Site investigation

Abandoned mine workings frequently occur beneath many urban areas in Britain and other parts of the world. On a site due to be developed it is essential that if abandoned mine workings are suspected, they are accurately located and suitable remediation measures taken to make them safe. A properly procured, supervised and interpreted site investigation should be undertaken to fully characterise the ground conditions.

A site investigation for an important structure(s) requires the exploration and sampling of all strata likely to be significantly affected by the structural loading, that is, to determine the geology and physical properties of the soils and rocks that have an influence on the structures erected, and the long-term stability of the ground.   Where a mineral (e.g. a coal seam) of workable thickness occurs at shallow depth (i.e. within about 30 to 50 m of rockhead), it should be assumed that it has been worked at some time, although this may not have been in a systematic way.  Even if there are no mine plans or records of past workings, the ground investigation should be planned on the assumption that mining has taken place until it is possible to prove otherwise.  In particular, the location of subsurface voids due to mineral extraction is of prime importance in this context.  In other words, an attempt should be made to determine the number and depth of mined horizons, the extraction ratio, the geometry of the layout, and the condition of any old pillar and stall workings.  The sequence and type of roof rocks may provide some clue as to whether void migration has taken place and if so, its possible extent.  Of particular importance is the state of the old workings, careful note should be taken of whether they are open, partially collapsed or collapsed, and the degree of fracturing, joint dilation and bed separation in the roof rocks should be recorded, if possible.  This helps to provide an assessment of past and future collapse that is obviously very important.

Each investigation should be designed to meet the requirements of the construction operations to be carried out.  The first stage of a site investigation involves a desk study and a reconnaissance survey, which are then followed by the necessary field exploration.  The desk study should include a survey of appropriate maps, data, documents, records and literature.  The presence on geological maps of mineral deposits that could have been mined suggests the possibility of past mining unless there is evidence to the contrary, and geological and topographic maps may show evidence of past workings such as old shafts, adits and spoil heaps.  All the geological and topographic maps of the area in question, going back to the first editions, should be examined. The presence of any faults should be noted.  Abandoned mines record offices, when they exist, represent primary sources of information relating to past mining activity such as old mine plans.  Other sources include public record offices, museums, libraries, specialist contractors and consultants, private collections, local interest groups and geological surveys. Unfortunately, however, it is often the case that the extent, age and condition of old abandoned workings are poorly documented or else unknown.  Even when former mine workings have been recorded the plans are often incomplete and inaccurate, therefore it may not be possible to determine their precise locations accurately in all situations.  Furthermore, old abandonment plans rarely show information about the condition of the workings such as whether the mine openings have been stowed to reduce the risk of collapse, or the type of support systems used.  In fact, sometimes in Britain mine plans may be misleading showing shafts and adits in areas where a mineral was not mined.  In yet other cases, old mine plans may show numerous faults that do not exist.  This practice was to avoid the payment of royalties, the faulted mineral being tax-free.  Hence, the prediction of instability problems associated with abandoned mine workings cannot always be aided by old mine plans.

 

Reconnaissance (‘walk over’) survey

The reconnaissance survey involves a walk-over visit of the site to allow familiarization, to find any new evidence of former mining activity and perhaps to confirm any anecdotal and documentary evidence for past mining.  This usually takes place as part of the initial ‘desk study’. Subtle variations in the topography may be observed together with evidence of past land use.  If sufficient information has been gathered at this stage, it may be possible to pass straight into a field investigation involving direct exploration of the ground by drilling.  In other words, the information derived from the desk study and reconnaissance survey hopefully can be used to help plan the field exploration programme.

 

Remote sensing (Earth Observation)

The use of remote sensing (known also as Erath Observation) imagery and aerial photography for the detection of surface features caused by subsidence to a large extent is restricted to rural areas and scale is a critical factor. Landsat imagery, SLAR imagery and high-altitude aerial photography may provide data on fractures and lineaments that could be used to locate zones of potential subsidence. High resolution airborne geophysical surveys involving gamma spectrometry, magnetic and very low frequency electromagnetic sensors are improving the ability to produce maps. These techniques help to map high conductivities that might be related to abandoned mine sites. Laser and radar sensors on airborne platforms are being used to produce high resolution (centimetre to metre) digital terrain models. The Light Detecting And Ranging (LIDAR) system sends a laser pulse from an airborne platform to the ground and measures the speed and intensity of the returning signal. From this, changes in ground elevation can be mapped. Similarly, the Wide-Angle Airborne Laser Ranging System (WA-ALRS) provides a means of measuring land subsidence with millimetre accuracy.  Radar systems use radar rather than lasers to achieve the same end. Satellite radar measurements are becoming increasingly sophisticated, with the potential to measure millimetric movements in urban areas where hard surfaces act as ‘permanent’ reflectors. Synthetic Aperture Radar Interferometry (InSAR), for instance, provides a means of mapping from satellite continuous displacements, over large areas (100 km x 100 km), with a spatial resolution of 10 m and an accuracy of a few centimetres.  Differential interferogram images derived from repeat pass spaceborne InSAR systems afford the possibility of mapping surface deformations of small extent, as well as providing a method of monitoring their development.  A similar satellite technique known as Permanent Scatterer Interferometry (PSInSAR) also uses radar and produces maps showing rates of displacement, accurate to a few millimetres per year, over extensive time periods, currently up to a decade long.  The process provides the millimetric displacement histories for each reflector point across the entire time period analyzed, as calculated at every individual radar scene acquisition. Small incremental ground movements therefore can be detected that might be caused by subsidence.

The resolution necessary for the detection of relatively small subsidence features (1.5 - 3 m across) is provided by aerial photographs with scales between 1:25 000 and 1:10 000.  Colour photographs may be more useful than black and white ones in the detection of past workings since they can reveal subtle changes in vegetation related to subsidence and, if there are differences in thermal emission, then infra-red (false colour) photographs should show these differences.  The detail obtained from aerial photographs should be represented on a site plan at a scale of 1:2500 or larger.

 

Geophysics

There are several types of geophysical instruments which are available to assist in the search for shallow mine workings and mine entries. The types of survey, methodology and interpretation will depend upon several complex factors such as the types of target buried, the geological conditions, the anticipated depth of burial, the age of the burial and the experience and skill of the geoscientist.  Geophysical surveys can be carried out by one person, or a team. They may be non-invasive or invasive and the individual surveys may take hours to weeks to complete.

Geophysical surveys are usually undertaken once reconnaissance geological surveys have been undertaken. The assessment of geological data, maps, and a reconnaissance walk-over survey will be required before the types of techniques are recommended. Typical geophysical surveys usually operate on a linear traverse or a grid with readings taken every 0.1–1.0 m. However, this is determined by the size and dimensions of the buried target, and on the prevailing topography and ground conditions. The use of automated data logging systems, computerised data processing, graphics technology combined with global positioning surveys (GPS) have made it possible for relatively large areas of land to be investigated in a single day. 

It should be emphasised however, that the use of geophysical methods does not preclude the use of other site investigation methods (like drilling trenching), but are complimentary and supportive techniques.  The comparison of a geophysical interpretation with directly obtained geological data is known as 'ground-truthing.'  This methodology enables the geophysical survey results to be extrapolated into areas where little or no ground truth information is available so that more confidence can be placed in the interpretation of the geophysical survey data.

 

Geophysical survey types

Geophysical survey data on its own measures the vertical and lateral variation of the physical properties, such as electrical conductivity, microgravity, magnetic and electromagnetic properties of the geological materials or buried objects at the site (or crime scene) under consideration. These data can only be interpreted in the light of some knowledge of the likely ground conditions, obtained from the conceptual geological model that will give meaning to the data set measured.  In this respect there are two main approaches to carrying out a geophysical survey:

 

·         Measurement of a physical property on a grid basis over the ground surface. Contouring of this data will locate anomalous zones, which may be associated with further investigation of the anomalous areas is required.

·         Measurement of a physical property along a detailed horizontal profile, such that details of the vertical variation of that property are determined. In this case the geophysicist attempts to produce a mathematical model of the geological structure, which will give rise to the measured geophysical data set.  Again, the accuracy of the model is largely dependent on the ground-truth information that is available either from historical sources or from excavations and exploratory inspection pits, or from probes.

 

Geophysical investigations rarely require any contact with the ground surface (ie. they are non-invasive) for their operation. The final objective of the geophysical survey must be clearly specified such that it can be designed and costed appropriately to achieve the required objective.

One of the main advantages of indirect geophysical methods of investigation over intrusive methods is that information is obtained for much larger volumes of ground at lower cost.  This is an important consideration because the probability of finding a small target within a large volume of ground is very low using point-sampling methods (such as drilling). Be that as it may, geophysical methods are not a substitute for direct methods but should be regarded as complimenting them. Because the geophysical data gathered relates to the variation of physical properties of a volume of ground, then the data must be processed and interpreted based upon a conceptual ground model and some ground control data.  Over the past 30 or so years attempts have been made to develop geophysical methods for the location and delineation of abandoned mine workings.  Unfortunately, no one geophysical method has yet been developed that completely resolves all problems of this nature.  A variety of surface traversing techniques are available that provide readings at close station intervals for the location of shallow voids where the lateral dimensions of the void are at least of the same order as the depth of burial. 

Considerable care should be exercised at the planning stage of a geophysical survey for the location of subsurface voids because of the variable nature of the target.  The selection of the most appropriate technique necessitates consideration of four parameters, namely, penetration, resolution, signal to noise ratio and contrast in physical properties.  The size and depth of the workings and the character of any infill control the likelihood of the workings being detected as an anomaly.  With the information obtained from the desk study and the reconnaissance survey, many of the available geophysical methods can be assessed at the selection stage, using a model study, and accepted, or rejected, without any requirement for field trials.  Generally, it is possible to detect a cavity whose depth of burial is less than twice its effective diameter.  However, since the presence of a void is likely to affect the physical properties and drainage pattern of the surrounding rock mass, this can give rise to a larger anomalous zone than that produced by the void alone.

The nature of the environment around a site affects the success of geophysical surveys.  For instance, traffic vibrations adversely affect the results obtained from seismic surveys, as do power lines and electricity cables in the case of electromagnetic and magnetic techniques.  Of particular importance is that there should be sufficient physical property contrast between the void and the surrounding rock mass so that an anomaly can be detected.

The success of geophysical methods in locating old shafts depends on the existence of a sufficient contrast between the physical properties of the shaft and its surroundings to produce an anomaly.  If no contrast exists a shaft cannot be detected.  Moreover, a shaft will frequently remain undetected when the top of the shaft is covered by more than 3 m of fill.  The size, especially the diameter, of a shaft influences whether or not it is likely to be detected.

Common geophysical techniques which may potentially be used as part of a site investigation to locate and characterise shallow, abandoned mine workings and associated voids include the following:

 

·         Seismic refraction: This technique has not been used particularly often in searching for voids at shallow depth created by previous mining since such voids are often too small to be detected by this method because of attenuation of seismic waves in the rock mass.  Furthermore, if the workings are dry, then high attenuation of the energy from the seismic source occurs so that penetration in to the rock mass is poor.  There is little likelihood of a cavity producing a measurable anomaly where it is buried at a depth greater than its diameter.  However, the disturbed zone around an anomaly may increase its effective size.  Voids in coal seams above a competent sandstone sometimes can be detected by localized increase in travel time and decrease in amplitude of the seismic event, provided that the anomalous region is comparable in dimensions with the seismic wavelengths.  For instance, a seismic wave having a dominant frequency of 100 Hz and a velocity of 2000 m s^-1 has a wavelength of 20 m.  As a consequence voids less than 20 m in width probably will not be recognized. 

