Science 7–10 · Year 8

Porosity and permeability of gravel, sand and clay

Science understanding: Earth and space sciences (NSW Stage 4 focus area: Change)

Practical, model not builtLow risk

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The idea

Porosity is how much water a sediment can hold and permeability is how fast it lets water through; both come from the size and packing of the grains, which is why an aquifer is sand or gravel and clay seals it.

Safety card

Low riskAn adult supervises

Hazards

water spills
cut bottle edges
fine dust from dry potter's clay powder, which contains crystalline silica

Controls

tape the cut edges
work over trays
an adult fills the clay column, pouring the powder slowly and low in a ventilated room with learners standing back; the column stays capped until it is wet; spills are wiped with a damp cloth, never swept dry

Note

Potter's clay powder contains crystalline silica, which the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update, Figure 1.8-1) lists as a substance requiring health surveillance; handled wet it raises no dust. Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) or through RiskAssess.

What you need

3 identical clear columns per group (cut 1.25 L bottles, neck down, with a square of fine mesh in the neck and a screw cap on the neck) holding gravel (2 to 5 mm), coarse sand and potter's clay or fine silt, each filled to the same 200 mL mark (dry; an adult fills the clay column)
3 100 mL measuring cylinders and 1 250 mL beaker; 1 stopwatch
water, about 1 L per group

How to do it

Porosity: with the cap screwed on, pour water slowly from a measuring cylinder into the dry gravel column until the water just reaches the top of the sediment; record the volume added and subtract the neck allowance (the water that fills the capped neck up to the mesh, measured once in an empty column). Porosity = (volume added - neck allowance) / 200 mL. Repeat for sand and clay (the clay takes minutes).
Permeability: hold each saturated column over the beaker, remove the cap and let the free water drain out; once the dripping stops, pour 100 mL onto the top and time how long the first 50 mL takes to collect in the beaker; record the time.
Repeat each measurement with a fresh dry fill to check reproducibility.
Rank the three sediments by porosity and by permeability; compare the ranks.
Compare the porosity results with the two packing limits from the simulation.

What you should see

Gravel and coarse sand each take a volume of water between a quarter and a half of the dry sediment volume, so porosities between 0.26 and 0.48, the limits for uniform spheres packed as tightly as possible (1 - pi / (3 root 2) = 0.260) and as loosely as a cubic grid (1 - pi / 6 = 0.476); the clay column holds water too but so slowly that the pour must be paced. For permeability 50 mL drains through gravel in seconds, through sand in tens of seconds and through clay in minutes or longer, sometimes not within the lesson. Porosity ranks the three close together while permeability separates them by orders of magnitude. The learner knows it worked when repeat fills agree within 10 mL and the permeability order is gravel, sand, clay.

What changes

What you change: sediment grain size
What you measure: porosity (volume of water held) and drainage time
What you keep the same: dry sediment volume
column shape
water volume poured
packing method (tapped ten times)

Common misconceptions

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

Clay is impermeable because it has no pores; it has high porosity but pores too small to pass water quickly.
Groundwater flows in underground rivers; most sits in the pores of sand, gravel and porous rock.
Bigger grains mean more pore space; porosity depends mainly on packing and sorting, while permeability depends strongly on grain size.

Curriculum references

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

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-CHG-01 SC4-WS-01 SC4-WS-04
Australian Curriculum v9AC9S8U04AC9S8I02AC9S8I04AC9S8I05

Sources

The pages the author read to write this activity.

All Concept Studio activities