Earth and Environmental Science 11–12 · Year 11

Porosity and permeability of sand, gravel and clay columns, with Darcy’s law

Module 4: Human Impacts

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Pore space stores groundwater and connected pores let it flow, so the same measured columns show why an aquifer holds water and a clay layer confines it.

What you need

three clear columns (50 mm internal diameter, 30 cm long, cloth over the base and a bung with an outlet tube and clip, the tube ending level with the base) packed to 20 cm with washed sand, gravel and a clay-rich soil
250 mL measuring cylinders, stopwatch, ruler, funnel
water supply with a constant head (an overflow bottle) for the permeability runs

How to do it

Porosity: with the outlet clipped shut, pour water slowly into each dry column until it just reaches the surface; porosity = water added / bulk volume of the packed column. Open the clip and let the column drain before the permeability runs.
Permeability: keep a constant 20 cm depth of water above the 20 cm column with the outflow free at the base, and collect the outflow for one minute; repeat three times.
Calculate hydraulic conductivity K = Q L / (A Δh) for each material in m/s, where Δh is the head difference from the water surface to the outlet (here 20 cm + 20 cm = 40 cm).
Rank the materials by porosity and by K and note that the rankings differ.
Relate the ranking to an aquifer (gravel, sand) and an aquitard (clay), and to how much of the rain on a paddock reaches the water table.

What you should see

Uniform sand gives a porosity between the packing limits computed for equal spheres, 0.26 (closest packing) and 0.48 (simple cubic); clay-rich soil can hold as much water or more yet drains slowest. For an example run of 10 mL/min through a 50 mm column 20 cm long with a head difference of 40 cm, K = 4.2 × 10⁻⁵ m/s; gravel runs so fast that the cylinder fills in seconds, and clay may yield only drops in a minute.

What changes

What you change: material (gravel, sand, clay)
What you measure: porosity (fraction) and hydraulic conductivity (m/s)
What you keep the same: column length and diameter
constant head
packing method
collection time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Groundwater flows in underground rivers.
Clay holds no water (it holds much but releases it slowly).

Safety card

Low riskLearners carry it out

Hazards

water on the floor
soil handling

Controls

trays under columns
wash hands

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-11 EES11-8 EES11/12-1 EES11/12-2 EES11/12-3
Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities