Tunnelling

Front matter

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Contents

Preface Notation Introduction

1.2 Tunnelling in antiquity 9

1.3 Development of rationale 12

Preface

Every author will claim that his or her book will cast light on aspects of the chosen topic not previously illuminated by his (or her—to be understood throughout) precursors. Where then are the dark unexplored recesses of the underground world that justify the promised light at the end of the tunnel of this present account?

The immediate spur to writing this book is that the author has lived and worked through a period of revolutionary change in tunnelling, with several components:

change from traditional craft to technological art; spectacular advances in site investigation techniques and in geotechnical analysis;

e great strides in technological development in all aspects of tunnel construction;

e emphasis on the teachable elements of science applied to tunnelling;

e recognition of the interplay of opposites: opportunity and risk, in the development of tunnelling strategies;

e institutional recognition of tunnelling as a specific branch of engineering.

But costly mistakes—possibly costing more than the original estimate of the project—are now more common occurrences, usually of a foreseeable and preventable nature. Overall, therefore, the industry is nowhere near optimum potential, to the frustration of those who work in it, the wasting of personal effort, the thwarting of the objectives of the promoters of projects who, in the most egregious failures, have themselves through lack of understanding established conditions unconducive to success. Where lawyers earn far more from the failure of projects than do the most skilled engineers from success, clearly there are fundamental systemic faults.

From the surface, no single explanation for this contradictory situation is apparent but deeper digging indicates a common set of system failures. A primary purpose of this book is therefore to see tunnelling as a system and to viii Preface develop principles for success based on effective understanding and operation of the system of interplay of specific tunnelling skills.

The views expressed in this book are the author’s own but he acknowledges his debt to his many immediate colleagues in Halcrow and to so many tunnellers and others around the world for several of the thoughts which have prompted the book. The author accepts full responsibility for any misunderstanding. The account may be criticised as unduly orientated towards the British and European examples. This is justified by selecting examples for which the circumstances are generally familiar; lessons learned may well have more universal application. The Channel Tunnel, for example, provides many examples of meritorious engineering with less meritorious management. An exemplary account of its engineering geology (Harris et al. 1996) attracts numerous references on account of its depth and breadth.

Notation

Against each symbol is a brief definition and a reference to the most appropriate Section or Appendix of the book for further explanation.

a major semi-diameter of ellipse 93 a tunnel radius App. 5A A cross-sectional area of tunnel 5.3 b minor semi-diameter of ellipse 9.3 c cohesion SL c, concentration of gas in air 8.3 c, undrained shear strength of soil 5.1.1 c, coefficient of consolidation SA1 C„ concentration of gas dissolved in water 8.3 C construction hazard 6.1.3 C circumference 9.3 C, velocity of compressive wave 4.1.2 C, velocity of shear wave 4.1.2 d,) size of tenth smallest fraction 6.3 D design of project hazard 6.1.3 E Young’s modulus for lining App. 5A E, Young’s modulus for ground App. 5A F flux rate for transfer of gas between air and water 8.3 b depth of tunnel axis App. 5G b depth of compensation grouting 53 H height of column of rock supported by tunnel App. 5D H head difference App. 5G H geological hazard 6.1.3 H Henry’s constant 8.3 i distance of point of inflexion from axis 5.3 I second moment of area App. 5A k hydraulic permeability App. 5G k, hydraulic permeability (horizontal) App. 5G k, hydraulic permeability (vertical) App. 5G x Noteation

K, earth pressure coefficient at rest

K, coefficient of passive pressure

K,” coefficient of effective passive pressure m radial spacing of rock bolts at rock face m, coefficient of volume compressibility

M bending moment in lining per unit length of tunnel

M, maximum value of M max n longitudinal spacing of rock bolts at rock face n period of years

N circumferential load in lining per unit length N, stability ratio

p V/A

p probability

p; internal support pressure

q surface surcharge pressure

q flow per unit area

q, unconfined compressive strength

O weight of explosive charge

O, inflow (outflow) per unit length of tunnel Q, inflow per unit length of probe-hole

r rate of interest

r radial coordinate

rọ radius of tunnel

R geological risk

R, radius of curvature of ellipse on major axis R, radius of curvature of ellipse on minor axis R, competence ratio (q,/6,)

