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Unit - 2

Permeability and Seepage

Q1) Explain the classification of soil water?

A1) Classification of soil water are as follows:

1. Broad classification:

Free water

Held water

2. Phenomenological basis:

Ground water

Capillary water

Absorbed water

Infiltered water

3. Structural aspect:

Pore water

Solvate water

Absorbed water

structural water

Q2) What is the definition of permeability?

A2)

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".

If water flow easily through voids of soil, then soil is highly pervious or permeable and when the permeability is extremely low it is called impervious or impermeable.

Q3) Explain Darcy’s law?

A3)

In 1856 Darcy studied and demonstrated experimentally, the velocity of laminar flow through homogeneous soil mass, which states that, "The rate of flow or the discharge per unit time is proportional to the hydraulic gradient."

q= K.i.A

V=q/A

Vi, V=Ki

Where,

q= discharge per unit time

A= total c/s area of soil mass, perpendicular to the direction of flow

i =hydraulic gradient K Darcy's coefficient of permeability

v =velocity of flow, or average discharge velocity

if a soil sample of length 1 and cross-sectional area A, is subjected differential head of water hA-hB, the hydraulic gradient i will be equal to (hA-hB)/L and

we have,

q=KA (hA-hB)/L

when hydraulic gradient i = 1, then k=v

Q4) What are the factors which affects permeability?

A4) Following are the main factors that affect permeability:

Grain size:

Grain size of the soil, or the effective size Dio is one of the factors which affect permeability. Allen Hazen gave the relation.

K = C(D10)2

where, K Coefficient of permeability in cm/s and D10 is the effective grain size of the soil, C= constant (between 100 to 150)

The permeability of coarse-grained soil is more than that of fine-grained soil

Properties of pore fluid:

Permeability is directly proportional to the unit weight of pore fluid and inversely proportional to the viscosity of the pore fluid.

Temperature:

Since viscosity of the pore fluid decreases with the temperature, permeability increases with temperature, as unit weight of the pore fluid does not change much with change in temperature.

Void ratio:

Increase in void ratio increases the area available for flow, hence the permeability increases for critical conditions.

Ke2

Stratification of soil:

Stratified soils are those, which are formed by layer upon layer of earth or dust deposited upon one another.

If the flow is parallel to the layers or stratification, the permeability is maximum while the flow in perpendicular direction to the stratification occurs with minimum permeability.

Entrapped air and organic impurities:

Organic impurities and entrapped air obstruct the flow and coefficient of permeability is reduced due to their presence.

Adsorbed water:

Adsorbed water means a thin, microscopic film of water surrounding individual soil grains.

This water is not free to move and hence reduces the effective pore space and thus decreases the coefficient of permeability.

Degree of saturation:

The permeability of partially saturated soil is less than that of fully saturated soil.

Shape of particles:

Permeability is inversely proportional to the specific surface e.g., the angular particles have more specific surface as compare to rounded particles. Therefore, the soil having angular particles is less permeable than soil of rounded particles.

Structure of soil mass:

For same void ratio the permeability is more for flocculant structure as compare dispended structure.

Q5) Explain Constant Head method?

A5)

It is based on measuring the volume of water flowing through a soil sample of known cross sectional area A and length L in time t under a constant head h of water.

The arrangement is shown in Fig.

$H:\unit2\IMG_20210518_181333.jpg$

Fig.: Constant head permeability test

The soil sample is enclosed in a perspex cylindrical tube. A number of manometer points are provided of the side of the cylinder. These can be used in pairs.

One such pair is shown in the figure.

Water is allowed to flow through the soil at a constant head 'h' as indicated by the level difference in manometer tubes.

The quantity of water or volume of water V collected in time t is measured, from which discharge q is calculated. The coefficient of permeability is then calculated as,

K= =

Where q=v/t=discharge in m3/s

V=volume of water collected in m3 in time t sec

A=cross sectional area of specimen in m

h =level difference between the manometer tubes in m.

