Soil water: Water present in the void spaces of a soil mass is called soil water

Soil water may be In the form of free water , gravitational water and held water

Free surcace water

Gravitational water – water that drains freely through the soil by force of gravity

HELD WATER: 1. adsorbed water is thin film of around the mineral grain

2. Structural water: water that is chemically combined as a part of the crystalline structure ofthe mineral of the soil grains.

Capillary tightly in small capillary pores by H bonding

Height of capillary rise inversely related to radius of the tube

Smaller pore diameter, greater the movementwater – held

• Capillary water & Saturation Percentage

– Capillary water – held tightly in small capillary pores by H bonding

• Water in minute tubes that will rise through soil matrix to needed areas

• Height of capillary rise inversely related to radius of the tube

– Smaller pore diameter, greater the movement

What is permeability?

• Property of a soil which permits the flow of water

• Permeability is defined as the property of a porous

material which permits the passage or seepage of water

through its interconnecting voids.

• It is a very important Engineering property

gravels highly permeable

stiff clay least permeable

• Calculation of uplift pressure under hydraulic structure and

their safety against piping

• Ground water flow towards wells and drainage of soil

• Calculation of seepage through the body of earth dams and

stability of slopes

• Determination of rate of settlement of a saturated

compressible soil layer

Permeability through soil is important for the

following engineering problems:

Flow of water through soils may either be a laminar flow or a turbulent flow

Each fluid particle travel along a definite path which never crosses the path of any other

particle

Paths are irregular and twisting, crossing at

random

Permeability

• Three types of permeability

• Absolute permeability - the permeability of a porous medium with only one fluid present (single-phase flow).

• When two or more fluids are present permeability of the rock to a flowing fluid is called effective permeability (ko, kg, kw).

• Relative permeability is the ratio of absolute and effective permeabilities kro=ko/k, krg=kg/k, krw=kw/k.

Coefficient Of Permeability

Depends not only on the

properties of soil but also on the properties of water

Absolute permeability

Independent of the

properties of water

It depends only on the

characteristics of soil

The absolute permeability

only depends on the

geometry of the pore-

channel system.

Relative permeability is the ratio of

effective permeability of a particular fluid to its

absolute permeability.

DARCY’S LAW

The law of flow of water through soil was first studied by Darcy in 1856.

The Darcy’s law is,

“For laminar flow through saturated soil mass, the discharge per unit time is

proportional to the hydraulic gradient”.

q = k.i.A

= k.i = v

v = k.i ……. Darcy’s Law

Where, q = Discharge per unit time (rate of flow)

A = Total c/s area of soil mass

i = Hydraulic gradient = h/L

k = Darcy’s coefficient of Permeability

v = Velocity of flow (discharge velocity)

FACTORS AFFECTING PERMEABILITY OF SOILS

The following factors affect the permeability of soils:-

1. Particle size

2. Properties of pore fluid.

3. Void ratio of soil.

4. Shape of particles.

5. Structure of soil mass.

6. Degree of saturation.

7. Absorbed water.

8. Impurities in water.

Variable-head Permeability

Test

Determination Of Coefficient Of

Permeability

Laboratory Methods

Constant-head Permeability

Test

Typical Soil Permeability

The constant head test method is used for permeable soils (k>10-4

cm/s), and the falling head test is mainly used for less permeable soils

(k<10-4 cm/s).

Test Apparatus• Permeameters

• Ruler

• Tamper

• Balance

• Watch (or Stopwatch)

• Thermometer

• Filter

Constant Head Permeability Test

Test Procedure:-1. Measure internal dimensions of the mould. Apply a little grease on the inside to the

mould.

2. Take about 2.5kg of the soil, from a thoroughly mixed wet soil, in the mould.

Compact the soil at the required dry density using a suitable compacting device.

3. Remove the collar and base plate. Trim the excess soil level with the top of the

mould.

4. Clean the outside of the mould. Find the mass of the soil in the mould. Take a

small specimen of the soil in container for the water constant determination.

5. Saturate the porous stones.

6. Place the porous stone (disc) on the drainage base and keep a filter paper on the

porous stone.

7. Place the mould with soil on the drainage base.

8. Place a filter paper and a porous stone on the top of specimen.

9. Connect the constant head tank to the drainage cap inlet.

10. Open the stop cock, and allow the water downward so that all the air is removed,

then close the stop cock.

11. Now, again open the stop cock and at the same time start the stopwatch. Collect

the water flowing out of the base in a measuring flask for some convenient time

interval.12. Measure the difference of head (h) in levels between the constant

head tank and the outlet in the base.

•c/s area of specimen = A = (π/4) x D2 (cm2)

•Volume of mould = V = (π/4) x D2 x L (cm3)

•Mass of wet soil in the mould = M = M2 – M1

Where, M1 = mass of empty mould.

M2 = mass of mould + wet soil

•Bulk density of soil, = _____ gm/cm3

• Dry density of soil, = _____ gm/cm3

• Quantity of water that flows under a given hydraulic gradient

through a soil sample of known length & cross sectional area in a

given time

• Water is allowed to flow through the cylindrical sample of soil

under a constant head

• For testing of pervious, coarse grained soils

K = Coefficient of permeability

Q = total quantity of water

t = time

L = Length of the coarse soil

Aht

QLk

FALLING HEAD PERMEABILITY TEST :-

Object:-

To determine the coefficient of permeability of a soil specimen by falling head

method.

Equipments:-

All the equipments required for the constant head permeability test.

Test Procedure:-

1. Prepare the remoulded soil specimen in the permeameter and saturate it.

2. Keep the permeameter mould in the bottom tank and fill the bottom tank with

water up to its outlet.

3. Connect the water inlet nozzle of the mould to the stand pipe filled with water.

Permit water to flow for some time till steady state of flow is reached.

4. Now open the valve of stand pipe and record the time (t) to fall the head from h1

to h2. Repeat this step at least twice.

Normally,

Constant head permeability test is used for more permeable soils like sand.

Falling head permeability test is used for less permeable soils like clay.

• Relatively for less permeable soils

• Water flows through the sample from a standpipe attached

to the top of the cylinder.

• The head of water (h) changes with time as flow occurs

through the soil. At different times the head of water is

recorded.

t = time

L = Length of the fine soil

A = cross section area of soil

a= cross section area of tube

K = Coefficient of permeability

2

110log

30.2

h

h

At

aLk

Permeability in Stratified Soils

H

HkHkHkk

iHkHkHkiHkq

kiAq

nnx

nnx

2211

2211 )(

Flow parallel to the plans of stratification

Flow normal to the plans of stratification