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Tunnelling32A M Muir Wood FRS, FEng, FICEConsultant, Sir William Halcrow & Partners

Contents

32.1 The options for a tunnel route 32/3

32.2 Costs of tunneling 32/332.2.1 Principal factors 32/332.2.2 Effect of tunnel size 32/3

32.3 Systematic site investigation 32/432.3.1 Geological data 32/432.3.2 Objects 32/432.3.3 Means 32/4

32.4 Tunnelling methods related to the ground 32/432.4.1 Historical background 32/432.4.2 Shield tunneling 32/432.4.3 The bentonite shield 32/632.4.4 Rock-tunnelling machines 32/6

32.5 Tunnel construction 32/932.5.1 Drilling and blasting 32/932.5.2 Spoil handling 32/932.5.3 Tunnel lining 32/932.5.4 Thrust boring 32/1132.5.5 Waterproofing 32/1232.5.6 Temporary support 32/1332.5.7 Advance by full face or by heading 32/14

32.6 Aids to tunnelling 32/1432.6.1 Compressed air 32/1432.6.2 Ground treatments 32/1432.6.3 Freezing 32/1532.6.4 Dewatering 32/15

32.7 Ground movements 32/15

32.8 Tunnel design 32/1632.8.1 Stresses around a tunnel 32/1632.8.2 Stability ratio, rock competence and

classification 32/1832.8.3 Stiffness of the tunnel lining 32/1832.8.4 Towards a better understanding 32/19

References 32/19

Many concerned with tunnelling continue, at their own periland that of others, to underestimate the need for practicalunderstanding of the behaviour of the ground, the essence ofgood tunnelling. No two tunnels are the same; experience, andreal insight of the value of that experience, are necessary totransmute particular experience to more general understandingand thus to transmit the experience of one tunnel appropriatelyto another.

Advances in tunnelling usually arise not from research somuch as from innovations in methods of design and construc-tion. Monitoring of the results in the field may then follow,supported by research where existing knowledge fails to explainthe findings.

Two essential elements to economic tunnelling are:

(1) The tunnel (unless permanently unlined) must be consideredas a composite ground-lining structure. Not only does thelining support the ground but the ground in its turnsupports the lining.

(2) The design of the permanent tunnel must be considered inassociation with the methods of construction. The overallcost of the process requires to be minimized and the finishedgeometry is only one of many factors.

There are many barriers to a full understanding of the be-haviour of the ground around a tunnel: (1) the three-dimensio-nal, time-dependent nature of the problem; (2) the complexity ofthe stress-strain relations in soft ground; (3) the effects of theinitial state of stress, discontinuities and joints upon the be-haviour of a rock; (4) the dependence upon the method ofexcavation; (5) the standard of workmanship and (6) inhomo-geneity of the ground.

Full-scale tunnels provide the one reliable laboratory fortesting theory against practice.

32.1 The options for a tunnel route

The ground is the principal determinant of the cost of a tunnelof a given size. For this reason great economic benefits mayderive from the capability of selecting a favourable and rela-tively consistent type of ground for tunnelling. Until the geolo-gical structure is known, the object should be to keep theoptions for a tunnel route as open as possible.

For each type of tunnel there are certain geometrical con-straints and other specific factors affecting cost. For a roadtunnel, for example, acceptable gradients and curves will berelated to the design speed and, hence, to traffic costs.1 For apressure tunnel, on the other hand, there is little direct geometri-cal constraint and the differential cost of construction in relationto the ground would need to be considered against the capita-lized head losses.

A general knowledge of the geological structure will indicatewhether or not the most direct route conforms to a favourablegeological horizon or whether, on the contrary, it mayencounter unstable ground such as squeezing rock, runningsand, major fault zones, decomposed rock, karstic limestone orsimilar hazards which may only be penetrated at great expense.

Where there is a possibility of adopting an economic methodof tunnelling, related specifically to a type of ground withlimited variation, there may be the greatest benefit from diverg-ing from the most direct route, in order to situate the tunnelthroughout in such ground.

At the earliest stage in planning, such factors should beconsidered so that the options may be described, systematicallytested and reduced as information arises from the first stage ofsite investigation.

32.2 Costs of tunnelling

32.2.1 Principal factors

Attempts are made periodically to set out tunnelling costs in asystematized form, with costs per unit length of a certain size oftunnel related to a few generalized ground types and to a fewother simplified categories of accessibility and tunnel length.Except for specific areas in which the ground can be reliablydepended upon, there is no valid way of expressing tunnellingcosts on a simple unit cost basis.

From a knowledge of the ground a system of tunnelling maybe selected and the costs evaluated on an assumed average rateof progress. The rate of progress may be assessed from ex-perience in similar ground elsewhere, taking account of anyinnovation in the tunnelling method, and not forgetting thecosts of ground treatments or similar ancillary operations. Ingeneral, the extent of variability in the cost of tunnelling isincreasing for these principal reasons:

(1) Tailor-made tunnel systems to suit a particular type ofground permit increasing economies in construction.

(2) The cost of labour-intensive tunnelling systems adopted fordifficult ground or in congested circumstances will naturallyreflect the trend of labour costs including incentive pay-ments.

(3) The demand for tunnels in urban development tends toreduce the options available for a tunnel route.

As the result of these factors, at the present time there is at leastan order of magnitude between the unit cost of constructing thecheapest and the most expensive tunnel of the same size. Hence,there is an increasing benefit to be derived from undertakingstudies appropriate to choosing the most economic expedient ineach situation.

A feature that may be overlooked in comparing the costs oftunnels concerns the means of access during construction. Whilea shallow urban tunnel or a short tunnel through a hill may beapproached directly from the ground surface, long and sub-aqueous tunnels usually require working shafts and accessheadings, adding not only to the direct cost of the project butalso to the cost of all the consequent tunnelling operations.

32.2.2 Effect of tunnel size

The cost of a given tunnel is specific to its situation and itstiming, on account of the varying differences in prices, varyinglocal skills and technical capabilities. There is therefore nosimple factor to be applied to the cost of a tunnel in order todetermine its hypothetical cost at a different place or time.

Neither is there a simple formula to determine the cost of atunnel by consideration of another tunnel in the same groundand conditions but of different size. As a simplification, wherevariation in size does not entail a change in basic techniques, wemay consider each factor in construction as entailing a unit costU expressed as:

U= A +Bd+Cd* (32.1)

where A, B and C are constants and d is the finished diameter

For a highly mechanized system, A will be high, while for alabour-intensive system C will be high. For excavation there willbe an appreciable element in spoil disposal costs for which C willpredominate while for temporary tunnel supports A and B willbe the principal factors.

As the size of tunnel is reduced, the increasing congestionleads to reduced efficiency in working. In consequence, there is a

size of tunnel for which the costs will be a minimum; the greaterthe degree of mechanization, the greater will be the size dmin forminimum cost (i.e. B and C->0 as d-+dmin). For a long length oftunnel in the London Clay the minimum cost is obtained for atunnel diameter of about 2.5 m while for certain machine-driventunnels in soft rock the optimum diameter has been found to beabout 3 m, and about 2 m for a hand-driven tunnel in hard rock.

32.3 Systematic site investigation

32.3.1 Geological data

The scheme for determining the geological conditions shouldwork from the general towards the particular. This will entail astudy of geological maps and papers, first on a regional and thenon a local basis. In the UK there are normally available sheets atscales of 1:50000 and 1:10000 with explanatory memoirs,produced by the Institute of Geological Sciences. Where geolo-gical maps do not exist, aerial photographs often provide usefulinformation on the geological structure.

32.3.2 Objects

According to the apparent options for the tunnel the scheme ofsite investigation may then be designed with these main objects:

(1) To test geological data at doubtful points.(2) To explore particular areas of tunnelling difficulty.(3) To obtain information necessary to complement available

data on important aspects of geology and geohydrology.(4) To obtain samples for testing and to undertake in situ tests

in order to establish the suitability of ground for alternativemethods of tunnelling.

(5) To determine design and construction parameters.

Far too often a site investigation is undertaken without ad-equate thought to its purpose; in consequence, information vitalfor good tunnelling is overlooked at the expense of acquiringmuch irrelevant material. The site investigation should besupervised by those with a direct practical understanding of theassociated techniques of tunnel design and construction.

