Wednesday, 4 April 2018

Permeability of Soil,Darcy's law-Limitation,Co-Efficient of Permeability,Importance of Study of Seepage Analysis


Permeability is defined as the property of a porous material which permits the passage or seepage of water (or other fluids)through its interconnecting voids.
A material having continuous voids is called permeable .Gravels are highly permeable ,while stiff clay is the least permeable.


                                                                                                 The study of seepage of water through soils is important for the following engineering problems:
1)Determination of rate of settlement of a saturated compressible soil layer 
2)Calculation of seepage through the body of earth dams,and stability of slopes.
3)Calculation of uplift pressure under hydraulic structure and their safety against piping 
4)Ground water flow towards wells and drainage of soils.


                               The law of flow of water through soil was first studied by Darcy.This law states that for laminar flow conditions in a saturated soil mass, the rate of flow or discharge per unit time (q) is proportional to the hydraulic gradient (i). If q is the discharge per unit time through a soil mass of cross sectional area A under hydraulic gradient i ,then 
                                             q ∞ i.A
                                        or, q= K.i.A 
Where ,
             K is a constant ,known as co-efficient of permeability.
If a soil sample of length 'L' and cross sectional area 'A' (measured perpendicular to the direction of flow) is subjected to differential head of water (h1-h2), then
                                                               h1 - h2
                                                        i =------------
                                               h1 -h2
                           &    q = K.------------- .A

LIMITATION OF DARCY'S LAW:- Darcy's law is valid only for laminar flow condition in the soil mass.


                                                                        The co-efficient of permeability is defined as the average velocity of flow that will occur through the total cross sectional area of the soil mass under unit hydraulic gradient.It is denoted by 'K' Its unit is the same as the unit of velocity such as Cm/sec or M/sec or M/Hr. etc .

MORE:-Factors Affecting The Permeability

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