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 & wheelbarrows.Largescale excavation works will require heavy plant such as multipurpose excavator,poclain & bulldozers.
Excavation may be classified as follows:
- Topsoil excavation
- Earth excavation
- Rock excavation
- Muck excavation
- Trench excavation Basement excavation
- Basement excavation
Generally, the choice of excavating plant will depend on:
- The quantity of material to be excavated.
- The type of condition of the soil
- The weather conditions
- The speed of excavation required
- The depth of the excavation
- The material disposal method (side cast or cart away)
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.