R, compressibility factor

T tensionin rock-bolt

u radial convergence

u, radial convergence at radius a

u. shear velocity

û maximum value of u

V, area of surface settlement trough

V, ground loss into tunnel per unit length

w surface settlement

W „ax Maximum value of surface settlement

y coordinate along axis of tunnel

z vertical coordinate 5.3

Zo depth from surface to tunnel axis

Ar change in radius, r AC change in circumference, C y unit weight of soil

Noteation xi unit weight of water strain

radial strain

axial strain circumferential strain angular coordinate radial stress parameter coefficient of ground reaction Poisson’s ratio

density

compressive stress stress at radius a horizontal stress normal stress

radial stress

vertical stress

bending stress longitudinal stress circumferential stress initial stress in ground and far-field stress shear stress

shear stress in plane 7,0 angle of friction

Airy function equipotential line

flow line

Introduction

Tunnelling, along with much civil engineering in Britain, particularly of projects of high potential risk, has suffered during the late 1980s and 1990s from the unsuccessful experiment—doubtless influenced by the prevailing politics of the time—to apply crude free market principles to the procurement of projects in fragmented elements, each element at least cost, centrally administered but not integrated. To many, the paradox has not been lost that a commercially motivated doctrine applied in an inappropriate manner to an essentially professional field reserved its hardest blows for the commercial interests themselves, as ‘management’ and ‘engineering’ were condemned to part company.

The jargon of ‘systems’ is unfamiliar to tunnelling. In consequence, in this book, a familiar term ‘design’, usually used in construction in far too narrow a sense, is used in the broad sense in which it is applied, for example, in manufacturing engineering (Chaplin 1989). Where the term ‘design’ is used in this comprehensive sense, as defined in Chapter 2, it is printed throughout as ‘design’ to avoid confusion with the narrow sense of ‘scheme design’ and ‘element design’ traditional to tunnelling. One particular objective of this book is the emphasis on the essential interaction between product design (the design of the finished project and of its operation) and process design (the design of construction and its means) for success. The dominant role of design in this broad sense is discussed in Chapter 2.

The book is thus largely concerned with the features of operation of the design process to allow the development and execution of the optimal scheme by the interactions, often iterative, between the several contributors. Along the way, examples, many within personal experience, illustrate success and failure, and underscore the benefits of design to all concerned, not least the ‘clients’ and their financial supporters. There are those who maintain that design equates to engineering and that there is merit in emphasising this correspondence. The author has much sympathy with this view but ‘engineering’ already has too many connotations. The main virtue in design is that the engineer will be working with other disciplines who will be able to apply the same term design for their combined, and for their separate, activities. The term will therefore bond and not divide the team.

What is so special about tunnelling? Each particular property is shared to some degree with other forms of construction; many of the principles discussed have far wider application. Tunnelling may however be characterised by these features:

e extreme dependence on the ground, the interpretation of its characteristics in terms of risk;

e high degree of interdependence between planning and project design, arising from provisions for containing the ground and excluding groundwater;

e domination of the methods of construction on the design of the project;

e effect of restrictions of access on logistics, particularly in dealing with construction problems;

e interdependence of a long chain of control between the intention and its execution.

To use current jargon, the broad claim is the attempt to provide the first holistic account of tunnelling. Much has been written on design—conceived as analysis in determining the geometry and constituent elements of the finished tunnel, somewhat less on techniques of construction. There are also many accounts of underground planning from the last 20 years, a few monographs on site investigation for tunnels and briefer accounts of the elements of the overall process. Many Papers and Conference Proceedings describe specific case histories but few attempt to trace misfortunes to their fundamental causes and fewer yet attempt to provide analysis of the explanations for success or to provide clues as to how to avoid unfortunate repetition in the future. A certain amount has been written by tunnellers on the contractual and management aspects for success but this seems largely to have passed by those who set standards from the top-down style of much that passes for engineering management.

