reading

6.7 Checking the tunnelling machine for suitability and acceptance based on a risk analysis

Prof. Dr.-Ing. Bernhard Maidl
mtc – Maidl Tunnelconsultants GmbH & Co. KG
Fuldastr. 11
47051 Duisburg
Germany

Prof. Dr.-Ing. Markus Thewes
Lehrstuhl für Tunnelbau, Leitungsbau und Baubetrieb
Ruhr Universität Bochum
Universitätsstr. 150
44801 Bochum
Germany

Dr.-Ing. Ulrich Maidl
mtc – Maidl Tunnelconsultants GmbH & Co. KG
Rupprechtstr. 25
80636 München
Germany

Project: Neubaustrecke Deutsche Bahn Berlin – Nürnberg VDE 8/Tunnel Reitersberg

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Former holder of the Chair of Construction Technology, Tunnelling and Construction Management at the Institute for Structural Engineering of the Ruhr University, Bochum

Holder of the Chair of Tunnelling and Construction Management, Ruhr University Bochum

Dipl.-Ing. Michael Griese (overall coordination), Maidl Tunnelconsultants GmbH & Co. KG

The “black book of tunnelling” has become a standard work in German-speaking countries since its first edition in 1984. It can be found on every tunnel site and in every design office – whether contractor or consultant. Students at universities and technical colleges use it as a textbook.

For many years, colleagues from abroad have been asking me for an English edition. Now the time has come to publish the two-volume book in English. An important step was that the publisher of the first German edition, VGE, gave their permission for the publishing of the English edition by Ernst & Sohn, Berlin. Special thanks are due to Dr. Richter from publisher Ernst & Sohn for his successful negotiations. However, preparation of the text for the translation showed that the 3^rd German edition required updating and extending. In particular, the standards and recommendations have been revised. This will all be included in a 4^th German edition, which will be published soon. Changes to the standards and recommendations are given in this edition, with the references stating the latest version.

As with all books, the first English edition has also required the collaboration of colleagues. Professor Dr.-Ing. Markus Thewes, who has succeeded me as the holder of my former university chair, and my son, Dr.-Ing. Ulrich Maidl, managing director of MTC, have joined me in the team of authors. Dipl.-Ing. Michael Griese from MTC is the overall coordinator, assisted by Dr.-Ing. Götz Vollmann and Dipl.-Ing. Anna-Lena Hammer from the chair of Prof. Thewes. I thank all those involved, also the translator David Sturge and the employees of the publisher Ernst & Sohn in Berlin.

Almost 20 years after the first appearance of the Handbook of Tunnel Engineering and about 10 years after the 2^nd German edition, a complete revision was necessary for the 3^rd German edition. Detailed investigation only made clear what enormous developments had taken place in tunnelling.

In conventional tunnelling, progress was predominantly in the field of advance support methods such as pipe screens and jet grouting, which enabled the scope of application to be extended to include larger cross-sections and better mechanisation. Further mechanisation, particularly of muck clearance, enables parallel operation of excavation, support, muck clearance and transport in conventional tunnelling, which has also improved advance rates.

In mechanised tunnelling, an even greater leap of progress has taken place as a result of the wider experience that has come with increasing application. The traditionally established limits to the application of EPB and hydroshield machines are no longer so clear in practice. Mixshields and combishields can be used in various modes.

Particularly worth mentioning are developments of support methods installed directly behind the machine. The requirements connected with the construction of the large Alpine tunnels under the Lötschberg and the St. Gotthard have given a spur to this development.

Since the extent of the text in the chapters concerning “support methods” and “construction processes” has also increased, Chapter X “Implementation of construction projects” has been omitted. This has been integrated under other headings in Volume II. The chapter “Waterproofing and drainage” has also been included in Volume II.

As has already been the case with other books, I once again required intensive assistance from my employees for this book. Even when we could refer to the other books about shotcrete, steel fibre concrete, shield and TBM tunnelling, this still involved an enormous amount of work, for which I wish to thank all, and also the authors who worked on the former books.

