PART A: CONSTANT HEAD PERMEABILITY TEST

1.0 OBJECTIVE

To determine permeability of sands and gravels containing little or no silt.

2.0 LEARNING OUTCOME

At the end of this experiment, students are able to:

Describe the procedure to determine the coefficient of permeability of sands and gravels

based on ASTM D2434.

Identify the relationship between permeability and pore size of the coarse grained soils.

Measure the coefficient of permeability of sands and gravels containing little or no slit.

3.0 THEORY

The most common permeability cell (permeameter) is 75mm in diameter and is intended

for sands containing particles up to about 5mm. A larger cell, 114mm, can be used for testing

sands containing particles up to about 10mm, i.e. medium gravel size. As a general rule the

ratio of the cell diameter to the diameter of the largest size of particle in significant quantity

should be at least 12.

The constant head permeability cell is intended for testing disturbed granular soils which

are recompacted into the cell, either by using a specified compactive effort, or to achieve a

certain dry density, i.e. void ratio.

In the constant head test, water is made to flow through a column of soil under the

application of a pressure difference which remains constant, i.e. under a constant head. The

amount of water passing through the soil in a known time is measured, and the permeability of

the sample is calculated by using Equation (1).

If the connections to the cell are arranged so that water flows upwards through the

sample, the critical hydraulic gradient can be determined after measuring the steady state

permeability, and the effects of instability (boiling and piping) can be observed. It is important

that use only air-free water, and measures for preventing air bubbling out of solution during

these tests is very crucial.

………..Eqn (1)

Where: q = rate of flow,

A = area of sample,

i = hydraulic gradient,

=

h1 - h2 = head difference between 2 reference points

L = distance between 2 reference points

3.0 TEST EQUIPMENTS

1. Constant head permeability cells, fitted with loading piston, perforated plates, flow tube

connections,

2. piezometer nipples and connections, air bleed valve, sealing rings. Figure 1 shows

permeameter

3. cells that commonly used in laboratory testing.

Figure 1: Permeameter cells for constant head test: (a) 75mm, (b) 114mm

(Courtesy of ELE International, 2007)

5.0 PROCEDURES

1. Prepare permeameter cell,

a. Remove the top plate assembly from the cell.

b. Measure the following dimensions:

i. Mean internal diameter (D mm),

ii. Distance between centres of each set of manometer connection points

along the axis of the cell (L mm),

iii. Overall approximate internal length of cell (H1 mm),

c. Calculate the following based on measured dimensions:

i. Area of cross-section of sample, A = D2/4 mm2

ii. Approximate mass of soil required, to fill the permeameter cell,

V = A H1/1000 cm3

iii. Approximate mass of soil required, if placed at a density Mg/m3,

mass = A H1/1000 g

2. Select sample,

a. Air-dry the soil which the test sample is to be taken.

b. Sieve the soil sample and any particles larger than 5 mm need to be removed by

sieving.

c. The material needs to be reduced by the usual riffling process to produce several

batches of samples each about equal to the mass required to fill the

permeameter cell

3. Prepare sample,

a. The sample may be placed in the permeameter cell by one of three methods:

i. Compacting by rodding,

ii. Dry pouring,

iii. Pouring through water

4. Assemble cell

a. Place a second porous disc (if one has already been used) and the second wire

gauze disc on top of the soil, followed by about 40mm thickness of glass balls or

gravel filter material,

b. The level of the top surface of the filter should be within the limits required to

accommodate the top plate,

c. Slacken the piston locking collar on the cell top, pull the piston up as far as it will

go, and re-tighten the locking collar,

d. Fit the cell top on the cell and tighten it down into place by progressively

tightening the clamping screws,

e. Release the piston locking collar and push the piston down until the perforated

plate bears on the filter material,

f. Hold it down firmly while the locking collar is re-tightened

5. Connect up cell

a. Connect the nozzle at the base of the cell to the de-aired water supply, and close

the inlet cock,

b. Connect each piezometer point that is to be used to a manometer tube and close

with a pinchcock close to the cell,

c. Connect the top outlet of the cell to the vacuum, fitted with a water trap, using

rigid plastic or thick-walled rubber tubing

d. Close the air bleed screw on the cell top

6. Saturate and de-air sample

7. Connect up for test

8. Run test

a. Turn on the supply of de-aired water to the constant head device, which be at a

low level initially,

b. Open water supply valve that connect it to the cell, and the base outlet cock

c. Allow water to flow through the sample until the conditions appear to be steady

and the water levels in the manometer tubes remain stationary

d. Adjust valve on the supply line to the constant head device so that there is a

continuous small overflow; if this is excessive, the de-aired water will be wasted.

