V acne:icsurveyinc 'or ouriecminestra':s anc we sby WYNFORD HOOPER, BSc, MSc, CEng, MIMM, FGS, and PETER McDOWELL, BSc, MPhil, CEng, MIMM, FGS"

The engineering implications of buriedmine shafts and wells, and the methodsof locating such features. are outlined.Indirect geophysical techniques are com-pared to direct investigation by drillingand trenching. Particular emphasis isplaced on the magnetic surveying meth-ods. Two examples of the use of mag-netic techniques, in apparently unfavour-able environments, are detailed.

IntroductionFOR MANY CENTURIES it has been com-mon practice to sink vertical shafts in

the ground for water supply and miningoperations. In the United Kingdom, waterwells are common and mine shafts havebeen constructed for the abstraction ofmetalliferous ores, salt, coal and otherrock materials. These wells and shaftscan all produce engineering problemslocally, but, because of the abundanceof coal mining areas, coal mine shaftsaccount for most of the problems that arerecorded.

There have been many cases recordedof the sudden collapse of old abandonedcoal mine shafts, which were either un-known, or presumed to be adequatelyfilled. It is generally known that many

'ecturer and Senior Lecturer, respectively, in En-gineering Geology, Department of Geology,Portsmouth Polytechnic, Portsmouth PO1 3QL

of the shafts abandoned in the early partof this century were inadequately cappedand filled. These and similar earlier shaftsare, therefore, a potential engineeringhazard. Several cases of the dramaticcollapse of mine shafts in North Stafford-shire and West Lancashire, involving lossof property and life, are described byDean (1966).

Unfortunately, a considerable number ofthese earlier coal mine shafts were sunkand abandoned without any record beingleft of their position and, since theirabandonment, all visible traces of theirexistence has vanished. The National CoalBoard keeps a record of all known shaftpositions, but it is an incomplete record.Mining activities have been in operationfor centuries and it was not until 1850that legislation was introduced makingit compulsory to keep plans of all work-ings, and not until 1872 that these planshad, by law, to be deposited with acentral agency. A list of the main sourcesof information for the location of aban-doned coal workings is given by Price,Malkin and Knill (1969). Even whererecords have been kept, however, theyare often on small-scale plans withouteasily identifiable points of reference andcan only provide the approximate positionof a shaft.

Methods of explorationVarious methods have been used to

locate the precise positions of abandonedmine shafts and wells. These methodsinclude observation of the ground fordepressions, trenching, drilling and geo-physical surveying. Stripping the groundof top-soil and vegetation, and the inspec-tion of the stripped surface for signs ofinfilling material has produced goodresults, but this method is not alwayspracticable. Trenching has also been usedon many sites to locate shafts, but ex-tensive excavation is again usuallyrequired, and careful backfilling is requiredto restore the ground condition to itsoriginal state. Drilling is usually an ex-pensive operation because the location ofa suspected shaft could involve sinkingholes on a grid as small as one metre.

It would, therefore, be more expedientto reduce the number of boreholesrequired, by using rapid indirect geophy-sical methods, which can indicate thepresence of shafts and wells.

Geophysical methods have been suc-cessfully used in the past, particularlytbe electrical resistivity and magneticmethods. Barker and Worthington (1972)and Aspinall and Walker (1975) giveexamples of the successful use of elec-trical resistivity traversing to locateshafts under a shallow cover of over-burden. Resistivity techniques are, how-ever, far slower and consequently moreexpensive than modern magnetic survey-ing methods.

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Fig. 1. The proton precession magnetometer in use

March, 1977 21

Magnetic methodsMagnetic methods have been used

successfully to locate mine shafts inrecent years because of the developmentof proton precession magnetometers.The outmoded magnetic variometers arefar too inaccurate and time-consumingand should not be used for this purpose.Raybould and Price (1966) discuss thesuccessful location of brick-lined andtimber-lined infiilled shafts, near Leeds,using the proton magnetometer.

The development of continuous record-ing fluxgate gradiometers has enabledsurveying time to be cut down even fur-ther. This is particularly advantageous forthe initial investigation of large areas.For detailed surveys a continuous record-ing of variations in magnetic gradient inprofile form can be provided from thecombined use of the gradiometer and anX-Y p'lotter (Phil pot, 1973) . Develop-ments in magnetic tape recording of datato produce contour maps is being activelypursued (Clark and Haddon-Recce, 1973).It should be noted, however, that compat-ible position fixing systems are necessarywhich may be very time-consuming forcomparatively large areas of investigation.Clearly these systems warrant further in-vestigation for the location of shafts.particularly as the fluxgate gradiometercan be used in areas of higher magneticgradients than the proton magnetometer.

