To determine the coefficient of permeability of coarse-grained soil by Constant Head Method

Object: To determine the coefficient of permeability of coarse-grained soil by Constant Head Method

Standard:

AASHTOT215-86

ASTMD234-70 BS1377

Apparatus:

1. Permeameter mould.
   
2. Compacting equipment.
   
3. Drainage bade.
   
4. Drainage cap.
   
5. Constant head tank.
   
6. Graduated glass cylinder to receive the discharge.
   
7. Stopwatch to note the time.
   
8. A meter scale to measure the head differences and length of specimen.
   

Need and Scope:

The knowledge of this property is much useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal bank affected by seepage, settlement etc.

Constant Head Permeability Test:

The constant head permeability test is a laboratory experiment conducted to determine the permeability of soil. The soils that are suitable for this test are sand and gravels. Soils with silt content cannot be tested with this method. The test can be employed to test granular soils either reconstituted or disturbed.

Definition of Coefficient of Permeability:

The rate of flow under laminar flow conditions through a unit cross sectional area of porous medium under unit hydraulic gradient.

The coefficient of permeability (k) is obtained from the relation,

Where,

q= discharge,

Q=total volume of water, t=time period,

h=head causing flow, L= length of specimen, A= cross-sectional area.

Procedure:

Specimen Preparation:

1. Remove the collar of the mould. Measure the internal dimensions of the mould. Weigh the mould with dummy plate to the nearest gram.
   
2. Apply a little grease on the inside to the mould. Clamp the mould between the base plate and the extension collar and place the assembly on a solid base.
   
3. Take about 2.5kg of the soil sample, from a thoroughly mixed wet soil, in the mould.
   

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

1. Take a small specimen of the soil in a container for the water content determination.
   
2. Remove the collar and base plate. Trim the excess soil level with the top of the mould.
   
3. Clean the outside of the mould and the dummy plate. Find the mass of the soil in the mould.
   
4. The mould with the sample is now placed over the permeameter. This will have drainage and cap discs properly saturated.
   

Test Procedure:

1. Through the top inlet of the constant head reservoir, the specimen is connected.
   
2. The bottom outlet is opened, and a steady flow is established
   
3. For a particular time interval, the quantity of flow can be collected.
   
4. Measure the difference of head (h) in levels between the constant head reservoir and the outlet in the base.
   
5. For the same interval, this is repeated three times.
   

Observation and Calculations:

Initially, the flow is very slow. It later increases and will become constant. The constant head permeability test is best for cohesion less soils.

Table: Observations and Calculations

Results

The coefficient of permeability of given soil, k =mm/sec

Comments: