TO DETERMINE THE PERMEABILITY OF A SOIL SPECIMEN BY THE VARIABLE HEAD PERMEAMETER
Theory:
The variable head permeameter is used to measure the permeability of relatively less pervious soils. The coefficient of permeability is given by
Where 
= initial head, 
= final head, t= time interval, a= cross-sectional area of the liquid stand pipe, A=cross-sectional area of the specimen, L= length of specimen.
Equipment:
All equipment required for constant head Permeability test and the following:
1. Graduated glass stand pipe and the clamp
2. Supporting frame for the stand pipe and the clamp
Procedure:
Steps 1 to 14 same as Constant head permeability test.
15. Connect the stand pipe of suitable diameter to the inlet at the top. Fill the stand pipe with water.
16. Open the stop cock at the top and allow the water to flow out till all the air in the mould is removed.
17. Close the stop cock and allow the water from the stand pipe to flow through the soil specimen.
18. Select the heights, ![clip_image002[1]](http://static6.theconstructor.org/wp-content/uploads/2010/09/clip_image002110.gif "clip_image002[1]"){kind=small}
and ![clip_image003[1]](http://static4.theconstructor.org/wp-content/uploads/2010/09/clip_image00316.gif "clip_image003[1]"){kind=small}
measured above the centre of the outlet such that their difference is about 300 to 400mm. mark the level corresponding to a height 
.
19. Open the valve and start the stop watch. Record the time interval for the head to fall from ![clip_image002[2]](http://static6.theconstructor.org/wp-content/uploads/2010/09/clip_image00222.gif "clip_image002[2]"){kind=small}
to ![clip_image004[1]](http://static6.theconstructor.org/wp-content/uploads/2010/09/clip_image00416.gif "clip_image004[1]"){kind=small}
and also from ![clip_image004[2]](http://static4.theconstructor.org/wp-content/uploads/2010/09/clip_image00421.gif "clip_image004[2]"){kind=small}
to ![clip_image003[2]](http://static6.theconstructor.org/wp-content/uploads/2010/09/clip_image00321.gif "clip_image003[2]"){kind=small}
. These two time intervals will be equal if the steady conditions have established.
20. Repeat the steps (19) atleast twice after changing the heights ![clip_image002[3]](http://static7.theconstructor.org/wp-content/uploads/2010/09/clip_image00231.gif "clip_image002[3]"){kind=small}
and ![clip_image003[3]](http://static7.theconstructor.org/wp-content/uploads/2010/09/clip_image00331.gif "clip_image003[3]"){kind=small}
.
21. Stop the flow of water. Disconnect all the parts.
22. Take a small quantity of the soil specimen for water content determination
Fig: Variable Head Permeability Test
DATA SHEET FOR VARIABLE HEAD PERMEAMETER TEST
Length of specimen=
Diameter of specimen =
Volume of specimen =
Water content =
Diameter of stand pipe =
Area of stand pipe =
Specific gravity of solids =
Sl. No.
| Observations and Calculations
| Determination No.
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1
| 2
| 3
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Observation
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1 | Mass of empty mould with base plate |
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2 | Mass of mould, soil and base plate |
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3 | Initial head |
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4 | Final head |
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5 | Head |
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6 | Time interval |
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Calculations
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7 | Mass of soil = (2) – (1) |
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8 | Bulk Density |
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9 | Dry density |
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10 | Void ratio |
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11 |
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Result:
Coefficient of permeability of the given soil = ____mm/sec.
Good day. I just would like to ask if variable head permeability is the same as falling head permeability? I do testing on in situ permeability tests.. Looking forward to hear from you.
Test procedure is different. Variable head test is done in relatively less permeable soil, such as clay. While constant head can be done for soil which has more permeability, such as silty or sandy soils.
:)