32.3.3 Means

A few large boreholes or adits may be justified for directexamination, in situ testing and for subsequent inspection bytendering contractors and others.

There is no general rule on the spacing between boreholes. Atone extreme, for sedimentary rocks of a uniform character itmay only be necessary to be able to establish general continuityof the geological sequence by identification of marker beds orhorizons. At the other extreme, igneous intrusions and meta-morphosed rocks may present so complex a pattern as tonecessitate a tunnelling method highly tolerant to change,however well the ground may be investigated. A good generalrule is to establish during site investigation a set of hypotheses,concerning the geological structure and the properties of theground to be tested so that when a conflicting anomaly isindicated by a new borehole its significance is appreciated, i.e. isthe benefit of an additional borehole likely to exceed its cost?Where there is doubt concerning the practicability of adopting amechanical system of tunnelling, special care is required toensure exploration of the ground in sufficient detail to determinethe feasibility of the scheme.2

Geophysical methods of exploration may serve not only toextend the data from individual boreholes in the second andthird dimension but also to reveal specific features such as faults

and igneous intrusions. Without adequate 'fixes' geophysicalresults may permit widely different possible interpretations.

Benefits are usually to be found in undertaking a site investi-gation in two or more stages, depending on the initial know-ledge of the terrain, the magnitude of the project and thediversity of possible options. The investigation should bedesigned initially to investigate those features most likely todetermine the tunnel location; otherwise money and time arewasted on investigation too far from the selected line to be ofgreat value. However, in ground variable to a common pattern,information obtained away from the tunnel route may yet berelevant; the validity of such transference needs careful assess-ment.

Water constitutes a hazard encountered in many forms. Thesite investigation should, as appropriate to the circumstances, bedesigned to provide information about water-bearing faultzones, fault zones with a weak filling, open joints and the effectof tunnelling upon aquicludes whose rupture may expose thetunnel to water from aquifers. The geological structure and thepossible head of water will control the zone of ground aroundthe tunnel which calls for investigation.

32.4 Tunnelling methods related to theground

32.4.1 Historical background

The history of tunnelling is one of increasing diversification ofmethods with an increasing capability to explore and to under-stand the ground.

While Brunei used the first tunnelling shield for the ThamesTunnel in 1825-28, tunnels throughout the nineteenth centurycontinued generally to be constructed by means of one of thetraditional methods of excavation and timbered support.3

Although these are now largely of historical interest only, theEnglish method, widely and successfully used, sometimes in softground and in broken jointed rocks where other methods hadfailed, merits mention.

An essential feature of the English method concerned the useof longitudinal crown bars, supported at the forward end onprops and sill and at the rearward end on the last section ofcompleted permanent lining, which might be brickwork ormasonry. In this way continuous support was provided to theground over the tunnel from the time of first excavation and, inprinciple, the method may be considered as the forerunner ofthe tunnelling shield.

32.4.2 Shield tunnelling

Shield tunnelling is strongly associated with the name ofGreathead. He worked with the first circular shield designed byBarlow for the Tower Subway beneath the River Thames in1869. Greathead designed a shield (Figure 32.1) for the SouthLondon Railway in 1886-90 incorporating most of the essentialfeatures which have survived to the present day.4 Greathead notonly recognized that a shield reduced the risks in tunnelling inwater-bearing ground but he was also one of the few of his timeto appreciate that it permitted faster and cheaper tunnelling ingood ground.

The first shield with a mechanical cutting head was the Priceexcavator used in 1897 for the Central London Railway.

Since this time there have been rapid developments, predomi-nantly in Japan, followed by Western Germany, the US andCanada, of mechanical shields provided with means for (par-tially) balancing soil and groundwater pressures.5 For the mostopen-textured grounds, sands and gravels, the choice may bebetween a bentonite (Figure 32.2) or a type of earth-balanceshield. This latter may use a fully plated head with controllable

slots or (Figure 32.3), alternatively, the shield may be open-faced and the soil extracted at a controlled rate by an archime-dean screw conveyor as the shield advances. For finer-texturedground, a slurry shield may be used with natural clay mixed withwater and used for pumping the spoil to the surface. The hydroshield for yet finer ground maintains the face of the shield underhydraulic pressure with a supply of water mixed with the spoilwhich, again, is transported by pumping. Developments in thesedirections have incorporated a number of novel features. Oneparticular contribution has been the perfection of seals between

the shield tail and the enclosed lining; another, for the pressurebalancing shield, has been the use of an air-vessel in thepressurized face of the shield to dampen pressure fluctuations.All such special shields are designed for limited variability of theground and contingency measures may need to be incorporatedto deal with departures, such as the presence of large boulders.In Japan, developments are proceeding towards full automationso that all operation of advancing and steering the shield,excavating the ground and transporting spoil are controlledfrom the surface.

Figure 32.2 Bentonite tunnelling machine. (After National Research Development Council)

Pump

Wetsump Pump

Shield

Slurryextractiondevice

Sand

CycloneCleanbentonite Pump

Tank

Stones Vibrating screen

Figure 32.1 Hooded Greathead shield with platform rams suitable for 3.5-m diameter tunnel

Half elevations Section A-A

External diameter of lining plus 60mm

To hydraulicmanifold Hydraulicmain

32.4.3 The bentonite shield

It is possible, where frequent access is not required to the face ofa shield, to provide for ground support by compressed air, wateror mud confined to the face of the shield. Air is not, however,recommended for this purpose. The use of mud in this applica-tion offers considerable benefits in permitting an approximatelybalanced pressure over the full height of the face and inproviding a suitable medium for the pumping away of spoil. Thefirst such shield was used in Mexico City,5 utilizing the mudformed from the natural montmorillonitic clay spoil of the area.A true bentonite tunnelling machine was first used successfullyin London (Figure 32.2) in sands and gravels.6

32.4.4 Rock-tunnelling machines

Many tunnelling machines for rock have been evolved since1956, although here again the prototype machine belongs to thelast century, usually attributed to Beaumont,7 used for ChannelTunnel heading in 1881-82 and subsequently for the MerseyRailway Tunnel.8 There are several features of such machines9

which merit differentiation as shown below.

(1) Cutters (see Figure 32.4). For the softest rock the cutters arefixed picks which chisel the rock out as a succession ofgrooves. For harder rocks, generally in ascending order ofhardness, machines make use of single or multiple-disccutters, toothed cutters, roller cutters or cutters with tung-sten carbide insert buttons.

(2) Cutter heads. For the smaller machines a single full-dia-meter rotary head is adopted (Figure 32.5). As the machinesize increases so there is a tendency to introduce planetarycutters to share the work between cutters and reduce therange between minimum and maximum cutter speeds.

(3) Thrust of machine. For the softest rock, machines receivepurchase by rams thrusting against a gripper ring expandedagainst the periphery. With a few exceptions the remainderof the machines obtain their forward thrust by means ofdiametrically opposed thrust pads jacked against theground. All such machines advance by periodically with-drawing and repositioning the thrust ring or pads. Onemachine uses a central pilot drill which is firmly anchoredinto a hole ahead of the face. Not only does this provide ameans for pulling the machine forward but it also estab-lishes a firm forward bearing for the cutter head which maybe a valuable feature where rock variation in the face causesuneven loading on the head.

The road header, a machine developed originally formining, has a rotary milling head on a telescopic boom,attached to the body of the machine by a universal joint(Figure 32.6). Thus a typical machine may excavate a galleryup to 4.5 m high and 5.8 m wide. The cutter head usuallymounts picks in the pattern of a conical scroll. Operation isby means of a 'sump' formed in the face, extended by lateralpressure on the rotating head. Generally the loading on eachpick will be less, and less controllable, than for a full-facemachine and hence for the limitations on rock strength foreffective application.