e. To start a test run, empty the measuring cylinder and start the timer at the instant

the measuring cylinder is placed under the outlet overflow.

f. Record the clock time at which the first run is started.

g. Read the levels of the water in the manometer tubus (h1, h2, etc) and measure

the water temperature (TC) in the outlet reservoir.

h. When the level in the cylinder reaches a predetermined mark (such as 50ml or

200ml) stop the clock, record the elapsed time to the nearest half second,

9. Repeat test

a. Emtpy the cylinder, and make four to six repeat runs at about 5 minutes

intervals.

10. Dismantle cell

11. Calculate results

12. Report

Figure 2: General arrangement for constant head permeability test (downward flow)

(Courtesy of ELE International, 2007)

6.0 RESULTS

Constant Head Permeability test

Location: Sample no:

Operator: Date:

Soil description:

Method of

preparation:

Sample diameter: 80 mm Sample length: 232 mm

Sample area, A: 5026 mm2 Sample volume: 1166 cm3

Sample dry mass: 1844 g Sample dry density: 16.19 kN/m3

S.G. measured/assumed: 2.7 Voids ratio:

Heights above datum: inlet mm

Heights above datum: outlet mm

Manometer a: 857 mm

Manometer b: mm

Manometer c: 814 mm

Head difference a to c: 43 mm Distance difference: 145 mm

Flow upwards/downwards Hydraulic gradients: 0.297

Temperature:

Reading:

Time from start

min.

Time interval, t

min.

Measured flow, Q

ml

Rate of flow, q = Q/t

ml/min

Remarks

0.30 0

0.60 0.30 200 666.67 1.83

0.90 0.30 200 666.67 1.83

1.20 0.30 200 666.67 1.83

1.52 0.32 200 625.00 1.77

1.82 0.30 200 666.67 1.83

2.12 0.30 200 666.67 1.83

2.40 0.28 200 714.29 1.89

2.73 0.33 200 606.06 1.74

7.0 CALCULATION

Permeability, k =

Hydraulic gradient, i = (h1 – h2) / L

= 0.043 / 0.145

= 0.297

Ai = 5.026 x 10-3 x 0.297

= 1.49 x 10-3 m2

q = 6.67 x 10-4 m3 / 60 s

= 1.11 x 10-5 m3/s

k = 1.11 x 10 -5 m 3 /s

1.49 x 10-3 m2

= 7.45 x 10-3 m/s

q = 6.25 x 10-4 m3 / 60 s

= 1.04 x 10-5 m3/s

k = 1.04 x 10 -5 m 3 /s

1.49 x 10-3 m2

= 6.98 x 10-3 m/s

1ml = 1cm3

1m3 = 100cm3

Average of permeability = ∑ K / number of k = 0.059 / 8

= 7.375 x 10-3 m/s

q = 7.14 x 10-4 m3 / 60 s

= 1.19 x 10-5 m3/s

k = 1.19 x 10 -5 m 3 /s

1.49 x 10-3 m2

= 7.99 x 10-3 m/s

q = 6.06 x 10-4 m3 / 60 s

= 1.01 x 10-5 m3/s

k = 1.01 x 10 -5 m 3 /s

1.49 x 10-3 m2

= 6.78 x 10-3 m/s

8.0 DISCUSSION

The coefficient of permeabilty for this sample of soil is 7.375 x 10-3 m/s. The value we

get is suitable because during test there should be no volume change in the soil and the factor

that influence the value such as size of the sample and effective air void. Therefore, the time we

have taken is fast because the water permeability of the soil is less. The rate of flow we get is

higher value.

The constant-head method is limited to disturbed granular soils containing not more than

10% passing the No. 200 sieve.

