Soil water, Capillarity, and permeability

# Held water
The water held by the forces that is other than the force of gravity is called held water.
# Held water is also classified as :
1. Structural water
2. Hygroscopic water
3. Capillary water

# structural water :
Its forms is crystal structure of soil minerals in chemically combine state, such water cannot removed by normal drying process in oven.

# Hygroscopic water :
This type of water can be removed by normal drying process.

# Capillary water :
As we know capillary is th rising phenomenon. Similarly, capilary water is also rising of water from the water table. Such water is called capillary water .

# height of capillary = (4T / dYw) * cosø
Where,
T = surface teinson force in dyne
Ø = Angle of contact
Yw = unit weight of water, gm / cc
d = Diameter of capillary tube in cm

NOTE :  Capillary height is denoted by hc.
At room tempr 20°c
T = 73 dyne/cm  = 73/1000 gm/cm =      0.073 gm/cc.

Yw = 1 gm / cc
Then

->    hc = 0.3/d
Where d is diameter of capillary tube.
From this we cleared that height of meniscus is inversily with diameter. Therefore when the diameter is small then height of capillary may increase.

# An equivalent diameter of the tube is estimated from,
 
      d = 0.2 D10
Where,, D10 is effective size.

From above that is phenomenon of capillary. Then what is the work of capillary in the soil may also we have to cleared. Therefore the void in the soil is act as capillary tubes in the soil.
Therefore in the case of soil,
Height of capillary is given by;

->    hc = C /e x D10

Where,
C = Empirical constant in square of unit of length
e = Void ratio
D10 = Effective diameter

Note :
Water  capillary tube under the negative pressure ie, teinson. At down of the water table there is compressive and it will +ve pressure. Above the water table, due to suction there is - ve pressure is teinson.

Now cleared that, incase of soil the teinson is -ve and compressive is +ve.

(Fig will post soon)

#Soil suction
As before said that the pressure at the above of water table is -ve pressure due to suction.
Soil suction is denoted by Pf. And it is given by:

->    Pf = log10 (hc)

#capillary potential
Capillary potential is the work required to take away unit mass of water from the unit mass of the soil.

        Ų = -p
It is in the negative pressure due to teinson.

# Hydraulic head
Hydraulic height is the difference of the point of two water level ie; from starting point and exist point.

Hydraulic head is given by
h = hp + he + hv
Where, hp is pressure head
       he is the potential head
       hv is velocity head.

Velocity head (hv) is negligible nearly equal to zero that is neglected. So the equation will be
    h= he + hp

# Permeability
You have seen the water falling from voids of any material may be granite bed, stiff cement etc. The water fall from the void is known as the permeability. As a civil engineer we must have all concept of permeability. Because in any structure you have seen seepage. That is due to permeability. Due to seepage pressure the permeability occurs.

# Darcy law
Water flow through the pipe may be laminar or turbulant. If the water flow straightly then we easily called it laminar or if zig zag direction we called tubulant. Also by reynolds number we can evident about laminar flow and turbulant flow.
From reynolds number,
        Re = V×D/v
Where,
   V = avg velocity
  D = diameter
  v = kinematic viscosity = density / dynamic viscosity

The Re is less than and equal to 2000 is called laminar flow

The Re is greater than 2000 is called turbulant flow.

Now, Darcy in 1856, given law. One of the limitation of darcy law is that it is only applicable in the laminar flow not in the turbulant flow.
He said that the velocity is directly propotion to the hydraulic gradient.

         v = Ki
Where, k is proportionality cinstant is called coefficient of permeability.

i = hydaulic gradient
q = A× kx i
Where, A is area of normal to flow.
       And q = discharge

Note : The velocity which is refered by the darcy is called seepage velocity.