Figure 32.3 Shield with fully-plated head with controllable slots

The selection of a tunnelling machine must take account of therock types to be encountered along the length of drive. Theefficiency of the machine is related to the inherent properties ofthe rock, principally to strength, the extent of jointing, strainmodulus and abrasion. The aim is to minimize the specificenergy needed to fragment a rock by keeping the size of particlehigh and by provoking brittle fracture. The cutter action aims toset up a high local difference in principal stresses and high-enough tensile stresses to induce cracking. Each type of cutterand pattern of cutters operates most efficiently in a rock whoseproperties lie within a limited range. Difficulties may arise fromseveral causes, e.g.: (1) from excessive wear of the cutters in hardrock which grooves instead of fragmenting; (2) from inefficientfracture of rock too soft or plastic for the type of cutter; (3) fromexcessive wear due to overheating in the presence of highcontent of silica; (4) from excessive bearing loads where rockvaries appreciably in the face; and (5) from jamming of the headwhere hard rock is heavily jointed and tends to collapse on tothe machine or at the face.10

A simple criterion for the economics of machine excavationconcerns the cost of repairing and replacing cutters. In soundsoft rock this will be found to be a trivial sum in relation to othercosts of excavation. For the hardest rock, the cost will be foundto climb to a figure of several pounds per cubic metre, withstoppages every few metres for replacement, and this stagerepresents the present economic limit.

The tolerance of a machine to the full range of rock types tobe encountered should be considered in weighing the overallmerits and costs of its introduction. Another essential questionconcerns prediction of the need for temporary support close tothe face, a process that presents greater difficulty for the full facemachines and for which object certain machines make specialprovision in their design."

Figure 32.4 Types of cutters for tunnelling machines32.4a 'Series 12' tooth cutter-MNX32.4b 'Series 12' HHIX cutter32.4c Bolt-on-disc cutter, type DGX (Courtesty:Hughes Tool Co.)

Figure 32.5 Cutter head of hard-rock machine (Courtesy: The Robbins Co.)

Figure 32.6 Road header in iron mine (Courtesy. Anderson Mavor Ltd)

32.5 Tunnel construction

32.5.1 Drilling and blasting

The traditional scheme of advancing rock tunnels has been bydrilling and blasting and this method continues to be generallyadopted for short tunnels, hard rock tunnels and for tunnels invariable ground. At the present date, for example, machinetunnelling is unlikely to be economic in shattered rock or in rockof strengths greater than 200 MN/m2.

The principle behind blasting in a tunnel is to obtain thegreatest 'pull' for the minimum explosive charge and for theminimum damage of the rock around the tunnel. Secondaryobjectives are: (1) to fragment the rock adequately; and (2) toform a compact stock pile against the face.

The pattern of drill holes is designed to suit the rock and theexplosive. Cut holes are arranged towards the centre of the face,usually inclined towards each other in order to remove a cone orwedge. One or more central unloaded holes of larger diametermay be used to assist the cut. The remainder of the holes aredrilled parallel to the tunnel axis. Delays of a few millisecondsare used between groups of drill holes, from the cut outwards, sothat the excavation is enlarged with the travel of the shock wave.

Considerable effort has been applied to establishing theneatest periphery to the excavation by the trimming holes,which may be charged or uncharged. In the technique of pre-splitting, the trimming holes are fired before the remainder withdistributed charges to cause cracking around the peripherybetween adjacent holes. Another technique which has been usedin tunnelling is termed smooth-blasting, whereby the line oftrimmer holes is required to coincide with the periphery of theexcavation, each being loaded with a reduced distributed chargeand fired with a short delay after the remainder. It may be wellworth considering means for reducing overbreak by carefulcontrol of the spacing, line and charging of the trimmer holes.The geometry of the drills or the drill carriage should bedesigned to permit the trimming holes to be drilled as parallel aspossible to the tunnel axis. Care in these respects may showconsiderable benefit not only in reduction of direct overbreakbut also in the reduced extent of the surrounding zone ofcracking and displacement of the rock, with consequent savingsin the extent of temporary support.

Sectional drawings of tunnels have often indicated the peri-phery of the 'minimum section' and the 'payment line' whichallows payment for overbreak to be assessed in relation to thevolume of excavation and the volume of concrete lining. Occa-sionally a 'limit line' is also shown, beyond which a leanerconcrete mix may be used for filling. Overbreak in a tunnel isfrequently expressed as a percentage of sectional area but,without knowledge of the size of tunnel, this designation haslittle significance.

The present tendency12 is to indicate surface areas of differentsizes of tunnel, possibly subdivided into different types ofground, in order to facilitate translation of overbreak into thecorresponding additional volumes of ground to be excavated.

For small tunnels, hand-operated drills are used on telescopicair-legs. For larger tunnels there is usually a wider choice,including ladder drills, light mobile boom-mounted drills orheavier drills mounted on a jumbo. The latter may provideadvantage in controlling the drill pattern and with the speed ofdrilling, also in protection close to the face for other operations;the main disadvantage arises from inflexibility in the event ofdeparture from full-face driving. For the Mont Blanc Tunnel,for example, it was fortunate that a jumbo was used only fromthe French end, since difficulties encountered along the Italiandrive compelled the enlargement from headings over a consider-able length of tunnel.13

A more recent development has been the introduction of the

hydraulic drill, offering a rate of drilling some 50 to 100%greater than the corresponding pneumatic rotary percussiondrill, at a considerable reduction in noise level of 10 to 15dB.

32.5.2 Spoil handling

The handling of spoil from the face cannot be consideredseparately from the method of excavation. Mechanical shieldsand tunnelling machines have built-in chain or belt conveyorsloading to a hopper or to another conveyor. The same operationis achieved in a drill-and-blast tunnel by means of a mechanicalloader, often with composite face shovel and conveyor. Thegeneral trend is to use rail wagons for transport for tunnels up toabout 7 m diameter and for tunnels worked from vertical shaftsand to use dump trucks for large tunnels directly accessible fromthe surface or for tunnels at a gradient of more than about 2.5%.

Many solutions to the problem of loading rail cars at the facehave been adopted. One currently used in relatively smalltunnels (say 3 m diameter) is to use a long transit car with anarmoured conveyor floor so that spoil loaded at one end may beevenly distributed. Another system for rather larger tunnels (say4 to 5 m diameter) uses an overhead conveyor capable ofloading in turn each of a train of six or more (or fewer) rail cars,preferably to contain the spoil from a complete round. Analternative uses a long sliding platform with rail track andturnouts in consequence maintained close to the working face.

Conveyors are also used for dry materials and where access isby inclined shaft. The pulverizing of spoil and its discharge bypipe as a slurry has been adopted for suitable soft rock.Frequently the bottle-neck in materials handling is found tooccur at the foot of a working shaft and here mining practice hasintroduced the use of automatic tipping of tunnel wagons intolarge hoppers from which shaft skips are rapidly loaded. Theentire process of excavation and removal of spoil merits con-siderable study at an early stage as to its adequacy, withcontingency plans to overcome foreseeable causes of break-down.

32.5.3 Tunnel lining

The method of tunnel lining is essentially related to the nature ofthe ground and to the scheme of excavation. General-purposetunnel lining has economic application to small tunnels invariable ground. Recent progress and attendant economy havebeen demonstrated to result from the capability for designingthe lining specifically to the condition of the ground and theoverall tunnelling system.

The first subdivision in type of lining results from whether ornot the need exists for an immediate support at the face. InNorth America the common practice in tunnelling in softground has been to tunnel by hand, to erect continuous supportin timber sets or steel liner plates and subsequently to place an insitu concrete lining. In the UK, and generally throughoutEurope, shields have been more widely used together withpermanent primary segmental linings.

The traditional lining over more than 100 yr has been the ringof bolted cast-iron segments built within the protection of thetail of the shield, with the external annulus often grouted withlime or cement. Improvements in site investigation procedurehave allowed the development of alternative types of liningwhich can be adopted in certain restricted types of soft ground.

Reinforced concrete segments14 have been preferred to cast-iron segments for reasons of cost since 1938 except whereloading is heavy or where watertightness is an essential object.

Another general type of tunnel lining is built in rings ofsegments immediately behind the shield. Each ring is thenexpanded directly against the ground with elimination of theprocedures of bolting and grouting. Evidently the system can

only be used where the ground around the tunnel is self-supporting over the width of a ring for a short period and thus acertain minimum apparent cohesion of the ground is necessary.Such techniques were developed predominantly in LondonClay, sufficiently stiff (i.e. with a low enough stability ratio - seepage 32/18) and homogeneous for a specifically designed sys-tem.15 Two types of lining based on this principle merit mention.

The Donseg lining is created from rings of tapered segments,expansion against the ground being achieved by the process ofinserting alternate segments, as longitudinally tapered keys, intothe ring by the shield rams (Figure 32.7). This is a highlyeconomic method, limited to tunnels of diameter not exceedingabout 3 m, because of the geometry of the lining.