Successful tunnelling requires the blending of many skills, the acquisition of experience and judgement, and the transmission of the benefit of this experience to newcomers into the underworld. Much benefit results from encouragement to exchange views and to compare experiences. This is achieved at several levels, including the learned society activities of professional and technical bodies, at an international level by the activities—and with the encouragement of—the International Tunnelling Association (ITA), the International Society for Rock Mechanics (ISRM), the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) and their specialist groups, also the Technical Committee on Tunnelling of PIARC (Permanent International Association of Road Congresses). The most valuable contributions demonstrate the degree to which expectations based on analysis and experiment correspond to experience in projects, particularly where the account includes statements as to what has been learned. Certain classes of engineer appear to avoid exposing their approaches to their work, preferring instead to spread a deliberate air of mystery around their work as an esoteric art confined to initiates of a particular doctrine. This is nonsense and their own degree of success suffers in consequence. It is noticeable, for example, that sprayed concrete lining (SCL) tunnelling in soft ground (see Chapter 1) has profited immeasurably from the recent open comparison between theory and practice arising from many participants of its application to British clays.

The principal players in a tunnelling project may be imagined to be assigned as the members of an orchestra. Each needs to be able to master his own instrument, each needs to have a good ear for the contributions of others in order to be able to engage in the counterpoint of dialogue. The conductor, the leader of the project, needs to understand how to blend the contributions by the players, requiring an appreciation of the range of pitch and tonalities—the specific element—of each instrument. Too often, the tunnelling players are each following unrelated scores, with the conductor confined to the role of the orchestral administrator, without insight into the essence of the enterprise, the manager without understanding of what is managed. No wonder if the result is too frequently cacophonous.

This book is directed towards the interests and needs of each member of the tunnelling orchestra and of its impresarios, the Owners, to assist each in playing his own part and to become better attuned to the contributions of others. The objective is to help not only those engaged in the direct functions of achieving the underground project but also those in associated functions such as physical planning and transport economics, particularly those concerned in financing and management from a distance, who play vital parts in achieving the optimal project. Furthermore, the key messages on the management of tunnelling projects should have a wider audience, not only because of common features of different types of major project but also to influence the ‘decision makers’ who far too often have made the wrong decisions of policy, incapable of retrospective correction, through failure to understand the criteria for success. These may include the Development Banks, Government Ministers, Local Government officials, and both public and private owners of Utilities. The hope is that there may be some influence towards steering an initial course so that those who come to put policy into action do not need to expend unnecessary energies on attempts partially to mitigate consequences of original misdirection. The fundamental criteria for success have a broad application to any project with a significant element of natural risk.

At the central point of initial decision-making, the ‘conductor’ is familiar with tunnel design and construction, also with the operational and financial objectives. During the early stages, particularly before commitment to the nature of the project, or even whether a tunnelled option may be preferred, the functions of the ‘conductor’ may be undertaken in whole or in part by a ‘surrogate client’ or ‘surrogate operator’ who brings the requisite understanding and who represents the client (Owner, promoter or operator) in commissioning whatever studies and investigations may be expedient to reach a preliminary view on the options and to advise on the formation of the team who will have the duty to carry forward in an orderly process the project design. Too often, the first professional recourse by the client is to a lawyer who sets out to erect the barricades for protection against contractual conflict, casting other participants as antagonists to be attached to contractual chains and to be exposed to legal minefields. This is the worst possible starting point for a project which, for success, essentially demands much cooperation from the contributing Parties.

Many skills need to be harnessed for the successful tunnelling project. Whether or not the appropriate skills are recruited depends on the management of the project. Here we encounter the first set of problems.

All civil engineering projects have a purpose beyond civil engineering. Tunnelling is no exception but occupies a rather esoteric position in that the ultimate objective is not, as a general rule, directly associated with an underground solution. The extension of a metro system or certain forms of hydropower are exceptions to this general rule. Those who commission projects of these latter types may be expected to have greater familiarity with the criteria for success of underground projects than those, for example, who operate main-line railways or water supply who only venture deep below the surface once in a generation. Whatever the purpose, the total management of the project will determine whether the potential for success survives the initial phase.