·         Electrical resistivity: Generally, where dry pillar and stall workings occur at a depth greater than 5 m, it is unlikely that resistivity profiling will detect their presence.  In addition, where the depth of a cavity is equal to its diameter, the maximum disturbance in the resistivity profile is only about 10%, in which case, cavities at depths greater than twice their average dimension usually are not recorded.  Electrical resistivity depth sounding can be applied to the location of voids where the width to depth ratio is large.  Mine workings that produce an air-filled layer often can be identified on the sounding curve as an increase in apparent resistivity. A resistivity survey may be successful where there is a significant contrast in electrical resistance between a shaft and its surroundings. Anomalies are detectable if the depth of cover around a shaft is less than its radius and success has generally been achieved where the cover was less than 1 m in thickness. Both sounding and profiling surveys should be undertaken, the latter are used to detect vertical anomalies that could be attributable to shafts. An isoresistivity map can be drawn from the results of profiling and may indicate the presence of a shaft or shafts.

·         Electromagnetic: Electromagnetic methods can be effective in the detection of mine shafts when shafts contain a mixture of materials with highly contrasting conductivities.  .

·         Conductivity: Terrain conductivity meters have several advantages over conventional electrical resistivity equipment when used for locating small, near-surface anomalous features.  For example, in the depth range up to 30 m, terrain conductivity surveys are more effective than resistivity traversing.  Conductivity values are taken at positions set out on a grid pattern and the results can be contoured to indicate the presence of any anomalies.  Penetration into the ground achieved by electromagnetic radiation can be limited by excessive attenuation in ground of high conductivity.  However, problems with detecting an extremely small secondary field in the presence of a strong primary signal can be overcome by the use of pulsed radiation.

·         Ground penetrating radar: Ground probing radar is capable of detecting small subsurface cavities, especially for surveys in urban areas.  The method is based upon the transmission of pulsed electromagnetic waves, the travel time of the waves reflected from subsurface interfaces being recorded as they arrive at the surface so allowing the depth to an interface to be obtained.  The high frequency of the system provides high resolution and characteristic arcuate traces are produced by air filled voids.  Depths to voids can be determined from the two-way travel times of reflected events if velocity values can be assigned to the strata above the void.  The conductivity of the ground imposes the greatest limitation on the use of radar probing in site investigation.  In other words, the depth to which radar energy can penetrate depends upon the effective conductivity of the strata being probed.  This, in turn, is governed chiefly by the water content and its salinity.  Furthermore, the value of effective conductivity is a function of temperature and density, as well as the frequency of the electromagnetic waves being propogated.  The least penetration occurs in saturated clayey materials or when the pore water is saline.  Useful data can be obtained from sites where clayey topsoil is more or less absent because wet clay and silt, in particular, cause great attenuation of electromagnetic energy frequently giving penetration depths of less than one metre.  On the other hand, the technique appears to be reasonably successful in sandy soils and rocks in which the pore water is non-saline.  The penetration of radar energy can be increased by using a lower frequency but unfortunately this reduces its resolution, which means that subsurface anomalies have to be correspondingly larger if they are to be detected.

·         Microwave: The microwave methods has been used as a means of locating  subsurface voids.  There are two microwave techniques, namely, the pulsed type and the continuous wave type.  In each technique an electromagnetic wave is sent through the rock material under investigation.  The wave is partly reflected back to the surface by any interface with different electrical properties, and the amplitude of the reflected wave is recorded.  It would appear that the continuous wave microwave technique is the more successful.

·         Magnetic: Generally speaking, voids in shallow abandoned mine workings are too small and located at depths too great to be detected by normal magnetic or gravity surveys.  However, the fluxgate magnetic field gradiometer permits surveys of shallow depths to be carried out.  It provides a continuous recording of lateral variations in the vertical gradient of the Earth's magnetic field rather than giving the total field strength.  The gradiometer tends to give better definition of shallow anomalies by automatically removing the regional magnetic gradient.  Quantitative analysis of the depth, size and shape of an anomaly generally can be made more readily for near-surface features, using the gradiometer, than from total field measurements, obtained with a proton-magnetometer.  The sensor of a gradiometer is small (0.5 m or 1 m) in order to record features within two to three metres of the ground surface.  On the other hand, a proton-magnetometer can more easily detect larger and deeper features, and yields results that are more suitable for contouring.  Magnetic surveys, especially those using the proton-magnetometer and, more recently, the fluxgate magnetic field gradiometer, have had some success in locating old mine shafts.  Abandoned mine shafts may be detected where a shaft is lined by bricks or iron tubbing, bricks containing a certain amount of iron. Similarly, if any shaft filling contains metallic objects such as rails or boggies, then they may be detected by a magnetic survey. An isomagnetic contour map is produced from the results.