Many thanks also to the many other helpers and also to those involved at the publishers.

Many letters and comments from Germany and abroad have confirmed that the Handbook of Tunnel Engineering from 1984 has been well accepted as a textbook and also in design offices and on tunnel sites. Positive developments in tunnelling are also good news, and this has led to the call for specific planning regulations for underground structures already becoming a reality in some countries.

When the new edition was being investigated, it became clear that a great variety of technical innovations have been introduced in the last ten years, sufficient to justify a new edition. But time is limited and cannot be multiplied, so only the most important revisions have been undertaken. This includes revisions and extensions of the standards listed in the references. The sections concerning shotcrete and steel fibre shotcrete in Chapter II “Support methods” have been revised, as well as Chapters VI “Mechanised tunnelling” and VII “The driving of small cross-sections”. Chapter IV “Shotcrete tunnelling using the New Austrian Tunnelling Method” has been rewritten and renamed and also part of Chapter X “Quality assurance in tunnelling”.

For the production of the new edition, I was once again dependent on the intensive support and experience of my colleagues at the University Chair and in the consultancy. In particular, I wish to thank Dipl.-Ing. Feyerabend for the overall coordination, ably assisted by Dipl.-Ing. Gipperich and Dipl.-Ing. Berger.

I also have to thank Helmut Schmidt for the production of drawings and naturally also the publisher, for whom Dr. Jackisch has supported us at all times.

The art of the Engineer is to avoid high ground pressure, that is not to permit it to occur, a much more difficult task than to overcome ground pressure after it has occurred.

And let us dare to resist the former with intellectual, the latter with raw material work.

Leopold Müller, my teacher, said with slight resignation at the 28^th Geomechanics Colloquium in 1979 in Salzburg: “Experience on construction sites and at congresses gives cause to reflect on the state of development in geomechanical research related to construction in rock, but experience shows that this development is largely uncontrolled and not always coordinated with the needs of practitioners and theoretical progress. Many results of scientific research remain unknown in practice or do not become accepted, while in the other direction, the research needs of practicing engineers are not recognised satisfactorily or indeed not at all”. It remains for us as his former students to consider the reasons for this development and to attempt to divert progress to a sensible path.

On tunnel projects in recent years, the “new Austrian tunnelling method” has become very successful in construction practice. But it has still today been verified by astonishingly few calculations, and that despite many Geomechanics Colloquia. However, the opinion is becoming ever more common that tunnels should be designed and built using refined calculations and with a lot of work and paper. Refined calculations demand particularly from the consultant the mastery of model formation and the application of the results of calculations in practice on site; the responsible site manager on the other hand should be able to judge how the parameters of rock mass, support and construction process affect the results of the calculation, particularly under varying geological conditions. But how is he meant to do that when the calculation has been performed externally and the given results are scarcely understandable for him?

If we investigate the failures, losses, schedule and cost overruns in tunnelling in recent years, then the calculations do not normally turn out to have been performed too little or too roughly, and there have been plenty of expert reports. The causes are rather more inadequate studies of the ground parameters, insufficient care in the selection of a construction process, inadequate adaptability to changing geological conditions, gross construction errors in important details, lack of skilled personnel, insufficient measurements; the list could be continued. Perhaps we feel too safe and become careless having put too much trust in too much paper. In addition, the structural design of tunnels cannot be compared to the structural verification of other engineered structures. The construction of a tunnel has more to it and thankfully most tunnellers appreciate this.

On every tunnel construction project, the correct selection of a construction process is a precondition for technical and commercial success. The factor time has not yet been considered in calculations in tunnelling, but it can only be influenced through the construction process with the various excavation processes on agreed schedules and the effect of the support. The surrounding ground belongs to the structure; the sequence of operations influences the loading on the support and the load-bearing behaviour of the rock mass. The literature of tunnelling is today aligned into specialist areas, which are often aligned with the specific activities of the chairs of university lecturers, like rock mechanics, foundation engineering, structural engineering, construction management and transport. Construction process technology should in this case be understood as a structural subject, which includes the influences of construction on the design, including the consideration of construction states. Such a systematic way of thinking has only been taught for a few years as an individual scientific subject in civil engineering; the number of publications is not yet too large.

