CIVL2410 Chap.3 Fine grained soils: clay minerals and Atterberg limits
Fine grained soils: clay minerals and Atterberg limits
Below about two microns a soil particle stops behaving like a small rock. Clay particles are plates with an enormous surface area for their volume and a chemically active surface, so surface forces dominate gravity and water is adsorbed rather than merely stored. Adding water therefore changes what the material is, not just how wet it is.
This chapter defines the water contents at which those changes happen and the three indices built from them.
What this chapter covers
- 01
Why clay behaves unlike sand: plate shape, specific surface and surface charge
- 02
Kaolinite, illite and montmorillonite, and what the interlayer bonding decides
- 03
Swelling and shrinkage, and why some clays crack as they dry
- 04
The four consistency states from solid through to liquid
- 05
The shrinkage, plastic and liquid limits as boundaries on one water content axis
- 06
Plasticity index as the width of the plastic range, and its three bands
- 07
Liquidity index, and what it says about the specimen you are holding
- 08
Activity, and how dividing by the clay fraction isolates mineralogy
- 09
The Casagrande cup and the fall cone as two definitions of one limit
- 10
The three millimetre thread test, and where operator scatter enters the index
Four laboratory numbers turned into a description
- +1Plasticity index is the width of the plastic band: PI = 52 - 22 = 30, which is above 17 and therefore high plasticity.
- +1Liquidity index locates this specimen inside that band: LI = (34 - 22) / 30 = 0.40.
- +1That value lies between 0 and 1, so the clay is in its plastic range and would be firm and mouldable rather than soft.
- +1Activity removes the quantity of clay from the picture: A = 30 / 28 = 1.07, which falls in the normal band.
Key terms
- Plasticity index
- Liquid limit minus plastic limit, so it measures the width of the water content range over which a clay is mouldable. Below 7 is low plasticity, above 17 is high.
- Liquidity index
- Where the natural water content sits inside the plastic range. Above one the soil is wetter than its liquid limit and will be very soft; below zero it is stiff.
- Activity of clays
- Plasticity index divided by the percentage finer than two microns. It removes the quantity of clay and isolates how reactive that clay is.
- Casagrande cup test
- A dynamic determination of the liquid limit, defined as the water content at which a standard groove closes over 13 mm in 25 blows of the cup.
- Plastic limit
- The water content at which a thread of soil rolled to three millimetres diameter just crumbles. It is the least repeatable of the limit tests, and its scatter lands entirely on the plasticity index.
- Montmorillonite
- A clay mineral whose sheet packets are not locked together, so water can enter between them. It swells strongly on wetting and shrinks and cracks on drying, and it drives high activity values.
Fine grained soils: clay minerals and Atterberg limits FAQ
What is the difference between plasticity index and liquidity index?
One describes the soil and the other describes the specimen. Plasticity index is the width of the water content band over which that clay is mouldable, and it is a property of the mineral and the specific surface. Liquidity index says where the sample you are holding sits inside that band on the day it was taken, so it moves with the weather and with depth while the plasticity index does not.
A liquidity index near one warns you that a clay is close to its liquid limit and will be soft, weak and highly compressible.
Why divide the plasticity index by the clay fraction?
Because a soil can be highly plastic for two quite different reasons: it may contain a great deal of ordinary clay, or a little of an extremely reactive one. Dividing by the percentage finer than two microns removes the first effect and leaves a statement about mineralogy.
Two soils with an identical plasticity index can therefore carry very different risks under a lightly loaded slab, where seasonal moisture change rather than the applied load is the governing action.
Exam move
Do not try to memorise a single definition of a clay, because the point of this chapter is that one clay has a different strength and stiffness at every water content it can hold. Learn the consistency axis as a picture with the three limits marked on it, and practise dropping a given water content onto it.
Keep the three indices separate by the question each answers: how wide is the plastic range, where in it is this specimen, and which mineral is responsible. When a result sits close to a classification boundary, repeat the plastic limit rather than the liquid limit, since the thread test carries the operator scatter and the index is a difference of the two.
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