L =distance between manometer points in m

Vacuum is used at the start of the test to ensure that all entrapped air from soil sample is removed keeping valves A and B closed.

Then the valve C is closed. Valve A is used to control the flow so that a constant head flow is achieved as indicated by constant level difference 'h' between the manometer tubes.

This test can be used for highly pervious soils where falling head test cannot be used due to very rapid fall in head, which cannot be measured.

This test cannot be used for soil with very little permeability or highly impervious soils like clay as rate of collection of water may be very low, sometimes even days, so that the rate of evaporation may be higher.

This will give wrong results as much of the volume of collected water will evaporate.

Q6) Explain Falling head method?

A6)

This test is used to determine the permeability of fine-grained soils with very low permeability.

Due to low permeability, the collected water will evaporate and will lead to wrong results in constant head test. Hence, this: test is used.

The arrangement is shown in Fig.

$H:\unit2\IMG_20210518_181400.jpg$

Fig.: Falling head permeability test

A cylinder containing the soil sample is placed on a base, i.e., a perforated disc, fitted with a fine gauze. The cylinder has a rubber stopper at top in which a graduated stand pipe is fitted.

The test is conducted by filling the standpipe with de-aired water. It is allowed to flow through the soil sample.

During the test, the water level in the stand pipe continuously drops and the height of the water in stand pipe is measured at several time intervals and it is then recorded. Any one pair of measurements can be used to calculate the coefficient of permeability from the following formula:

K =

Where, a =area of c/s of stand pipe.

A =area of soil sample.

L = length of soil specimen

and h1, h2=heights of water measured in the stand pipe at time t1 and t2

If t2-t1 then

K = 2.3

Several pairs of readings are used in this way and average of 4-5 values gives the average coefficient of permeability.

Q7) Explain Open end test?

A7)

In open-end test, a pipe casing is inserted into the hole of bore up to the required depth and then pipe casing is entirely cleaned out.

Fig. Show the open-end test.

$H:\unit2\IMG_20210518_181521.jpg$

Fig.: Open end test (constant discharge)

At the time of cleaning, the hole is kept under water if it extends below the level of water table, which prevents the possibility of squeezing of the soil into the bottom of the pipe casing at the time of removing i the driving tool.

After cleaning the hole, hole is filled with water by a metring system. When the steady conditions are developed, that time the constant rate of flow or discharge (q) is found

The coefficient of permeability is then found by the following expression.

K =

Where, K = coefficient of permeability

H = difference of level between the inlet of casing and water table

q = discharge

r= inner radius of pipe casing

As per the requirements, the discharge 'q' can be increased by pumping-in water under pressure 'p' as shown in Fig. 2.6.6 and then the value of H becomes equal to (H+

)

Note that for better results, the lower end of the pipe must be at a distance not less than 10r from the top and bottom of the strata.

The open-end test can also carry out above W.T i.e., water table as shown in Fig.

When this test is carried out above water table, it is very difficult to make up the constant water level in pipe casing and hence some surging of this vel has to be tolerated.

(k=

can also used in this case, However, H is equal to the difference of inlet level and the bottom end of the pipe.

As per the requirement, discharge can be increased by pumping in water under a pressure 'p' and that time total head becomes equal to (H+

)

Q8) Explain Seepage pressure?

A8)

There is an energy transfer between the water and the soil due to the viscous friction exerted on water flowing through the soil pores.

The pressure exerted by water on the soil through which it percolates is known as seepage pressure (Ps).

It is given by,

Ps=h

Ps = ×L

Where, h=Hydraulic head,

Z= Length over which the head (h) is lost,

i = Hydraulic gradient,

=Unit weight of water

Seepage force (F) is given by,

Fs= Ps. A= i.Z.A

Where A= Total cross-sectional area of the soil mass

=Unit weight of water

The seepage pressure always acts in the direction of the flow.

The effective pressure (P) in the soil mass is given by

Pe=Z± iz

For upward flow,

Pe=Z- iz

In upward direction, the effective pressure is decreased hence-ve sign.