Volume I “Details and construction processes” puts the emphasis on construction process technology as a constructive area of tunnelling. Support materials and construction, excavation and advance processes and their directions of development are also dealt with. Some sections have been dealt with in more detail though the research work carried out at the Chair of Construction Technology, Tunnelling and Construction Management at the Institute for Structural Engineering at the Ruhr University, Bochum in the fields of shotcrete, steel fibre shotcrete an the driving of small-diameter tunnels, through my experience on the tunnels of new lines for German Railways and also through my former work into the drilling of blast holes. An extensive provision of illustrations shows numerous practical examples and the tables contain much technical data.

This handbook is based on my lectures for the specialised course “Construction process technology and operations”. I would like to thank the managing director of the publisher Verlag Glückauf GmbH, Dr.-Ing. Rolf Helge Bachstroem, for the encouragement to develop this handbook from my lecture notes on Underground Construction. In the course of long discussions, he advised me about the final version of the text, assisted in the selection of illustrations and tables and made constructive suggestions for many improvements as publisher and expert. I have also received valuable support from my employees at the Construction Technology, Tunnelling and Construction Management and in the consultancy. I wish to thank Oberingenieur Dr.-Ing. Dietrich Stein, my brother Dipl.-Ing. Reinhold Maidl and particularly Dipl.-Ing. Harald Brühl for their intensive collaboration. I thank Agatha Eschner-Wellenkamp for her inexhaustible industry with the writing work, and Helmut Schmidt and Walter Zamiara (publisher) for the preparation of many drawings.

Volume II will have the subtitle “Basics and auxiliary works in design and construction”; the volume should include geotechnical aspects, rock classification, stress states in the rock mass, structural verifications, monitoring instrumentation, dewatering, surveying and scheduling.

Introduction

1.1 General

Tunnelling is one of the most interesting, but also the most difficult engineering disciplines. It unites theory and practice into its own construction art. For the weighting of the many influential factors, practice is sometimes more important, and at other times theory, according to ones own state of knowledge. Tunnel engineering is normally performed by civil engineers. Everyone, however, should be aware that knowledge about structural analysis and concrete engineering alone is not sufficient. Geology, geomechanics, mechanical engineering and particularly construction process technology are equally important.

1.2 Historical development

Tunnels and caverns already existed in nature before mankind started to create them artificially to meet vital interests.

Tunnel engineering in the 20^th century could also make use of existing specialised knowledge from mining. One of the founding fathers was Georg Agricola, whose 1556 work De Re Metallica, Libri XII covered mining and metallurgy.

Drill and blast

The building of significant tunnels in the Alps had already led to a first heyday of tunnelling before 1900, which explains why the railway engineer Franz Ržiha, mining superintendent of the duchy of Braunschweig, considered tunnel engineering as a separate discipline from mining in his 1867 textbook of tunnelling. This heyday continued to the start of the 20^th century, after which there were only a few spectacular tunnel projects (Table 1-1) until 1960. The building of the Mont Blanc Tunnel was the start of a new phase in Europe, which continued with the construction of the Tauern Autobahn Tunnels, the Arlberg Tunnel and the new Gotthard Tunnel. The construction of more than a hundred tunnels by the German Railways (Deutsche Bundesbahn, later: Deutsche Bahn AG) continued the development. A new phase opened with the Seikan Tunnel, the Channel Tunnel and the base tunnels through the Alps.

The extent of the enormous development in tunnelling enabled by currently available support materials and machinery is illustrated in Fig. 1-1 and Fig. 1-2 for conventional tunnelling. The introduction of shotcrete as a means of support introduced a new phase of development, which made much greater use of machinery. Only years later did mechanisation take off again, permitting simultaneous working at the face and removal of the excavated material. The development of tunnel boring machines was even more impressive, and this is dealt with in detail in Chapter 6.

