What it shows
Before any road, school building or pond is built, the soil must be tested, but lab tests cost money. A student kit of sieves, a cone for plasticity and a small compaction mould can classify soils from three real sites. The project compares them using the same standard ideas as the geotechnical lab.
The science
Sieve analysis separates a dry soil by particle size, and the percentages passing give the grading curve. Fine soils are classified by plasticity: the plastic limit is the water content at which a thread of soil just crumbles at 3 mm, and the liquid limit is where it flows like a liquid. Compaction increases density, and for each soil there is an optimum water content that gives the maximum dry density. Soil classification links to its use as fill or foundation.
Materials
Cost: under Rs 5,000. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A set of kitchen and hardware sieves of 4.75 mm, 2 mm, 1 mm, 0.5 mm, 0.25 mm and 0.075 mm (or the nearest mesh available)
- A digital kitchen scale with 1 g resolution
- A drying tray, a sunny day or a domestic oven used by an adult at low heat
- A plastic cone and 80 g weight for a simple fall-cone test, or a glass plate for thread rolling (plastic limit)
- A small mould (a 10 cm diameter, 12 cm tall steel can) and a 2.5 kg rammer made of a pipe with weights
- Soil samples from three places
- Water spray and a measuring cylinder
Safety
For this project: Wear a dust mask and gloves while handling dry soil and take permission before digging. An adult must handle the oven and heavy rammer. Do not dig near pipes or cables, or deeper than 30 cm.
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
12 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.
Collect 2 kg of soil from each of three sites, noting depth, colour and location, and dry them in the sun for two days.
Crush any lumps without breaking grains and weigh 500 g of each sample.
Pass each soil through the stack of sieves, shaking for 10 minutes, and weigh the amount retained on each sieve.
Calculate the percentage passing each sieve and plot the grading curve on a semi-log graph.
For the fine part, mix soil with water, roll 3 mm threads on a glass plate and note the water content at which they crumble to find the plastic limit.
Find an approximate liquid limit with a fall cone or the flow test and calculate the plasticity index.
Place the soil in the mould in three layers, give 25 blows to each layer with the rammer and weigh the mould with soil.
Repeat compaction at five water contents and find water content of each by drying a small sample.
Calculate the dry density at each water content and plot a compaction curve.
Identify the optimum water content and maximum dry density for each soil.
Classify each soil by a simple system as sandy, silty or clayey and suggest its suitability for a road base, brick or pond lining.
Write the report with all tests, curves, comparison with typical values and the limits of home-made equipment.
What you should see
The three soils give clearly different grading curves, plasticity values and optimum moisture contents. The sandy soil has a high maximum density at low moisture, and the clayey soil has high plasticity and lower density. The report states how close the kit is to standard tests.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does a grading curve show?
The percentage of the soil finer than each particle size.
2What is the plasticity index?
The liquid limit minus the plastic limit, a measure of how clayey a soil is.
3Why is there an optimum moisture content?
A little water lubricates the grains so they pack closer, but too much water takes up space and lowers density.
4Why test soil before building?
Its strength, settlement and drainage decide the type and size of the foundation.
Take it further
Make bricks from each soil with 0 and 10 percent cement or lime and compare their crushing strength using weights on a plate.
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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