of the International Association of ENGINEERING GEOLOGY ae rAssocation Internationale de GEOLOGIE DE L'INGCNIEUR

I BULLETIN

No 33

PARIS

1986

I

LOCATION OF ABANDONED WORKINGS IN COAL S E A M S REPI~RAGE

DE CHANTIERS

ABANDONNi~S

DANS

DES EXPLOITATIONS

DE CHARBON

F.G. BELL *

Abstract Coal m i n i n g has gone on in many parts of Western Europe and North America frequently for 200 years or more. Consequently in many urban areas there are abandoned v, orkings at shallow depth which often are unrecorded. These may present a potential hazard when such areas are redeveloped. Investigation of abandoned coal mine ,aorkings is no easy task and requires some knowledge of past methods of mineral exploitation. Such an investigation involves assessing the nature of old mine workings. The desk study will include a survey of appropriate maps, documents, records and literature, and, at times, aerial photographs. Generally speaking the usual methods of geophysical exploration have not proved very successful in revealing the layout of shallow old mine workings. However, some relatively new methods of geophysical surveying do appear have had some success. Of these, ground probing radar would seem to offer the most potential. The location of old workings has generally been carried out by exploratory drilling. Workings may be e x a m i n e d by using borehole camera or closed circuit television, or by gaining direct access from headings or shafts.

Resume L'exploitation de la houille se poursuit depuis au moins deux cents ans dans plusieurs regions de I'Europe occidentale et de l'Amerique du Nord. En consequence il existe dans les alentours de beaucoup de villes, des chantiers d'exploitation a b a n d o n n e s et peu profonds dont souxent on ne trouve aucune mention. I1 se peut que ceux-ci presentent une situation dangercuse lors de l ' a m e n a g e m e n t de telles regions. La reconnaissance de ces chantiers abandonnes est assez difficile et d i e necessite pas real de connaissances sur les anciennes methodes d'exploitation minera[e. En faisant une telIe reconnaissance, il faut estimer la nature des chantiers anciens. Le travail en bureau dolt c o m p r e n d r e un examcn attentif de tous les documents necessaires : plans, registres, et parfois photographies aeriennes. En general les methodes habituelles d'exploration geophysique n'ont pas bien reussi & decouvrir la disposition des chantiers anciens peu profonds. Cependant il parait bien que certains procedes assez recents employes par les geophysiciens aient eu du succes. Parrni ceux-ci, le radar pour sonder [e terrain semble etre le procede le plus interessant pour l'avenir, G e n e r a l e m e n t on a decouvert les anciens chantiers en effectuant de nombreux forages explorateurs. II est possible d ' e x a m i n e r les chantiers en employant des appareils photographiques speciaux ou la television '3. circuit fermd, ou en se procurant l'acces direct par les gaieties d'avancement ou les puits.

1. Introduction Subsidence at the surface can be regarded as ground movement which takes place due to the extraction of mineral resources. It is an inevitable consequence of mining activities and reflects the movements which occur in the mined out area. Unfortunately subsidence can and does have serious effects on surface structures, services and communications, can be responsible for flooding, can lead to the sterilization of land or call for extensive remedial measures or special constructional design in site development. Old a b a n d o n e d coal workings occur at shallow depth beneath the surface of many urban areas in Western Europe and North America. Indeed the presence of

coal was one of the m a j o r reasons for the urban development in the first instance. Because many of these old workings were u n r e c o r d e d they can represent a potential hazard to those engaged in subsequent redevelopment since such workings may give rise to subsidence problems. F u r t h e r m o r e the detection of unrecorded a b a n d o n e d workings frequently has not been very successful. Even if records exist, they often are inaccurate.

2. Resume of past working methods In the United K i n g d o m coal mining began to be carried out on a significant scale in the thirteenth century. Drifts and adits into shallow workings were usually

* Department of Civil Engineering, Teesside Polytechnic, Middlesbrough, TSI 3 B A, Grande-Bretagne.

124

situated at the base of quarries and open pits or along the coal outcrops in hilly country. The workings extended as far as natural drainage and ventilation permitted. However. by the fourteenth century outcrop workings had largely given way to bell pits. The shafts of bell pits rarely exceeded 12.2 m in depth and their diameter was usually about 1.3 m. They are, therefore, a feature of coalfield areas where the drift cover is thin. Extraction was carried on around the shaft until such times as roof support became impossible, another shaft was then sunk nearby. Hence, where such mining went on. the number of bell pits may be very numerous (Figure 1). If bell pits were backfilled, then the state of compaction of the fill is generally unsatisfactory.

Increased demand for coal in the sixteenth c e n t u ~ led to the development of the pillar and stall method of extraction. Underground workings were shallow and not extensive, for example, t h e y rarely penetrated more than 413 m from the shaft Indeed, when such limits were reached, it was usually less costly to abandon a pit and sink another shaft nearby. Workings extending 200 m from the shaft were exceptional even at the end of the seventeenth century, t h e shaft itself usually being less than 60 m deep (see Bell, 1979). In the pillar and stall method of working, pillars are left to support the roof thus t h e y have to sustain the redistributed weight of the overburden which means that they and the rocks immediately above and below are subjected to added compression (Figure 2). In very

Where a coal seam occurred at more than about 7 m below the surface, bell pit mining tended to be replaced by headings which radiated into the coal seam for short distances around the shaft. The pillars of coal between the headings generally represented the only type of support to the overlying strata. The layout of a mine was unplanned and simply consisted of a complex of interconnected headings. Hence the support pillars were irregular in shape and size.

SurfQce

Column of S t r a t a Supported by P i l l a r

r t

P'/////////////A

("/////////////,r ELEVATION

0

30

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~} ...

b

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9 8e~l-oits ~ossincJ throuc~h unnamed SIT c0al ~

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Ironslocle . ~

D i a g r a m illustrating e x t r a c t i o n r a t m (ie p r o p o r t i o n of coal minedl. The extraction ratio (r) is d e r i v e d f r o m the expression :

R

:-."7": : ' : ' ) " : . . . . 9

Fig. 2 :

~

2 a b - b2 (a + b) 2

A c c o r d i n g to W a r d e l l a n d W o o d (19651 d e t e r m i n a t i o n of the loading o n pillars is best a p p r o x i m a t e d by a v e r a g i n g the load o n a given pillar d u e to the weight of strata above. The latter is equal to the w e i g h t of t h e c o l u m n of strata over an area equal to (a a- b):. It is a s s u m e d t h a t the load acts vertically a n d is u n i f o r m l y d i s t r i b u t e d over the cross sectional a r e a of the pillar, It c a n be s h o w n that the average loading l m / on a pillar c a n be o b t a i n e d from :

Top Vict~no Seam(T) ~ 4 . 6 - 7 - 6 m - - - ~ J Relofionsh=p Of bell-p~,Is to exl:~oited ;ronstone horizon and cOOl seams

Fig. 1:

Intensity of bell pits exposed at Sproats o p e n c a s t site. Northumberland

Z 1

-

r

w h e r e Z is the d e p t h (A s u m m a r y of o t h e r expressions w h i c h c a n be u s e d for d e t e r m i n i n g the stress on pillars is given in Bell F G . F o u n d a t i o n E n g i n e e r i n g in Difficult G r o u n d . B u t t e r w o r t h s . L o n d o n . p. 326-327. 19811

125 early mining the remnant pillars were rather haphazard in size and arrangement (Figure 3), but with time mining became systematic and pillars of more or less uniform shape were formed. Generally the main galleries were driven in parallel lines, between which were left ribs of coal. Then narrow headways were cut at intervals through the ribs to connect adjacent galleries, thus leaving a series of pillars which were approximately square or rectangular in plan. The size of the pillars and stalls was often influenced by the thickness and quality of the seam, and the character and thickness of the overburden. The future working intentions also influenced their proportions, if the pillars were to remain, then the object was to procure the greatest

possible proportion of the coal at the first working, whilst if they were to be removed later it was usual to provide fairly adequate support to protect the workings and haulage roads until their removal on retreat. There was a general tendency for the size o f stalls to increase, for example, in the sixteenth century they were usually less than 2 m wide whereas in the eighteenth century they began to exceed 3 m in width. Wardell and Wood (1965) noted that in the nineteenth century the normal width of stall varied from 1.83 to 4.75 m, the extraction ratio varying from 30 to 70%. Similar developments took place in the United States. For instance, Bruhn et al (1981) recorded that during the eighteenth and nineteenth centuries mines in the .• ,<,\

.'.-~:" ? 9,

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Early methods of pillar and stall working in different parts o[ Britain : a) Stoop and room (17th Century). Scotland: b) Bord and pillar (17th Century). north east England; c) Post and Stall (17th Century). South Wales; d) Square work (18th Century). Staffordshire. England 9 Shaded areas represent pillars and unworked coal. circles represent shafts

126

Pittsburg Coal were small and shallow, with rather arbitrarily spaced pillars of differing sizes. Pillar arrangement became more systematic during the second half of the nineteenth century. The stalls were 1.5 to 1.8 m wide and alternated with pillars of equal width so that the extraction ratio did not exceed 50~ In the 1890's the width of the stalls increased to between 6 and 7.3 m and pillars dimensions to between 3 and 5.4 m. Pillars often experience local failures whilst mining is taking place. If a pillar is highly jointed then its margin may fail and fall away under relatively low stress. Such action reduces and ultimately removes the constraint from the core thereby subjecting it to increasing stress. This can lead to pillar failure. Collapse in one pillar can bring about collapse in others in a sort of chain reaction because increasing loads are placed on those remaining. Stephenson and Augenbaugh (1978) remarked that the yielding of a large number of pillars can bring about a shallow broad subsidence over a large area. This type of subsidence usually is very slow and so difficult to detect. Slow deterioration and failure of pillars may take place years after mining operations have ceased, although observations at shallow depth and the resistance of coal to weathering suggests that this is a relatively uncommon feature at depths less than 30 m. In other words, at such depths the size of the pillars affords sufficient strength to support the overburden and any additional surface loading. However, old workings affect the pattern of groundwater drainage which in turn may influence pillar deterioration. On the other hand the small pillars of earlier workings may be crushed out once the overburden exceeds 50 to 60 m (see Piggott and Eynon, 1978). Old pillars at shallow depth have occasionally failed near faults and they may fail if they are subjected to the effects of subsequent Iongwall mining. Carter et al (1981) described subsidenses in Bathgate, Scotland, which were attributable to the failure of eighteenth century pillar and stall workings with a high extraction ratio, located at shallow depth. Thirty per cent of the subsidence took place immediately and 50 % had occurred after the first two weeks. After one year probably 90 to 95 % had taken place. The subsidence took the form of broad troughs with a deep central area and affected a surface area of more than 7 500 m 2. The maximum recorded subsidence exceeded 300 mm. The collapse of pillars in the mine workings threw extra loading onto the up-dip pillars which subsequently failed some two years later. Russell et al (1979) also recognised areas of broad subsidence in the Pennsylvania Coalfield, which in some cases resembled the pillar and stall workings in the mined areas beneath. The surface lowering was frequently quite uniform and amounted to between 0.3 and 3 m. They also noted linear subsidence which they presumed was associated with fracturing of rock on a large scale and with slight rotation of some blocks. These linear depressions develop from vertical cracks which often represent discontinuities which have gaped due to subsidence movements. Where little of no

superficial material is present these cracks range up to 4.5 m in width and some are up to 3 km long. Very often pillars were robbed on retreat. For example, Thorburn Reid (1978) mentioned the technique of quartering, used in Scotland, whereby the dimensions of pillars were reduced to a quarter of their former size, which meant that the extraction ratio could be as high as 85 %. Extraction of pillars during the retreat phase simulates the longwall surface condition although it can never be assumed that all pillars have been removed. As noted, at moderate depths pillars, particularly pillar remnants, are probably crushed and the goal compacted, but at shallow depths lower crushing pressures may mean that closure is variable. Squeezes or crushes sometimes occur in a mine as a result of the pillars being p u n c h e d into either the roof or floor beds, which might have become weakened or altered by the action of water or weathering (see Piggott and Eynon, 1978). Once again surface subsidence adopts a trough-like or basin form and minor strain and tilt problems occur a r o u n d the periphery of the basin thereby produced (see Bruhn et al, 1981). Briggs (1929) reported that when pillars are forced into a yielding pavement subsidence may restart. This could take place many years after mining had ceased and could damage property. He quoted an example in Wallsend, England, where the surface subsidence amounted to 1.2 m. Even if pillars in old shallow workings are relatively stable the surface can be affected by void migration (Figure 4). Void migration develops when roof rock falls into the worked out areas and represents the main problem in areas of shallow a b a n d o n e d mines. It can occur within a few months, or a very long period of years after mining has ceased. The factors which influence whether or not void migration will take place include the width of the u n s u p p o r t e d span; the height of the workings; the nature o f the cover rocks, particularly their shear strength and the incidence and geometry of discontinuities: the thickness and dip of the seam; the depth of overburden; and the ground water regime, When void migration occurs the material involved in the fall bulks, which means that migration is eventually arrested, although the bulked material never completely fills the voids. Nevertheless the process can, at shallow depth, continue upwards to the surface leading to the sudden appearance of a crown hole. According to Tincelin (1957) the height to which a void usually migrates can be estimated from the following expression: H

=

t

Pl

P

"t-

1

-

P~

P

where: t

p p~ H

-= = =

thickness of the seam bulk density of the roof rocks bulk density of collapsed roof materials height of migration

127

Russell et al (1979) described potholes which have formed at the surface in the Pennsylvania coalfield as a result of subsidence. These are usually circular or rectangular depressions in the soil which are from 1 m wide by 1 m deep to 61 m wide by 15 m deep. Newly formed potholes may have walls with 80-900 slopes while slumping reduces slope angles in older potholes to 35-40 ~, depending upon the type of soil. Pothole subsidence often occurs in a linear pattern. Some potholes exhibit openings at their base that indicate collapse into mine workings.

3. Investigations in subsidence areas The primary object of a site investigation is to assess the suitability of the site for the project concerned (see Anon, 1981). It first of all involves a desk study, which is then followed by field exploration (the desk study may continue into the period of field exploration). In the case of sites located in areas underlain by shallow abandoned mine workings, the investigation of subsurface conditions such as the occurrence of voids and bedding plane separation, as well as the detection of any old mine shafts, is of particular importance. 3.1 Desk Study

Fig. 4 :

Face showing arch closure beneath about 12 m of shale cover at Pethburn opencast site. County Durham. England. Workings are in the Five Quarter seam (approx. 2 . 4 m thick). The distance between pillars is 4.9 m and height of arch. 7.62 m (Courtesy of Dr R K Taylor)

It can be seen that in all cases the maximum height of void migration is directly proportional to the thickness of seam mined and inversely proportional to the change in volume of the collapsed material. It would appear that the height of collapse is independent of the width of the excavation although it is a limiting factor. In other words the larger the span, the more likely is collapse to occur. The maximum height of migration in exceptional cases might extend to 10 times the height of the original roadway, however, it generally is 3 to 5 times the roadway height. If a competent bed occurs in the roof rocks, which is thicker than 1.75 times the span width, it will arrest the collapse. Site investigations frequently reveal partially choked voids in abandoned pillar and stall workings. For example, the voids may be less than a metre in height and are occupied by waste which may have suffered some amount of decomposition (see Thorburn and Reid, 1978). On the other hand the self-choking process may not be effective if dipping seams are affected by running water which redistributes material fallen from the roof. Indeed Carter et al (1981) attributed the development of supervoids to such a process.