Figure 1-1 Construction of the Semmering Tunnel in 1848 using the old Austrian method [302]

Figure 1-2 Construction of the Westtangente Tunnel, Bochum in 1982 with shotcrete [343]

Comparison of the data from the various Gotthard Tunnels in Table 1-2 shows that extremely short construction times were already possible many years ago, but the number of accidents has been greatly reduced by the modernisation of tunnelling technology.

Large underground structures have also been built for hydropower stations outside Europe, for example at Tarbela in Pakistan and Cabora Bassa in Mozambique. Many projects are still urgently needed, even if these have not yet been implemented due to financing problems. Underground and urban rapid transit lines, but also road tunnels will have to be extended to solve environmental and traffic problems in the cities. New developments in tunnel boring machines and in microtunnelling will assist the requirement in cities for environmentally friendly construction of extensive transport works, water supply and drainage, district heating, post and other utilities.

In German coal mining, development headings and drifts have amounted to annual totals of more about 100 km in rock and 400 km of coal roads in the past. As coal mining in Germany is discontinued, this will no longer be needed, although mining will maintain its significance outside Germany.

Tunnel boring machines

The development history of the first tunnel boring machine (TBM) has featured many tests that failed due to various problems, with the exception of the successful work of the Beaumont Machines in the Channel Tunnel. Sometimes the limitations of the available materials had not been considered, or else the ground to be driven through was simply not suitable for a TBM. Early applications proved successful where the ground offered ideal conditions for a TBM drive.

As early as 1851, the American Charles Wilson developed and built a tunnel boring machine, although he didn’t patent it until 1856. This machine already showed all the features of a modern TBM and can thus be described as the first tunnel boring machine; see also Chapter 3.4.

Shield machines

Tunnel builders learnt long ago to support unstable rock or loose soil with timbering followed by a masonry lining. This was also successful in rock with seepage or joint water, but working below the water table in permeable soil or particularly under open water remained impossible well into the 19^th century. The situation changed in 1806, as the ingenious engineer Sir Marc Isambard Brunel in London discovered the principle of the shield machine and later obtained a patent. The purpose of the invention was the building of a link across the Neva in St. Petersburg that could remain open in winter. As this project was not built, the machine was only developed on paper for the patent. Brunel was only able to try out his ideas in practice on the Thames Tunnel in London (see Chapter 3.4 “The Classic Shield Machines”).

1.3 Terms and descriptions

In order to describe and understand underground structures, knowledge of the most important specialist terms and descriptions is essential. As there is not always a generally accepted term from the variety of terms derived from mining, preference has been given to those that have become most widely used and most precisely describe the subject (Table 1-3).

Underground structures can be categorised according to the purpose of the completed structure:

Tunnels are extended, flat or only slightly sloping underground cavities with excavated cross-sections of over 20 m². They are mostly intended for road or rail transport. Each tunnel has two openings to the surface.

Adits, drifts or galleries are extended underground cavities, horizontal or sloping at less than 25° to the horizontal, with small diameters. They house pipes or cables or provide access and serve as auxiliary structures during the construction phase or for permanent use. They often only have one opening to the surface.

Shafts are extended, underground, vertical or inclined (more than 25° to the horizontal) cavities to overcome level differences. They serve similar purpose to adits, drifts or galleries.

Underground pipes mostly have inaccessible cross-sections. They serve to transport liquids, heat or gases and to house cables (ducts).

Caverns are underground cavities with large cross-sections and relatively short lengths. They serve for the storage of solid, liquid or gaseous goods, to house machinery and vehicles, underground generation plant, assembly halls and military facilities. They are normally connected to the surface through tunnels, adits or shafts.

Chambers are small compact underground cavities. They serve for the storage of goods during construction work or permanently.

