Lesson 5: How do engineers describe and classify soil?

You will learn

Putting it all together

In Lesson 3 you measured grain sizes. In Lesson 4 you measured how a soil responds to water. Now you will use those results the way engineers do every day: to give a soil a short classification symbol, and to write a clear description that anyone in the industry can read.

Keep your answers from Lessons 3 and 4 handy. We reuse those soils here.

Why it matters

Classification and description do different jobs

Borehole logs, lab reports and design reports all rely on these two. A designer who has never seen the soil should be able to picture it from the description, and know roughly how it behaves from the classification.

Classifying

The Unified Soil Classification System

The Unified Soil Classification System (USCS) grew out of a system devised by Arthur Casagrande in 1948 and is now used on projects all over the world. Every soil gets a two-letter symbol: the first letter says what it mainly is, and the second adds detail.

The first decision is always the same: is the soil mainly coarse or mainly fine? If more than half of it is coarser than the fines boundary, it is a coarse soil (G or S). If half or more is finer, it is a fine soil (M, C or O). For a coarse soil, it is a gravel if there is more gravel than sand, and a sand otherwise.

Classifying

Classifying a gravel or sand

For a coarse soil, the second letter depends on how much fines it contains:

Remember the well-graded rule from Lesson 3: gravel needs C_u of at least 4, sand needs C_u of at least 6, and both need C_c between 1 and 3.

Classifying

Classifying a silt or clay

For fine soils, everything comes from the plasticity chart you met in Lesson 4:

• Above the A-line: clay (C). Below it: silt (M).
• LL below 50: low plasticity (L). LL of 50 or more: high plasticity (H).
• That gives four main symbols: CL, CH, ML, MH.
• If the soil plots in the narrow band with PI of 4 to 7 on or above the A-line, it is CL-ML.
• Organic soils (OL, OH) plot below the A-line. They are usually dark, smell of rotting plants, and their liquid limit drops sharply after oven drying.
• Peat (Pt) is mostly plant remains. It is spongy, fibrous, dark and very compressible. It is recognised by eye, not by tests.

Worked example

Classifying the soils from Lessons 3 and 4

Using it

What a symbol tells an engineer

The real value of a classification is what it says about behaviour. A few broad patterns:

These are general tendencies, not design values. A soil’s state in the ground (how dense, how stiff, how wet) matters just as much, and that is what the description records.

Describing

A description has a fixed order

So that everyone writes and reads descriptions the same way, each part has a fixed place. In the UK the order comes from BS 5930, which follows BS EN ISO 14688-1:

Two examples in the right order:

Describing

The main soil and its secondary constituents

For a coarse soil, the main type is simply the biggest part by mass. For a fine soil, you name it CLAY or SILT by how it behaves (its plasticity), not by the mass of clay-sized grains. A soil with only 25% clay-sized particles can still be a CLAY if it plots above the A-line.

Secondary constituents are described with set words. The amount needed depends on whether the main soil is coarse or fine:

Notice how much sand a clay needs before it is even called “sandy”. A little clay changes how a soil behaves far more than a little sand does.

Where two soils are present in roughly equal amounts, both can be named, for example SAND and GRAVEL. Check the current edition of BS 5930 for the full set of terms.

Describing

How firm, how dense, what colour, what shape

Consistency of fine soils

Density of coarse soils

Sands and gravels are described as very loose, loose, medium dense, dense or very dense. This is usually judged from in-situ tests such as the SPT, because a sand sample loses its natural density once it is dug up.

Colour, fabric and particle shape

Record colour on a freshly broken surface, because some soils change colour soon after they meet the air. Use simple words: light or dark, a tint such as reddish or greyish, then the main colour, for example dark greyish brown. Note fabric such as fissured (fine cracks) or laminated (thin layers) where you can see it.

Particle shape hints at history: angular grains have not travelled far, while rounded grains have been worn down by water or wind.

Worked example

Building a description, step by step

Back to the Lesson 4 clay. On site, a thumb pressed into it left an easy impression, it had fine cracks, and its fresh surface was greyish brown. The lab found about 25% sand, LL = 46 and PI = 23.