The desk study includes a survey of appropriate maps, documents, records and literature (see Symons, 1978). Geological maps indicate the probable positions where coal seams outcrop and these, together with topographic maps, may show the positions of old shafts and adits. All the geological and topographic maps of the area in question, going back to the first editions, should be looked at. The National Coal Board and the Abandoned Mines Record Office represent primary sources of information relating to past mining activity in the United Kingdom. Other sources include county record offices, public record offices, museums, libraries, private collections and the Institute of Geological Sciences. Where records of past mining are available, they must be treated with some reserve as they are frequently inaccurate and incomplete. Nevertheless such records provide useful information relating to which seams were worked and to the extent and method of mining. The use of remote sensing imagery and aerial photography for the detection of surface features caused by subsidence is more or less restricted to rural areas. If the site has to be flown, the maximum information can be obtained from photographs taken in winter when the low angle of the sun produces larger shadows thereby emphasizing the topographic expression, and the vegetation cover is low. Colour photographs prove more useful than black and white ones in the detection of past workings and if there are differences in thermal emission, then infra-red (false colour) photographs should be able to pick up these differences. The detail obtained from aerial photographs should be represented on a plan 1:2500 or larger. Russell et al (1979) used a number of remote sensing systems to evaluate their usefulness in this context. They concluded that scale

128 was a critical factor. In other words the resolution necessary for the detection of the relatively small subsidence features 11.5 to 3 m across) which are common in many parts of the Northern Anthracite Field o f Pennsylvania. is provided by aerial photographs with scales between 1:30000 and 1:10000. These authors, however, did note that Landsat imagery, S L A R imagery and high altitude aerial p h o t o g r a p h y provided data on fractures and lineaments which could be used to locate zones o f potential subsidence. 3.2 Indirect Subsurface Exploration Except in special circumstances, the c o m m o n methods of geophysical exploration have not proved very successful in detecting and revealing the layout o f shallow a b a n d o n e d mine workings. Seismic refractions has not been used particularly often in searching for voids created by previous mining in shallow coal seams since such voids generally are too small to be detected by this method. What is more the detection of subsurface voids becomes even more unlikely if they are situated at a depth greater than three times their diameter or if their diameter is less than several geophone spacings (which are c o m m o n l y not less than l m each). The depth limitation is due to the attenuation o f waves in the rock overlying the void. However, recent advances in seismic reflection techniques have improved the potential for detection o f shallow voids (see Benson, 1979). Howell and Amos (1975) maintained that they had located subsurface cavities, using a seismic reflection technique. After extensive field work, Maxwell (1975) c o n c l u d e d that, except for workings with a depth of cover less than 5 m, it was unlikely that resistivity profiling would detect the presence of dry pillar and stall workings. Moreover if the coal seam possessed an infinite resistance, then workings could not be detected. This conclusion was s u p p o r t e d by Burton and M a t o n (1975) who indicated that where the d e p t h o f a cavity is equal to its diameter, the m a x i m u m disturbance in the resistivity profile is only about l0 % of the b a c k g r o u n d level. As noise about the b a c k g r o u n d level frequently exceeds 10%, then cavities at depths greater than twice their average dimension usually are not recorded. Current-path electromagnetic prospection has been described by Howell and A m o s (1975) who claimed it could be used to detect the presence of subsurface cavities. They c o n t e n d e d that this technique could locate features when the d e p t h - d i a m e t e r ratio was 8:1 or smaller. According to M c D o w e l l (1981) terrain conductivity meters have several advantages over conventional electrical resistivity equipment when used for locating small near-surface a n o m a l o u s features. The conductivity o f the g r o u n d is measured by an inductive electromagnetic method. M c D o w e l l went on to describe a conductivity meter which is carried along traverse lines across a site and thereby provides a direct continuous read-out. Hence surveys can be carried out rapidly. Conductivity values~ however, are taken at positions set out on a grid pattern, although the continuous r e a d - o u t