·         Microgravity: Microgravity meters, accurate to 2 mgal (1 mgal = 10 g.u. i.e. gravity units), may be successful when the voids have a significant lateral extent, as in some pillared workings.  A series of traverses can be used to map the lateral extent of such features but there are still size/depth constraints and an inherent ambiguity in the interpretation of results.  An open shaft offers a gravity contrast but unfortunately most old shafts are too small to produce a difference in density that is sufficient to yield a gravity anomaly.  The maximum depth at which an open shaft can be located by a gravity survey is about 1.5 times the diameter of the shaft.  If the shaft has been backfilled the depth is less.  The production of a gravity contour map hopefully allows any anomalies to be recognized.

·         Cross hole seismic tomography Seismic tomography uses two or more drillholes, and possibly the ground surface, for the location of sources and detectors, the object being to derive one or more two-dimensional images.  In this way it utilizes a multitude of wave paths to enable the location, shape and velocity contrast relating to an anomaly, such as a void, between drillholes to be delineated. The quality of the results obtained is a function of the nature of the ground, in particular the physical property contrasts, the number of drillhole pairs, the distances apart of the drillholes and their location in relation to the target. In seismic tomographic surveys, the wavelength of the seismic event needs to be less than the average dimensions of the target. Seismic surveys generally are not used to locate old mine shafts.  However, crosshole seismic testing, which involves sending both compressional and shear waves between drillholes, can be used to detect shafts (see above). Crosshole techniques can be used when the depth of burial of a void is more than two or three times the diameter of the void.  Generally, the source and receiver are at the same level in the two drillholes and are moved up and down together.  Drillholes must be spaced closely enough to achieve the required resolution of detail.  The method can be used to detect subsurface cavities if the cavity is directly in line between two drillholes and has at least one tenth of the drillhole separation as its smallest dimension.  Air filled cavities are detectable more readily than those filled with water.

·         Electrical resistance tomography: Electrical resistance tomography is a relatively new geophysical imaging technique that uses a number of electrodes in drillholes, and sometimes at the ground surface, to image the resistivity of the subsurface.  Electromagnetic techniques also have been used to produce tomographic imagery. Drillhole radar tomography has been used to assess foundation conditions in which voids occurred and concluded that crosshole methods are probably the best tools available at present to provide the detailed information for good foundation design.

 

Drilling

The location of old workings generally has been done by exploratory drilling, the locations of drillholes being influenced by data obtained from the desk study and/or the data gathered by indirect methods.  However, it must be admitted that although frequently successful in locating the presence of old mine workings, exploratory drilling is not necessarily able to establish their layout.  Nevertheless, when the results from drilling are combined with a study of old mine plans, if they exist, then it should be possible to obtain a better understanding of methods of working, sizes of voids, and directions of roadways and galleries.

Rotary drilling to prove the existence of old mine workings usually is done by open holes, which allow relatively quick probe drilling.  Rotary percussion drilling with a cruciform bit may be used.  The drillholes should be taken to a depth where any voids present are not likely to influence the performance of the structures to be erected.  For sites where there is little data available drillholes should be sunk to a depth of 60 m, especially if substantial structures such as multi-storey buildings are to be erected. If a grid pattern of drillholes is used some irregularity should be introduced into the pattern to avoid holes coinciding with pillar positions. The presence of old voids is indicated by the free-fall of the drill string, the loss of flush, the presence of ferruginous or ochrous water, or wood remains in the drill returns (from roof supports and pit props).

