soils
The water flow through soils takes place due to the total head difference between two different points within the soil mass. The total head = Matric suction head + Osmotic suction head + gravity head.The water flow through soils can be either Steady-state or Transient. The steady-state flows in unsaturated soils depend on the boundary conditions and type of soil. In a steady-state flow through soils, the water flux does not vary with time.
For the hydraulic conductivity of an unsaturated soil to be obtained, particular water content and suction head must be maintained.
Steady-State & Transient Flow When there are no chemicals present in the pore solution of soil, the osmotic head component is 0. Also, if there is horizontal flow in a soil, its elevation head component is 0. So, if a soil has no chemical present and undergoes horizontal flow, its total head will be only matric suction head. In order to obtain how the matric suction head varies with elevation for a steady-state vertical flow in unsaturated soils, the steady flux, Soil-Water Characteristic Curve, and Hydraulic Conductivity Function must be known. The transient flows in unsaturated soils are time-variant flows. In this transient flow, the flux varies with time. The transient flow condition in soils occurs before the steady-state flow condition is achieved.
Analytical Methods for Transient Flow
In 1907, the work of Buckingham on the movement of soil moisture was published. It involved establishing the rate of capillary rise in soils. In 1943, Terzaghi derived an analytical solution to determine the rate at which the capillary rise occurs in soils. Researchers found that Terzaghi's expression over estimates capillary rate in unsaturated soils because of its limitations. One of such limitation is that it assumes the saturated hydraulic conductivity for the soils. In 2004, Lu and Likos derived an expression that better predicted the rate of capillary rise in coarse-grained soils. Richard’s expression for transient flow through unsaturated soils is one that is commonly in use now. The expression is based on generalized Darcy’s law and mass conservation expression.