For larger tunnels, the Halcrow lining provides for articulat-ing joints between segments. In this way, a part ring of segmentsmay be assembled clear of the extrados. The insertion andexpansion of jacks between special segments cause the ring toexpand against the ground, accompanied by relative rotationbetween adjacent segments (Figure 32.8). A special feature of alining of this type is that secondary stresses are limited to a lowlevel with consequent savings in the structural thickness of thelining. For the Cargo Tunnel at Heathrow11 a lining 300mmthick has been used for a 10.3-m diameter tunnel.

One of the most highly developed linings of this type has beendeveloped by Holzmann and used inter alia by Wayss andFreytag for metro tunnels in Antwerp in open water-bearingground, using a slurry shield and necessitating high standards ofwater tightness.16 Each ring (Figure 32.9) comprises eight longi-tudinally tapered segments, the width of the ring itself beingtapered so that all segments are built without packings, correc-tions to line and level being achieved by relative rolling of thering. This is one example of recent concrete and (ductile) iron

Figure 32.7 Donseg tunnel lining

rings which depends on extruded plastic seals compressed intorecesses to achieve watertightness.17

Steel linings of two basic types are used for soft-groundtunnels. Pressed liner plates with a maximum sheet thickness ofabout 8 mm serve as a primary lining for hand-driven tunnels.18

Such a lining is inadequate for accepting the thrust from a shieldbut, for particularly arduous conditions, fabricated steel liningsmay be used here. These conditions may arise from excessivevariation in loading around the lining, on account of the nature

Segments after beingpushed home by rams

Ring of segmentsas placed by hand

152 mm normally191 mm where coveris greater than about 40 m

'Don - Seg' segment

Arrangement of segments in ring(arrangement symmetrical aboulvertical centreline)

Figure 32.8 Lining for cargo tunnel at Heathrow Airport, London. (After Muir Wood and Gibb (1971) 'Design and construction of thecargo tunnel at Heathrow Airport, London.' Proc. lnstn. Civ. Engrs, 48,11-34)

Segment Mk 3

Jackingrecess

25x25mmchambers

Liftingholes

317-5cmradius

Taplow terrace gravels and brickearth

Water table Top of London clayJacking spaces packedwith earth dry concretein two stages after stressing,firstly between horns andsecondly in recesses following

removal of jacks

Jackingrecesses

Three-dimensional viewof jacking space

Jackingrecesses

317-5cmradius

Liftingholes25x25mmchambers

Segment Mk 1

lntrados

Longitudinal section of segmenton A -A

Figure 32.9 Precast lining with neoprene gasket seal

of the ground, low top cover, confined side clearance orproximity to foundations.

Several types of flush lining have been designed for initialerection around a central spider but these are suitable only forsmall-diameter tunnels. For the Mersey Tunnels 3A and 3B,lining segments (Figure 32.10) were made in mass concrete withan internal steel face.10 Each ring is attached to the previous ringby means of long bolts inserted into threaded sleeves. Water-proofing of the lining is achieved by welding cover plates acrossthe joints. The concrete expanded linings result in a flushinterior surface, which may be beneficial for tunnels serving asconduits.

Where a lining is built in any but very weak ground, boltingbetween segments has no permanent structural significance.Fastenings are therefore required primarily to control shapeduring erection prior to filling the extrados, the space betweenground and lining. One effective cheap system uses tapered elmdowels to achieve alignment of adjacent circumferential jointswhile each radial joint is located by a longitudinal tube insemicircular channels; the tube collapses under load to avoidexcessive local pressures.

Ductile (spheroidal graphite) iron has been used for tunnellinings. While this material allows a considerable saving inweight by comparison with grey iron, the reduced depth ofsegment is a disadvantage for obtaining purchase for the thrustrams but generally such linings are found to offer economicbenefits as alternatives to steel linings where high loading andappreciable tensile stresses are expected.

Tunnel segments are erected in rings and the width of the ringdetermines the stroke of the propelling ram and hence the lengthof the shield. In the UK the tendency has been, in soft ground,

to maintain tunnel linings to a width of no more than 70 cmwhile on the Continent segment width is generally greater, with1 m as a common standard and this trend is generally extendedas new shields are built.

Rock tunnels are usually lined in situ with concrete placedbehind shutters. The lining may be cast in discrete lengths orcontinuously behind shutters travelled forward in a retractedmode. Concrete is usually pumped, with placers used withdecreasing frequency for filling the crown. Subsequent contactgrouting is usually necessary to fill shrinkage cracks and voidsbetween lining and rock.

For many years, attempts have been made to form a conti-nuous in situ lining immediately behind a shield in soft ground.Success requires synchronization of advancing the shield andfilling the concrete annulus. This has been achieved by Hochtief,Holzmann and Wayss and Freytag in Hamburg for the metro.The shield thrust is transmitted through internal shutters toavoid pressure on newly placed concrete.19

32.5.4 Thrust boring

Thrust boring of tunnels20 has developed from pipe jacking,whereby lengths of steel pipe are pushed through the ground,from a jacking pit, with the addition of a new length of pipe atthe rearward end after each extension of the jack. Thrust-boredtunnels are frequently in the form of reinforced concrete ele-ments or layered materials incorporating fibre reinforced plas-tic. The limiting distance of thrust boring depends upon theground, the geometry of the tunnel and the capacity of the jacks.This may be extended by the use of an external lubricant such as

Recess forneoprenegasket

Extrados

Fixing and lifting details omitted

Detail of neoprene gasket

bentonite or by using intermediate jacking points to control themaximum length to be advanced at a time. For small pipesexcavation is often by continuous-flight auger; for larger tun-nels, excavation may be by hand or by small mechanicalexcavator.

Various types of cutting head or shield are used with jackedtunnels which, for longer drives, may be designed to help correcterrors in alignment. The extent of support to the face willcompare with that necessary for shield tunnelling in similarground. Thus, very weak silt may be extruded into the tunnelthrough a ported head.

The most variable features of jacked tunnels concerns thejoint between elements. Traditionally, for a concrete lining, aspigot was used with clearance between the internal parts of thejoint which would subsequently be sealed. Such an arrangementreduces the surface area available to transmit thrust. Severaltypes of flush external sleeve are now used in association with abutt joint, usually associated with an external annular seal toexclude the ground.

For relatively short lengths of tunnel through soft groundthrust-boring offers the benefit of erecting all lining at the thrustpit directly accessible from the ground surface, in lengths of 2 mor more, thus reducing manufacturing costs and the aggregatelengths of joints to be sealed. Furthermore, in weak ground thepipe form provides improved circumferential strength. Tunnelsmay be jacked in continuous easy curves using tapered pipes (ortapered packings) at the expense of increased thrust. Experienceshows that a well designed and engineered jacked tunnel, built

to fine tolerances, considerably reduces thrust loads and inconsequence extends the total length of tunnel, or spacingbetween intermediate jacks. Measurement of the build up ofthrusts in the initial period of jacking will help to establishappropriate spacing between jacking stations.

32.5.5 Waterproofing

The availability of new sealing materials provides a wide choiceof waterproofing systems for the joints between preformedtunnel elements.23 Selection will normally be on the basis of costand durability, to meet particular criteria concerning:

(1) Capacity to tolerate relative movement between elements.(2) Hydraulic pressure.(3) Application to wet surfaces and under pressure.

The first barrier is normally provided by annular or contactgrouting. Thereafter there are fundamentally three choices: (1) asealant provided in a liquid or plastic state; (2) a materialcaulked into the joint space; and (3) a preformed gasketcompressed between elements. The latter has found wide accep-tance as a high-performance seal used with segments cast to finedimensional tolerances. Where practicable, seals should beformed at a radius beyond that occupied by bolts or otherfastenings so that these do not require separate treatment for theexclusion of water.

