Permeability is the ability of soil to allow the flow of water through the pore spaces between solid particles. The degree of permeability is determined by applying a hydraulic pressure gradient in a sample of saturated soil and measuring the consequent rate of flow. The coefficient of permeability is expressed as a velocity.

Uses of Permeability of Soil

• Solving problems involving pumping seepage water from construction excavation.
• Estimating the quantity of underground seepage.
• Stability analysis of earth structures and earth retaining walls subjected to seepage forces.

Factors Affecting Permeability of Soil

• Grain size or Particle size

Permeability depends on the shape and soil of soil particles. Permeability varies with the square of particle size diameter.

• Void Ratio

If the presence of voids is more then the permeability is also more.

• Composition

For gravels, sand, and silts presence of mica can decrease the permeability of the soil. For clay, water attracted between clay particles reduces the permeability.

• Structural Arrangement

Remolding of natural soil reduces permeability. If soil contains more rounded particles, the permeability is more.

• Stratification

When the flow of water is parallel to the strata, permeability will be more when compared with the flow perpendicular to the strata.

• Presence of foreign particles and entrapped air

This affects the permeability as it reduces void space and it blocks the interconnectivity between the pores.

• Degree of saturation

If the soil is dry or partly saturated the permeability of soil is always less.

Required equipment

•        Permeameter cell

•        Two discs of wire gauze (or porous disc) with a diameter equal to the internal diameter of the cell body

•        A vertically adjustable reservoir tank

•        A supply of clean water

•        A discharge reservoir with overflow to maintain a constant level

•        A set of transparent manometer tubes

•        A pinch cock for each manometer tube

•        Filter material of suitable grading for placing adjacent to the perforated plates at each end of the permeameter

•        Measuring cylinders of 100 ml, 500 ml, and 1000 ml

•        A scoop

•        A flat-ended tamping rod

•        A thermometer

•        A stop clock

•        A balance readable to 1 g

Sample Preparation

1. Remove oversize particles larger than 8 mm in diameter. The volume of the sample shall be about twice that required to fill the permeameter cell. The sample shall not be dried.
2. Take two or more representative samples from the prepared material for the determination of Moisture Content and Particle Density.
3. Weigh the remainder of the prepared sample to 1 g (m1).

Test Procedure

• Assemble the base plate, with perforated base, to the permeameter cell body.
• Place the graded filter material in the bottom of the cell to a depth of about 50 mm. Level the surface and place a wire gauze (or porous disc) on top.
• Place the soil in the permeameter in at least 4 layers, each of which is of a thickness about equal to half the diameter.
• Tamp each layer with a controlled number of standard blows with the tamping rod. Level the surface of each layer before adding the next.
• Place the upper wire gauze (or porous disc) on top of the sample.
• Place the graded filter material on top of the disc to a depth of minimum 50 mm.
• Release the piston in the top plate and withdraw it to its fullest extent.
• Fit the top plate to the permeameter cell and tighten it down in position.
• Lower the piston carefully and bed the perforated plate onto the filter material. Hold the piston down firmly and tighten the locking collar in this position.

Measurements

1. Determine the mean length of the test sample, L1 (in cm), to 1 mm, by measuring at three locations around the perimeter.
2. Dry the soil left over and weigh it to 1 g (m2), so that the dry mass of the test sample can be obtained by difference

Saturation

1. Connect the control valve on the base of the permeameter to the water supply. Open the top connection and the air bleed to the atmosphere, and close the connection to the manometer tubes.
2. Allow water to enter the cell and slowly percolate upwards through the sample until it emerges first from the air bleed, which is then closed, and then from the top connection.
3. Measure the length of the sample again (L2) and record the average measurement, L (in cm) = ½ (L1 + L2).
4. Close the control valve. Connect the water supply to the permeameter top connection, and connect the control valve at the base to the discharge reservoir, without entrapping air.
5. Set the inlet reservoir at a level a little above the top of the permeameter cell and open the supply valve. Open the manometer pinch cocks one by one and ensure that no air is trapped in the flexible tubing as water flows into the manometer tubes. The water in all tubes shall reach the level of the reservoir surface.
6. The permeameter cell is now ready for test under the normal condition of downward flow. Iƒ a test with upward ƒlow is required, e.g. ƒor investigating piping eƒƒects, ƒit the control valve, connected to the discharge reservoir, to the top oƒ the cell and connect the water supply to the base.

Downward water flow

• Adjust the height of the inlet reservoir to a suitable level with regard to the hydraulic gradient to be imposed on the sample.
• Open the control valve at the base to produce flow through the sample under a hydraulic gradient appreciably less than unity (1.0). Allow the water levels in the manometer tubes to become stable before starting test measurements.
• Place a measuring cylinder of suitable capacity under the outlet from the discharge reservoir and simultaneously start the timer.
• Measure the quantity of water collected in the cylinder during a given time interval. Alternatively, record the time required to fill the cylinder up to a given volume.
• Record the water levels in the manometer tubes.
• Record the temperature of the water in the discharge reservoir.
• Repeat four more times, or until consistent readings are obtained.

Calculations

1)       Calculate the rate of flow, q1, q2, etc. (in ml/s) during the period of each observation of flow from the equation:

etc.    where

Q1, Q2, etc. (in ml) is the volume of water collected from the outlet reservoir during each time period, t (in s).

Calculate the average rate of flow, q, for the set of readings at one hydraulic gradient.

2)       Calculate the hydraulic gradient, i, between the uppermost and lower manometer gland points from the equation:

Where

h is the difference between the two manometer levels (in cm);

y is the difference between the corresponding gland points (in cm).

3)       Calculate the coefficient of permeability, k (in cm/s), for one set of readings from the equation:

Where

A is the area of the cross-section of the sample (in cm2);

Rt is the temperature correction factor for the viscosity of water

4)       Calculate the Dry Mass, m3 (in g), of the initial sample from the equation:

Where

m1   is the initial mass of the sample (in g)

w is the moisture content (in %)

5)       Calculate the Dry Density, pd (in kg/m3), of the test sample from the equation:

where

m2 is the mass of dry soil remaining after setting up the test sample (g)

D is the sample diameter (in cm)

L is the overall length of the sample (in cm).

6)       Calculate the Void Ratio, e, of the test sample (if required) from the equation:

Where

Ps is the Particle Density (in kg/m3)

Permeability Range of Different Soil Mass

• Gravel  –  100  cm/s
• Sand     – 100   – 10-2  cm/s
• Silt         – 10-2  – 10-4  cm/s
• Clay      – 10-4  – 10-6  cm/s