is observed. The results c a n be c o n t o u r e d in order to indicate the presence of ans' anomalies. Generally speaking voids i n shallow a b a n d o n e d mine workings are too small and located at depths too great to be detected by n o r m a l m a g n e t i c or gravity surveys. Recently, however, M c D o w e l l (1981) referred to the fluxgate magnetic field g r a d i o m e t e r which permits surveys of shallow depths t o be carried out. It provides a continuous recording o f lateral variations in the vertical gradient of the E a r t h ' s magnetic field rather than giving the total field strength. The readings taken at fixed stations by the g r a d i o m e t e r have. according to McDowell, several a d v a n t a g e s over the proton-magnetometer. Strong magnetic g r a d i e n t s are not a problem and the effects o f magnetic variations with time, including the effects o f m a g n e t i c storms, are effectively removed. In addition, the g r a d i o m e t e r tends to give better definition o f shallow a n o m a l i e s by automatically removing the regional m a g n e t i c gradient. Quantitative analysis of the depth, size and s h a p e of an a n o m a l y generally can be m a d e m o r e readily for near surface features using the gradiorneter, than from total field measurements obtained w i t h a p r o t o n - m a g n e t o m e t e r . The sensor spacing o f g r a d i o m e t e r s is small (0.5 m or 1 m) in order to record features within two to three metres of the g r o u n d surface. On the other h a n d a p r o t o n - m a g n e t o m e t e r can m o r e easily detect larger and d e e p e r features and y i e l d s results which are more suitable for contouring. A microgravity method, a c c o r d i n g to McDowell (1981), is capable of locating m e d i u m size cavities which are near the surface and l a r g e r ones at greater depth. Recently, a n u m b e r o f i n d i r e c t techniques have been developed which have had s o m e success in subsurface exploration. Some o f t h e s e techniques are still at the development stage but w i t h time they may prove of value (see K o e r n e r et al, 1982). O f these techniques. ground probing r a d a r p r o b a b l y has had the most success. Indeed a c c o r d i n g to Benson (1979) ground probing r a d a r is c a p a b l e o f detecting small subsurface cavities directly. The method is b a s e d u p o n the transmission o f pulsed electromagnetic waves in the frequency range 1-1000 MHz (30 to 100 M H z was f o u n d to be the most effective frequency range b y Ballard, 1983, for detecting cavities in limestones). In this m e t h o d the travel time o f the waves reflected from subsurface interfaces are recorded as they arrive at the surface and the depth (Z) to an interface is d e r i v e d from Z = vt/2 where v is the velocity o f the r a d a r pulse and t is its travel time. The relative dielectric constant o f the ground material (~r) is r e l a t e d to the r a d a r velocity as follows er = (c/v) 2 where c is the velocity o f t h e electromagnetic waves in air (3 x 108 m/s). If the relative dielectric constant of this material is known, t h e n the d e p t h (Z) to the reflecting interface can be d e r i v e d from Z = t/2 x c/V~

129

Previously C o o k ~1974~ had described the use of ground probing r a d a r to explore a b a n d o n e d coal mines. The exploration was done from a shaft. Moffat and Puskar (1976) also referred to impulse r a d a r as a means of exploring shallow old mine workings.

distances of h u n d r e d s of metres. The p e n e t r a u o n of radar energy can be increased by using a lower frequency but this unfortunately reduces its resolution which means that subsurface anomalies have to be correspondingly larger if they are to be detected.

Recently Darracott and Lake t1981~, and Leggo and Leach (1982). described a trolley m o u n t e d system of ground probing r a d a r which could be towed either by hand or behind a vehicle. Rates o f towing usually vary between 1.5 to 6.5 k m / h ~the slower the progress, the better the resolution). They noted that the conductivity of the ground imposes the greatest limitation on the use of radar probing in site investigatmn. In other words, the depth to which radar energy can penetrate d e p e n d s upon the effective conductivity of the strata being probed. This. in turn. is governed chiefly by the water content and its salinity (Table 1). As the table shows.

As with any investigation the m o r e that is known about the soils and rocks present at the site. and their geological setting, the easier it is to interpret the data obtained. Consequently subsurface information obtained from boreholes, trenches or pits offer a measure of control which ensures more accurate interpretation. In addition. Darracutt and Lake ~1981) r e c o m m e n d e d that the conductivity o f g r o u n d material should be determined before a ground p r o b i n g r a d a r survey is carried out. By using such data it is possible to estimate the radar performance. As few sites are completely radar opaque, trial runs yield useful data.

Tab. I : A p p r o x i m a t e A f t e r M o r e y , 1974}

E l e c t r o m a g n e ~ t c P a r a m e t e r s of E a r t h M a t e r i a l s

Material

Air

...............

Fresh water Sea water S a n d , << d r y ~ .......... Sand, saturated ( f r e s h w a t e r ) .... Silt, s a t u r a t e d ( f e s h w a t e r ..... Clay, saturated ( f r e s h w a t e r ) .... D r y s a n d y coastal l a n d ........ G r a n i t e . d r y ..... . Limestone, dry .......

