A strong intuitive understanding of the different types of soils is useful to understand their strengths and weaknesses, as well as design risks. We cover some basics below.
Soil Classification
There are six major soil classifications:
- Clay Less than 0.002 mm
- Silt 0.002 to 0.06 mm
- Sand 0.06 to 2.0 mm
- Gravel 2 to 60 mm
- Cobbles 60 to 200 mm
- Boulders More than 200 mm
There is another type of ground called "made ground". If we quote the HS2 London-Midland Environmental Statement 2013 (source), it can be defined as:
| Land where natural and undisturbed soils have largely been replaced by man-made or artificial materials. It may be composed of a variety of materials including imported natural soils and rocks with or without residues of industrial processes (such as ash) or demolition material (such as crushed brick or concrete). |
※ What is the Bearing Stength of Different Soils?
Broad Description
The soils above (excl. made ground) may be classified into two main types:
- Fine Soils
- Coarse Soils
Clay and Silt are fine soils (smaller than 0.06 mm in size). The larger soils (from sand upwards) are described as coarse soils. Cobbles and Boulders may be described as very coarse soils.
Soil Behaviour
It is difficult to comprehensively summarise soil behaviour in a brief description. However, broadly speaking, fine and coarse soils can be described respectively as follows:
- Cohesive Soils (Clay and Silt)
- Granular Soils (Sand, Gravel, Cobbles, etc.)
Cohesive soils are small enough that the soil particles adhere to each other, hence the name. The adhesion that exists gives them good strength and prevents sudden failures.
How to Determine the Soil Type
Soil types are determined by undertaking a soil investigation of the site and taking borehole surveys. By looking at the borehole samples and performing particle distribution tests, we can determine the composition of the soil. Usually, multiple boreholes will be taken from a single site since the ground conditions across the site may vary.
※ What is a Geotechnical Investigation? Significance, Methods and Findings.
※ What is a Particle Size Distribution Test?
An example of findings from a particle size distribution test is shown below. This represents a well-graded soil (a soil sample which displays the full distribution of soil types).
(source: Elementary Engineering Library)
Other Types
Expandable Soils
Engineering Soils
Limestone, Ash, etc.