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Types of soil

Introduction

The term 'soil' refers to 'The upper layer of the earth's crust, in which plants grow. It consists of weathered rock, organic matter, air spaces and water. Descriptions usually identify the relevant characteristics of its (usually) horizontal layers in terms of their significance for soilcharacteristics and crop growth, usually to a depth of 1.2m.'

Soil structure is the combination or aggregation of soil particles into larger compound units (or peds) with pore spaces and channels between that allow the flow of air and water and the penetration of roots. The secondary units are characterised and classified on the basis of size, shape and degree of development.

Soil texture is the relative proportion of the various soil particle size fractions in a soil (sand, silt and clay).

A soil profile is a vertical cross-section through a soil.

Soil wetness is classified according to the depth and duration of waterlogging in the soil profile. Six wetness classes are identified, ranging from ‘very well drained’ to ‘very poorly drained’. A soil wetness limitation exists where the soil water regime adversely affects plant growth or imposes restrictions on cultivations or grazing by livestock.

Ref The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.

Many different types of soil can be encountered on construction sites. The importance of the characteristics of soil, such as the size and nature of particles, its density and structural properties, means soil surveys are often required to inform design and construction decisions.

A soil survey will:

Classify the soil according to a standard classification system.
Record the boundaries of soils on drawings.
Provide assessments how soils are likely to behave.

Basic soil types

Very coarse

Boulders

Particle sizes: Anything above 200 mm.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Compactness: Loose.
Strength field test: By inspection of voids and particle packing.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Cobbles

Particle sizes: 60-200 mm.
Particle shape: Angular, sub-angular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Compactness: Dense.
Strength field test: By inspection of voids and particle packing.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Coarse soils

Gravels

Coarse gravels:

Particle sizes: 20-60 mm. Easily visible to the naked eye meaning that grading can be described. Well graded means there is a wide range of grain sizes; poorly graded means there is a limited range.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Clay or silt content of under 5%. Classified as ‘slightly clayey’ or ‘slightly silty’.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Medium gravels:

Particle sizes: 6-20 mm. Easily visible to the naked eye meaning that grading can be described. Well graded means there is a wide range of grain sizes; poorly graded means there is a limited range.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Clay or silt content of 5-15%. Classified as ‘clayey’ or ‘silty’.
Compactness: Loose.
Strength field test: Can be excavated with a spade; 50 mm wooden peg can be easily driven.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Fine gravels:

Particle sizes: 2-6 mm. Easily visible to the naked eye meaning that grading can be described. Well graded means there is a wide range of grain sizes; poorly graded means there is a limited range.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Clay or silt content of 15-35%. Classified as ‘very clayey’ or ‘very silty’.
Compactness: Dense.
Strength field test: Excavation requires a pick; 50 mm wooden peg is difficult to drive.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Sands

Coarse sands:

Particle sizes: 0.6-2 mm. Visible to the naked eye. When dry there is little to no cohesion. Grading can be described.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Classified as ‘sandy gravel’ or ‘gravelly sand’. Clayey composites are described as plastic or cohesive. Silty composites are described as non-plastic or of low plasticity.
Compactness: Slightly cemented.
Strength field test: Visual examination; pick removes soil in lumps.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Medium sands:

Particle sizes: 0.2-0.6 mm. Visible to the naked eye. When dry there is little to no cohesion. Grading can be described.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Classified as ‘sandy gravel’ or ‘gravelly sand’. Clayey composites are described a plastic or cohesive. Silty composites are described as non-plastic or of low plasticity.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Fine sands:

Particle sizes: 0.06-0.2 mm. Visible to the naked eye. When dry there is little to no cohesion. Grading can be described.
Particle shape: Angular, subangular, rounded, flat, elongated.
Texture: Rough, smooth, or polished.
Composite soil types: Classified as ‘sandy gravel’ or ‘gravelly sand’. Clayey composites are described a plastic or cohesive. Silty composites are described as non-plastic or of low plasticity.
Structure: Homogenous, inter-stratified, heterogeneous, or weathered.

Fine soils

Silts

Coarse silts:

Particle sizes: 0.02-0.06 mm. Barely visible to the naked eye.
Particle nature: Non-plastic or of low plasticity.
Compactness: Soft or loose.
Strength field test: Easily moulded or powdered between fingers.
Structure: Fissured, intact, homogeneous, inter-stratified, or weathered.

Medium silts:

Particle sizes: 0.006-0.02 mm. Not visible to the naked eye.
Particle nature: Non-plastic or of low plasticity.
Compactness: Firm or dense.
Strength field test: Can be moulded or powdered between fingers with strong pressure.
Structure: Fissured, intact, homogeneous, inter-stratified, or weathered.

Fine silts:

Particle sizes: 0.002-0.006 mm. Not visible to the naked eye.
Particle nature: Non-plastic or of low plasticity.
Composite soil types: Sand or gravel content of 35-65%. Classified as ‘sandy’ or ‘gravelly’.
Compactness: Very soft.
Strength field test: Exudes between fingers when squeezed in hand.
Structure: Fissured, intact, homogeneous, inter-stratified, or weathered.

Clays

Dry lumps can be broken but not powdered between fingers; smooth to the touch; shrinks on drying which usually leaves cracks.
Particle nature: Intermediate plasticity through to high plasticity.
Composite soil types: Sand or gravel content of less than 35%.
Compactness: Soft.
Strength field test: Soft (moulded by light finger pressure), firm (moulded by strong finger pressure), stiff (can be indented by thumb), very stiff (can be indented by thumb nail).
Structure: Fissured, intact, homogeneous, inter-stratified, or weathered.

Organic soils

Organic clay, silt or sand

Particle sizes: Varies.
Visual identification: Contains substantial amounts of organic vegetable matter.
Composite soil types: Sand or gravel content of 35-65%. Classified as ‘sandy’ or ‘gravelly’.
Compactness: Firm.
Strength field test: Fibres already compressed together.
Structure: Fibrous (plant remains recognisable and retains some strength); amorphous (recognisable plant absent).

Peats

Particle sizes: Varies.
Visual identification: Predominantly plant material which remains dark brown or black, usually with a distinctive smell and low-bulk density.
Compactness: Spongy, plastic.
Strength field test: Very compressible and open structure; can be moulded by hand.
Structure: Fibrous (plant remains recognisable and retains some strength); amorphous (recognisable plant absent).

Alluvial deposits

Natural materials deposited within and adjacent to rivers. For more information see: Alluvium.

Others

Calcareous soil

Calcareous soil is soil which contains accumulations of calcium and magnesium carbonate, formed by the weathering of calcareous rocks and fossil shell beds.

Plastic soil

Plastic soil is soil with a relatively high proportion of silt and clay that, within a certain moisturecontent range, is capable of being moulded or deformed without rupture.

Subsoil

Weathered soil layer extending between the natural topsoil and the unweathered basal layer(geological parent material) below, or similar material on which topsoil can be spread. Subsoilhas lower organic matter and plant nutrient content than topsoil. In most cases topsoils require a subsoil to perform one or a number of natural soil functions.

Ref The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.

Topsoil

The upper layer of a soil profile, usually darker in colour (because of its higher organic matter content) and more fertile than subsoil, and which is a product of natural biological and environmental processes. ibid DETR 2013

Superficial deposit

A geological deposit that was laid down during the Quaternary period. Such deposits were largely formed by river and glacial processes but can also include wind-blown deposits known as loess. ibid DETR 2013

Till

An unsorted glacial sediment deposited directly by a glacier. ibid DETR 2013

External references

‘Introduction to civil engineering construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)
The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.
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