ie 8'i Ran~ne ec:ureGround support for tunnels in weak rockA short report by A. D. M. PENMAN", DSc, CEng, FICE

THE GEOTECHNICS DIVISION of theBuilding Research Station is justifiablyproud that Dr. W. H. Ward was chosento give the 18th Rankine Lecture. Peopleoften ask why the national expertise onthe soil mechanics aspects of large civilengineering structures such as dams, largefoundations, deep basements, offshore oilproduction platforms and tunnels shouldlie in the Building Research Station. Thosewho do not know, do not think of theBuilding Research Station as a place ofcivil engineering research. In fact, the Geo-technics Division is, in effect, the 'BritishGeotechnical Institute', with internationalconnections and a world-wide reputation.Perhaps, one day the name may bechanged.

Readers will, no doubt, recall that theestablishment of the Rankine l.ecture fol-lowed the rich days left by the success-ful 4th International Conference on SoilMechanics and Foundation Engineeringheld in London in 1957. The policy was tohave an 'expensive'verseas lecturer, fol-lowed during alternate years by a

'cheap'ome

man. A list of previous lectures isgiven in Table I.

The ball was started rolling by a con-troversial lecture by Professor ArthurCasagrande, who chose to point out thelimitations of grout cut-offs under damsand caused a fairly violent reaction fromthe well-known grouting specialists Sole-tanche. It also stimulated Dr. Ambraseys(1961, Geotechnique vol 11, pp. 362-4) towrite a note on the efficiency of groutcurtains, shying that an increased effec-tive thickness of a cut-off could substan-tially improve its theoretical behaviour.

The second Rankine Lecture was givenby the late Dr. L. F. Cooling, then Headof our Geotechnics Division at BRS. Hechose for his subject "Field measure-ments in soil mechanics", an importantaspect of the subject in which the Geo-techn'ics Division has always excelled.

Dr. Ward, late Head of the Division andnow Deputy Chief Scientific Officer, is thesecond Rankine Lecturer to come from ourGeotechnics Division. Throughout his car-eer, Dr. Ward has taken a keen interest in

the geotechnical problems associated withtunnelling and it is natural for the subjectof h'is Rankine Lecture to be "Groundsupport for tunnels in weak rock".

Dr, David Henkel, Chairman of the Geo-technique Advisory Panel, in his introduc-tory remarks said that he had known Dr.Ward for more than 30 years and recalledtheir first meeting in the depths of BorthBog where Dr. Ward was studying insta-bility problems in banks of new drainagechannels. Dr. Ward was born in 1917 andobtained a first class honours degree in

engineering from the City and Guilds Col-lege in 1939. He had come to the BuildingResearch Station through an advertise-ment put in Engineering by Dr. HughGolder, who wanted to leave BRS to set

'Geotechnics Division, Building Research Estab-lishment, Building Research Station, Garston,Wafford, Herts.

38 Ground Engineering

up Soil Mechanics I td, in 1942 but, be-cause of war-time employment restric-tions, was unable to do so until he founda replacement. At BRS Dr. Ward hadearned an international reputation throughhis work on the effects of vegetation onclay soils, the deformation and strengthproperties of London Clay and for hisinformal studies on the deformation ofglaciers in Norway, Switzerland and BaffinIsland. He was awarded the degree ofDSc by London University in 1961.

Dr. Ward (right) in the Kielder experi-mental tunnel

Dr. Ward began his lecture by pointingout that the Bntish civil engineenng pro-fession excelled in soft ground tunnelling.Tunnels that act as drains in silts andsoft clays have to carry a ground pres-sure approaching the overburden and theirlinings have to be designed to supportthis load. In weak rocks the strength ofthe rock may be used to help support thetunnel, but its behaviour is controlled bythe joint and fissure system in the rock.To illustrate this point, Dr. Ward showedslides of a timber model made up ofseveral prisms representing intact rock be-tween fissures and joints, with a holebored through it to represent the tunnel.The joints and fissures were more or lessorthogonal to each other and were made,for simplicity, with equal spacings in eachgiven direction. The hole cut out partsof several of the prisms, leaving a varietyof size of pieces free to fall out fromwhat would be the roof of the tunnel andrepresenting substantial rock falls in actualtunnels.