Figure 32.10 Cross-section of Mersey Kingsway Tunnel. (After McKenzie and Dodds (1972) 'Mersey Kingsway Tunnel: construction' Proc.lnstn. Civ. Engrs, 51, 503-533)

305mm thick precast concretesegments 1-22 m wide with6 mm thick mild steel skinanchored to inner face with13mm dia. hoop studs

Grouting space

Holes for longitudinali tie rods

203 mm dia.pumping main fromWallasey portalto mid-river sump

•Extent of mild steelsteel skin

Fresh-air duct

Grouting space

Fire mainvalve152 mm f iremainExtent of mildsteel skinSide entry gulley

Fresh-air duct152mm dia. road drain

TVcamera Tunnel

lighting

Wig-wag sign Wig-wag signSegments above deck levelbreak joint by half a segmentas indicated by dotted lines

All joints made watertightby continuous welding ofmild steel cover strips

Caulking grooveSecondary lining

Fireproof or electricaldistribution cabinet

Horizontal axis

C02 detectorVisibil i ty detectoi

Secondary lining

Inspectionwalkway

7-32 m carr iageway

Crossfall

Prestressed precastconcrete beams filledwith in situ concrete

32.5.6 Temporary support

In rock tunnelling, the permanent lining cannot be consideredseparately from the scheme of excavation and temporary sup-port. The initial stability of the excavated ground depends notonly upon the inherent quality of the rock but also on themethod and quality of the excavation process. Generally,mechanical excavation will not only provide a better shapedarch around the tunnel but, more important, also much lessdisturbance of the surrounding rock. Recent studies have indi-cated that blasting may cause cracking of the rock up to adiameter outside the tunnel.

The essence of good tunnelling in jointed rock is to provideadequate support to incipiently collapsing rock as soon aspossible. The means for achieving this end are directly related tothe nature of rock and its jointing. The situation may besummarized thus:

(1) Where the rock is highly shattered or with frequent openjoints, effective support may require the use of heavy arches.These must be provided with adequate foot supports toavoid punching into the invert and must be blocked off therock sufficiently frequently to avoid excessive bendingstresses.22 One means of achieving an even or virtuallycontinuous blocking is by the use of porous bolsters placedbehind the arch into which a weak element/flyash grout ispumped.23 Arches of the yielding type,23 designed originallyfor colliery support, are now widely used in tunnels inrecognition of their ease in erection and the virtual equiva-lence of their major and minor second moments of area, andhence greatly reduced tendency to distort, in conditions inwhich their higher cost may be justified.

(2) Where the rock is subject to progressive deterioration or tosurface weathering, an immediate application of concrete ormortar may provide great benefit. A thin application ofpneumatically applied mortar (gunite) or fine concrete(shotcrete)24 will often serve in this respect, applied prefera-bly to enter open crevices between blocks so that anadequate arch is provided around the tunnel. Shotcrete isfrequently reinforced with steel mesh attached to the rockface by rockbolts or pins. Alternatively, the shotcrete maybe applied with a wire staple or fibre reinforced content. Asomewhat heavier and more expensive version with thesame general object may be provided by an initial concretelining placed against the newly exposed rock, possiblybehind perforated steel sheeting supported by arches.26

There are often great advantages in the reduction of over-break if support of this nature can be applied so close to theface as to receive benefit of the three-dimensional dome thatoccurs here. There is also a certain time dependence of thetendency for collapse from a tunnel roof; thus a great deal ofthe barring down of an unstable tunnel roof can frequentlybe avoided by immediate support.

(3) The action of rock bolts in supporting the ground around atunnel depends upon the nature of the jointing.27"29

For a regular pattern of sets of joints, the areas around atunnel arch may be identified from which unsupported blocksmay tend to fall or slide. Rock bolting will be designed in aregular pattern to create a reinforced rock arch, taking accountof the strength characteristics of the joints or the stress-strainbehaviour of the rock mass, where unacceptable rock conver-gence may otherwise develop. Special circumstances for rocksupport may arise from:

(1) High horizontal ground stresses, recognizing the need forappropriate disposition of support.

(2) Strongly laminated rock, for which rock bolts may serve totie together the laminations.

(3) Presence of dominant pattern of open joints, necessitatinggreat care in design of support.

(4) Weak filling of joints which may further weaken or beeroded as a result of tunnelling.

Large blocks of rock adjacent to rock caverns, bounded byjoints of low strength, have called for special measures ofanchorage in order to ensure stability, by means of anchoredtendons and cables.

There are many types of rock bolt but these may convenientlybe considered in two groups: (1) those which rely upon endanchorage, usually by some method of mechanical expansion ofthe end of the bolt, and (2) those which are keyed along theirlength. The latter type may be deformed bolts, set in cement orin epoxy resin or similar adhesive. The cement is introducedeither as a mortar introduced in a split expanded metal cage, oras a grout through a perforated bolt or a separate tube. Theepoxy resin is usually in the form of cartridges inserted ahead ofthe bolt, with twisting of the bolt used to burst the cartridgesand mix the two-part resin. The form of keying depends, interalia, on the ability to drill true regular holes in the rock. Wherethis is possible a bolt in the form of a hollow split sleeve may bedriven into the rock.

The end anchorage bolt is generally the cheaper expedientand is more readily stressed but, in soft or weathered rock theanchoring should be achieved by a resin bonding. The head ofthe bolt should be fitted with plate washers or a short length ofchannel to spread the load adequately over the surface of a softrock. Progressive failure of a jointed rock may be controlled bya wire mesh between bolts acting as a containing cage which isalso a useful safety measure. Evidently the effective depth of abolted rock arch or slab depends upon the bolt size, length andspacing, the length usually requiring, for overall economy, to betwice the spacing or more.28

Rock bolting and shotcreting are often used in association,the former providing the major support, the latter controllingsurface deterioration without which aid the bolts would beeffective for a short time only. Surface cracking of shotcreteprovides early warning of continuing movement of the ground.

Dowels generally represent reinforcement placed in the rockwithout tensioning. They may be preferred to bolts in thefollowing circumstances: (1) where subsequent movement of therock will suffice to stress the dowel but would be otherwise liableto overstress or dislodge a prestressed bolt; (2) where light(possibly temporary) support only is required; and (3) wheresubsequent tunnelling or mining will excavate through thesupported area, favouring the use of wooden (bamboo) orreinforced plastic dowels. Spiles are driven into the ground,ahead of the face or around the tunnel periphery, providingsupport by shear stress mobilized along the spile. In conse-quence, spiles are frequently of rolled steel sections providing ahigh superficial area per unit weight. They may be drivenobliquely ahead of the tunnel face, first to support the face itselfand, subsequently, by successive redriving as the face advances,to support the periphery of the tunnel.

There has been much development in recent years in tunnel-ling with support designed to reinforce the rock (or stiff soil)such that minimum applied support is required to achievestability. Such support is often in the form of rock bolts andprojected concrete (shotcrete) but may also include the use ofgrouting, compressed air and similar expedients. The principleof such a method is that observations should be undertaken toconfirm that the support is adequately stabilizing the surround-ing grourld, so that the rate of convergence towards the tunnel isperceived to be approaching an asymptotic value. Where, afterinitial support, this is not assured, further support may beapplied incrementally. Thus, the principle of such a system maybe described as that of 'incremental support5. The most widely

known application of this approach is the New Austrian Tun-nelling Method (NATM). Confusion has arisen by the incorrectapplication of the term NATM to all forms of support by rockbolts and shotcrete or, more widely yet, to forms of tunnellingwhich do not adopt formal linings.

The essence of design of a system of incremental support is tounderstand the stress-strain properties of the ground and of thetunnel support since, essentially, controlled strain of the groundhas to occur in order to develop a changed stress field in theground around the tunnel compatible with the degree of sup-port.

32.5.7 Advance by full face or by heading

A first consideration in excavating a large tunnel concerns thepracticability of full-face excavation. This will depend upon thestability of the rock in relation to the tunnel size and upon theneed for any advance heading to explore the ground and toprovide an opportunity for undertaking ground treatmentahead of the main excavation. In very large tunnels it may beeconomic to excavate a top heading first in order to insertsupports for the crown and then subsequently to work the invertsection as a vertical bench; a variation to such a method mayutilize a bottom heading in addition, serving for drainage andfor removal of spoil.30

In swelling ground (i.e. in rock containing a montmorilloniticclay) the difficulty in support may be roughly expressed asproportional to the area of the tunnel. In consequence there maybe considerable benefits in utilizing a series of small headings ordrifts around the periphery of the tunnel in which the permanentlining for the full arch and invert is cast section by section.