Approximate Conductivity (mho/m)

Approximate Dielectric constant

0 I0 -4 to 3 x 1 0 - : 4 10 -7 to 10 -3

1 gl 81 4to6

10 4 to 10 -e

30

10 -3 to 10-2

10

10 ~ t o l 2 x 1 0 -3 i 0 -~ 10 -9

8to

12

t0 5 7

the nature of the pore water also exerts the most influence on the dielectric constant. Furthermore the value o f effective conductivity is also a function o f temperature and density as well as the frequency of the electromagnetic waves being propagated. The least penetration occurs in saturated clayey materials or where the moisture content is saline. F o r example. Leggo (1982) noted that useful data can be o b t a i n e d from sites where clayey topsoil is more or tess absent as wet clay and silt, in particular, mean the greatest attenuation of electromagnetic energy so that depth o f penetration frequently is less than one metre. On the other hand the technique appears to be reasonably successful in sandy soils and rocks in which the moisture content is non-saline. Leggo remarked that a cavity in dry granular material produces a m a r k e d a n o m a l y particularly if it occurs in relatively homogeneous material. Rocks like limestone and granite can be penetrated for distances of tens o f metres and in dry conditions the penetration may reach 100 m (see Cook. 1975). Benson (1979) obtained a maximum depth o f penetration of 26 m in the Miami Limestone and he pointed out that a depth of 6 to 9 m was c o m m o n in many areas. This more or less agrees with Rubin and Fowler (1978) who showed that subsurface features in rock could be detected at ranges exceeding 8 m. Dry rock salt is radar translucent, permitting penetration

Koerner et al (1982) investigated a microwave method as a means of locating subsurface voids. This is an electromagnetic m e t h o d and the frequency o f the waves used is around 1 GHz. 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 u n d e r investigation. The wave is partly reflected b a c k to the surface by any interface with different electrical properties, and the amplitude of the reflected wave is recorded, It would a p p e a r that the continuous wave microwave technique is the more successful. It allows the depth of an interface to be obtained. Most of the geophysical methods have a down-the-hole counterpart which can be used to log a hole. For example, Ballard et al (1983) used crosshole r a d a r tests to detect cavities in karstic limestone. As expected they found that attenuation o f the r a d a r signal was related to specific geological anomalies, cavities occupied by clay showing a very sharp decrease in the strength of the signal. Anomalies varying u p w a r d s from 0.7 m in vertical extension were d e t e c t a b l e up to distances of 33 m. In interborehole acoustic s c a n n i n g an electric sparker. designed for use in a liquid filled borehole, produces a highly '~epetitive pulse. This signal is received by a h y d r o p h o n e array in an a d j a c e n t borehole, similarly occupied by liquid. G e n e r a l l y the source a n d receiver are at the same level in the two boreholes and are moved up and down together (see M c C a n n et al, 1975). Boreholes must be spaced closely enough to achieve the required resolution o f detail. This may be up to 400 m in some clays (in soft o r g a n i c clays, by contrast. it may only be a few metres) a n d up to 80 m in sands and gravels. These distances are for saturated materials, the effective range being c o n s i d e r a b l y reduced in dry materials (see Grainger a n d M c C a n n . 1977). The method can be used to detect subsurface cavities, if the cavity is directly in line between two boreholes and has at least one tenth of the b o r e h o l e separation as its smallest dimension. Air filled cavities are more readily detectable than those filled with water. Ballard et al (1983) found that acoustic crosshole tests were ~able to distinguish cavines in karstic limestone which had diameters around 500 mm.

130

Crosshole seismic testing has been used, employing two or more boreholes, to detect near-vertical subsurface anomalies (see Ballard 1976). However, according to Ballard et al. (1983) this technique has not proved particularly successful in locating cavities although it can recognise major zones of weakness attributable to faulting. 3.3 Direct Subsurface Exploration During the initial inspection of a site beneath which old mine workings are believed to exist, in addition to the general requirements of a preliminary survey, note should be taken of any surface evidence of past mining acti')ity such as old buildings associated with mining (eg pump house, winding house), ventilation systems, railway tracks, spoil heaps, abandoned shafts etc. Existing buildings should be inspected for signs of damage caused by subsidence. A site plan should be produced from the data gathered by the desk study and preliminary survey. The location of old workings has generally been done by exploratory drilling, the locations of drillholes being influenced by data obtained from the desk study (see Stephenson and Augenbaugh, 1978) or from the data gathered by using indirect methods. However, it must

be admitted that exploratory drilling frequently is not able to establish the layout of old mine workings (see Piggott and Eynon, 1978), although some site investigations have been relatively successful. For example, Carter et al (1981) described a site investigation at Bathgate where almost half the drillholes encountered voids at seam level. Generally these voids were around 0.5 m in height and overlay waste material. Drillholes which penetrated pillars indicated that coal was oxidized, in fact some pillars had collapsed. Drilling to prove the existence of old mine workings is frequently done by open holes, which allows relatively quick probe drilling (Figure 5). The drillholes should be taken to a depth where any voids present are note likely to influence the performance of the structures to be erected. Rotary percussion drilling with cruciform bit may be used. The stratal sequence should be established by taking cores in at least three drillholes. If a grid pattern of drillholes is used some irregularity should be introduced to avoid holes coinciding with pillar positions. Furthermore the grid axes should not run along the dip and strike directions of the coal seam(s) involved. The presence of old voids is indicated by the free-fall of the drill string and the loss of flush (the use of air flush provides independence of water supply). In some cases, once the basic pattern of the old workings has been revealed, hand drills have been used to fill in the details (see Scott, 1957). Below surface workings may be examined by using borehole cameras or closed circuit television (see Ackenheil and Dougherty, 1970), 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. What is more there are strict regulations concerning their use, as with any other electrical equipment. Occasionally smoke tests or dyes have been used to aid the exploration of subsurface cavities (see Scott, 1957; Benson, 1979). Access to shallow abandoned coal mines is rare. Consequently, if old workings which have remained open are to be explored directly, then access must be gained either by driving a heading from the outcrop of the seam, if this is close at hand, or by sinking a shaft to the coal seam. Thorburn and Reid (1978) described a shaft, with a diameter o f 1.2 m, which was sunk into old mine workings in Lanarkshire. Once the shaft reached the base of the coal seam, an adit was driven along it into the workings. When such workings are entered, strict safety precautions must be adhered to because of the possible danger resulting from their partially collapsed or partially flooded condition or from the presence of noxious or explosive gas.

