Last edited 22 Nov 2016

Types of soil

Contents

[edit] Introduction

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 a map.
  • Provide predictions about how soils will behave.

[edit] Basic soil types

[edit] Very coarse

[edit] Boulders

Boulders.jpg

  • 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.

[edit] Cobbles

Cobbles.jpg

  • Particle sizes: 60-200 mm.
  • Particle shape: Angular, subangular, 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.

[edit] Coarse soils

[edit] Gravels

Gravel.jpg

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.

[edit] Sands

Sand.jpg

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.

[edit] Fine soils

[edit] Silts

Silt.jpg

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.

[edit] Clays

Clay.jpg

  • 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.

[edit] Organic soils

[edit] 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).

[edit] Peats

Peat270.jpg

  • 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).

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki:

[edit] External references

  • ‘Introduction to civil engineering construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)