Much, possibly too much, has recently been written on project management. The engineer concerned with underground projects must be familiar with the elements of good management, of special concern to his type of project, including the available tools based on information technology or IT, and will find it advantageous, but not necessarily for clear thinking, to be familiar with the jargon and the acronyms—but at all costs must avoid the dangers of lapsing into management-speak. The essentials of project management start from an understanding of the qualities of leadership, and the criteria for overall project optimisation. Management by remote direction is fatal for tunnelling; some of the fatalities are described in this book. Tunnelling, as with any complex activity in a previously unexplored environment, contains elements of uncertainty that need to be understood and controlled. Optimisation in consequence entails the control of risk and the counterpart of exploiting opportunities for innovation. A primary essential of the manager is therefore to understand the features of the tunnelling process, to influence the overall strategy from the moment of first recognition of the possibility of a tunnelling solution through to the operation of the finished project. Chapter 7 describes the most essential functions of the manager and, for those contemplating the commissioning of a possible tunnel, this may be the point of entry, leading into Chapter 3 which describes the process of project definition, from the outset, when the project is no more than a gleam in the eye or a hypothetical solution to a previously unsolved problem. Chapter 5 follows the evolution of the project, identifying the principal features which should influence decisions at each stage, to avoid later untoward consequences.

The scheme of project management has no unique structure. The principles need to be respected within the evident requirement for compatibility with the organisation of the Owner (or Commissioner) of the Project. Good project management may readily accommodate the several forms of Private and Public Finance of Projects. It is nevertheless recognised that the rigidity of the rules of certain Public—and some Private—bodies needs to be relaxed. The fragmentation of projects by the separate commissioning of different aspects of the work must also be abandoned since such practices inhibit the essential interactive features of design. Can the Owner afford to engage people at the initial stages with operational knowledge and the appropriate skills to interact with those who are to define the tunnelling option? If not, then he cannot afford to consider a tunnelled option.

Those who continue to visualise tunnels as examples of linear procedures may find a certain logic in the ordering of Chapters 3, 4, 5 and 6, titled respectively: Planning; Studies and Investigations; Design of the Tunnel Project; Design of Construction. This is partly an illusion, however. Constant reminders are provided of the fact that this arrangement is little more than a convenience for reference; the important feature is that these apparent phases should be viewed as continuous processes with many crossconnecting functions, the essence of the holistic approach. Following another line of thought, the different types of tunnel for different purposes might have been developed coherently through planning to operation. Traces of this cross-dimension will be found in elements of each chapter.

The interlinkage of activities may be illustrated by consideration of risk, defined in Chaper 2. The principal technical elements of uncertainty of a tunnelling project derive from the ground. The initial route planning may introduce more or less features of uncertainty, which will need investigation or control or at least strict definition in relation to possibilities of construction. Tunnel design is dominated by the ground; so too is construction. Different forms of construction will be vulnerable to different aspects of uncertainty. Hence optimisation is a continuing activity, taking account of all such considerations. While the perception and anticipation of risk is fundamental to successful design, it should be understood that risk in tunnelling seldom lends itself to statistical calculation but is more of a pragmatic Bayesian nature (whereby assessments of future risk may be constantly updated from data compiled from immediate past experience), for several reasons.

Firstly, geological variability is so wide that the degree of uncertainty of the ground can usually only be expressed between limits, progressively narrowed as the project develops. For example, the precise nature of possible water inflows and the positions of important ground interfaces may be sufficiently unpredictable to demand a form of observational approach (see Chapter 2).

Secondly, the risks are consequences of interactions of a site- and project-specific nature. Even, therefore, if averaged statistics existed of incidents with particular combinations of the several factors, their application to a specific project would have little significance. The most significant factor will be the degree of awareness of the possible consequences of the particular combination of circumstances and their anticipation.

Thirdly, as every tunneller will know, there is a prevailing ethos of every project, partially derived from the degree of shared interests in the definitions for success between the parties concerned. This ethos may profoundly influence the attitude to risk and the appropriate preparedness. This is the social dimension of tunnelling, a source of light or of darkness.

