Engineering geology in practice in Britain: 1 1

=nc ineerinc ceo ocyanc minincby PETER R. RANKILOR*, MSc (Eng), BSc (Geol), CEng, MICE, FGS, LTI

In this article the author has taken theopportunity of sketching in some of thefunctions fulfilled by the EngineeringGeologist in a mine working operation.Comments are generally limited to under-ground operations and, although refer-ence is occasionally made to opencastworkings. comment has not been extend-ed to quarrying.

IntroductionIn the United Kingdom and Europe,

much evidence exists to show that min-ing has been undertaken since Palaeolithictimes. For example, at Bury St. Edmunds

«Present Chairmen of the Association of Engineer-ing Geologists (UK Section); Chairman of Man-stock Geotechnical Consultancy Services, 1, NorthParade, Parsonage, Manchester

in England, Palaeolithic man excavatedBell Pits for flints. Elementary Geologyand Engineering came together as farback as those times when the siting ofpits would be enhanced by experience interms of locating productive ground, andthe size of each pit would be governedby a fundamental 'engineering'udgementof safety. This decision-making can beobserved to have been relatively success-ful by virtue of the now known positionof extractions, and the long-lasting stabil-ity of these ancient excavations.

Moving from the early dawn of min-ing to the present day in a single step,one finds that a third 'practice' econom-ics—has entered the scene. At first, thelimitations in geological and mechanicalknowledge were the limiting factors gov-

erning the existence and extent of mineworkings, but quickly skill in both ofthese subjects grew to the point whereEconomics had to be considered as themajor controller. Therefore, the authorviews mining as a combination of the Artsof Geology and Mechanical Engineeringworking within a limiting framework setby economic criteria.

It is important to see this framework inorder to appreciate the working situationof the engineering geologist within themining industry and thereby appreciatethe contribution he can make to theoverall scheme of any particular miningproject. The economic boundaries withinwhich he operates are usually clear, andoften rigid —being in many cases set inaccordance with decisions made as part

Fissure revealed at rockhead subsequent to mining subsidence

22 Ground Engineering

step develops

„~ET,IN.„SOii.,i..:':.::.",'-'":.:-':,'::.":::::.:',„::,:,':,::,.::::::,''.h

sa s

3C 35

of a larger, overriding plan.Since the rate at which Mechanical En-

gineering adjustment can be made withina project is slow, the engineering geo-logist must be able to provide amplewarning of variation from expected con-ditions. This is a critical on-stream func-tion which must be fulfilled effectively in

order to maintain a state of high workingefficiency. Against this setting, the authorpoints out the more-detailed contributionsmade in an Engineering sense.

TABLE I

Geologicalresearch

Geostructuraldata

Statisticaldata

Extractedsamples

'Mechamcalengineering analysis

of feasibility andmining techmoue

apphcation

Factors fromother sciencesand fields

Analysis of potentialvolume of reserves

Economical analysisand study for the

entire project

Physical and chemicalanalysis

Analysis ofnuality of

reserves

Socialfactors

It is really surprising that, even within recentyears, the premature failure of some majormining operations has been caused by projectanalysis being undertaken without the MechanicalEngineering study being made to determinewhether 'theoretical'eserves were

'attainable'eserves.

Data acquisition at the pre-planningstage

Individual mining projects usus'llybegin to crystallise at the data acquisitionstage of general exploration programmes.Today, the major mining exploration com-panies have continuous on-going explora-tion programmes to assess reserves ofnew potential mining areas. Within anysuch large programme, the geological datatend to produce discrete zones of min-ing potential. Each of these zones mayultimately comprise a mining project in

its own right. Within each zone, geo-logical information is required for thebreakdown analysis set out below andshown as Table I.

Considerable geological expertise is re-quired in the combined desk study/practice programme needed to provideadequate geostructural information. Exist-ing sources of information must be ex-haustively checked, including both out-of-house institutions and in-house recordsof past mining activities. Beyond this,data must be added from surface map-ping, drilling programmes, and hydrogeo-logical studies. Field studies are aimed in

fissures propagatethrough superf icialdeposits

~

::,:THICK,'.'":::;:::,:::'::;.j.';

saafissures openedby tension 'tgjt~mFffajliÃllll~

f. Effect ofFig superficial deposits over faults and fissuresZONE OF TENSIONAT THE SURFACE

ill.";I';:it'i':;;':;:,:,ii,';:::,''" Ri k

h'."!::.';:,',.i;::;::,'".'.".:,' ~r. "~,

ZONE OF COMPRESSIONAT THE SURFACE

nce trough in plan view

Compressionone Zone Vertical Scale

Advances Exoggeroted

DI

FA

RECTION OFCE ADVANCE

Fig. 2. Subside

Tension Z

Advances

AREA OFCOAL EXTRACTIONUNDERGROUND

G.L

RAPID STRATACOLLAPSE .

