Last edited 24 Nov 2016

Types of excavation

Excavator.jpg

Contents

[edit] Introduction

In construction terms, excavation is the process of removing earth to form a cavity in the ground.

On small sites or in confined spaces, excavation may be carried out by manual means using tools such as picks, shovels and wheelbarrows. Larger scale excavation works will require heavy plant such as bulldozers and backactors. For more information, see Excavating plant.

[edit] Material types

A common method of classification is by the material being excavated:

[edit] Topsoil excavation

This involves the removal of the exposed layer of the earth’s surface, including any vegetation or decaying matter which could make the soil compressible and therefore unsuitable for bearing structural loads. The depth will vary from site to site, but is usually in a range of 150-300 mm.

[edit] Earth excavation

This involves the removal of the layer of soil directly beneath the topsoil. The removed material (referred to as 'spoil') is often stockpiled and used to construct embankments and foundations.

[edit] Rock excavation

This is the removal of material that cannot be excavated without using special excavation methods such as drilling (by hand or with heavy machinery) or blasting with explosives.

[edit] Muck excavation

This is the removal of excessively wet material and soil that is unsuitable for stockpiling.

[edit] Unclassified excavation

This is the removal of a combination of the above materials, such as where it is difficult to distinguish between the materials encountered.

[edit] Excavation purpose

Excavation can also be classified according to the purpose of the work:

[edit] Cut and fill excavation

This is the process of excavation whereby the material that is cut or stripped. The removed topsoil and earth can be used as fill for embankments, elevated sections, and so on. It can also be used to form a level surface on which to build, as elevated sections of the site are ‘cut’ and moved to ‘fill’ lower sections of the site.

[edit] Trench excavation

A trench is an excavation in which the length greatly exceeds the depth. Shallow trenches are usually considered to be less than 6 m deep, and deep trenches greater than 6 m.

Trench, or footing, excavation is typically used to form strip foundations, buried services, and so on. The choice of technique and plant for excavating, supporting and backfilling the trench depends on factors such as; the purpose of the trench, the ground conditions, the trench location, the number of obstructions, and so on.

The common techniques that are used include:

  • Full depth, full length: Suitable for long narrow trenches of shallow depth, such as pipelines and sewers.
  • Full depth, successive stages: Suitable for deep trenches where works can progress in sequence, reducing the risk of collapse.
  • Stage depth, successive stages: Suitable for very deep trenches in confined areas, deep foundations and underpinning.

[edit] Basement excavation

A basement is part of a building that is either partially or completely below ground level. For more information, see Basement excavation.

[edit] Road excavation

This typically involves stripping topsoil and cut-and-fill. For more information, see Road construction.

[edit] Bridge excavation

This typically involves the removal of material for the footing and abutments of bridges. The work may be subdivided into wet, dry and rock excavation. Underwater excavations may require special methods of drill and blast. For more information, see Bridge construction.

[edit] Dredging

Dredging is the process of excavating and removing sediments and debris from below water level, typically from the bottom of lakes, rivers, harbours, and so on. For more information, see Dredging.

[edit] Excavation support

Materials have different stability characteristics during excavation works. The ‘angle of repose’ of the material describes the steepest angle at which it will remain stable without support. The exact angle of repose will depend on the presence of groundwater, but some typical angles are:

  • Drained clay: 45-degrees.
  • Wet clay: 16-degrees.
  • Gravel and dry sand: 40-degrees.
  • Wet sand: 22-degrees.

The type and extent of temporary support that is required will depend on the following factors:

  • The stability and angle of repose of the subsoil.
  • The proximity of the excavation to vehicles, services and buildings.
  • The level of the water table.
  • The type/s of subsoil.
  • The depth of the excavation.
  • The length of time the excavation will be left open.
  • The time of year and weather conditions.

The types of support that can be used include:

  • Timber supports: Commonly used for low risk, narrow trenches, shafts or headings.
  • Trench boxes: Can be placed in pre-excavated trenches in low-risk situations.
  • Trench sheets: Can be overlapping or interlocking, and are used to provide continuous support for deeper trenches.
  • Ground anchors and rock bolting.
  • Caissons.
  • Cofferdams.

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