The terms for individual parts of a structure that are normally used in underground construction are shown in Figs. 1.3 and 1.4 in cross-section and Fig. 1-5 along the tunnel. The basic terms are explained in more detail below.

Tunnel driving denotes the entirety of excavation works to advance underground cavities.

Temporary support denotes the temporary support of the excavated cavity until the complete installation of the permanent lining. The temporary support can also provide part of the structural function of the permanent lining if it is integrated. The temporary support may also be described as:

– excavation support.

– temporary lining.

– outer lining.

– primary lining.

– shoring, timbering or lagging.

The lining provides the structural support of the cavity and waterproofing measures. It may also be described as the secondary lining, inner lining or permanent support.

Installations are structures and fittings required for the operation of the completed tunnel. This can include dividing slabs and partitions, wall linings, cable ducts and channels and technical equipment.

The construction process denotes the entirety of the technical and organisational measures used for the implementation of the tunnel drive, the temporary support and the lining. The construction process is characterised by the construction method and the operational method.

The construction method is the sequence of construction activities in the excavated cross-section and refers to the division of the excavation cross-section into partial sections for excavation, temporary support and lining (see Fig. 1-1 to 1.3).

The operational method is the sequence of construction activities along the tunnel and refers to excavation and support activities, but also to supply and disposal activities along the entire length of the tunnel (see also Fig. 1-1 and Fig. 1-2).

Tunnel portals are the structure provided at the ends of a tunnel to support against slope sliding, lateral earth pressure and falling rock (Fig. 1-5). Tunnel portals should also fit the tunnel into the landscape.

Waterproofing is the description of measures to protect the structure against water ingress from outside and also to prevent the escape of liquids to the outside.

Isolation denotes measures to protect the structure, adjacent buildings and cables against unwanted electrical effects.

Support methods and materials

2.1 General

For the construction of structures in rock, it should generally be the intention to preserve the load-bearing capacity of the surrounding rock mass. Support measures, both temporary or permanent, are intended to enable, assist and favourably influence the load-bearing contribution of the surrounding rock mass, and should ideally serve to reinforce the surface of the rock mass. Support measures possess more or less support stiffness, which can be calculated. The following preconditions are necessary to preserve the load-bearing capacity of the rock mass:

– The selection of a suitable shape for the cross-section, which considers the rock mass properties encountered.

– The selection of suitable methods for construction and operation.

– The selection of a suitable support procedure.

– Consideration of the time factor for rock mass and support.

– The use of excavation methods, which are as gentle as possible to the rock mass so as to reduce its strength as little as possible.

The boundaries between temporary and final support will become blurred rather than exactly defined in the future. Future developments are trending towards single-layer construction, with the temporary support having to be integrated into the overall final support system. In order to reflect this development, support materials are considered together in this chapter, with the emphasis on a complete description of materials and stages in development history, even if some of these are no longer up-to-date. In tunnelling, which has a wealth of tradition, some old ideas have already been revived.

2.2 Action of the support materials

The structural behaviour of the support materials is determined by their deformability or stiffness, the degree of bonding between support and rock mass and the time of installation (Fig. 2-1 and Table 2-1).

Figure 2-1 Categorisation of shotcrete processes according to rock mass classes [243]

2.2.1 Stiffness and deformability

Support systems are regarded as highly stiff in bending (rigid) when they can stand up freely without affecting the rock mass and only show negligibly small deformation under load (Table 2-1). As deformations are small when highly stiff support systems are used, restoring forces in the rock mass to reduce the bending moments in the lining can not be assumed. For the loading on the lining, the prevailing vertical and horizontal ground pressures, and particularly their relationship, are of great significance. As this is however generally not known, many limit cases of loading have to be investigated. After a highly stiff support system has been installed, subsequent stress redistribution in the rock mass is prevented. Because the loading that occurs is larger than with a deformable lining, highly stiff support systems should if possible only be installed after stress redistribution has largely taken place, in order to be cost-effective. Linings that are highly stiff in bending are today mostly made of reinforced concrete and relatively thick. Considering the cost, they should only be used for the final lining.

