Skip to main content

Developed and maintained
by the NFCC

Unstable or collapsed excavations

Hazard Knowledge

Excavations may become unstable or collapse; some may be defined as, or become, confined spaces. Excavations may include trenches, pits and tunnels. 

Excavations should be adequately supported, or be sloped or battered back to a safe angle. Excavation support systems (also referred to as shoring systems) are designed to stop the collapse of an excavation wall. The type and strength of the support system will be influenced by factors such as the type of material being excavated and the height and angle of the excavated face. 

Support systems used for excavations need to meet and maintain various standards, as defined in guidance and regulations such as: 

Excavations may become unstable or collapse due to: 

  • Vibration from vehicles or machinery 
  • Weather conditions and surface water 
  • Severe impact including explosions
  • Loads, such as vehicles, machinery or building materials, being positioned close to an edge 
  • Failure of supports 

Signs of collapse may include distortion or deflection of supports, tension cracks or soil movement. 

The incident may involve: 

  • Collapse of the sides or roof 
  • People or objects falling into the excavation 
  • Materials falling onto people working in the excavation 
  • The undermining of nearby structures 
  • Damage to utilities 
  • Water ingress 

An unstable or collapsed excavation may involve many tonnes of material, such as soil, leading to risks including entrapment under the material, or physical injuries.