One of the principal objectives of investigations of abandoned mine workings is to determine their extent and condition.  Accordingly, core material may need to be obtained.  In addition, the stratigraphic sequence should be established by taking cores in at least three drillholes.  Double barrel sampling tubes with inner plastic liners can be used to obtain core, which then can be photographed and logged, and the rock quality designation (RQD) or fracture spacing index recorded.  Drilling penetration rates, water flush returns and in situ permeability tests may be used to assess the degree of fracturing.  The degree of fracturing is important in that it tends to increase as old workings are approached.  Determination of groundwater conditions is necessary, especially where a grouting programme is required to treat the workings. 

 

Trenching and trial pitting

Excavation of superficial deposits, soils and made ground using mechanical excavator may prove an effective method for locating mine entries. Although, these methods may only be suitable where the depth to bedrock is less than about 3 to 4 metres. As this may be a hazardous task, necessary safety precautions should be taken.  When a possible position of a shaft has been determined, a mechanical boom-type digger can be used to reveal its presence.  The excavator is anchored outside the search area, and harness and lifelines should be used by the operatives.  A series of parallel trenches are dug at intervals reflecting the possible diameter of the shaft.  If excavation to greater depth than can be achieved by an excavator is required, resort may be made to a dragline.  Again the dragline should be anchored safely outside the search area. Drilling rigs should be cited on a drilling platform that extends well beyond the zone-of-influence of the shaft. Consideration therefore needs to be given to the cratering of superficial deposits in the vent of collapse induced by drilling.

 

Mapping of mine workings

Detailed mapping of galleries is best made by driving a heading from an outcrop if this is close at hand or by sinking a shaft to the level of the mineral deposit to obtain access to the workings.  Sometimes access can be gained via old shafts.  Radial holes may be drilled from a shaft to establish the dimensions of pillars and stalls.  As old workings may prove dangerous, exploration should be undertaken with the advice and aid of experts.

Below surface workings may be examined by using drillhole cameras or closed circuit television, information being recorded photographically, or on videotape, and used to assess the geometry of voids and, possibly, the percentage extraction.  However, their use in flooded old workings has not proved very satisfactory.  Occasionally, smoke tests or dyes have been used to aid the exploration of subsurface cavities.

It is possible to study the interior of large abandoned mine workings that are flooded by using a rotating ultrasonic scanner.  The ultrasonic survey can be carried out within the void, the probe being lowered down a drillhole to mine level.  Horizontal scans are made at 1 m intervals over the height of the old workings and then tilting scans at 15° intervals are taken from vertical to horizontal.  The echoes received during the horizontal scan are processed by computer to provide a plan view of the mine.  Vertical sections are produced from the vertical scan.  Hence, it is possible to determine the positions of pillars and the extent of the workings.

 

Thematic maps

In recent years thematic maps have been produced in Britain of both urban and rural areas with a view to benefiting planners and civil engineers concerned about ground stability and associated land use.  Early thematic maps produced by the British Geological Survey depicted areas of undermining assumed to be within 30 m of the surface on the one hand and at depths exceeding 30 m below the surface on the other.  This 30 m depth is based on limited information and therefore is subject to interpretation.  It assumes that bulking factors of 10 to 20% will affect the strata involved in void migration.  Initially, known and suspected mining areas were not differentiated on these maps.  However, only areas of mining shown on mine plans were represented on the map of the Glasgow district and no areas of suspected mining were shown.  No attempt was made to infer the extent of working beyond the limits defined by mine plans other than to plot relevant drillhole data.  The recognition of single and multiple seam working led to the requirement that areas of shallow working (i.e. less than 30 m below rockhead) should be identified in terms of seams worked.  Separate maps were prepared illustrating areas of total known mining; current mining; known mining within 30 m of rockhead together with the locations of shafts and drillholes encountering old shallow workings; and mining for minerals other than coal and ironstone.  In Fife, known and inferred old shallow mining were differentiated on the same map.  Each modification has reflected an attempt to clarify the presentation of known and inferred past mining.  An indication of the area in which old mine workings might be expected can be obtained by plotting all drillholes that encounter spoil outside areas of workings known from abandonment plans.  In areas where mineral outcrops are reasonably well known, areas of suspected workings can be mapped as a separate category, although it then is necessary to assume that all workable beds have been exploited, at least in the near surface area.