The model made it clear that any tun-nel in fissured weak rock must createoverbreak and rockfalls unless excavationof the face was followed very closely bya ground support system. The amount ofindividual rock prisms left by the exca-vation of a tunnel clearly depends on theorientation and spacing of the fissuresand the size of the tunnel, but the situ-ation is bound to occur when a small partof a prism is left in which the stresses be-

TABLE I. A LIST OF THE RANKINE LECTURES%

1. Prof. A. Casagrande 1961

2. Dr. L, F. Cooling 19623. M. A, Mayer 19634. Prof. A. W. Skempton 19645. Prof. N. M. Newmark 1965

6. Prof. A. W. Bishop 19667. Dr. L. Bjerrum 1967

8. Mr. R. Glossop 1968

9. Prof, R. B. Peck 1969

10. Prof. K. H, Roscoe 197011. Prof. J. C. Jaeger 197112. Prof, R. W. Rowe 1972

13. Prof, T. W. Lambe 197314. Prof. R, E. Gi'bson 197415. Prof. J. Kerisel 197516. Dr. A, C. Meigh 1976

17. Prof. V. F. B. de Mello 1977

18. Dr, W. H. Ward19. Prof. H. B. Seed

19781979

Control of seepage through foundations andabutments of dams.Field measurements in soil mechanics.Recent work in rock mechanics.Long-term stability of clay slopes.Effect of earthquakes on dams and embank-ments.The strength of soils as engineering materials.Engineering geology of normally-consolidatedmarine clays as related to the settlement ofbuildings.The rise of geotechnology and its influenceon engineering practice.Advantages and limitations of the o'bserva-tional method in applied soil mechanics,The influence of strains in soil mechanics.Friction of rocks and stability of rock slopes.The relevance of soil fabric to site investiga-tion practice.Predictions in soil eng'ineeringThe analytical method in soil mechanics.Old structures in relation to soil conditions.The Triassic rocks, with particular referenceto predicted and observed performance ofsome major foundations.Reflections on design decisions of practicalsignificance to embankment dams,Ground support for tunnels in weak rock.Subject not yet announced.

'The Rankine Lecture is an annual lecture of the British Geotechnical Society—the nationalbody of the ISSMffrFE.

Reading an extensometer

come high enough to cause rock crushing.Rock crushing and yield causes con-

vergence which the tunnel engineer at-tempts to control by provid'ing groundsupport. In fact some yield into the tun-nel relieves the pressure and the rate ofconvergence decreases with time. In somecircumstances a limiting value may bereached when all the pressure is carriedby the strength of the rock itself and alining is only required to prevent looserock pieces from falling into the tunnel.But there is the danger that the tunnelengineer is caught in a Catch 22 situation.Convergence of his tunnel may occur in-itially so quickly that he reacts by in-creasing the stiffness of his support sys-tem. This, in turn, causes more load tobe carried by the supports so that eventhough stiffened, they still yield. The en-gineer reacts by adding further stiffnessuntil he has spent a lot of money andproduced a very thick rigid support thatmay almost have begun to encroach onthe finished internal dimensions requiredfor the final tunnel. It is then too late tostrip out the support and remove furtherrock to allow for the convergence andany further yield of the, by now, massivesupport may seriously endanger the use-fulness of the tunnel by reducing, for ex-ample the clearance around trains to un-

acceptable values.The importance of allowing some con-

vergence before a lining is constructedwas emphasised during the discussion ofthe lecture, which took place next dayat the Institution of Civil Engineers. Tun-nels for water and sewerage have ap-parently been constructed successfully in

limestone in Canada using the traditionalm'ining methods, supporting the groundwith steel arches and lagging until theconcrete lining was cast some time later.Recently tunnels have been dug by mach-ine, followed closely by the finished con-crete lining, thereby speeding up the rateof construction and saving the cost ofsteel arches and lagging. But the linings

have spalled and smashed —something un-known with the older method of working.