For 'larger areas requiring investigationthe authors have found it profitable touse both the fluxgate gradiometer andproton magnetometer. The fluxgate gradio-meter was used to locate anomalousareas. These anomalies were subsequentlyre-surveyed with the proton magnetometerto provide detailed absolute magneticfield strength variations. In this way thearea was completely covered quicklywithout recourse to elaborate positionfixing devices.

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Magnetometr rstations + + + + + + +

Fig. 2. Magnetometer survey at Mangotsfield, Bristol

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The magnetic anomalyThe limited avai'lable data on the mag-

netic anomalies associated with aban-doned mine shafts indicates a widevariation in anomaly magnitude and shape.Anomalies, from ten gammas to hundredsof gammas, have been recorded. Largeamplitude anomalies are usually associ-ated with ferrous metal masses, but thesealso require investigation as the metalmay form the capping or infilling of ashaft. This type of anomaly is usuallyof the 'dipole'ype, i.e. a 'high'ithan adjacent 'low', Smaller

'monopole'ype

anomalies, with an amplitude of lessthan 100 gammas, are usually producedby the mineshaft itself.

The nature of the shaft itself, and themagnetic properties of the surroundingrock, infilling material and lining material,will effect the shape and magnitude ofthe anomaly. The anomaly may be nega-tive or positive, depending on whetherthe shaft is empty or filled with materialof a higher magnetic susceptibility thanthe host rock. The positive anomalies maybe increased and the negative anomaliesdecreased by the presence of a bricklining. Various combinations of the pres-ence or absence of lining and infiilling tothe shaft will give different anomalymagnitudes. Particularly disadvantageouscombinations will arise when the shaftis infilled with the same material as thehost rock, and when the shaft is linedbut left unfilled. However, other combina-tions can give rise to identifiable anomaly

22 Ground Engineering

magnitudes when the magnetic survey-ing is properly carried out.

The magnitude of anomalies will alsodepend on the height of the recordinghead above the top of the mine shaft.A surveying height of one metre aboveground level 'has proved convenient andis adequately free from magnetic varia-tion effects of the topsoil. The thicknessof overburden will have a significanteffect on anomaly magnitude. Generallyspeaking, mine shafts buried at depthsgreater than three metres will be verydifficult to locate unless they have asso-ciated ferrous metal.

Mine shafts may, therefore, only haveassociated small magnitude anomaliesand care must be exercised in the fieldmeasurement. A sensitive magnetometeris required. such as a proton magneto-meter, with a sensitivity of less than ~1 gamma. Furthermore, readings shouldbe taken on a grid pattern, with a stationinterval of one metre, to allow for thesmallest diameter shaft. Correctionsshould be applied for diurnal and secularvariations of the earth's magnetic fieldwhen using the proton magnetometer.

Small anomalies, associated with mineshafts, can also be distorted or obscuredby the effects of buried ferruginuousobjects and local high magnetic gradients.This is a particular problem in urban areas.An unacceptable magnetic gradient ofover ten gammas per metre would existat 12m from a one tonne concentratedmass of iron, or in the proximity ofpower cables.

A site with buried ferrous metalThis problem was encountered in the

investigation of a site of a proposedthree-storey block of flats in the Mangots-field district of Bristol. It was known,from the National Coal Board, that thearea had been mined for coal and thatshafts and shallow workings might existat the site. Although the ground surfacewas cleared there was clearly some metalremaining in the fill.

A detailed magnetic survey was com-pleted, using the Elsec proton preces-sion magnetometer at 1.2m (4ft) intervalsalong traverses spaced 1.8m (6ft) apart.The magnetometer was held 1.2m (4ft)above the ground.

The magnetic anomaly map (Fig. 2)shows several isolated anomalies whichhave been analysed and further investi-gated. It was considered that some ofthese anomalies were the result of buriedmetal objects.

The largest anomaly. labelled D, isclearly a large amplitude 'dipole'nomalyof the type normally associated withburied metal. Subsequent excavation dis-closed a 2m dia. brick-lined shaft.Although not recorded, there clearly musthave been ferrous metal in the infillingof this shaft. Another shaft which wasstone lined and of 1m dia. was revealedat location A beneath a cover of 1.3m.Although this discovery would seem tojustify the use of the meth43d, the ampli-tude of the anomaly is more consistentwith the flanking 'low'f a dipole pro-duced by buried metal. Unfortunately the

M.T. M.T. M.T. MT. MT. MT. MT. IVLT. MT.

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MagnetometerTraverse

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(a)

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47590-

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Distance along MT5.in metres (b)

Fig. 3. (a) Magnetometer survey at Kingswood, Bristol. (b) Magnetic field strengthprofile over the anomaly found at the Kingswood site, along the line of MT 5

rule (Breiner, 1973), the approximatedepth of burial of the mine shaft was as-certained. The results indicated that thetop of the mine shaft was approximately0.25m below the ground surface. Trenchingwas, therefore, quickly started on the siteof the anomaly and proved the existenceof a sharp steep contact between in-situCoal Measure strata and backfill of mine-waste, including ash, coal and mudstone.Further investigation, on removal of thetop soil only, showed the infllling to havea circular cross-section, 4m in diameter. Aborehole which was subsequently drilledat this location reached the intact rockat a depth of 10m.

