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Unstable surfaces

Hazard Knowledge

For the purposes of this guidance unstable surfaces, includes any surface that may give way, break, collapse or allow people, equipment or vehicles to sink or become stuck. This includes but is not limited to:

  • Ice
  • Mud
  • Gravel
  • Sand
  • Slurry
  • Bulk storage of materials that do not provide a firm footing such as rubble, grain or rice

The hazards and associated control measures for unstable surfaces also apply to steep slopes, embankments, trench walls, cliff tops and roofs. Incidents may combine working at height, confined space and unstable surfaces.

A surface may appear to be stable, but the effects of the incident or operational activity may cause it to become unstable. Examples include peat or waste burning away underfoot or the application of large volumes of water to a soluble or semi-soluble substance such as mud. There is frequently little or no indication of the transition between firm and unstable surfaces.

Sudden collapse or the failure of unstable surfaces may be caused by adverse weather conditions or naturally occurring erosion that has undermined the ground’s integrity and it can fail without warning. Vibration from nearby machinery, severe impact and heavy loads close to an edge can also affect the stability of the ground. Partial or full building collapse may be caused by or cause unstable surfaces and may indicate that the area surrounding the collapsed structure is unstable.

The natural environment including pits, cliffs, steep ground, or free-flowing solids may be subject to instability.

Trenches and excavations are present in building works and utilities maintenance. 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 or plant. See National Operational Guidance: Sub-surface, height, structures and confined spaces – Unstable natural or built environment.

Geological weakness such as earthquakes and subsidence may cause buildings to collapse through movement of the strata on which the foundations are laid. This movement can place excessive stress on a structure, overloading it and causing collapse. Alternatively, the ground on which the building is constructed may weaken to the extent that it is unable to support the weight of the building. Liquefaction, where the water content in the soil increases to such an extent that the soil loses all cohesiveness and strength and the building sinks into the ground, is one of the most common forms of failure.

Unstable surfaces can be hazardous; an unstable or soft surface may give way when downward pressure is applied. The surface may be so soft that humans or animals can sink until movement becomes impossible. Surfaces can have a layer of relatively firm ground covering a softer surface beneath. A thin layer of firm ground may break through when pressure is applied causing personnel to sink into softer ground below.

A serious risk of injury exists at incidents involving an unstable surface; soil, for example, 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 from height
  • Falling into a trench, pit or opening
  • Drowning and asphyxiation

Depending on the site involved, ground conditions can become unstable for many reasons. Certain sites present increased risks of ground becoming unstable:

  • Landfill sites: deep-seated fires burning waste deep under the crust can create underground voids with little or no warning. These voids present a significant hazard to onsite vehicles as well as to responding fire and rescue service personnel.
  • Poorly maintained or illegal industrial sites and byways: these sites are less likely to have designated hard standing or purpose-built vehicle routes. Ground conditions can quickly deteriorate as a result of firefighting and vehicle movement.
  • Farmland: tracks and fields may not be suitable for fire service vehicles. Slurry pits may appear to be hard waste ground, or covered in grass or weed growth.
  • Naturally occurring hazards such as sink holes and liquefaction can make a surface that appears solid to collapse or become unstable.


Ice should never be considered safe to walk on. The thickness may vary depending on water depth, temperature and microclimates. It can be difficult to determine the depth and flow of water underneath the ice. Personnel can fall through ice and travel a significant distance below the surface or become trapped beneath it.


It may be difficult to access casualties through deep mud, as walking becomes impossible without specialist equipment such as mud shoes. The distance to the casualty can make the situation more difficult. Incidents can be particularly hazardous at night or during periods of poor visibility such as dense fog.

Working on unstable surfaces

Personnel should be aware of the physical condition of the surfaces they are working on. Roofs may be weak, embankments and cliff tops may be steep, slippery or have loose surface materials and some built environments may be unstable and weaker than they first appear.

Fire and rescue personnel should remember that while the equipment they use is regularly inspected, tested and maintained, the areas they traverse and attach to are not. These areas should be stringently examined and suitable precautions taken.

Fire and rescue service vehicles may become trapped or stranded on unstable, soft or uneven ground. On sites that do not have designated hard standing or purpose-made vehicle routes, ground conditions can quickly deteriorate. Weather conditions, firefighting operations and vehicle movements can cause ground conditions to deteriorate.

The hazards and associated control measures for bodies of water and unguarded edges apply when working near unstable surfaces.