/Mining Face Advances

Fig. 3. Subsidence trough, longitudinal sectionSURFACE SUBJECT TO ROTATING TENSIONSTRAINS AND LEFT IN TENSION.THEREFORE NOT PRACTICAL TO DESIGN

SURFACE SUBJECT TO FLEXIBILITY IN ONE DIRECTIONLINEAR TENSION STRAINAND LEFT IN TENSION. eTHEREFORE PRACTICALTO DESIGN FLEXIBILITY EIN ONE DIRECTION r::;:::"':::::.'-euti

rI.:"::::.::::::::::.+~~„.:i' 'N 'THIS ZONE, SURFACE

I " "'3:::,:',:.:nlj': SUBJECT TO LINEAR TENSION '. LjMfTOF'I.. - THEN COMPRESSION. BUT -

. EXTRACT(ONa. COAL

LEFT .UNSTRAINED

TENSION AND THEN EVO COMPRESSION.LEFT IN COMPRESSION.

SURFACE SUBJECT TO LINEAR TENSION~ AND ROTATING COMPRESSIVE STRAINS,

THEN LEFT IN COMPRESSIONFig. 4. Zones of differing surface behaviour above a modern mine working face

October, 1976 23

Distant Surface

ExtractionSeam

stage I stage 2S s 1s oc O'Uoc

s'tage 3

CollapseDebris .o",oxyo'„

stage 4

Distant SurfaceCrown SurfaCe

Near Workings

cFi

stage 7

staqe S

Fig. 5. Surface instability owing

NARROWARCHINGOCCURS

stage 6

—:CLAY ".'::.:S'AND'.";:-.,',I.:,:;,.

~ t

swatei flow';<~t

'ashes.:~~away sand

ap..'o ~'ypa

~ y

S<m

pss

%oo

yew

wate

Fig. 6. Effectworkings

of superficial deposits on type

away into oldmine

vol<sfand area of instability from shallow

TOFIDEN CECTS

Bool

" DEECHBUVA

Boul

Sand'::::;:,':::::;:,'::::.";""

C

C I ay

VELOPMENTLUDED

E AREA

Fig. 7. Effect of thick sand on subsidence area

SuperficialCLAY deposits

geologica I

fault inbedrock

SuperficialSAND deposits

Ffg. e.

24 Ground Engineering

ZONE OF ACCENTUATED STRAIN INGENERAL SUBSIDENCE AREA IS WIDERIN SAND DEPOSITS THAN IN CLAY.

to abandoned shallow mine workings

BROAD ARCHING'ITH SPREADINGOCCURS

particular at defining the existence ofmajor discontinuities such as faults; theexistence and frequency of fissures; thestate of the rockhead weathering; and thetype and distribution of superficialdeposits within the area likely to be in-fluenced by the mine workings (see Figs.1, 2, and the photographic illustration onpage 22.

Statistica'I data to be drawn up at thisstage include statistically meaningful sam-ples of mineral dimensions, and also theresults of testing (of both mineral andbedrock) for strength and rippability. Pro-per engineering logging of rock cores isof paramount importance since

'mineral'ogging

alone cannot provide the requiredquantitive data needed to assess therelative properties of the major rock zonesinvolved in the works. The Report by theEngineering Group of the GeologicalSociety'hows how usefully quantitativesuch information can be, and how it canbe displayed for analytical purposes. Apaper presented in the Quarterly Bulletinof the Association of Engineering Geo-logistss by the author suggests one sys-tem of field core logging which suppliessuitable quantitative information for bothengineering and hydrological purposes.

Hydrological studies must be exten-sive, both in terms of undergroundaquifers, and surface flows. Subsidenceconnected with mining extraction cancause severe disturbance of natural hy-draulic networks, and apart from this, aknowledge of surface and near-surfaceflows is vital when consideration is givento the the tipping requirements at thesurface". Since Aberfan, it has not beenpossible to overlook this aspect in theUnited Kingdom.