9.0 CONCLUSION

From the experiment, we get the time is found to be constant at volume of water. The

time we get is faster. This is because the permeability of the gravel soil absorbs the water is

low. This gravel soil has a large molecular space. Therefore, the water diffusion rate is low. It

appears to be a function of three factors for a constant paste amount and character: effective air

void content, effective void size and drain down. From the coefficient of permeability for the

given sample of soil value, we can say that the rate of flow the sample has get the value higher.

10.0 REFERENCE

1. http://www.slideshare.net/xakikazmi/constant-head .

PART B: FALLING HEAD PERMEABILITY TEST

1.0 OBJECTIVE

To determine permeability of soils of intermediate and low permeability (less than 10 -4 m/s), i.e.

Silts and clays.

2.0 THEORY

In the falling head test a relatively short sample is connected to a standpipe which

provides both the head of water and the means of measuring the quantity of water flowing

through the sample. Several standpipes of different diameters are normally available from which

can be selected the diameter most suitable for the type of material being tested.

In permeability tests on clays, much higher hydraulic gradients than are normally used

with sands can be applied, and are often necessary to induce any measurable flow. The

cohesion of clays provides resistance to failure by piping at gradients of up to several hundred,

even under quite low confining or surcharge pressures. Dispersive clays however are very

susceptible to erosion at much lower gradient.

The falling head principle can be applied to an undisturbed sample in a sampling tube

and to a sample in an oedometer consolidation cell. The equation used in determine the

permeability of fine grained soils is given in Eqn (1).

………..Eqn (1)

The time difference (t2-t1) can be expressed as the elapsed time, t (minutes). The heights

h1 and h2 and the length, L are expressed in millimetres, and the areas A and a in square

millimetres. Eqn (1) then becomes Eqn (2).

………..Eqn (2)

To convert natural logarithms to ordinary (base 10) logarithms, multiply by 2.303. If k is

epxressed in m/s, the above equation becomes Eqn (3).

………..Eqn (3)

Where: a = area of cross-section of standpipe tube,

A = area of cross section of sample

h1 = heights of water above datum in standpipe at time t1

h2 = heights of water above datum in standpipe at time t2

L = heights of sample

t = elapsed time in minutes

3.0 TEST EQUIPMENTS

1. Permeameter cell, comprising:

Cell body, with cutting edge (core cutter), 100 mm diameter and 130 mm long.

Perforated base plate with straining rods and wing nuts.

Top clamping plate.

Connecting tube and fittings.

Figure 1: Compaction permeameter

(Courtesy of ELE International, 2007)

4.0 PROCEDURES

1. Assemble apparatus,

a. The apparatus is set up as shown in Figure 2. The volume of water passing

through a sample of low permeability is quite small and a continuous supply of

de-aired water is not necessary, but the reservoir supplying the de-airing tank

should be filled with distilled or de-ionised water

2. Calibrate manometer tubes,

a. The areas of cross-section of the three manometer tubes should be determined

as follows for each tube:

i. Fill the tube with water up to a known mark near the top of the scale,

observed to the nearest mm,

ii. Run off water from the tube into a weighted beaker, until the level in the

tube has fallen by about 500mm or more,

iii. Read the new water level on the scale, to the nearest mm,

iv. Weigh the beaker containing water from the tube (weighings should be

to the nearest 0.01g)

v. The diameter of the manometer can be calculated as follows:

mm2

If mw = mass of water (g),

h1 = initial level in tube (mm),

h2 = final level in tube (mm),

A = area of cross-section of tube (mm2)

vi. Repeat the measurements two or three times for each tube, and

average the results.

3. Prepare cell,

a. Dismantle the cell,

b. Check the cell body is clean and dry, and weigh it to the nearest 0.1g,

c. Measure the mean internal diameter (D) and length (L) to the nearest 0.5mm

4. Prepare sample,

a. Undisturbed sample can be taken by means of core cutter.

b. Make sure that the sample is a tight fit in the body and there are no cavities

around the perimeter through which water could pass,

5. Assemble cell

6. Connect cell

7. Saturate and de-air sample

8. Fill manometer system

9. Run test

a. Open screw clip at inlet to allow water to flow down through the sample, and

observe the water level in the standpipe,

b. As soon as it reaches the level h1, start the timer clock,

c. Observe and record the time when the level reaches h3, and when it reaches h2,

then stop the clock,

d. Close screw clip at inlet

10. Repeat test

11. Calculate permeability

12. Report result

Figure 2: Falling head permeability cell with manometer tubes

(Courtesy of ELE International, 2007)