A structure for systematically listing the factors contributing to risk should nevertheless be devised. How to circumscribe risk and how to apportion responsibility for control and management feature throughout this book. Risk as a main cause of increased cost and delay is cultivated by absence of continuity of purpose in project development, the associated fragmentation of responsibility, the failure early in the life of a project to address risk and its attempted evasion by transmission to other parties further down the line, involved with only a part of the project, who have remote prospects of definition, investigation or control. This book is concerned in developing the culture for enhancing opportunities, for the encouragement of good practices to permit the potential for successful projects and for the full realisation of satisfying project experiences.

While certain aspects of the relationships between the parties to a tunnelling project need to respect commercial criteria, in essence success depends upon the mutual recognition of the professional standards of the participants. The nature of design emphasises that these are paramount, between promoter, engineer, contractor and specialist. Recognition of this mutual respect is more highly cultivated in European countries other than Britain than in Britain and the USA, where the engineers have less control over the practices in procuring the contributory elements of tunnelling projects. The consequent loss is shared by all parties, with greater risk of loss and less innovation in practice. There are solutions to this sub-optimal culture, whose exposure and amelioration are the principal purposes of this book.

All of these aspects have a bearing upon the education and training of engineers. Tunnelling provides an excellent example of the need to combine depths—the understanding and expertise in several quite different disciplines—with breadth, the ability to fit all these elements together. Some suggest that technical design in the ‘sharp’ parts of engineering is quite a different activity from managing construction. This is a dangerous myth which promotes the separation between the two activities of design and management. The import of this book is that the two activities merge imperceptively and that it is the ability to achieve this merger which is a principal criterion for success.

Chapter 1

If we can think of one bit of time which cannot be sub-divided into even the smallest instantaneous moments, that alone is what we can call the ‘present’. And this time flies so quickly from the future to the past that it is an interval with no duration.

The reader will find here no attempt to provide a comprehensive history of tunnelling. The objectives are more modest and more targeted, namely, to set the scene from the earliest known tunnelling, to review developments which have contributed to present-day understanding of the criteria for success, with references to exemplary projects.

On the negative side, the objective is to attempt to explain historical factors contributing to the current lack of general appreciation for these same criteria for success. In so doing, it is necessary to understand how circumstances have changed with time and with the increased complexity of the tunnelling operation. The many contributory factors include the increasingly specific technical requirements to satisfy the objectives of an underground project, developments in the techniques and the means of tunnelling, also in the associated improved capability to explore the ground, to measure and model its characteristics.

This account is in consequence deliberately selective, using particular examples, from the several periods of development of tunnelling, as landmarks, generally to emphasise the varying degrees of intuition, craftskill and technology which have characterised the several periods into which this chapter is divided. There is no clearly defined boundary between tunnelling history and current developments. The most important features of current practice rest upon concepts which have been developing over many years. The sub-division between past and present is therefore one of convenience in telling a reasonably continuous tale.

A broad survey of tunnelling rapidly reveals how tunnelling has progressively been challenged to penetrate ground of increasing degrees of inherent difficulty. With time, in consequence, the relationship between the main parties concerned with each tunnel has assumed greater importance. A contrary more recent development, during a period in which inappropriate contract relationships have been increasingly imposed, adds to a mismatch between what these relationships should be and what in fact they have been. A pervading objective of this book is to help to guide towards good practice for the future, in the management aspects of engineering as much as the technology, the two being indivisible in good practice. A historical perspective provides a guide, with the view that knowledge of the point of departure is necessary in order to chart the course to our destination. For general accounts of the history of tunnelling, the reader may turn to Sandström (1963), Széchy (1970) and Harding (1981) although it will be found that more recent information causes the account below to make some modification to these texts. Background reading into earlier times may start from Singer et al. (1954).

1.2 Tunnelling in antiquity

Since tunnelling, especially tunnelling for mining minerals and for water supply, precedes recorded history, we can only conjecture as to the thought processes of those in the earliest times who discovered that tunnelling provided the solution to perceived requirements. We need to respect the intelligence of these early tunnellers of 3000 years ago and more for the magnitude of their achievements, whose art depended entirely on trial-anderror (heuristics) to learn what could be done and how best to achieve results, with neither the benefit of underlying technology nor with access to specialised tools. All ancient civilisations have left behind examples of tunnelling at varying scales.