Dr. Ward described the work in an ex-perimental tunnel 'in Weardale in connec-tion with the Tyne-Tees water tunnelcurrently under construction. He said theyhad opened up a section of the tunnelwith no support system, but had con-structed a protective cage from whichthey could observe rock behaviour. Thishad proved most valuable in showing thedominant fissure and joint systems andallowed attention to be paid to the partsof the rock likely to cause trouble, Variousdegrees of support stiffness had been usedin further sections of the tunnel and mea-surements made of the convergence andmovement of the rock over a radial dis-tance outside the tunnel of about twodiameters. The extensometers measured atpositions 0.3, 1, 2, 3 and 6m into therock. In the sides of the tunnel theyshowed that beyond the first joint therewere negligible movements of less than0.1mm. In the roof, convergence amount-ed to 35mm after 400 days and thenpieces of rock began to fall out. Afterabout 780 days a collapse occurred ex-tending 1.85m above the tunnel and laterthere was a second collapse extendingto 3.2m. The shape of the cavity causedby these collapses is controlled largely bythe rock structure.

Theoretical analyses based on the as-sumption that the tunnel is being cut inan homogeneous, isotropic elastic materialcan indicate the form of the displacementsto be expected as the tunnel face ad-vances. Strains in a direction tending toclose the tunnel occur in the ground be-fore the face reaches it, and immediatelybehind the face, convergence is occurringat its fastest rate. Calculations can bemade of the load that would be carriedby a rigid support put into the tunnel atvarious distances behind the advancinglace, and it is very clear that the laterthe support is put in, the lower the loadit will have to carry.

Dr, Ward gave numerous examples ofthe behaviour of tunnels in the Alps andHimalayas, including the Giri tunnel whichhad a measured convergence of 1.2m. He

gave examples of load measurements onsteel linings placed at different distancesfrom a tunnel face which showed clearlythe effect of allowing some convergencebefore installing the lining —the ring near-est the face carried three times the load ofthat in the furthest ring. This emphasisedhis point that there is no such thing as aunique support load —it all depends on theconstruction method and sequence.

At present tunnel supports tend eitherto follow soft ground practice, when theymay be too stiff, or mining practice usinqtemporary flexible supports, What thecivil engineering industry needs is todevelop a support system for weak rockswhich permits a much closer degree ofcontrol of convergence and support.

Tunnelling needs careful monitoring andDr. Ward recommended that a keen youngengineer, free of the politics of construc-tion, should be given the task of studyingthe behaviour of a tunnel from the begin-ning. He should be armed with a 35mmstereo flash camera to obtain the fissurepattern of the rock and have the facil-ity for installing extensometers. It wouldpay to begin the work with an unsup-ported excavation, using a protective cageto make observations of the rock's behav-iour, then to advance initially with veryflexible supports, taking measurementsall the time to establish the pattern ofbehaviour of the rock. Displacement mea-surements are those of greatest value—often load measurements are made in rockbolts, but wh'ile this is going on, the rockbolt may be moving into the tunnel at analarming rate —information unknown w'ith-out measurements. These observations willallow the most economic support systemto be used, will show up changing rockconditions as the tunnel advances andwill reveal potential collapse conditionsin time to avert serious trouble.

Such an approach soon becomes re-spected by engineer and contractor alike.The sooner the British tunnelling industryadopts this approach for work in softrocks, the sooner they will reap the bene-fits of speed and efficiency that result.

In giving a vote of thanks Mr. DavidCoats, Vice-Chairman of BNCOLD, saidthat Dr. Ward had drawn our attention tofeatures we'e all seen, but never noticed.He had shown clearly the benefits of re-search and that monitoring during con-struction could reap great benefit. Wewere to some extent converting art intoscience but, as Dr. Ward had said, softrock tunnelling would continue to requirea substantial ingredient of art.

The Lecture, given in the Great Hallat Imperial College, attracted an audienceof more than 650, including several pastRankine Lecturers, the President and Sec-retary of the Institution of Civil Engineers,two past Presidents and the SecretaryGeneral of the International Society for SoilMechanics and Foundation Engineeringand the Director of the Building ResearchEstablishment. The meeting was chaired byProfessor A. N. Schofield, Chairman of theBritish Geotechnical Society, The lecturewill be printed in the June issue of Geo-technique

Readers may like to know that the firstten Rankine Lectures have been reprintedin "Milestones in Soil Mechanics" availablefrom the Institution of Civil Engineers.

April, 1978 39