Support systems that are stiff in bending (semi-rigid) are stable without the surrounding rock mass, but also deformable so that they can evoke structural response from the rock mass by stress redistribution. They can be used as temporary or final support, although mainly for the final support. Good bonding with the rock mass is important.

Some examples of the application of support systems that are highly stiff or stiff in bending are:

– Tunnels, which are constructed in an open excavation and subsequently backfilled (except for special linings like Armco Thyssen).

– Tunnels in heavily faulted zones.

– Tunnels with very shallow cover.

– Tunnels with weak bonding or no bonding between support and rock mass.

– All examples, where no structurally active ring can form in the surrounding rock mass.

– Low deformations and settlements.

Support systems that are weak in bending (highly deformable) are not stable under load on their own, but require the interaction with the surrounding rock mass. The support should be considered at the limit state as a reinforcement of the edge; it resists normal forces and shear forces, but no bending or only very slight bending. The normal application should be as yielding temporary support. It mostly consists today of unreinforced or reinforced concrete or shotcrete, also steel fibre shotcrete. Deformations are only permissible to such a magnitude that the assumption of restoring forces is justified. The bond with the rock mass is particularly important. The structural design should check the shear forces. The deformation capability of the lining can also be improved by deformation gaps and yielding elements (see Chap. 2.8.7) in special cases where deformation could otherwise not be controlled.

2.2.2 Bond

The bond between the support and the rock mass should ensure the transfer of radial forces over the whole area and the continuous transfer of tangential forces. Except for rare cases, the transfer of radial forces remains the most important requirement, as the shear transfer between the support measures and the rock mass is not normally assumed in calculations. If tangential forces are assumed, then the magnitude of the shear strength of rock mass and concrete has to be estimated or determined in tests. If a waterproofing membrane is installed between rock mass and support, or between temporary and final support, then no tangential forces can be transferred.

Support measures consisting of timber, steel, precast concrete elements or other assembled parts only support the rock mass at points, so there is no bonding effect. If shotcrete or pumped concrete is used with subsequent grouting (crown filling), then it can normally be assumed that there is a bond at the edge of the excavation between support and rock mass, which greatly influences the transfer of forces between rock mass and support.

2.2.3 Time of installation

If the rock mass is unstable, temporary support should be provided during or even before excavation. If the rock mass will stand up temporarily, the support is installed after excavation. In a stable rock mass, support is not generally necessary, although head protection may have to be considered. If the elastic condition of the rock mass can be largely preserved, then the support is installed early. If, however, the creation of large areas with plastic deformation in the rock mass is unavoidable, then installation after a controlled delay can lead to limited deformation of the rock mass and thus to reduced and bearable ground pressure.

Deformation of the rock mass leads to a reduction of the cavity cross-section, and this has to be taken into account in the design of the cross-section. If it is assumed that a deformable edge reinforcement increases the load-bearing behaviour of the structurally active ring in the surrounding rock mass, then it should be installed as early as possible as shotcrete or steel fibre shotcrete with high early strength. The jointed body bonding of the structurally active ring is also preserved; rock falls that could disturb the geometry of the load-bearing vault are avoided and the rock mass is protected against weathering. Immediate sealing of the exposed surface is especially important in ground susceptible to swelling.

For the time of installation, not only the support of the crown and sides is important but also the closure of the ring. This can be achieved through support measures like concrete, shotcrete, steel fibre shotcrete, precast reinforced concrete elements, steel ribs or rock bolts; or in the appropriate ground conditions by the rock mass itself. The ring closure time and the ring closure distance should be differentiated. The ring closure time is the time from the opening of the face to the installation of support measures to produce a load-bearing ring. The ring closure distance is generally the distance between the face and the location of the load-bearing ring closure. The ring closure distance is determined by operational considerations and mainly ensues from the construction method and the support materials used. For example, steel fibre shotcrete can favourably reduce the ring closure time and distance due to the reduced number of working steps and its high early strength behaviour.