In an assessment of the degree of risk due to subsidence incidents associated with abandoned mine workings in South Wales, found that of the 388 events recorded, 64% occurred in open land and so posed no threat to person or property.  21% cent had occurred when people were nearby or property threatened.  The remainder caused damage to highways, buildings or other property and only one of these events resulted in minor injuries.  In the context of the South Wales Coalfield this represents a low level of hazard.  Assuming that a typical incident affects an area of 5 m², then the probability of collapse occurring on any 25 m² plot is of the order of 10-7 per year.  Even if the number of subsidence incidents that have remained undiscovered increased the above total figure by a factor of 3, then the overall risk would still be low.  It has been noted that over 90% of the incidents occurred within 100 m of the outcrop of the seam concerned.  The maps make contributions towards regional planning policies by taking account of a possible development constraint at an early stage and offer an early warning on the likely scale of ground investigation required at specific sites. However, it must be borne in mind that thematic maps that attempt to portray the degree of risk of a hazard event represent generalized interpretations of the data available at the time of compilation.  Therefore, they cannot be interpreted too literally and areas outlined as ‘undermined’ should not automatically be subject to planning blight.  Obviously, there is a tendency to assume that the limits of old mine workings represented on a map indicate the full extent of the workings but the interpretation of their location is based on scanty information and includes assumptions, some of which may be unfounded.  It should be recognized that engineering problems in areas of past mining only occur if buildings are not properly planned, designed and constructed with reference to the state of undermining.  Also, zoning based entirely upon depth of cover above workings cannot be relied on completely, since occasionally subsidences have occurred in zones labelled ‘safe’.

Thematic geological maps of past mining areas therefore have their limitations due to factors such as scale of presentation, reliability and availability of data.  They are transitory and require amendment as new data becomes available.  These maps must not be used as a substitute for site investigation.

 

Hazard zoning

A hazard involves a degree of risk, the elements at risk being life, property, possessions and the environment. Risk involves quantification of the probability that a hazard will be harmful and the tolerable degree of risk depends upon what is being risked, life being much more important than property. The frequency of a particular hazard event can be regarded as the number of events of a given magnitude in a particular period of time at a certain location.  The risk to society can be regarded as the magnitude of a hazard multiplied by the probability of its occurrence. Risk, as mentioned, should take account of the magnitude of the hazard and the probability of its occurrence. Risk arises out of uncertainty due to insufficient information being available about a hazard and to incomplete understanding of the mechanisms involved. The uncertainties prevent accurate predictions of hazard occurrence. Risk analysis involves identifying the degree of risk, then estimating and evaluating it.

Assessments of mining hazards usually have been on a site basis, regional assessments being much less common.  Nonetheless, regional assessments can offer planners an overview of the problems involved and can help them avoid imposing unnecessarily rigorous conditions in areas where they really are not warranted.

Any spacial aspect of a particular hazard can be mapped providing there is sufficient information on its distribution. Hence, when hazard assessment is made of an area, the results can be expressed in the form of hazard maps. An ideal hazard map should provide information relating to the spacial and temporal probabilities of the hazard mapped. However, hazard maps, like other maps, do have disadvantages. For instance, they are highly generalized and represent a static view of reality. They therefore need to be updated periodically as new data becomes available. Hazard maps of areas where old mine workings are present ideally should represent a source of clear and useful information for planners and developers.  In this respect, they should realistically present the degree of risk (i.e. the probability of the occurrence of a hazard event).  Descriptive terms such as high, medium and low risk must be defined, and there are social and economic dangers in overstating the degree of risk in the terms used to describe it.  Ideally, numerical values should be assigned to the degree of risk.  However, this is by no means an easy matter for numerical values can only be derived from a comprehensive record of events, which unfortunately in the case of abandoned mine workings is available only infrequently.  Furthermore, many events in the past may not have been recorded, which throws into question the reliability of any statistical analysis of data.  Indeed, subsidence is affected by so many mining and geological variables that many regard it as, to all intents and purposes, a random process. In addition, the matter of risk assessment is complicated further in terms of its tolerance, for example, people are less tolerant in relation to loss of life than to loss of property.  Hence, the likelihood of an event causing loss of life would be assigned a higher value than loss of property (e.g. the probability of a 1 in 100 loss of life would presumably be regarded as very risky whilst very risky in terms of loss of property may be accepted as 1 in 10).  Nonetheless, if planners can be provided with some form of numerical assessment of the degree of risk, then they are better able to make sensible financial decisions and to provide more effective solutions to problems arising. 

