Determination of transmissivity of a highly permeable soil in an aquifer
INTRODUCTION
Transmissivity (or hydraulic conductivity) refers to the ease with which water can flow through a soil. This property is necessary for the calculation of seepage through earth dams or under sheet pile walls. Transmissivity is useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal banks affected by seepage, settlement etc. The falling head method of determining transmissivity is used for soil with low discharge, whereas the constant head permeability test is used for coarse-grained soils with a reasonable discharge in a given time. For very fine-grained soil, a capillary permeability test is recommended. Usually, transmissivity of soils is determined by two methods:
- Constant head method
- Falling Head method
As we are dealing with soils in and around aquifers, where the soil is usually highly permeable, we will perform constant head test.
Darcy’s Law
Darcy’s law studied the laminar flow of fluid in a homogeneous soil profile and demonstrated that the velocity of flow (v) is directly proportional to the hydraulic gradient (i). i.e. v = ki, where k is a constant called coefficient of permeability. This velocity of flow is also called as superficial velocity or discharge velocity. If the discharge velocity (v) is known, the discharge (q) can be obtained as: q = vA = kiA, where, ‘A’ is the cross-sectional area of soil including both the solids and voids. When, the hydraulic gradient is equal to 1, k = v;
Hence, the coefficient of permeability can be defined as the velocity of flow occurring in the soil for a unit hydraulic gradient. The unit of coefficient of permeability is mm/sec or cm/sec or m/day.
Formulas Used:
Where, Q=amount of water collected in graduated cylinder,
L = Distance between base of manometers,
t = fixed time interval for collection of water,
A = Area of the mould,
∆H = Difference in energy between both manometer levels
Where, μT = coefficient of viscosity at T degree Celsius,
μ27 = coefficient of viscosity at 27 degree Celsius
Where K = coefficient of permeability,
D = Aquifer thickness