Permeability Test

Transport of water through soil media depends on the pressure head, velocity head, and the potential head due to elevation. In most cases, the most important parameter is the potential head due to elevation.
See Fig. 1.43. Water travels from point A to point B due to high potential head.

The velocity of the traveling water is given by the Darcy equation.

v = k x i

where

v - velocity
k = coefficient of permeability (cm/sec or in./sec)
i= hydraulic gradient = h/L
L = length of soil
Q -- A x v = volume of water flow
A -- area
v- velocity

Design Example
Find the volume of water flowing in the pipe shown in Fig. 1.44. The soil Permeability is 10 -5 crn/sec. The area of the pipe is 5 cm 2. The length of the soil plug is 50 cm.

Solution:

Apply the Darcy equation

v = k x i
v = k x (h/L)
v - 10 -5 x 20/50 - 4 x 10 -6 cm/sec
volume of water flow- A x v - 5 x 4 x 10 -6 cm3/sec
= 2 x 10 - s cm3/sec

Liqued limit and Plastic limit

Water flowing through soil

Water flowing through soil

Water flowing due to gravity

Seepage Rate:

Water movement in soil occurs through the voids within the soil fabric. The more voids there are in a soil, the more water can flow through. This seepage path can be seen in Fig.

Seepage Path

The velocity of water seepage through a soil mass is dependant upon the void ratio or porosity of the soil mass. This seepage velocity can be seen in Fig. 1.46. The volume of water traveling through the soil is

Seepage Velocity

where

A = area

v = velocity

The porosity (n) of a soil is defined as

n= Vv / V

where

Vv - volume of voids = L x Av

Av - area of voids

V = total volume -- L x A

L -- length

A --total cross-sectional area

Hence

n -- (L x Av)/(L x A) = Av/A

Av--n x A

Q=v x A

The velocity of water traveling through voids (v~) is known as the seepage velocity.

Q=vs x Av

Q = v x A = vs x Av = vs X (n x a)

v x a - vs X (n x a)

Vs - v/ n