And the Lesson 3 sand, which an SPT showed to be medium dense, with light brown subrounded grains of all sizes:

Your turn

Classify and describe

Quick check

Five quick questions

This is not a test. It simply checks the main ideas from this part of the course. Choose an answer to see the explanation.

Key points

What to take away

• Classification puts a soil in a group. Description records this particular soil. Each checks the other.
• USCS symbols have two letters: the main soil (G, S, M, C, O, Pt) and a detail (W, P, M, C, L, H).
• Coarse soils: under 5% fines, use grading (W or P); 5 to 12%, use a dual symbol; over 12%, use the type of fines (M or C).
• Fine soils: above or below the A-line gives C or M; LL below or above 50 gives L or H.
• Descriptions follow a fixed order, ending with the MAIN SOIL TYPE in capitals, then extra detail and geological origin.
• Fine soils are named CLAY or SILT by how they behave, and need far more sand before they are called “sandy”.
• Always check that the description agrees with the lab results.

Assessment (optional)

Test yourself properly

Unlike the practice questions, this assessment has no answers on the page. Attempt it on your own, as if it were an exam.

Section A: Concepts (12 marks)

1. Explain the difference between a soil classification and a soil description. [2 marks]
2. State what each of these USCS letters means: G, S, M, C, W, P, L and H. [4 marks]
3. Explain why a fine soil is named CLAY or SILT by how it behaves, rather than by its percentage of clay-sized particles. [2 marks]
4. List the parts of a UK engineering soil description in the correct order. [4 marks]

Section B: Classification and description (16 marks)

5. Give the USCS symbol for each soil, and show your reasoning. Use the A-line, PI = 0.73 × (LL − 20), where needed. [3 marks each, 12 total]

   Soil	Test results
   P	Gravel 62%, sand 35%, fines 3%. Cu = 12, Cc = 1.8
   Q	Gravel 10%, sand 70%, fines 20%. Fines: LL = 30%, PI = 12%
   R	Fines 85%. LL = 62%, PI = 22%
   S	Gravel 5%, sand 87%, fines 8% (non-plastic). Cu = 3.0, Cc = 1.1

6. A borehole log reads: Brown sandy CLAY, firm, fissured.

   (a) Rewrite the description in the correct order. [2 marks]

   (b) Laboratory tests show the soil contains 20% sand. Which word should replace “sandy”, and why? [2 marks]

Section C: Applied case (12 marks)

7. A field log describes a layer as Loose brown slightly silty fine SAND. Laboratory tests on a sample from the same layer give 28% fines, with LL = 34% and PI = 14% for the fines.

   Write a short technical note to the project engineer (maximum 300 words) that:

   • classifies the soil from the laboratory results, showing your reasoning [3 marks]
   • explains why the field description and the laboratory results disagree, and what you would check [3 marks]
   • gives a corrected description that agrees with the laboratory results [3 marks]
   • notes the sieve sizes behind the USCS symbol [1 mark]

   Clear, well-organised communication. [2 marks]

How your work is judged

References

• ASTM International (2017) ASTM D2487-17 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). West Conshohocken, PA: ASTM International.
• British Standards Institution (BSI) (2018) BS EN ISO 14688-1:2018 Geotechnical investigation and testing. Identification and classification of soil. Identification and description. London: BSI.
• British Standards Institution (BSI) (2020) BS 5930:2015+A1:2020 Code of practice for ground investigations. London: BSI.
• Casagrande, A. (1948) ‘Classification and identification of soils’, Transactions of the American Society of Civil Engineers, 113, pp. 901–930.
• Clayton, C.R.I., Matthews, M.C. and Simons, N.E. (1995) Site Investigation. 2nd edn. Oxford: Blackwell Science.
• Knappett, J.A. and Craig, R.F. (2019) Craig’s Soil Mechanics. 9th edn. Boca Raton, FL: CRC Press.

Further learning

• Practise on real logs: in the UK, the British Geological Survey’s free GeoIndex map (bgs.ac.uk) shows thousands of real borehole logs. Read the descriptions and check they follow the order in this lesson.
• Describe a soil yourself: take a sample from a garden or roadside cutting, and write a description using only what you can see and feel.
• Read: Chapter 1 of Craig’s Soil Mechanics covers classification with further examples.