The earliest tunnels would have been modelled on natural caves, selecting rock types capable of penetration with crude tools, and having innate strength and tight joint structure conducive to a natural stable form without need for artificial support. Much of the tunnelling for mining was initially of this nature following lodes with near-surface exposures. Early tunnels reflect the change of social priorities with time. Whereas at the present day a tunnel has one or more special purposes as conveyor of people or substance, storage or protected housing, many of the most spectacular tunnels were for ceremonial and religious purposes. It is remarkable that the first known subaqueous tunnel was built in about 2000 BC to connect the royal palace of Queen Semiramis to the Temple of Jove (or his equivalent) beneath the River Euphrates. The tunnel was about 1 km long, had a section of 3.6 mx4.5 m, and was lined in brickwork with a vaulted arch set in bituminous mortar. The river was diverted to permit cut-and-cover construction. While this may not constitute tunnelling in the purest sense, nevertheless the sophistication of the techniques point to a preexisting skill and assurance in undertaking such works.

It also deserves record that, for city drainage systems during the Akkadian supremacy (BC 2800-2200) of Mesopotamia, vaulted sewers were built in baked brick, with inspection chambers and rodding eyes, lined in bitumen. To wander a little further from our scene, there were bath-rooms paved in bitumen-faced brick, also closets with raised pedestals with the occasional refinement of a shaped bitumen seat (Singer et al. 1954). Rings of brickwork in vaults for wide spans were inclined to avoid the need for centring.

Tombs and temples provide other examples of tunnelling. The Royal Tombs at Thebes in Egypt and at Ur in Mesopotamia date from around 2500 BC; Abu Simbel in Egypt dating from around 1250 BC is another such example. These tunnels were in limestone, requiring high standards of craftsmanship in their form but presenting no problems of stability. Another later example, mentioned by Sandström (1963) among many examples in India, concerns the caves of Ellora, near Bombay, excavated between 200 and 600 AD, tunnels aggregating to a length of more than 9 km, cut by chisel in fine-grained igneous rock.

A celebrated, but by no means unique, water supply tunnel of the ancient world is the Siloam tunnel (also known as Hezekiah’s tunnel), mentioned at 2 Kings 20:20, at 2 Chronicles 32:30 and elsewhere in the Old and New Testament of the Bible. The oldest part of the city of Jerusalem, dating from 2000 BC or earlier had a source of water from the Gihon spring on the Ophel ridge to the east of the city. The name Gihon in Hebrew (and its Arabic form, Umm al-Darah) relates to its gushing siphonic nature, with high flows for a duration of 30 minutes or more at intervals of 4-10 h (depending on the season and the source of information). As early as about 1800 BC, a short length of tunnel connected the spring to a well shaft nearly 30 m deep within the city wall. In around 950 BC Solomon connected the spring by an open channel outside the city wall to the pool of Siloam within the wall. Under threat of siege by the Assyrians under Sennacherib, King Hezekiah secured the water supply by resiting the pool of Siloam and feeding it through a tunnel 533 m long, following a sinuous route about 60% longer than the direct length from spring to outlet portal. The tunnel is roughly 1 m wide and 2 m high, increasing to a considerably greater height near the portal, presumably as a result of correcting an error in level. The reasons for the devious course of the tunnel have long been the cause of speculation. Most probably the explanation includes geological influences (natural fissures in the rock) combined with the avoidance of areas of royal tombs. The tunnel was advanced from both ends and a sharp Z-bend near mid-length probably marks the point of break-through. A stone memorial tablet found in 1880 provides, in one of the oldest example of cursive Hebrew script, a brief account of the tunnel and the occasion of break-through, which reads in translation: ‘While the hewers yielded the axe, each man towards his fellow, and while there were still three cubits to be hewn, there was heard a man’s voice calling to his fellow, for there was a crack in the rock on the right and on the left.’