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by the NFCC


Unstable or collapsed natural or built environments

Hazard Knowledge

Fire and rescue services may be called to many natural or built environments, some of which can be defined as confined spaces; this includes trenches, excavations and pits.

Personnel should be aware that some of these environments require additional resources and skills beyond the scope of non-specialist responders.

The natural environment, such as a trench, excavation, pit, cliff or steep ground, or free-flowing solids, may be subject to instability, for example due to:

  • Excessive rainfall
  • Vibration from nearby heavy vehicles or machinery
  • Severe impact
  • Loads, such as vehicles, machinery or building materials, being positioned close to an edge
  • Failure of supports

A serious risk of injury exists at incidents involving an unstable natural environment because soil can weigh up to 1.7 tonnes per cubic metre. Even small collapses may be fatal. The hazards to people include:

  • Becoming trapped or buried
  • Being crushed by the movement of soil and any subsequent loading
  • Falling into a trench, pit or opening
  • Drowning

Trenches and excavations are present in building works and utilities maintenance, and pits can be found in a variety of locations, such as used or abandoned mines or quarries, and agricultural or industrial sites. An incident involving a trench or excavation may require shoring or the removal of soil, along with having to relocate heavy machinery or other objects.

Any unsupported trench or excavation may be subject to collapse and there are many factors that will influence stability, such as:

  • Height of face
  • Angle of face
  • Type of soil
  • Adjacent loading
  • Vibration from nearby machinery or vehicles
  • Water content
  • Surface water
  • Buried services or other obstructions – refer to Utilities and fuel
  • Changes in soil type or make-up
  • Previously worked-on ground
  • Weather conditions
  • Length of time the trench or excavation has been exposed

Loading at ground level adjacent to the trench increases the likelihood of unstable faces collapsing, as does vibration from machinery or vehicles; this area should be kept clear.

The incident may involve:

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