Variable head permeability test is one of several techniques by which the permeability of soil is determined. It is used to evaluate the permeability of fairly less previous soil. Permeability is the measure of the ability of soil to allow water to flow its pores or voids.
Permeability is one of the most crucial soil properties of interest to geotechnical engineers. This is because it affects the rate of settlement of saturated soil under loads, the design of earth dam is mostly based on the permeability of soil used, and it greatly influences the stability of retaining structures and slopes. Soil permeability is also significant for estimation of underground seepage and stability analysis of earth structures suffered from seepage forces.
Apparatus
1. Mould Assembly
The mould assembly including drainage base and drainage cap which need to be conform to IS: 11209-1985
2. Compaction Hammer
3. Set of Stand Pipes
Glass stand pipes varying in diameter from 5 to 20 mm, suitably mounted on stand or otherwise fixed on wall.
4. Constant Head Tank
A suitable water reservoir capable or supplying water to the permeameter under constant head for constant head test arrangement.
4. Miscellaneous Apparatus
For instance, IS sieves, mixing pan, graduated cylinder, meter scale, stop watch, 75micron wire gauge, thermometer, and a source of de-aired water.
Soil Specimen Preparation
Disturbed Soil Sample
- Take 2.5-kg soil from a thoroughly mixed air-dried or oven-dried material and evaluate its moisture content.
- Remove the collar of the mould. Measure the internal dimensions of the mould. Weigh the mould with dummy plate to the nearest gram.
- 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.
- Place soil specimen in the mould, and compact it at the required dry density using a suitable compacting device.
- Take a small specimen of the soil in a container for the water content determination.
- Remove the collar and base plate. Trim the excess soil level with the top of the mould.
- Clean the outside of the mould and the dummy plate.
- Find the mass of the soil in the mould.
- The mould with the sample is now placed over the permeameter. This will have drainage and cap discs properly saturated
Undisturbed Soil Sample
- Trim the specimen in a form of cylinder not larger than 85cm in diameter, and having a height equal to that of the mould.
- Place the specimen over porous disc of the drainage base fixed to the mould.
- Use impervious material like cement slurry to fill the space between mould and the specimen.
- Fix the drainage cap over the top of the mould.
Procedure
- Connect the specimen to the selected stand-pipe through the top inlet.
- Open the bottom outlet and record the time interval required for the water level to fall from a known initial head to a known final head as measured above the center of the outlet.
- Refill the stand-pipe with water and repeat the test till three successive observations give nearly same time interval; the time intervals being recorded for the drop in head from the same initial to final values, as in the first determination.
- Alternatively, after selecting the suitable initial and final heads, h1, and h2, respectively, observe the time intervals for the head to fall from h1 to (h1*h2)^0.5, and similarly from (h1*h2)^0.5 to h2.
- The time intervals should be the same; otherwise the observation shall be repeated after refilling the stand-pipe.
Data Sheet
The following values shall be recorded in the data sheet of variable head permeability test:
Length of specimen (L)
Diameter of specimen (D)
Volume of specimen (V)
Water content (W)
Diameter of stand pipe (d)
Area of stand pipe (a)
Specific gravity of solids
Table 1 Typical table for recording observations and calculations
| Sl. No. | Observations and Calculations | No. of Trials | ||
| 1 | 2 | 3 | ||
| Observation | ||||
| 1 | Mass of empty mould with base plate | |||
| 2 | Mass of mould, soil and base plate | |||
| 3 | Initial head h1 | |||
| 4 | Final head h2 | |||
| 5 | Head (h1*h2)^0.5 | |||
| 6 | Time interval | |||
| h1 to (h1*h2)^0.5 | ||||
| (h1*h2)^0.5 to h2 | ||||
| h1 to h2 | ||||
| Calculations | ||||
| 7 | Mass of soil = (2) – (1) | |||
| 8 | Bulk Density = mass/volume | |||
| 9 | Dry density (Pd)= P/(1+W) | |||
| 10 | Void ratio (e)= (Pw G)/ Pd | |||
| 11 | k=(QL)/(Aht) |
Calculations
At temperature T of water, the permeability kT is calculated using the following expression:
kT= ((2.30aL)/(At))Log10(h1/h2) Equation 1
Where
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.
Result
The permeability values at temperatures T and 27°C are reported as numbers with units as cm/s. The state of the sample is also reported in terms of water content, void ratio and degree of saturation.