TO DETERMINE PERMEABILITY OF SOIL BY CONSTANT HEAD PERMEAMETER.
The coefficient of permeability is equal to the rate of flow of water through a unit cross section area under a unit hydraulic gradient. In the constant head permeameter, the head causing flow through the specimen remains constant throughout the test. 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.
1. Permeameter mould, internal diameter = 100mm, effective height =127.3 mm, capacity = 1000ml.
2. Detachable collar, 100mm diameter, 60mm height
3. Dummy plate, 108 mm diameter, 12mm thick,
4. Drainage base, having porous disc
5. Drainage cap having porous disc with a spring attached to the top.
6. Compaction equipment such as Proctor’s rammer or a static compaction equipment
7. Constant head water supply reservoir
8. Vacuum pump
9. Constant head collecting chamber
10. Stop watch
11. Large funnel
13. Weighing balance accuracy 0.1g
14. Filter paper.
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.
4. Remove the collar and base plate. Trim the excess soil level with the top of the mould.
5. Clean the outside of the mould and the dummy plate. Find the mass of the soil in the mould.
6. Take a small specimen of the soil in a container for the water content determination.
7. Saturate the porous disc (stones).
8. Place a porous disc on the drainage base and keep a filter paper on the porous disc.
9. Remove the dummy plate and place the mould with soil on the drainage base after inserting a washer in between.
10. Clean the edges of the mould. Apply grease in the grooves around them.
11. Place a filter paper, a porous disc and fix the drainage cap using washers.
12. Connect the water reservoir to the outlet at the base, and allow the water to flow upwards, till it has saturated the sample. Let the free water collect for a depth of about 100mm on the top of the sample.[Alternatively the soil of low permeability can be saturated by subjecting the specimen to a gradually increasing vacuum with bottom outlet closed so as to remove the air from the voids. Increase the vacuum gradually to 700mm of mercury and maintain it for 15minutes or more, depending upon the type of the soil. Follow the evacuation by a process of slow saturation of the sample from the bottom upward under full vacuum. When the sample is saturated, close both the top and bottom outlets.]
13. Fill the empty portion of the mould with desired water without disturbing the soil.
14. Disconnect the reservoir from the outlet at the bottom.
15. Connect the constant head reservoir to the drainage cap inlet.
16. Open the stop cock and allow the water to flow downward so that all the air is removed.
17. Close the stop cock, and allow the water to flow through the soil till a steady state is attained.
18. Start the stop watch and collect the water flowing out of the base in a measuring flask for some convenient time interval.
19. Repeat this thrice keeping the same interval. Check that the quantity of water collected is approximately the same each time.
20. Measure the difference of head (h) in levels between the constant head reservoir and the outlet in the base.
21. Repeat the test for at least two more different intervals.
Fig: Constant Head Permeability Test
DATA SHEET FOR CONSTANT HEAD PERMEAMETER
Diameter = 100mm, length =____, volume=______ ml, G=____, Area=_____
Observations and Calculations
Mass of empty mould with base plate
Mass of mould, soil and base plate
Hydraulic head (h)
Time interval (t)
Quantity of flow (Q)
(a) First time in period t
(b) Second time in period t
(c) Third time in period t
Mass of soil = (2) – (1)
Water content w
Coefficient of permeability of the given soil =_______ mm/sec.