The anomaly magnitude of this shaftis consistent with magnetic susceptibilityvalues for the host rock and infill, asmeasured in the field with the ABEMKappameter and as deduced from pub-lished data (Tite, 1961).

ConclusionsThese case histories show that the

magnetic method can be successfully usedto locate wells and mine shafts. Success,however, depends upon a large numberof factors, but some of these, such asinstrumentation and field procedures, canbe controlled. In general, it is recom-mended that a proton precession magneto-meter should be used at a station intervaland recording height of not more than onemetre in each case.

The ampl'itude and shape of the mag-netic anomalies is as variable as thenature and geometry of the wells andsffafts themselves. In some cases it ispossible to estimate the depth of burialof the shaft from the anomaly width.Where the depth of burial greatly exceedsthe diameter of the shaft, the magneticmethod can only be successful if the shafthas ferruginous metal in the capping orinfills. It should be also noted that theabsence of anomalies does not necessarilyindicate the absence of shafts but, untilnew techniques are developed, the mag-netic method appears to be the bestsuited for this particular problem.

All magnetic anomalies, even thosewhich are clearly the result of buriedferruginous metal, should be further in-vestigated by means of trenching andpossibly drilling. Magnetic surveys, evenin urban areas, where magnetic gradientsmay be high, can still be a useful inves-tigation tool.

site limits precluded a complete magneticinvestigation of this anomaly.

The other isolated anomalies such asB were suspected or known to be asso-ciated with buried metal. It is also ofinterest that at location C unusual groundvibrations were produced by the passageof mechanical plant.

A site with high magnetic gradientsA building site, in the Kingswood

area of Bristol, where high magnetic grad-ients were suspected was surveyed bymagnetic methods. The National CoalBoard plans provided the approximatelocation of a shaft known as Chas Brain'sPit and also reported the possibility ofother unrecorded shafts and/or seamopenings on the site.

A detailed magnetic survey was carriedout over an area of approximately 300mzaround the suspected location of ChasBrian's Pit using the Elsec proton preces-sion magnetometer. The magnetic tra-verses were 1.8m (6ft) apart and the

station interval was 1.2m (4ft) (Fig. 3a).The recording head was held 1.2m (4ft)from the ground surface. The geophysicalfieldwork, which included surveying of thethe grid and placing pegs at the magne-tic anomalies, was completed by thetwo authors in about eight hours.

The magnetic anomaly map (Fig. 3a)shows a large dipole magnetic anomaly,and the high magnetic gradient, producedby a power cable running vertically up apole, in the south-west corner of thesite. The gradient could clearly masksmall magnetic anomalies normally pro-duced by a mine shaft, but fortunately a40 gamma anomaly, marked X on Fig. 3a,was observable at a distance of 15m fromthe cable. This anomaly is a magnetic'high'f the monopole type and is thesort that could be expected for a shaftwhich has been backfilled with highermagnetic susceptibility material than thesurrounding rock.

From the profile, shown in Fig. 3b, andby utilisation of the anomaly

'half-width'eferences

Aspinall, A. and Walker, A. R. (1975): "The earthresistivity instrument and its application toshallow earth structures". Underground Services,3, No. 1, pp. 12-15.Barker, R. D. and Worthingron, P, F. (1972):"Location of disused mine shafts by geophysicalmethods". Civ. Eng. and Pub. Works Rev., 67,No. 788, pp. 275-276.Brainer, S. (1973): Applications manual for port-able magnetometers. California: Geometries, pp.1-58.Clark, A. J. and Haddon-Reace, D. (1972/3): "Anautomatic recording system using a Plessey flux-gate gradiometer". Prospezioni Archeologiche, 7-8,pp. 107-113.Dean. J. W. (1966): "Old mine shafts and theirhazards". Trans. Inst. Min. Engrs., 126, pp. 368-376.Philpot, F. V. (1972/3): "An improved fluxgategradiometer for archaeological surveys". Prospez-ioni Archeologiche, 7-8, pp. 99-105.Price, D. G., Malkin, A. B. and Knilk J. L. (1969):"Foundations of multi-storey blocks on the CoalMeasures witn special reference to old mineworkings". Q. Jl. Engng. Geol., 1, No. 4, pp.271-322.Raybould, D. R. and Price, D. G. (1966): "Theuse of the proton magnetometer in engineeringgeological investigations". Proc. Mech., Lisbon,1, pp. 11-14.rite, M. S. (1961): "Alternative instruments formagnetic surveying; comparative tests at the IronAge hill-fort at Rainsborough". Archaeometry,4, pp. 85-91.

March, 1977 23