The falling head permeability test is a laboratory testing method conducting to determine the permeability of fine grained soils with intermediate and low permeability. Soil permeability is the property by virtue of which the soil allows to transmit water through it. Soil permeability is one factors to be determined for any variety of soil. This is because different types of soils shows different ranges of permeability when water is allowed pass through them.
The permeability of some of the common soil types are given in the table below.
|S. no.||Soil Type||Permeability coefficient ,k|
|1.||Coarse gravel||Exceeds 10-1||High|
|2.||Sand Clean||10-1 – 10-3||Medium|
|3.||Sand Dirty||10-3 – 10-5||Low|
|4.||Silt||10-5 – 10-7||Very Low|
|5.||Clay||Less than 10-7||Impervious|
In the falling head permeability test method, the soil sample is first saturated under a specific head condition. The water is then allowed to flow through the soil without adding any water, so the pressure head declines as water passes through the specimen.
This testing method can be applied to an undisturbed sample.Some of the standards that are to be followed during conducting as of the experiment are :
- BS 1377-5:1990
- ASTM D2435 – 04 Standard Test Methods for One-Dimensional Consolidation Properties of soils Using Incremental Loading.
There are two laboratory tests to calculate the permeability of soil. These are
- Constant head permeability test.
- Falling head permeability test
TO DETERMINE PERMEABILITY OF SOIL SPECIMEN By FALLING HEAD PERMEAMETER Test
In this test flow of water through a relatively short soil sample connected to a standpipe which provides the water head and also allows measuring the volume of water passing through the sample. The permeability of soil is also known as the falling head permeameter. The falling head permeameter is used to measure the permeability of relatively less pervious soils. The coefficient of permeability is given by
h1 = initial head,
h2 = 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 Used In Falling Head Permeability Test of Soil :-
The falling head Permeability test apparatus are as following:
- Permeability cell
- Vacuum control panel
The falling head permeability test procedure is discussed below.
1. Remove the collar of the mould. Measure the internal dimensions of the mould. Weigh the mould with dummy plate to the nearest gram for permeability test of soil.
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. For permeability test of soil 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. Permeability test of soil is determined by saturating 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 100 mm 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 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 for permeability test of soil, h1 and h2 measured above the centre of the outlet such that their difference is about 300 to 400mm. mark the level corresponding to a height √h1h2.
19. Open the valve and start the stop watch. Record the time interval for the head to fall from h1 to √h1h2 and also from √h1h2 to h2. These two time intervals will be equal if the steady conditions have established.
20. Repeat the steps (19) atleast twice after changing the heights h1 and h2.
21. Stop the flow of water. Disconnect all the parts.
22. Take a small quantity of the soil specimen for water content determination
DATA SHEET FOR VARIABLE HEAD PERMEAMETER TEST OF SOIL :-
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.|
|1||Mass of empty mould with base plate|
|2||Mass of mould, soil and base plate|
|3||Initial head, h1|
|4||Final head, h2|
|h1 to √h1h2|
|√h1h2 to h2|
|h1 to h2|
|7||Mass of soil = (2) – (1)|
The result of Falling head permeability test is
Coefficient of permeability test of soil = ____mm/s
- In the falling head permeability test the soil should be completely saturated.
- Coefficient of permeability in falling head permeability test and other test for permeability is used to assess drainage characteristics of soil, to predict rate of settlement founded on soil bed.
To download this experiment ( prmeability of soil specimen by the falling head permeameter) as pdf. Click on below PDF button