32.6 Aids to tunnelling

32.6.1 Compressed air

As a tunnel advances, relaxation of the ground in the vicinity ofthe face will induce dilation. In fine-grained soils this can occuronly at the rate at which water can be drawn into the soil. As thesoil dilates, effective stress between the grains is reduced and thesoil may flow or ravel. The period during which the face remainsstable is known as the stand-up time and, in any particularcircumstances, the dominant controlling features are the soilpermeability and swelling modulus. A first aim of any one of theseveral aids to tunnelling will be to extend the stand-up time.

The application of the use of compressed air to soft groundtunnelling is another development associated with Greatheadand the South London Railway (1886-90).4

In soft clays, compressed air will provide direct support to theground. In silts and sands the compressed air displaces thegreater part of the pore water and causes cohesion betweengrains of the soil by surface tension. The effect allows runningsands to be treated in excavation as a soft rock. Another side-effect of compressed air in the ground is to reduce its permeabi-lity to the flow of water (by as much as an order of magnitudefor silts).

The use of compressed air necessitates a considerable outlayin low-pressure compressors, air coolers, air locks (including amedical lock) and the associated control and monitoring system.Even a momentary loss in air pressure might have fatal conse-quences and, hence, the need for a high degree of duplicationand standby equipment. The working conditions in compressedair owe a great deal to pioneering studies by Professor J. S.Haldane, leading to a set of recommendations by the Institutionof Civil Engineers later revised and issued as a set of regulationsunder the Factory Inspectorate.31 Comparable standards have

been evolved in other countries which undertake tunnelling orcaisson work in compressed air.

Compressed air introduces increased direct and indirect costs,the latter arising from the reduction in effective working timeand the period spent in 'locking out' which may for instanceincrease from about 25 to 45 min for a 6-h shift as the workingpressure (measured above atmospheric) rises from 1 to 2 atmos-pheres. The upper limit, without special air mixtures, is about 3atmospheres. It is recognized that it is necessary for strictmedical supervision to be provided for workmen in compressedair.32 For many years it has been known that the amount ofnitrogen dissolved in the blood is related to the period ofexposure to a given pressure so that if the pressure is loweredtoo rapidly bubbles are formed, particularly at the joints,leading to the condition known as 'the bends'. More recently,compressed air has become associated with a more seriouscomplaint, that of bone necrosis, which may leave the victimcrippled. One reaction to this discovery has been to resort toother forms of aid in order to dispense totally with the use ofcompressed air. A more reasonable attitude appears to be todiscover the causative process and to eliminate the offendingfactor, since alternatives for compressed air may not only entailhigh cost but also introduce new hazards. In the past, beforebone necrosis was associated with compressed air, medicalinspection of workmen passed them fit to work in compressedair without attention being given to any latent defect of thebones or joints, an oversight that should not recur for futurecompressed-air working.

Compressed air has been used for many subaqueous tunnelsin soft ground. The problem of balancing the external waterpressure increases with the depth, as well as the size, of thetunnel. The depth below the water surface and the texture of theground will determine the quantity of air required. In coarsesand a rule of thumb for determining the maximum demand inrelation to losses through the face has been stated as 7.5Z)2m3/min where D is the tunnel32 diameter in metres. To face lossesneed to be added losses through the lining and through airlocksand bulkheads. Lining losses, in the absence of special care insealing and caulking, can for a long length of tunnel represent ahigh fraction of total losses.

Where the ground comprises clay interbedded with thin layersof sand or silt it has frequently been found that a relatively lowratio between the pressure of air and the external head of wateris adequate to provide greatly improved stability to a tunnelface.

In open ground, air losses may be reduced by locally sealingthe exposed face, for which purpose bentonite dust has beenused. A further problem area arises, in the construction of asegmental tunnel lining behind a tunnelling shield, in theavoidance of collapse of the ground on to the lining immediatelybehind the tail of the shield. This has been countered bygrouting with bentonite through the skin of the shield in orderto increase the capability of supporting the ground by com-pressed air. An alternative method has been to fill the annularspace with pea gravel as the shield advances, by no means easyto perform satisfactorily.

32.6.2 Ground treatments

A wide choice of grouting media33 is now available for consoli-dating weak or water-bearing ground:

(1) Setting grouts containing cement, bentonite, fly ash andother materials may be selected, at the lowest cost compat-ible with adequate travelling capability for the dimensionsof pores and joints to be filled. Bentonite may also be usedon its own as a lubricant for the extrados of the shield skin,

Figure 32.11 Pattern of ground treatment for north end of secondBlackwall Tunnel, London (J. lnstn Civ. Engrs, 35, 19, October1966-with acknowledgement to the Council of the Institution ofCivil Engineers)

32.6.3 Freezing

Freezing has had a longer history as an aid to sinking mineshafts than for civil engineering applications.35 It has been usedin civil engineering works predominantly for situations ofunusual difficulty and for installations using vertical freezing-holes sunk from the surface. In each hole is inserted a U-tube or,alternatively, a composite freezing tube with concentric innerand outer tube through which cold brine is circulated, usuallydown the inner and up the outer tube. The brine is usually usedat a temperature of about - 2O0C but this may be reduced to-350C.

Freezing was widely used for construction of the MoscowUnderground in the early 1930s, for vertical shafts and forinclined escalator tunnels. For several lengths of recent sewertunnel in Germany freezing has been adopted with horizontalfreezing holes. A freezing operation with the use of brine usuallyoccupies several weeks after the installation of the tubes andequipment.

Freezing has been used for six shafts for the Ely-Ouse water

scheme,36 each requiring control of groundwater to a depth of25-65 m below its surface. The cost of freezing (1969) was about£560/m for a 4.5-m internal diameter shaft and £740/m for a7.5-m internal diameter shaft.

A new development has entailed the use of liquid nitrogen asthe freezing agent. Since the operating temperature may then belowered to — 15O0C the freezing operation occurs rapidly andthe process has frequently been used for penetrating relativelythin bands of water-bearing ground during the sinking of shafts.Freezing by liquid nitrogen has been used in tunnels in Switzer-land and South Africa in conjunction with shotcrete, to providea secure, if expensive, temporary support with low subsidence inweak ground.

32.6.4 Dewatering

Control of water for tunnelling may be achieved by lowering thewater table by pumping or by diverting the water as a tunnel islined. The first requires no further explanation here beyond theobservation that pumping continues to be adopted in associa-tion with urban tunnelling with inadequate appreciation of therisks of settlement to adjacent buildings, particularly whereorganic soils are concerned. In certain circumstances rechargingwells may be used to control the extent of the depression of thewater table.

If the water is permitted to flow freely into a tunnel there maybe a risk of ground settlement but this is not generally animportant consideration in rock tunnels. Exceptions to thisgeneral rule occur in crushed or altered fault zones, where weakjoint filling may be softened or washed out or where the rock isincompetent in relation to the pressure of groundwater. Particu-lar care in controlling water is demanded where weak, jointed,rock is associated with stronger rock serving as aquifers. Gypsi-ferous rocks in the presence of water may continue to swell overa long period. Provisions may be made to permit continuedswelling without excessive pressure on the tunnel; anotherexpedient may be to exclude water from the area by sealing ordrainage. In the Seelisberg Tunnel, Switzerland,36 such expe-dients were employed conjointly.

While major flow of water will require to be controlled inconsideration of pumping capacity and deterioration of theground, minor flow will only present problems for the liningoperation. Over a period of many years several expedients havebeen devised for the diversion and control of water to allowplacing of the lining. One of the successful methods has been toprovide a continuous protection of plastic sheeting around thetunnel supported on panels of steel mesh with longitudinalfrench drains along each side of the invert which are grouted upas a final operation. An alternative arrangement, where waterflow is general but not great, will use a quick-set mortarpneumatically applied on to steel mesh with pipes inserted atintervals to concentrate the water flow. The pipes are stoppedoff on completion of lining or, occasionally, allowed to flowwhere permanent drainage and pressure relief are intended.

Where water is confined locally to joints it may be adequate toform a stopping of flash-set mortar around a flexible tubularformer to provide a drainage path.

32.7 Ground movements

Excavation for a tunnel may give rise to associated groundmovements for two principal reasons. These may either becaused by over-excavation, leaving cavities beyond the spaceoccupied by the lined tunnel, or by release of original stresses inthe ground, giving rise to elastic or plastic deformation towardsthe tunnel.

for thrust-bored tunnelling and for shaft sinking. Bentonitemixtures are thixotropic, i.e. they form a gel in the absenceof shearing motion.

