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Unstable natural or built environment

Hazard Knowledge

Fire and rescue services may be called to many varied environments that can be defined as confined spaces. These could include trenches, excavations and pits.

Personnel should be aware that some of these environments require additional resources and skills beyond the scope of the fire and rescue service.

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

  • Excessive rainfall
  • Vibration from nearby heavy plant or machinery
  • Severe impact
  • A load (e.g. crane/heavy plant) positioned too close to an edge

A serious risk of injury exists at incidents involving an unstable natural environment because soil can weigh more than 1.25 tonnes per cubic metre square. Even small collapses may be fatal. The risks 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 old mines or quarries, agricultural and industrial sites. An incident involving a trench or excavation may require shoring or the removal of soil, along with having to move heavy machinery/plant, etc.

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¬†¬†see National Operational Guidance Utilities and fuel
  • Changes in soil type/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.

Other safe systems of work or procedures may be involved in rescue operations, such as those for confined spaces (a trench may collect flammable or toxic vapours or have an oxygen-deficient atmosphere) or safe working at height (for example, personal protective equipment (PPE) may be required for exposed edges). Access and egress may be difficult.