2.3 Timbering

2.3.1 General

Timber is only used for temporary support and should always be removed. The original forms of longitudinal and transverse timbering are hardly ever used today. This is a result of the development of newer, more suitable support materials like steel, shotcrete, steel fibre shotcrete and rock bolts. Special cases for the application of timbering could still be: partial collapses, transition profiles and particularly as emergency support after collapses (Fig. 2-2). Timber is also suitable as a reserve material for immediate measures due to its adaptability and should be available on all tunnel sites.

Advantages:

– Any critical increase of ground pressure is visibly and audibly indicated.

– Easy to transport.

– Easily worked and adaptable.

Disadvantages:

– No bonding with the rock mass.

– Large deformations under load.

– Seldom reusable.

– The support has to be removed, making re-bracing and underpinning necessary.

– Qualified craftsmen required.

Figure 2-2 Timbering as emergency support after a collapse in the Euerwang Nord Tunnel [136]

2.3.2 Frame set timbering

As the tunnel is advanced, poling boards are pushed over the cap pieces (Fig. 2-3) or driven into the ground. The space between boards and ground is wedged or stowed with stones. If high ground pressure is expected, then a sill piece is also installed. The normal spacing of frame sets is 1.0 to 1.5 m. In squeezing ground, the spacing can be so small that the sets are directly next to each other. This method is suitable for headings with partial face excavation and trapezoidal full-face excavations up to 9 m².

Figure 2-4 Trussed timbering. Fully braced cross-section (left) and strutted frame supported at the sides

2.3.3 Trussed timbering

Trussed timbering is a multi-part truss supported from the sills, a support construction or from the tunnel sides. As truss timbering is laterally unstable, bracing is important (Fig. 2-4).

2.3.4 Shoring and lagging

If the ground is friable, then the space between the sets or ribs has to be supported. This is done with lagging or forepoling boards, which rest on timber frames or a steel construction and are pushed forward, driven forward, pressed or simply installed in contact with the ground (Fig. 2-5). When the lagging can be pushed over an already installed frame, this is described as forepoling.

Timbering provides the support with driven lagging. The basic method is still in use today although with other materials as timber.

2.4 Steel ribs

2.4.1 General

Steel has been increasingly used instead of timber since the middle of the 20^th century, since steel enables standardisation of support elements and prefabrication of the support. Steel support ribs are made of rolled profiles, U profiles, special mining profiles or composite sections, as closed arches or open at the bottom [240].

– Prefabrication is possible.

– Immediate load-bearing if in contact with the rock mass.

– Can be installed vertically or inclined according to the form of the face.

– Heavy profiles are difficult to handle.

– Poor flexibility.

– Long ordering times.

2.4.2 Profile forms

In contrast to the profiles that are commonly used in steelwork, special profiles have been developed for mining, which were than used for engineered tunnelling. Fig. 2-6 shows some types from various manufacturers; the criteria for their development were: large moment of inertia with small cross-sectional areas and weights, high strength and easy handling.

The steel quality can be DIN 21530-3 [71] type 31 Mn 4 and S235JR and S355JO according to DIN EN 10027 [76].

Figure 2-6 Steel profile shapes commonly used in German tunnelling and mining

2.4.3 Examples of typical arch forms for large and small tunnels.

The geometrical form of steel arches is determined by the intended cross-section of the excavation. The ribs are normally made to size (Fig. 2-7), although there are standardisations for mining.

For better or simpler handling, ribs are made in two or three parts. A variety of butt connections have been developed to meet different requirements. Fig. 2-8 shows, for example, rigid and yielding butt connections. The manufacturers will have to be contacted for special requirements like defined yielding or friction forces. Such problems occur in mining for the overcoming of large convergences or also in tunnelling with large-scale plastic deformation of the rock mass.

Steel ribs are anchored with special anchor clamps, or the anchors can pass directly through the steel profiles. Some examples are shown in Fig. 2-9.

