Annex F


Ground investigations and development in ground potentially containing voids

F.1 General

Where ground has the potential to contain voids, and there is insufficient or inappropriate consideration of them, the presence of such voids can present geotechnical, environmental and/or health and safety hazards to proposed development, which adversely impact the existing use of land or prevent the safe occupation or enjoyment of land.

Such geohazards include:

  • formation of voids at the ground surface;
  • ground subsidence;
  • toxic and asphyxiating gases;
  • combustible materials;
  • rising water levels;
  • mines and natural cavities acting as pollution pathways; and
  • contamination.

As well as the risks that such geohazards could present to a new development or the existing use of the land, a cavity of any sort might have achieved some level of metastability, which can easily be disturbed, for example, by changes in stress brought about by ground investigation or construction.

In the United Kingdom, responsibilities are placed on planning authorities and developers via the planning legislation to ensure that the future development of land is safe, sustainable and appropriate.

NOTE Attention is drawn to the existence of local planning authorities and planning legislation.

The responsibility to mitigate potential hazards associated with land potentially affected by mining, quarrying and natural cavities remains with the developers of the land.

F.2 Classification of voids

To enable organizations, such as consulting engineers and contractors, to design appropriate ground investigations to assess the potential for voids to exist beneath development land, and then to enable the significance of any voids to be assessed, it is fundamental that consideration is given at the earliest stages of investigation to the possible sources of such voids.

A classification of voids, based on their mode of formation, can aid the understanding of the associated hazards and identifying suitable investigation strategies. The classification presented in this annex follows that proposed in Donnelly and Culshaw, 2012 [169].

Voids can be grouped as those formed from either natural processes or those formed from the actions of man, i.e. anthropogenic activities. The principal types of voids are presented in Figure F.1 and further information and references are provided in Table F.1.

Figure F.1 Principal types of void
Principal types of void
Table F.1 Principal types of void (1 of 2)
Void type Subdivisions Characteristics
Caves in more soluble rocks Caves in limestone Voids in limestone / dolomitic limestone.
Caves in gypsum/anhydrite Voids in gypsum/anhydrite.
Breccia pipes Choked near vertical pipes formed by the collapse of voids
Sinkholes (dolines) Solution
Dissolutional lowering of surface; up to 1 000 m across, 100 m deep.
Rock roof failure into cave; up to 300 m across, 100 m deep.
Soil collapse into soil void above bedrock fissure; up to 50 m across, 10 m deep.
Down-washing of soil into bedrock fissures; up to 50 m across, 10 m deep.
Soil infilling of earlier sinkhole in rock; up to 300 m across, 100 m deep.
Caves in insoluble rocks Sea caves
Sandstone caves
Marine erosion of fractures, faults and weak rock horizons.
Sandstones can weather to sand followed by piping.
Landslide fissures Landslide fissures Narrow, sub vertical fissures at the head of larger landslides; can become bridges.
Gulls formed by cambering Open
Periglacial conditions needed for formation; wide fissures (up to 10s of metres) open up as blocks of competent rock slide downhill.
Can become bridges by soil and debris leaving passages 1 m to 2 m wide
Other features Soil pipes
Frost involutions; ice wedges
Glacial over-riding fissures
Pipes, collapses and pseudo-sinkholes
Ephemeral features in some cohesive soils.
Small-scale features in soils in periglacial environments; almost always infilled. Hidden by soil veneer. Circular features 100 mm to 2 m in diameter.
Linear troughs up to 2 m wide and 50 m long
Mining by partial extraction Coal mines
Industrial mineral mines
Building stone mines
Evaporite mines
Metalliferous mines
Depths from less than ten to hundreds of metres, extraction can be over 70% in some cases.
Mines stable / meta-stable at first, but subsidence eventually takes place; this could take more than 100 years to complete.
Mining by total extraction Coal mines Depths from tens to hundreds of metres.
Extraction 100%.
Voids can be left after subsidence.
Subsidence could take many years to complete.
Table F.1 Principal types of void (2 of 2)
Void type Subdivisions Characteristics
Salt extraction Partial extraction
Dissolution mining
Wild brine pumping caused much subsidence.
Modern mining designed to avoid subsidence.
Void might be left filled with saturated brine.
Some subsidence might still take place.
Groundwater abstraction and disposal Groundwater resources supplies
Fluid waste disposal in boreholes
Can cause the reactivation of faults and fissures.
Mining induced fault reactivation Moorland slopes
Hillside slopes
Up to 3 m wide and 500 m long.
Can form complex interconnecting voids, or single isolated voids.
Sometimes bridges by surface vegetation or jointed rock.
Vein stoping Various metalliferous minerals Stopes usually a few metres to tens of metres wide and high angle.
Depths to hundreds of metres or more.
Subsidence could take more than 100 years to complete.
Surface mining Quarries
Pits in soils and softer rocks have inclined walls.
Quarries in stronger rocks have steep to vertical walls.
Both might be backfilled with natural materials or waste.
Mine entrances Shafts
Many not recorded.
Can be well capped, poorly capped or bridged by debris.
Can be unstable, particularly near entrance and when shallow.
Engineering Tunnels
Underground storage chambers
Utilities and pipelines
Linear, 1 m to 10+ m in diameter.
Shallow and deep; might be unrecorded.
Irregular; usually in stiffer soils or weaker rocks; usually at shallow depth.
Underground storeys of buildings.
Small voids less than 1 m to 5 m.
Military sites Bunkers, shelters and storage facilities
Munitions and explosive manufacturing and test facilities
Vary considerably in size, from less than 1 m2 to hundreds of metres in size, depending upon the design requirements.
Archaeological sites Graves
Hypocausts Wells
Other underground structures
Can vary from centimetres to tens of metres in size; often no record; might have deteriorated and be hard to identify.