For example, an attempt at hazard zoning in an area of old mine workings was made at a site in Airdrie, Scotland, underlain by shallow abandoned mine workings, it  was able to propose safe and unsafe zones. In the safe zones the cover rock was regarded as thick enough to preclude subsidence hazards (about 10 m of rock or 15 m of till was regarded as sufficient to ensure that crown holes did not appear at the surface) and normal foundations could be used for the two-storey dwellings that were to be erected.  On the boundaries between the safe and unsafe zones, the dwellings were constructed with reinforced foundations, or rafts, as an added precaution against unforeseen problems.  Development was prohibited in zones designated unsafe.   In effect, a thematic mining information plan of the site to facilitate its development. 

 

Geochemical techniques

Geochemical exploration depends on identifying chemical changes such as changes in mineral content or in the chemical character of the moisture content in the soil associated with old mine workings.  In addition, gases such as carbon dioxide, carbon monoxide, methane, hydrogen sulphide or stythe (nitrogen enriched air) may accumulate in open or partially filled shafts.  Indeed, the most effective geochemical method yet used in the location of abandoned coal mine shafts is that of methane detection.  Methane, being a light gas, may escape from old shafts and methane detectors can record concentrations as low as 1 ppm.  Anomalies associated with old coal mine shafts generally have ranged between 10 and 100 ppm.  The detector is carried over the site near the ground and should not be used on a windy day.  A contour map showing methane concentration is produced.

 

Mine gases

 

Mine gas emission may accompany subsidence. The principal gas associated with working coal mines is methane. During mining operations, methane is liberated into the mine ventilation system and vented to atmosphere. In addition, methane drainage above longwall panels by inclined underground boreholes is common practice at working mines. However, not all the methane is removed by these processes and unknown but probably substantial volumes accumulate and are stored in goaf areas and in fractured rock. During periods of low barometric pressure, this gas can migrate out into the mine roadways causing problems and potentially dangerous situations.

In abandoned mine workings where there is no longer any active ventilation, ‘blackdamp’ or ‘stythe gas’ - oxygen-depleted mine air, mainly CO2 and nitrogen - may accumulate in addition to methane. ‘Stinkdamp’ - a mixture of carbon monoxide and hydrogen sulphide, both of which are poisonous even at very low concentrations - is occasionally encountered in old workings. Radon may also occur, particularly in the vicinity of fault zones.

Since 1945, there have been over 70 recorded incidents of gas emission at the surface from abandoned mine workings. Over two-thirds of these were of methane, the remainder mainly blackdamp . To date, these emissions have been at locations on exposed coalfield areas, and in some instances appear to be related to the presence of solid sandstone at the surface. Joints and fractures, probably enhanced by mining subsidence, probably act as gas migration pathways.

The gas may travel laterally before reaching the surface; an emission may not be directly adjacent to a shaft. By contrast, argillaceous or drift covered areas seem to be at a lower risk. The emissions were often associated with sudden falls in barometric pressure, although minewater rebound due to general cessation of pumping is seen to be a regional driving mechanism.

Mine gas flow pathways to the surface are likely to be controlled by:

 

·         Mine shafts or other mine entries with no or ineffective seals.

·         Exploration boreholes or wells with no or ineffective seals.

·         Joints and fractures, natural and/or mining-induced.

·         Structural geological situations.

·         Minewater rebound following cessation of pumping.