Experiment: Permeability Test (Constant Head Method)
1. OBJECTIVE
The primary objective of this experiment is to determine the coefficient of permeability (k) for a given coarse-grained soil sample by using the constant head permeability test method.
2. APPARATUS REQUIRED
- Constant head permeameter apparatus with an overhead tank
- Soil sample
- Two porous stones or plates
- A measuring jar or graduated cylinder
- A stopwatch
- A ruler or measuring tape
3. THEORY
Permeability is the property of a soil that allows water to pass through its interconnected voids. The ease with which water flows through a soil mass is quantified by the coefficient of permeability, also known as hydraulic conductivity.
The flow of water through soil is governed by Darcy’s Law, which states that for laminar flow conditions in a saturated soil, the rate of flow (or discharge, q) is directly proportional to the hydraulic gradient (i).
Darcy’s Law:
\[ q = k \cdot i \cdot A \]
where:
- q = Discharge (volume of water flowing per unit time, Q/t)
- k = Coefficient of permeability
- i = Hydraulic gradient (h/L)
- A = Cross-sectional area of the soil sample
For the constant head test, the coefficient of permeability can be calculated using:
Permeability Formula:
\[ k = \frac{QL}{hAt} \]
4. PROCEDURE
- The standard dimensions of the permeameter (length and internal diameter) were measured and recorded.
- The soil sample was placed inside the permeameter, carefully compacted, and positioned between two porous stones.
- The sample was saturated by allowing water to flow in an upward direction.
- The permeameter was connected to the overhead tank to establish a constant water head.
- Water was allowed to flow through the sample until steady state was achieved.
- A measuring jar was placed at the outlet, and time was recorded to collect a known volume of water.
- The procedure was repeated for three successful trials.
5. OBSERVATION AND CALCULATIONS
Given Data:
- Volume of water collected (Q): 1000 ml = 1×10⁻³ m³
- Constant head loss (h): 150 cm = 1.5 m
- Length of soil sample (L): 13 cm = 0.13 m
- Diameter of soil sample (d): 10 cm = 0.1 m
Calculations:
Cross-sectional area of sample (A):
\[ A = \frac{\pi d^2}{4} = \frac{\pi (0.1)^2}{4} = 7.854 \times 10^{-3} m^2 \]
| Trial No. |
Volume (Q) (m³) |
Head Loss (h) (m) |
Time (t) (sec) |
Coefficient of Permeability (k) (m/s) |
| 1 |
1×10⁻³ |
1.5 |
37.39 |
2.95×10⁻⁴ |
| 2 |
1×10⁻³ |
1.5 |
36.40 |
3.03×10⁻⁴ |
| 3 |
1×10⁻³ |
1.5 |
38.20 |
2.89×10⁻⁴ |
Average Coefficient of Permeability:
\[ k_{avg} = \frac{2.95 + 3.03 + 2.89}{3} \times 10^{-4} = 2.96 \times 10^{-4} m/s \]
6. RESULT
The average coefficient of permeability (k) of the given soil sample was determined to be 2.96×10⁻⁴ m/s.
7. DISCUSSION AND CONCLUSION
The experiment successfully determined the coefficient of permeability for the soil sample using the constant head method. The resulting average value of 2.96×10⁻⁴ m/s indicates that the soil has moderate to high permeability, characteristic of fine sand.
The constant head method proved effective for this coarse-grained soil, where measurable water flow could be obtained. The test results are valuable for engineering applications requiring knowledge of soil drainage characteristics.
In conclusion, the permeability coefficient obtained classifies the soil as permeable, making it suitable for applications requiring good drainage, such as in foundations and filter materials.