(2) Chemical grouts are used in medium to fine sands, singlechemical systems having a time-dependent control of settingand two-chemical systems, of which the Joosten is the mostfamiliar process, depending on contact between the twocomponents.

(3) For silty sands, resin grouts may be used, low viscositygrouts being available for permeabilities down toabout 10~ 5m/s.

Generally, the finer the ground and the lower its permeabilitythe more expensive the grouting process. The principle ingrouting variable ground is therefore one of working throughthe available grouts from cements, clays, chemicals and resins asappropriate, so that the cheaper grouts are used to confine thetravel and hence the 'take' of the more expensive grouts (Figure32.11). In fine material, electrochemical grouting may be used intunnels in the future. It has already been used for foundations.34

In this process, electro-osmosis accelerates the rate of penetra-tion of the chemical agent through the ground.

Clay cementand silicabased grouts Gravel

5-6£mpilot tunnel

2 13m i.d.Pilot

tunnel

Silica basedgrouts

Resingrout

Silty fine sands ofWoolwich andReading beds

858mmain tunnel

Clay

In rock, over-excavation may occur from roof collapses orfrom failure to line solidly against the ground. Rock falls maydevelop domes, arches or chimneys depending upon the natureof the ground and the pattern of initial stresses. High horizontalstresses, for example, will normally tend to limit the extent of thecavity provided the rock is sufficiently competent in relation tothe maximum resultant stress around the periphery of thetunnel. Crush zones in a homogeneous rock around a tunnelmay indicate high stress; the same phenomenon occurs in therelease of strain energy by rock bursts when thin slabs of rockbecome violently detached from the periphery, in strong rocksat depth and in weaker rocks nearer the surface. As rockfractures it increases in bulk; in consequence, once a plug isprovided at the base of a cavity its upward extent will be limitedand may be approximately calculated.

Even small cavities immediately behind the lining are seriousin that they may lead to uneven loading on the lining andconsequential failure; hence the need for systematic contactgrouting. In soft incompetent ground, over-excavation willusually be transmitted in full to the surface approximately toequate to the volume of surface settlement. However, in densesand the total settlement at the surface will be reduced; in loosesand, settlement may occur as a result of disturbance bytunnelling even in the absence of any over-excavation. It is oftenimpossible in soft ground to subdivide the effects of over-excavation and of changes in the stress pattern, the lattertending to give rise to loss of ground towards the exposed face.

The shape of the 'trough' of settlement at the surface willusually be influenced by loss of ground along a length of tunnelsomewhat greater than its depth below surface, the influencefactors being highly dependent upon the geological structure. Inhomogeneous soft ground and for a tunnel advanced withconsistent standards of design, workmanship and progress, acharacteristic depression will develop over the tunnel which maybe described approximately in terms of the shape of statisticalnormal distribution curves.37 Approximately half the totalsettlement will have occurred immediately above the advancingtunnel face, for tunnels at no great depth.

Tests undertaken during the construction of tunnels in Lon-don Clay indicate that with increasing depth, there is a greatertendency for loss of ground arising from deformation towardsthe advancing face. In general, the contribution to loss ofground may be as set out in Table 32.1.

A special cause for settlement over a tunnel may occur wherea shield in soft ground can only be kept to correct line by means

Table 32.1 Contribution to loss of ground around a shield-driventunnel

Nature of ground loss Computation Normal limits(%)

Ground loss at face nd2h/4 0.1-?Ground loss behind

cutting edge ndt 0.1-0.5Ground loss along the

shield Ti/u/8 0-1Ground loss behind the

tail nd(d-d0)/2 0-4(7id(d—d0)/4 above

water table) 0-2

Where the loss per unit length is expressed as a percentage of area of tunnel faceand d is the diameter of the shield, ̂ 0 is the external diameter of the lining, t is therelief behind the cutting edge, u is the 'look up' of the shield measured as theextent of out of plumb on vertical diameter, / is the length of shield and h is thehorizontal movement of ground at the face per unit length of advance of shield.

of maintaining an appreciable 'look up' on account of atendency to settle at the cutting edge. This loss may be coun-tered to some extent by grouting above the shield as it advances,with fly ash or similar material.

32.8 Tunnel design

32.8.1 Stresses around a tunnel

The state of stress in real ground around a full-size tunnelduring the course of construction is too complex to analysefully. A more rewarding process is to idealize the problem to acertain degree and then, by inference and judgement, determinethe significance of inadequacies of the conceptual model.

We start by considering the two-dimensional problem of along unlined circular tunnel pierced instantaneously at greatdepth in perfectly elastic ground. We can in this instance buildup the overall stress pattern around the tunnel by superpositionof its constituents.38 The initial vertical loading will be redistri-buted and will set up the tangential and radial principal stressesae and ar shown in Figure 32.12 for the vertical and horizontalaxes. At the periphery,

<rr = 0 (32.2)

and at axis and crown level,

O8 =3a* and 0 = -o* (32.3)

respectively where cr* was the final vertical loading in theground.

Figure 32.12 Stresses around circular tunnel in elastic groundinitially stressed in vertical direction only

A similar set of relationships may be obtained for the horizontalloads Na*. For ground loaded from above and laterally con-strained, it can readily be shown that N=v/(l — v) where v isPoisson's ratio. For ground loaded and then subjected toreduction of vertical loading, N may be greater than unity and,indeed, in over-consolidated ground where appreciable surfaceerosion has occurred TV, according to the circumstances, may be2, 3 or more. Evidently if TV= 1, Equation (32.3) indicates bysuperposition that aB=2a* around the periphery. The factor JVmay vary in azimuth and be influenced, inter alia, by tectonicforces.

The most important departures from this simple model maybe caused by:

(1) Nonelastic behaviour of the ground.(2) Limiting ultimate strength of the ground.(3) Inability of the ground to accept tension.(4) Discontinuities in the ground.

The simplest nonelastic model is that for ground assumed tobehave elastically up to certain limiting differences betweenmaximum and minimum principal stresses (generally the stressparallel to the tunnel axis may be considered as intermediatebetween the other two) and thereafter to deform perfectlyplastically. For example, a jointed rock might be considered aselastic for stresses lying within the Mohr's envelope with plasticdeformation occurring at the limiting shear stress of:

T = C' + ON tan <p' (32.4)

The stress pattern around a circular tunnel39 might then berepresented as Figure 32.13. It will be noted that full develop-ment of the plastic zone will entail appreciable movement ofground into the tunnel and theoretical considerations suggestdelay in supporting ground to reduce to a minimum the load ontunnel supports (see page 32/13). In most tunnels, the object isto provide support as rapidly as possible and then to considermerits of systems that will yield noncatastrophically at excessloads. A diagram such as Figure 32.13 permits examination ofthe reduction in plastic movement as a result of increased ar atthe periphery of the tunnel by means of ground support.

IFigure 32.13 State of stresses around a circular tunnel in groundyielding to plastic and elastic strains (N=I). (After Kastner (1971)Statik des Tunnel- und Stollenbaues, (2nd edn) Springer-Verlag)

Many of the recent advances in economic rock tunnellinghave required the behaviour of the rock to be better understoodin two particular respects: (1) the determination of the approxi-mate shape of the Mohr or other form of yield envelope,determining the permissible changes of stress within the rock,i.e. the stress history, which may be tolerated without inevitabledamage of the rock structure;29 (2) an understanding of thestress-strain behaviour of the rock when it is stressed beyondyield. Leaving aside questions of nonhomogeneity and aniso-tropy, knowledge of these two characteristics would permitprediction of the (two-dimensional) behaviour of the rock withspecific schemes of support. In fact, the problem is not so simpleon account of the rotation of principal stresses which occurs inthe ground in the vicinity of a tunnel. While, therefore, such an

approach provides considerable insight into the behaviour ofthe rock, this cannot be in a fully quantitative sense, on accountof the sheer complexity of the problem even as simplified to twodimensions, and neglecting time-dependent effects. There arenevertheless great economic benefits in devising schemes oftunnelling which utilize the rock in the vicinity of the tunnel tosupport a high fraction of the ground load. The technique isthen one of making predictions, on the basis of experience andon tests on the particular rocks or types of rock, on expectationsof behaviour, and then systematically to monitor specific pre-dicted features. The simplest and most effective set of measure-ments concerns the convergence of the rock face as the tunnel isadvanced, possibly supplemented by measurements of internalmovements up to a diameter or so from the tunnel. Monitoringimplies that if movements are observed to be beyond tolerablelimits in magnitude, velocity or distribution, designed counter-measures may be introduced as reinforcement, to re-establishtolerable conditions. This is the application of the observationalmethod, at the heart of all techniques of tunnelling which useincremental support. The best-publicized but by no meansunique exponent of the technique is the NATM; while it hasattracted mythological accretions, most of the applications havebeen successful and many of these economic. Contributions tothe tunnelling techniques based on the observational methodhave evolved separately in numerous countries and continents.27

One essential feature lies in the recognition that design andconstruction become inseparable processes; for successful appli-cation the contractual relationships must reflect this interaction.