Contribution to the planning stageThe contribution of engineering geology

in this phase takes the form of aninterpretative feed-back responding tothe main dialogue between mining en-gineers and economic planners, since thefundamental Geological Statement hasbeen prepared and presented in the formof a report. As variable aspects of planningand operation are considered, the viewsand experience of the engineering geo-logist can play an important part inassisting and guiding the decision-makingprocess. The geologist can bring to bearhis training and knowledge of the depos'i-tional or formational environment of thedeposits in question. His fundamentalunderstanding of the 'like'ly variability ofa deposit (both dimensionally and interms of purity) can help to define limita-tions which could be set on equipmentchoice and performance, and also on min-ing systems to be adopted.

The engineering geology report willmake comment on special observationpoint requirements and ground measure-ments needed to check on geologicaldata, and it will describe the likely extentof zones of subsidence for damage esti-mates. Field work wi'll have allowedrecommendations to be made with respectto waste-disposal systems, locations,methods, and important geotechnical mat-ters such as tip and opencast settlementand 'instability. A longer-term contributionto be made at this stage is the advice

HIGH- RfSEBLOCK

G.L

the engineering geologist can give in pos-sible post-mining utilisation of the miningvoid —for example, the storage of oil,water, or gas.

.'::::::::SUPERFIClai DEPOSITS

)'

Data feed-back during operationsGeotechnical expertise and observation

is required consistently throughout the lifeof any mining operation, both within themine working area, and at the overlyingor surrounding surface.

In the sub-surface operations, monitor-ing of the deposit dimensions and qualityis necessary in order to confirm thatplanned operational procedures can takeplace. Additionally, regular observationand measurement must be undertakento record ground strains being generatedby the operations. Typically, traditionalmethods have included photoelastic plugs.electronic strain gauges, and physicalm eas urem en t.

Recent developments, however, havetended towards optical solutions such aslaser-illuminated tunnel profiling 'ndstereo-photographic analysis of rock facemovement in opencast workings. Newmethods are also being developed formonitoring the cuttability of undergroundrock faces ". The engineering geologistshould be aware of such developmentsand should bring them to bear on indivi-dual projects to ensure that optimummachine utilisation is being maintained.

'Exploratory drilling ahead of workingfaces is common in many mining projects.The object can vary from checking forwater inflows in subsea or deep mines, tochecking for continuity of deposits inorder to ensure forward working program-mes. In coal mining, for example, the pres-ence of a small fault can mean the seamis thrown out of the current working line.On a mechanised face, this can result insubstantial on-stream schedule alterationsand working adjustments.

The mechanisms of, and surface mani-festations of, subsidence are complexbut are fairly well understood for mostsimple cases. Figs. 3 and 4 describe thedevelopment of subsidence over a deepseam total extraction. Figs. 5 and 6 illus-trate the migrating collapse associatedwith isolated workings. Although theselatter diagrams refer to shallow mineworkings, this type of collapse can occurIn tunnel workings and in the loca'lisedextraction of emplacement minerals. Aknowledge of the influence of mine work-ings on the ground support of structuresis vital to the engineering geologist con-nected with mining projects. Some of themore elementary aspects are illustrated in

Figs. 7-12, and Fig. 13 is suggested by theauthor as a possible logical first steptowards the consideration of appropriatefoundation design. Fig. 14 is a metricversion of a graph published by theNational Coal Board in the United King-dom~ shovving the amount of damagelikely to be suffered by structures experi-encing different amounts of strain. Fig. 15illustrates the outlines of raft designrecommended by some NCB Areas.

The disposal of waste from undergroundmining activity has to be controlled geo-technically. Samples of dumps must betaken and stability analysis constantly up-dated by 'back~analysis'rocedures using

5cv

Fig. 9. Selection of pile termination point in areas of shallow room-and-pillar mineworkings

PREDICTEDINSTABILITY ZONE

PREDICTEDINSTABILITY ZONE

road, ~. Ir ~l I rl

=clay

.35'LAN

rackhead outcropof coal seams development site

Pl

factory buildingsin this area

pa&- jetttis area,::,

,>'+

wzzzzzzzzzi ~M vzii~zzzzzzzzzzzzzzzzzzzzzzzzzxzzzzzzzzzzzzzzzizzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzizzzziiEF/i'"'IT

zone of potential instabilityV ) P T

,5'~j '"

SECTION

Fig, 11.Effect of shallow mine workings on positioning of development structures

SHRUBBEDAREA

'einforccancret

SHRUBBEDAREA

~ ..''W''>'conical

.plunge - plug::.. type cap

EMPTYSHAFT

EMPTYSHAFT

Fig. 12. Effect of thickness of superficial deposits on instability areas arising frommine shafts

October, 1976 25

SAND-WASHFig. 10. Positioning houses off areas of predicted instability instead of grouting oldmine workingsis a satisfactory solution on clays, but not on sands

key.