For downward flow,

Pe=Z+iz

In downward direction, the effective pressure is increased hence +ve sign.

Q9) Explain Quick Sand phenomenon?

A9)

When flow takes place in an upward direction, the effective pressure gets reduced since the seepage pressure also acts in the upward direction.

When the seepage pressure becomes exactly equal to the submerged weight of the soil, through which the flow is taking place, the effective pressure becomes zero.

In this case, the soil with less cohesion loses all its shear strength and soil particles move up in the direction of flow. This lifting of soil particles is known as quick sand condition or boiling sand condition during this condition the effective pressure reduces to zero.

i= ic =

The hydraulic gradient of the quick sand condition is known as the critical hydraulic Gradient(ic).

Thus, quick sand condition is the particular flow condition which occurs when effective pressure reduces to zero during upward flow.

Fig.: Quick Sand Condition

Q10) What is flow net?

A10)

When water flows through Soil in laminar flow conditions, the paths along which the layers of water flow are called flow lines.

Equipotential lines are the lines joining points of equal pressure along the flow lines. These can also be defined as lines along which the points have equal head of water.

If piezometers or glass tubes are inserted along an equipotential line, water will rise to the same height in the tubes.

Equipotential lines are always perpendicular to the flow lines.

Analysis of seepage phenomenon is simplified by using the graphical solutions. One of the most use graphical tool is "Flow Net".

The grid, mesh or net formed by the intersection of equipotential lines and flow lines is called flow-net.

The portion of a flow net between two adjacent flow lines is called a low channel.

Every section of a flow channel between two successive equipotential line is called a field.

$H:\unit2\IMG_20210518_181959.jpg$

Fig.: Flow lines and equipotential line

$H:\unit2\IMG_20210518_182024.jpg$

Fig.: Flow net

Q11) Explain the properties of flow net?

A11)

In a flow-net, flow lines and equipotential lines intersect each other at right angles.

The quantity of water flowing through each flow channel is the same.

The drop of head or potential drop between any two successive equipotential lines is the same.

The fields are approximately squares.

The flow net is representative of the flow pattern and dissipation of the hydraulic head see fig below.

$H:\unit2\IMG_20210518_182056.jpg$

Fig.: Part of flow net illustrating characteristics

Q12) What are the applications of flow net?

A12) A flow net chart is used for following applications:

Determination of discharge.

Determination of total head.

Determination of pressure head.

Determination of hydraulic gradient.

For calculating quantity of seepage.

For stability of earthen dam.

For analysis of flow phenomenon.

For stability analysis.

Q13) Explain flow net construction for flow under sheet pile?

A13)

ABC is a sheet pile driven in pervious layer of depth d, the depth of embedment BC being d’.

UB and TV are potential boundaries with known heads. BCST and xy are flow line boundaries because these are impervious surface across which water cannot pass.

Flow lines will emerge at right angles from UB and end at right angles at TV. Because of symmetry flow not will symmetrical, about vertical axis:

Procedural steps:

Make a scale drawing showing the structure, soil mass, the pervious boundaries and the impervious boundaries.

Sketch the first trial flow line as F1f1, emerging at right angles at UB running round the sheet pile and meeting TV at right angles.

Sketch the first equipotential line p1p1 so as to make a square figure with the flow line f1f1 and the boundary BC and ending at right angles to the boundary flow line, Now sketch the second flow line ff again emerging at right angles with UB making square figure with P1p1 going round the sheet pile parallel to the first line f1f1and ending at right angles to TV.

Then sketch the next trial potential line, emerging at right angles from E, making square figures with neighboring flow lines and meeting xy at a right angle. Continue sketching to complete the flow net.

Now, check (by drawing circles touching all the sides of the square) the square figures and orthogonality everywhere in the flow net. The first attempt can hardly produce a good flow net. Make a second, third and if required more attempts, till a reasonably good flow net has been sketched.

$H:\unit2\IMG_20210518_182121.jpg$

Fig.: Flow net for sheet pile

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