Fig.

5 :

Probedrilling used to locate old mine workingsat a site in Sheffield. England

When old workings are located the number and depth of mined horizons should be recorded and the geometry and direction of the workings, should be assessed as accurately as possible. O f particular importance is the state of the old workings, careful note being taken of whether they are open, partially collapsed or collapsed. Whenever possible, the extraction ratio (the percentage of coal mined) should be estimated.

131

4 Conclusions In some parts of the world, Europe and North America in particular, coal has been mined for several centuries. Unfortunately, however, no records are available for many old abandoned workings. Because these are at shallow depth, they have frequently presented problems in foundation engineering. These problems are associated with the worked out areas which remain after mining operations have ceased. Collapse of roof rocks can. occur in these areas and at shallow depth the resultant voids may migrate up to the ground surface thereby producing crown holes. Pillars left in place to support the roof have to carry extra-loadings and so occassionally fail giving rise to broad subsidence at the surface. Void migration or pillar collapse can occur at more or less any time after the workings have been abandoned and there is no means of predicting when such events will take place. The situation is further complicated by the fact that such voids at shallow depth have proved difficult to detect. Conventional drilling and geophysical techniques generally have not been very successful. By themselves, the former are very much hit or miss and are costly. However, within the last few years developments in certain geophysical methods, especially ground probing radar, appear to offer a possible means of locating those old mine working which occur within the zone of interest of the foundation engineer. Ground probing radar may be able to provide, under suitable ground conditions, results which help locate the position of anomalous subsurface features. It also affords the opportunity of obtaining a continuous profile of the subsurface which reduces the likelihood of misinterpretation of the ground conditions between drillholes. Indirect methods are cheaper than drilling and therefore where old subsurface workings are not known with any precision can be used as a primary means of investigation. The information obtained by indirect methods can then be used to plan subsequent detailed investigation by direct methods, notably drilling. This can reduce the cost of the drilling programme significantly. Drilling should extend to a depth which provides the necessary data for the design of the structure(s) to be erected.

References 1 ACKENHEIL, A.C. and DOUGHERTY, M.T., Recent Developments in Grounting for Deep Mines, Journal of Soil Mechanics and Foundations Division, ASCE, Vol. 96, SMI, 1970, 251-261. 2 ANON, Code of Practice for Site Investigations, BS 5930, Bristish Standards Institution, London, 1981. 3 BALLARD, R.F., Method of Cross-hole Seismic Testing, Journal of the Geotechnical Engineering Division, ASCE, Vol 102, No GTI2, 1976, pp. 1261-1273. 4 BALLARD, R.F., Electromagnetic Techniques Applied to Cavity Detection, Technical Report GL-83-1, US Army Engineers Waterways Experimental Station, Vicksburg, 1983. 5 BALLARD. R.F., CUENOD, Y. and JENNI, J.P., Detection of Karst Cavities by Geophysical Methods, Bulletin of the