(2)(3)(1)

INTEGRAL RIGIDITY(I.I) With flat based raft.(1.2) With shallow ring beam.(I.3) With deep ring beam.

FLEXIBLE CONSTRUCTION UNIT.

MULTI-UNIT SPLIT INTO SEPARATE INTEGRAL UNITS.TRENCHING FOR STRAIN REDUCTION. 4-

rl

eE

SA NDsmallstructure

A (I.3)

B (I 2)

C (2)

D (I. I)

E (I. I)

LIJUZUJ

QEQ Z>0ti) N

Fig, 13. Table of design approach

largestructure

(I . I)

(I.I) I

I

3 I TI

I

2 I TI

2+3 I TI

I

CLAY6m a I I largestructure structure

(1.2) 3 I TI

(I.I) 3I

2 3 I TI

I

I

(I I) 2+3 I TI

(I.I) 2+3 I

I

I

EEe 3—

Z

txI-rn

0 03m

0 30 60 90 120I

150

LENGTH OF STRUCTURE(metree)

Fi .14.S'g.. everity of damage for differing strains andstructure lengths

180

timberfloor

Fig. 15. NCB recommended rafts

solidfloor

LI GHT

MEDIUM

HEAVY

planes, and the organisation of closingdown mine entrances, are among themany service jobs necessary.

I n the case of opencast workings, thegeotechnical engineer has a major rol t

lae o

p ay in land reinstatement. In many Plan-

fi

ning Permissions, this work will be spec'-ied to start early during the life of th

speci-

peration. However, soils analysis for in-c

filling and compaction work will go on afterextraction has been completed, togetherwith forecasts of settlement and monitor-ing of backfilled bedrock edges (Fig. 16).

Finally, a report on the entire projectrecording all aspects of the work includ-ing the final state of all tips and voidswill be required —at least by the mining

UK'

company, and in most cases (a 'h) by Government archives as well.

Tips and abandoned workings in the UKhave to receive official approval, and fu'll

mining records must be deposited withthe relevant authority.

-downwash sand failure plane-sand failure plane

-clay failure plane

AREA OF OPENCAST WORKING

instability area

jo

Fi .16.l

any slips or other failures which haveoccurred. In many instances, where nopotential loss of life exists, dump andslope safety factors are kept close tounity. Under such circumstances, freakweather or earthquake tremors can causefailure of rock or soil designs, which areusually very costly to the operation.

26 Ground Engineering

Geotechnical contributionsubsequent to completion ofextraction

Geotechnical involvement with ananyparticular mining project must extendwell beyond the actual completion ofextractive working. The monitoring oftips and subsidence settlement on fault

e,,~c:sujr& e„Ajt,!:ej r-»'.~ej '":0,,

quarriesg nstability features arising at the boundaries of ba kfll dc i e opencast pits or

ConclusionsIn brief, the role of the engineering

geologist in any particular operation is aswide as the operation itself. He starts in

tl'ithe planning and exploration stage; co-ributes to the design of the workings;

on-

continuously feeds information to themining engineers and planners during ex-traction to ensure efficient working; d

fa ter the extraction phase, the engineeringgeologist is involved in the closing-upand final report preparation. This is ademanding role, requiring personal abilita d a high standard of education —prefer-ably obtained from the study of engin-eering geology as a subject in its ownright.

References1. Report of the Engineenng Group of the Geo-

logical Society of London. Quarterly Jnl. EnGeology. Vol. 3, No. 1, Dec. 1970.

2. Rankiior, P. R.: "A suggested fsystem orogging o rock cores for engineering

purposes". Bulletin of the Association ofEngineering Geologists, Vol. XI, No. 3. Smer 1974.

o.. um-

3. Mather, J. D., Gray, D. A. & Jenkins D. G.:"Tho use of tracers to investigate the rela-tionship between mining subsidence andground water occurrence at Aberfan, SouthWales". Journal of Hydrology, 9 136-154(1969).

4. "Tunnel profrlrng by photographs''. Tunne/sand Tunnelling, May 1976.

5. Hudson, J. A. & Drew S. D.: "An impactpenetrometer for assessing the cuttability ofsoft rocks'' TRRL Report 685, Transport andRoad Research Laboratory.

6. Subsidence Engineer's Handbook, NationalCoal Board, London, 1966. (Revised 1975).