The strength of the rock in true plastic yielding may ultima-tely be represented as a purely frictional material, giving alimiting strength line, as in Figure 32.14, to be reproduced in ananalysis of the failure of the ground around a tunnel.

Figure 32.14 Failure limits for rock. (After Lajtai (1969) 'Shearstrength of weakness planes in rock/ lnst J. Rock Mech. Min. Sd.,6, 5, 499-515

From Equation (32.3) it is readily seen that 3>N> 1/3 willcause no tension in elastic ground. Outside these limits, tensionzones will occur for a circular tunnel (Figure 32.15).

The behavioural implications of an elementary feature of thisnature needs to be covered satisfactorily by any effective methodof stress analysis applied to a rock tunnel. A single discontinuitymay have a considerable influence upon the stress distribution inan otherwise sound rock.

The degree of knowledge of the ground, its homogeneity andthe extent of the tunnel will determine the length to which simpleanalysis, models and numerical methods may appropriately beapplied to defining the economic basis for tunnel design. Themethod of finite elements40 has almost unlimited potential forsolving this class of problem but the cost of the solutionincreases rapidly with increasing complexity. Great care isrequired for the nonlinear stress-strain conditions, since theresult depends upon the loading sequence.

The objectives of any such analysis should be stated at the

For no-tension condition(A/^)jHl-3A/)/2A/

Figure 32.15 Circular tunnel in no-tension material

outset. Often a small fraction of the cost of elaborate analysiswould have been better spent on a better understanding of therock structure without which the results of analysis are unreli-able. Increasing use is made of boundary integral methodswhich, for linear problems, entails superposition of familiarpatterns of stress distribution and will, hence, provide solutionsof complex geometry, including discontinuities and nonhomo-geneities, at modest cost. Often, the variability of the rock andthe imprecision of construction methods renders it appropriateonly to make qualitative assessments of stress distribution. Thenotion of 'stress flow' around a cavity41 helps to identify areas ofconcentration; important 'stress raisers' around a cavity mayalso be readily identified. Simple models have yielded muchinsight on the behaviour of nonuniform or jointed rock27 and,for qualitative solutions, inexpensive physical models shouldnot be overlooked.

Laboratory tests and their associated analyses have contri-buted to the development of rational methods of design fortunnels in soil,42 especially in determining factors of safety forstressing of the ground beyond elastic limits. The use of centri-fuge testing is then effective in scaling-up from model to full size,for which gravity assumes a more important role.40

At the present day, many techniques are available to theengineer for the analytical aspects of design; the matter is muchmore a question of selection of the appropriate technique inrelation to reliable knowledge of the ground. For rock, the mostubiquitous problem is that of presenting the rock properties in aform assailable by analysis and in identifying the relevantfactors.

32.8.2 Stability ratio, rock competence andclassification

A simple failure mechanism at the face of a tunnel in soil derivesa stability ratio TV as the ratio (in consistent units) of nettoverburden stress (yD — P) to undrained cohesive strength cu.Generally stability, in the absence of special measures, requiresAf to be no greater than 4 or 5. It will be noted that N may bereduced by increasing P-, the internal pressure in the face of thetunnel, by some such expedient as air or liquid under pressure.

The competence of a rock is a measure of its capacity to resistdeformation under a given loading. Since the loading is usuallydirectly related to the overburden, it appears helpful, in classify-ing the ground from the view point of tunnelling, to define acompetence factor43 as:

Fc = qjy" (32.5)

where qu is unconfined compressive strength of the ground and yis density of overburden of depth D above tunnel

(Note that, if qu = 2cu and P- = O, Fc = 2/N

Where Fc < 2, immediate support is required. Where 10 > Fc > 2,stability of unsupported ground will depend on the initial stateof stress and on stress-strain-time characteristics. WhereFc> 10, the ground will be competent, the strength of the rockstructure may become largely irrelevant, and the real problemconcerns discontinuities and joints, pre-existing or caused bytunnelling.

The problem then beomes that of describing a jointed rock ina form which can lead to rational designation of requirementsfor support. Several systems of rock classification have beendevised for this purpose which embrace selected factors affect-ing stability. There remain two major limitations. First, that thedominant factors vary from situation to situation; e.g. for weakjointed rock, rock strength may be important. Second, that eachfactor has such a wide degree of variability on account ofgeological history and structure that we can scarcely expect tofind unique combinations of two or more factors reliablyrepresented by a single classification index. The system ofBarton43 is the most comprehensive and is helpful in making afirst assessment; thereafter, for any particular situation ofreasonably consistent rock type, a local classification systemmay be devised, as a basis of different degrees of rock support.Often, the most difficult feature to be represented in anyclassification system concerns joint quality, including such char-acteristics as openness, continuity, nature (shear, tension, etc.),roughness, planarity, filling (and how liable to be affected bymovement, water, exposure) which cannot hope to be coveredby one or two numerical factors.

32.8.3 Stiffness of the tunnel lining

The tunnel lining and the surrounding ground should be treatedas a composite structure when considering states of stress anddeformations. A question of first importance concerns therelative stiffness of the lining and the ground it displaces. Forelastic conditions for the simplest, 'elliptical', mode of deforma-tion of a circular tunnel44 we may express this stiffness ratio as:

Rs = 3EI/a^ (32.6)

coefficient of ground reaction

,- 3^c(l + v)(5-6v)a (32.7)

Likewise, the compressibility factor

Rc = aEc(\-v$IAE(\ + v) (32.8)

where A represents lining area per unit length

The average hoop stress and the maximum bending moment inthe lining are then, respectively:

P0 =p/(l + Rc) where p = (pv +pJ/2 (32.9)

M=ifo -ph)a2[RJ(l + /y] (32.10)

where /?v and ph are initial vertical and horizontal pressures inthe ground

Initial state of stress

Limit of zone inarch requiring

support

The maximum and minimum hoop stresses are then:

p/(l+Rc)±M/a

and the radial departures from the original circle of radius a arefound to be, on the vertical and horizontal diameters, respec-tively:

P0a/E± Ma2/3E I

For most applications, Rs is considerably less than unity andlining stiffness then makes little contribution to the reduction ofdeflections. An objective in economic design in such circum-stances will be to minimize stiffness. Likewise, compressiblelinings need only to support a reduced fraction of hoop loading.

32.8.4 Towards a better understanding

The advance in techniques of tunnelling is a continuing processand the results of monitoring the behaviour of a tunnel bycomparison with prediction provide valuable correction todesign techniques and assumptions. Great skill is required indetermining the data to be acquired and in interpreting thequantities of data that may be obtained in a comprehensiblemanner without introducing the risks of oversimplification.Such monitoring will normally be concerned with strains in theground and with deformations of, and stresses in, the lining.56

Generally, the simplest and most robust instruments should beused to avoid disappointment from incomplete results arisingfrom damage.

Improvements in understanding the behaviour of the groundand in tunnelling techniques bring economies in cost and time.These also lead to better ground control and reduced subsidenceover the tunnel, a feature which has received much attention.45

One example of the specialized development of tunnellingrelates to storage of oil and liquefied petroleum gas (LPG). Thisapplication requires selection of sites of reliable rock quality, theconfidence to excavate caverns of great size and span with littleor no ground support and the ability to ensure containment ofthe product stored, by natural tightness of the rock, possiblysupplemented by a water curtain to ensure that any flow acrossthe boundary of the cavern is inwards not outwards.46

Where a tunnelling option exists, its optimization dependsupon its assessment by those knowledgeable in the potential oftunnelling at an early stage in planning. Too often the tunneloption is considered too late to be optimized, where it mightotherwise have provided the most economic solution withassociated social and environmental benefits.

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