5.0 RESULT

Falling Head Permeability test

Location: Sample no:

Operator: Date: 8/03/2012

Soil description: CLAY

Method of

preparation:

Sample diameter, D: 99.21 mm Sample length, L: 129.84 mm

Sample area, A: 7730.38 mm2 Sample volume, V: 1003.7 cm3

Mass of mould: 960 g Mass of sample+mould: 2820 g

Mass of sample: 1860 g

S.G. measured/assumed: Voids ratio:

Bulk density, : 16.43 kN/m3 Dry density, : 14.94 kN/m3

Mositure content: 20 % Test temperature: c

Standpipe diameter: 4.05 mm Standpipe area, a: 12.88 mm2

Reading:

Reference point

Height above

datum, y

(mm)

Height above

outlet, h

(mm)

Test

Height ratios

Permeability k

(m/s) x10-13

No. Time, t

(min)

1 900 850 3.50 0 1.059 9.82

2 850 800 7.72 4.02 1.063 4.71

3 800 750 11.98 4.26 1.067 3.24

4 750 700 16.57 4.59 1.071 2.50

6.0 CALCULATIONS

=

Example calculation for reference point 1 and 2

L = 129.84 x 10-3 m

A = 7730.38 x 10-6 m2

a = πd2 / 4

= π(4.05mm) / 4

= 12.88mm2

= 12.88 x 10-6 m2

Point 1

t = 3.50 min = 210 second

h1 = 900 mm

h2 = 850 mm

K = 2.303aL / (1000xAx60t) x log10 (h1/h2)

K = [2.303(12.88 x 10-6)( 129.84 x 10-3) / [ 1000(7730.38 x 10-6) x 60(210)] x log10

(0.90/0.85)

= 9.82 x 10-13 m/s

Point 2 t = 7.72 min = 463.2 second

h1 = 850 mm

h2 = 800 mm

K = 2.303aL / (1000xAx60t) x log10 (h1/h2)

K = [2.303(12.88 x 10-6)( 129.84 x 10-3) / [ 1000(7730.38 x 10-6) x 60(463.2)] x

log10(0.85/0.80)

= 4.71 x 10-13 m/s

Average of permeability = ∑ K / number of k = 2.027 x 10-12 / 4

= 5.068 x 10-13 m/s

7.0 DISCUSSION

The coefficient of permeability for this sample of soil is 5.068 x 10-13

m/s. The value we get is suitable because during test there should be no

volume change in the soil, there should be no compressible air present in

the voids of soil i.e. soil should be completely saturated. The flow should be

laminar and in a steady state condition. The coefficient of permeability of soil

is depend on several factor such as fluid viscosity, pore size distribution,

grain size distribution,void ratio and degree of soil saturation. In this

experiment the fluid viscosity can not be take as the factor becouse we use

the water.

Coefficient of permeability is used to assess drainage characteristics

of soil, to predict rate of settlement founded on soil bed. Generally, soils

which contain 10% or more particles passing the No.200 sieve are tested

using the falling-head method.

Several errors could have affected the test results:

air trapped in sample or sample not 100% saturated;

soil was washed from the sample;

some of the head loss occurred in the apparatus rather than in the

sample;

not starting and stopping stop watch at correct point;

sample settling during test;

sample disturbed by flowing water at inlet;

difficulty of accurately measuring heads relative to tail water and

significant figures

8.0 CONCLUSION

From the experiment, we get the time is increase at volume of water

is constant. The time we taken is slow. This is because the permeability of

the clay or silt soil absorbs the water is higher. This clay or silt soil has a

small molecular space. It is generally the pore sizes and their connectivity

that determines whether a soil has high or low permeability. Water will flow

easily through soil with large pores with good connectivity between them.

Therefore, the water diffusion rate is higher.

9.0 REFERENCE

1. http://www.civil.umaine.edu/cie366/permeability/default.htm .

APPENDICES

CONSTANT HEAD PERMEABILITY TEST