CIVL2410 Chap.4 Classifying soils: the Unified Soil Classification System
Classifying soils: the Unified Soil Classification System
Classification turns two cheap measurements, a grading curve and a pair of Atterberg limits, into a two letter symbol that lets an engineer anticipate drainage, settlement, workability and stability well enough for preliminary design.
This chapter walks the decision path from the 0.075 mm sieve through the coarse and fine routes to a group symbol, including dual symbols for borderline soils, and is honest about what the symbol may and may not be used for.
What this chapter covers
- 01
Why a cheap classification is needed when soil has to be used as found
- 02
The 0.075 mm split between the coarse and fine grained routes
- 03
The grammar of the symbol: G, S, M, C, O and PT, then W, P, H and L
- 04
The 4.75 mm split that separates a gravel from a sand
- 05
Clean, borderline and dirty coarse soils at 5 and 12 per cent fines
- 06
The grading criteria, and why a sand faces a stricter uniformity threshold
- 07
The plasticity chart, and reading both letters from one plotted point
- 08
The A line, and why the boundary slopes rather than sitting at a fixed plasticity
- 09
Dual symbols such as SW-SM, and when both routes must be evaluated
- 10
What a group symbol is evidence for, and where a measured value is required instead
A fine soil classified from its limits
- +1More than half the sample passes 0.075 mm, so the soil is fine grained and the grading curve is set aside entirely.
- +1The plasticity index is 52 - 22 = 30.
- +1At a liquid limit of 52 the A line sits at 0.73 x (52 - 20) = 23.4, and 30 lies above it, so the point plots above the A line and the first letter is C.
- +1The liquid limit exceeds 50, so the second letter is H, giving the symbol CH.
Key terms
- Group symbol
- The two letter code a classification returns. The first letter names the material and the second qualifies it, by grading for a coarse soil and by liquid limit for a fine one.
- Well graded
- A coarse soil containing a wide range of particle sizes with the intermediate sizes present, so the small grains can fill the voids between the large ones. It has to pass both a uniformity and a curvature criterion.
- A line
- The sloping boundary on the plasticity chart that separates clays above it from silts below it. Its slope encodes the fact that the useful quantity is plasticity relative to liquid limit rather than plasticity alone.
- Dual symbol
- A classification written with two group symbols, used when a coarse soil carries between about 5 and 12 per cent fines so that both the grading route and the plasticity route have to be evaluated.
Classifying soils: the Unified Soil Classification System FAQ
Why does a sand need a higher uniformity coefficient than a gravel?
Because the two materials achieve dense packing differently. A gravel spans a wide absolute range of sizes even at a modest uniformity coefficient, so a value above four already indicates a usefully broad distribution. A sand occupies a narrower band of absolute sizes, so the same coefficient describes a much tighter spread, and the threshold is raised to six to demand a comparable range in practice.
Both still have to satisfy the same curvature window, since a gap in the middle disqualifies either material.
Can I use a group symbol instead of a measured permeability?
Only for a first pass. A symbol is a prediction of behaviour from tests that can be run in an afternoon on material anyone can dig, and it is genuinely useful for deciding whether to expect free drainage or effectively none, and whether settlement will be immediate or take years. It is not a substitute for a measured permeability, strength or compressibility, all of which require the apparatus described later in this unit.
Quoting a symbol where a number was asked for is a reliable way to lose marks.
Exam move
Learn this chapter as a flowchart rather than as a set of rules, and rehearse it by classifying soils from data you invent yourself until the branch points come without hesitation. The three decisions are the 0.075 mm split, the 4.75 mm split and the fines content, in that order, and each one closes off half of what remains.
Write the deciding test beside every step of your answer, because the marks follow the reasoning rather than the symbol. Watch for points that sit close to the A line or to a fines boundary and say so explicitly: a borderline classification reported as borderline is a better answer than a confident one on the wrong side of a line the plastic limit test cannot resolve.
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