F.3 Natural voids

F.3.1 Soluble rocks

The dissolution of soluble rocks such as halite, gypsum, anhydrite, chalk and limestone can lead to a variety of types of natural voids. Natural dissolution processes are generally slow in limestone and chalk and such voids have typically developed over long periods of time, frequently thousands of years. In suitable hydrogeological situations, dissolution of gypsum can be rapid and dissolution of salt extremely rapid. Many dissolution voids are related to periglacial processes, but the triggering mechanisms of collapse commonly relate to localized infiltration.

Dissolution usually affects limestone, chalk, rock salt and gypsum-bearing rocks and these lithologies contain the majority of natural voids in the United Kingdom. Dissolution features associated with soluble rocks are, therefore, not uniformly distributed throughout the United Kingdom. Consideration of lithology and the processes involved in void formation assists in assessing the susceptibility of a particular area to contain such natural voids and their influence on a particular project.

Rocks are progressively dissolved on contact with water, forming a wide geometrical variety of voids. This process, although more dominant in the upper zones of soluble rocks, can result in the formation of voids at depth by dissolution along discontinuities. Dissolution enables the formation of a variety of void types including cave systems, open fissures or enlarged joints, sinkholes, dissolution breccias, "wet rockhead" and pipes. "Karst topography", the characteristic landscape formed as a result of dissolution processes, could be formed. Such karst might comprise grikes and dykes of exposed limestone pavements or by gently undulating dry valley terrain, e.g. the Pennines. The potential hazards represented by this and other sources of natural voids are summarized in Table F.2.

A map of the various karst areas in the UK is included at: www.bgs.ac.uk/caves [last viewed 24 June 2015].

Table F.2 Natural voids: potential hazards (1 of 2)

Potential hazard	Naturalvoids
Potential hazard	Soluble rocks	Mass move ment	Glacial and frost	Action of water	Faults and fissures
Near surface voids into which overlying ground collapses might generate voids at the ground surface.	X
Voids can collapse, resulting in subsidence of the land above. More commonly, changes in ground or surface water flow can flush out existing sediment-filled fissures, sinkholes and caves, leading to subsidence. This causes the formation of circular cylindrical or conical depressions at the ground surface known variously as sinkholes, swallow holes, shake holes or dolines.	X
Preferential flow of water (and high groundwater level) can control the hydrogeology of wide areas and might represent a transmission pathway for contamination. Preferential flow of water (and high groundwater level) can control the hydrogeology of wide areas and might represent a transmission pathway for contamination.	X
Where rocks become partially dissolved they might become brecciated, with a marked reduction in intact rock and rock mass strength characteristics.	X
In areas of rock salt the long-term dissolution processes can result in the formation of "wet rockhead", a "solution" surface below which the horizons of rock salt remain intact and above which they are dissolved.	X
Depth to rockhead in areas of karst might be variable and unpredictable.	X
The generation of a failed mass of material containing a high volume of voids. Such materials might have a high potential to collapse following the re-orientation and reduction in the void spaces or might potentially flow under the action of water.		X		X
Migration of voids generating localized loss of ground and voids at the ground surface.	X	X	X	X
Subsidence	X	X	X	X	X
Linear cracks (fissures or breaks) in soils or rock either formed where the materials have detached totally, or where the tensional stresses on the slope have formed tension cracks behind an unstable slope face. Such features might become blocked or choke filled at the ground surface and represent a hazard to existing or proposed development above.		X	X
Preferential migration pathways for hazardous gases and groundwater.	X			X

Table F.2 Natural voids: potential hazards (2 of 2)

Potential hazard	Natural voids
Potential hazard	Soluble rocks	Mass move ment	Glacial and frost	Action of water	Faults and fissures
Loss of ground generating voids at the ground surface.					X
Loss of drilling fluids during tunnelling and other forms of "no dig" technologies.	X				X

F.3.2 Mass movement

Mass movement is a generic term for downslope movement of soil, rock or a mixture of both materials under the force of gravity. The type of mass movement occurring at a particular location is dependent on various interrelated factors including slope angle, in-situ material characteristics, material mass characteristics, groundwater regime, loading conditions and the influence of external works, e.g. engineering activities. The result of mass movement is a failed mass of material, with the extent and characteristics of the failed mass a function of the type of mass movement. The causes of mass movement are varied and frequently interlinked.

F.3.3 Glacial and frost

During the Quaternary period significant parts of the UK were affected by cyclical periods of extreme cold weather (glaciations) followed by periods of warmer weather (interglacials). Since the end of the last glaciation, less extreme cyclical variations in temperature have occurred on a seasonal basis. There are numerous effects on soils, rocks and geomorphology associated with these climatic variations. Many of these do not generate voids of a significant size, but generate metastable soil structures, e.g. some loessic deposits.

The effects of freezing of water in discontinuities and voids, with a cyclical freeze-thaw process, generates extensional forces in these materials with the overall effect being the formation of voids, or the increasing in size of pre-existing voids. The scale of such processes varies between the smaller disintegrations due to frost shatter to larger scale displacements involved in foundering, cambering and valley bulging. Certain rocks are more susceptible to frost shatter, e.g. chalk.

F.3.4 Action of water

Surface and subterranean water flows can cause soils and rocks to become unstable, either generating voids by dissolution or physical weathering, or forming resultant deposits potentially containing voids. The chemical and physical weathering processes acting on rocks can be significantly increased by the action of water. The presence of pre-existing discontinuities, representing zones of increased permeability and water movement and preferential water flow, result in concentrated water flows.

Where the stratigraphy contains soluble rocks, such as limestone or chalk, dissolution processes over significant time periods result in the formation of natural cavities. In marine settings, the erosion of non-soluble rocks can result in the formation of sea caves and in such settings the location and lineation of the caves is frequently associated with pre-existing structural discontinuities and low strength horizons in the rock strata. Erosion of soils could result in the formation of scour hollows, which might represent areas of reduced density, voided ground.

F.3.5 Faults and fissures

Voids might be generated where compressive and tensional tectonic forces have broken the rock strata. This can result in either the formation of fault planes or zones (possibly containing fault breccias) or in open rock fissures. Faults can occur on a variety of scales from less than a metre to many hundreds of metres, with the nature of the voids generated along and adjacent to the fault similarly variable. Fault zones can contain a high percentage of voids or the voids might be infilled with finer grained materials and clay "gouge". Similarly, the development of fissures or breaks in rock which have not laterally moved, can result in the formation of vertical or steeply inclined voids, either single features or a number of features within discrete fissure zones.