International Association of Engineering Geology, No 26-27. 1983, pp. 153-157. 6 BELL, F.G., Old Excavations in Coal Seams, Chartered Land Surveyor/Chartered Mineral Surveyor, Vol I, No3, 1979. pp. 40-58. 7 BENSON, R.C., Assessment of t,ocalized Subsidence (Before the Fact), Proceedings of a Specially Conference, ASCE, Evaluation and Prediction of Subsidence, Edited by Saxena, S K. Gainsville, 1979, pp. 47-57. 8 BRIGGS, H, Mining Subsidence, Edward Arnold, London. 1929. 9 BRUHN, R.W., MAGNUSON, M.O. and GRAY, R.E., Subsidence over Abandoned Mines in the Pittsburg Coalfield, Proceedings of the Second International Conference on Ground Movements and Structures, Cardiff. Edited by Geddes, J D, Pentech Press, London, 1981, pp. 142-156. 10 BURTON, A.N. and MATON, P.I., Geophysical Methods in Site Investigation in Areas of Mining Subsidence, in Site Investigations in Areas of Mining Subsidence, Edited by Bell, F.G., Newnes-Butterworths, London, 1975, pp. 75-102. 11 CARTER, P., JARMAN. D. and SNEDDON, M., Mining Subsidence in Bathgate, a Town Study, Proceedings of the Second International Conference on Ground Movements and Structures, Cardiff, Edited by Geddes, J D, Pentech Press, London, 1981, pp. 101-124. 12 COOK, J.C., Status of Ground Probing Radar and Some Recent Experience. Proceedings of a Speciality Conference, ASCE, Subsurface Exploration for Underground Excavation and Heavy Contruction, Henneker, 1974, pp. 195-212. 13 COOK, J.C., Radar Transparencies of Mine and Tunnel Rocks, Geophysics, Vol 40, 1975, pp. 865-885. 14 DARRACOTI', B.W., and LAKE M.I.. An Initial Appraisal of Ground Probing Radar for Site Investigation in Britain, Ground Engineering, Vol 14, No 3, April 1981, pp. 14-18. 15 GRAINGER, P. and McCANN. D.M., Interborehole Acoustic Measurements in Site Investigation, Quaterly Journal of Engineering Geology. Vol I0, 1977, pp. 241-156. 16 HOWELL, M. and AMOS, G.W.. Improved Geophysical Techniques for Survey of Disturbed Ground, in Site Investigations in Areas of Mining Subsidence, Edited by Bell, F.G., NewnesButterworhts, London, 1975, pp. 103-108. 17 KOERNER. R.M., LORD, A.E., BOWDERS, J.J. and DOUGHER"fS", W.W. C W Microwave Location of Void Beneath Paved Areas, Journal of the Geotechnical Engineering Division, ASCE, Vol 108, No GT1, 1982, pp. 133-144. 18 LEGGO, P.J., Geological Applications of Ground Impulse Radar, Transactions of the Institution of Mining and Metallurgy, Section B -- Applied Earth Science, Vol 91, 1982, pp. B1-B5. 19 LEGGO, P..I. and LEECH, C., Subsurface Investigations for Shallow Mine Workings and Cavities by the Ground Impulse Radar Technique, Ground Engineering, Vol. 15, No 1, January 1982, pp. 20-24. 20 McCANN, D.M., GRAINGER, P. and McCANN, C., Interborehole Acoustic Measurements and Their Use in Engineering Geology, Geophysical Prospecting, Vol. 23, 1975, pp. 50-59. 21 McDOWELL, P.W., Recent Development in Geophysical Techniques for the Rapid Location of Near-Surface Anomalous Ground Conditions, Ground Engineering, Vol 14, No 3, April 1981, pp. 20-23. 22 MAXWELL, G.M., Some Observations on the Limitations of Geophysical Surveying in Locating Anomalies from Buried Cavities Associated with Mining in Scotland, Mining Engineer, Vol 134, 1975, pp. 277-285. 23 MOFFAT, D.L. and PUSKAR, R.J., A Subsurface Electromagnetic Pulse Radar, Geophysics, Vol. 41, 1976, pp. 506-518. 24. MOREY, R.M., Continuous Subsurface Profiling by Impulse Radar, Proceedings of a Speciality Conference, ASCE Subsurface Exploration for Underground Excavation and Heavy Construction, Henneker, 1974, pp. 213-232. 25 PYGOTr, R.J. and EYNON, P., Ground Movements Arising from the Presence of Shallow Abandoned Mine Workings, Proceedings of the First Conference on Large Ground Movements and Structures, Cardiff, Edited by Geddes. J D, Pentech Press, London, 1978, pp. 749-780.

132 26 RUBIN, L.A. and FOWLER, J.C., Ground Probing Radar for Delineation of Rock Features, Engineering Geology, Vol 12, 1978, pp. 163-170. 27 RUSSELL, O.R., AMATO, R.V. and LESHENDOK, T.V., Remote Sensing and Mine Subsidence in Pennsylvania, Transportation Engineering Journal, ASCE, Vol. 105, No "FEZ 1979, pp. 185-198. 28 SCOTT, A.C., Locating and Filling Old Mine Workings, Civil Engineering and Public Works Review, Vol 52, 1957, pp. 1007-101 I. 29 STEPHENSON, R.W. and ANGENBAUGH, N.B., Analysis and Prediction of Ground Subsidence Due to Coal Mine Entry Collapse, Proceedings of the First Conference on Large Ground Movements and Structures, Cardiff, Edited by Geddes, J D, Pentech London, 1978, pp. 100-118.

30 SYMONS. M.V. Preliminary Site Investigations in Old Coal Mining Areas - Problem of Correlating Coal Seam Names. Proceedings of the Second International Conference on Ground Movements and Structures, Cardiff, Edited by Geddes, J D, Pentech Press. London, [981. pp. 2[I-240. 31 THORBURN, S. and REID, W.H., Incipient Failure and Demolition of Two-Storey Dwellings Due to Large Ground Movements, Proceedings of lhe First Conference on Large Ground Movements and Structurex, Cardiff, Edited by Geddes. J D, Pentech Press, 1978, pp. 87-99. 32 TINCELIN, E., Pression et Deformations de Terrain dans les Mines de Fer de Lorraine, Jouve, Editeurs, Paris. 33 WARDELL, K. and WOOD, J.C., Ground Instability Problems Arising from the Presence of Shallow Old Mine Workings, Proceedings of the Midland Society of Soil Mechanics and Foundation Engineering, Vol 7, 1965, pp. 7-30.