An excavation pit (also called a construction pit or cofferdam) is a secured, excavated and dewatered space in the ground in which underground construction work is carried out. The walls of the pit are supported by sheet piles, diaphragm walls or soldier pile walls to prevent earth and water from entering.
Excavation Pit vs Open Cut
Although the terms are sometimes used interchangeably:
- Open cut — an open excavation, with or without sloped sides
- Excavation pit — an enclosed pit with watertight walls and active dewatering
Application
Excavation pits are used for deep underground structures:
- Basements — car parks beneath apartment complexes or offices
- Tunnels — the launch and reception shafts for tunnel boring machines
- Stations — underground railway and metro stations
- Locks and pumping stations — hydraulic engineering structures
Construction Sequence
- Install retaining walls — sheet piles or diaphragm walls are driven or constructed in the ground
- Excavate — the soil within the walls is removed layer by layer
- Prop — props or ground anchors hold the walls in place
- Dewater — groundwater is pumped out to allow dry working conditions
- Build — the basement or structure is constructed within the pit
- Backfill — after completion, the walls are removed if temporary and the remaining space is backfilled
Related Terms
- Sheet pile wall
- Dewatering
- Foundation
- Basement
- Ground anchor
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