What it shows
Buildings stand on foundations that spread the load over soil. The project loads small square footings on a tray of sand and shows that a wider footing carries more total load but the same or lower pressure, and that burying the footing raises its capacity.
The science
The soil under a footing fails when the pressure applied exceeds its bearing capacity. For sand, bearing capacity depends on the friction angle, the depth of the footing and its width, so q_ult = 0.5 gamma B Ngamma + gamma D Nq in a simple model. A wider footing spreads the load to a larger area, and burying the footing adds surcharge that confines the sand and raises capacity.
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 strong plastic tub or wooden box 40 x 40 x 30 cm filled with dry sand
- Square footings 3 cm, 5 cm and 7 cm cut from wood
- A loading rod and a bucket or plate
- Sand or water in bottles as weights
- A dial gauge or ruler with a pointer
- A kitchen scale
Safety
For this project: Dust from sand can irritate eyes and lungs: wear a mask. Keep the load stack low to avoid toppling. An adult must supervise.
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.
Fill the tub with dry sand in layers and level it, then compact each layer by tapping the sides equally every time.
Place the 3 cm footing on the surface and the loading rod on top with a bucket attached.
Fix a ruler and pointer to measure settlement as load is added.
Add load in 0.5 kg steps and record settlement after 1 minute at each step.
Continue until settlement suddenly increases or reaches 10 percent of footing width, which is taken as failure.
Repeat for the 5 and 7 cm footings, using freshly levelled sand.
Repeat for the 5 cm footing buried to depths of 2 and 4 cm.
Calculate pressure as load divided by area and plot pressure against settlement for each case.
Compare ultimate loads and pressures in a table.
Write the report on foundation types, the role of width and depth, and limits of a small model.
What you should see
The graphs show a curve that bends sharply at failure. Ultimate load rises strongly with width and with depth, and the larger footing carries more load although pressure at failure is comparable.
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 bearing capacity?
The maximum pressure the soil can take before it shears and the footing sinks.
2Why does a deeper footing carry more?
The soil around it adds a confining weight that resists the shear failure.
3What is settlement?
The downward movement of the footing as the soil compresses under load.
4Why is a model less accurate than a real foundation?
Soil strength depends on stress level and size, so small models do not scale exactly.
Take it further
Repeat with wet sand and with a clay-sand mix to show how water and cohesion change the result.
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.
Free ebooks for this project
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