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

Table F.1 Principal types of void (1 of 2)

Void type	Subdivisions	Characteristics
Natural
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 below.
Sinkholes (dolines)	Solution Collapse Caprock Dropout Suffusion Buried	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 Covered	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
Anthropogenic
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	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 Adits Drifts Headings Roadways	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 Basements 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 Kilns Shafts Other underground structures	Can vary from centimetres to tens of metres in size; often no record; might have deteriorated and be hard to identify.