What it shows
How fast water moves through soil decides whether a field drains, a recharge pit works or an earth dam leaks. A simple falling head permeameter measures the coefficient of permeability k for sand, silt and clay mixes using Darcy’s law.
The science
Darcy’s law says that flow through soil is proportional to the hydraulic gradient. In the falling head test, water in a thin standpipe falls from height h1 to h2 in time t while passing through a soil sample of length L and area A. The coefficient k = (a L / A t) ln(h1/h2), where a is the standpipe area. Sand gives k near 10^-2 cm/s, silt around 10^-5 and clay 10^-7 or lower.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear plastic pipe 5 cm across and 50 cm tall
- A nylon mesh and a rubber stopper with a short outlet tube
- A 1 cm diameter standpipe (burette or tall thin pipe) with a scale
- Dry sand, silt from a river bank, clay or red soil
- A stopwatch, a measuring cylinder
- A ruler and a bucket
Safety
For this project: Wear a dust mask when handling dry soil and wash hands after. An adult must cut the pipe.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix mesh on the lower end of the large pipe and pack a 10 cm layer of the soil in three layers, tamping each equally with a rod.
Measure the length L of the soil and the internal diameter of the pipe to get the area A.
Connect the standpipe to the top and saturate the soil by letting water rise slowly from below for 30 minutes to push out air.
Fill the standpipe to a marked height h1 and open the outlet.
Note the time for the water level to fall to h2 and repeat for at least three runs with the same soil.
Measure the standpipe area from its diameter, and calculate k for each run.
Repeat for fine sand, sand with 20 percent clay, and a red soil sample.
Make a table of k values and classify them using a standard chart of drainage quality.
Plot k on a log scale against the percentage of fine particles.
Write the report and comment on use in rainwater recharge pits and in embankments.
What you should see
Clean sand drains in seconds and k is about 10^-2 cm/s, whereas clay mixes may take hours and give k of 10^-6 cm/s or lower. The log graph shows k dropping sharply as clay content rises.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is permeability?
The ease with which water flows through soil, measured as a speed in cm/s or m/s.
2Why remove air bubbles?
Trapped air blocks pores and lowers the measured permeability.
3Why use falling head for fine soils?
Flow is too slow to measure a constant discharge, so the fall of the water level is timed.
4Where is permeability important?
In dam design, drainage layers, landfills and groundwater recharge.
Take it further
Compare three compaction levels of the same soil and show how denser packing reduces k.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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