Figure 2-7 Typical arch forms with open invert (top) and with closed invert (bottom)

Figure 2-9 Examples of anchoring clamps for rails , GI, SI, TH, and GP profiles

2.4.4 Installation

Steel ribs can be erected immediately after excavation of a round. Special equipment can be used to simplify installation and also permit the use of larger arch segments, which reduces the number of butted connections. Erectors for ribs are used particularly with tunnel boring machines and shield machines. A simpler way of assisting installation is the use of longitudinal beams fixed to the already installed ribs.

Sufficient longitudinal stiffening and bracing is important in order to avoid deformation, sideways buckling or movement of the ribs. The feet of the arches should also be secured with rock bolts into the rock mass or stiff connections to each other. Propping against the rock must be improved by packing or grouting in the course of further work.

Longitudinal stiffening or bolting is normally provided by a sufficient number of spacing bars, which locate the ribs to prevent movement and have to be able to resist tension and compression forces, particularly with ungrouted profiles. When welding on site is intended, it should be noted that the higher permissible stresses of tempered steel can no longer be fully exploited; the supervisory structural engineer would object to such a solution on site. It is therefore recommended to order a longitudinal bracing system together with the prefabricated steel arches. If the ribs are installed with shotcrete, then a reusable longitudinal bracing system can be used.

Figure 2-11 Structural detail of foot support with foot rails and starter bars

The advantages of immediate load-bearing capacity can only be exploited when the foot support is designed and constructed so that the loads can be transferred into the ground immediately in a suitable way. Details of the point of transfer of loading into the ground are shown in Fig. 2-10.

Contact with the rock mass is achieved by local wedging or bracing against the excavated surface, a technique used in German mining and in American tunnelling. The steel ribs are wedged behind with timber. Under load, the arch is loaded at points and only uses the advantage of continuous and immediate load-bearing around the arch to a limited extent. There is naturally no bonding effect between support and rock mass.

If settlement has to be kept very small, closed steel arches equipped with expanding presses can also be used (see Fig. 2-12).

Good contact with the rock mass is normally achieved by immediately bedding the steel rib in shotcrete. This wall-type structural action exploits the immediate contact with the rock mass and the associated bonding effect. Fig. 2-13 shows various ways of filling steel ribs with shotcrete or steel fibre shotcrete. In contrast to mining, immediate spraying is normal in European tunnelling. Shotcrete containing steel fibre reinforcement offers a simplification by reducing the number of working steps and the thickness of the layer. The load-bearing capacity of a shotcrete vault between the ribs in mostly much greater than the ribs on their own.

If unfavourable geological conditions on site mean that shotcrete alone will not provide adequate support, and an immediately load-bearing support system is required for safety reasons, steel arches are used as structural elements. The arches are erected, sprayed in and loaded by the rock mass through the shotcrete. As it hardens, the stiffness of the shotcrete increases and it resists the increasing ground pressure. Lighter ribs with low support resistance can also be used as a surveying aid and for better maintenance of the excavation profile. The concrete between the ribs then acts as an independent structural element. It serves to brace the ribs against each other and also ensures the structural connection between the ribs and the rock mass. Steel arches can be taken into account in the structural design of the outer lining (see Ril 853 [296] and ZTV-ING Part 5 [384]).

2.5 Lattice beam elements

Prefabricated reinforcement elements can be installed with shotcrete or cast concrete as temporary and permanent support. Like steel ribs, they also provide protection and assist profiling. For design purposes, they are considered as reinforcement, meaning that all DIN requirements apply and lapping bars are required at butted joints (see Fig. 2-14). Fig. 2-15 shows a detail of the construction of a lattice beam.

Figure 2-14 Lapping reinforcement at butted joints of between top heading and bench

Partial spraying-in produces a skeleton load-bearing element, and complete spraying produces a wall-type element (Fig. 2-16). The number of working steps and the wall thickness can be reduced if steel fibre-reinforced shotcrete is used. The solutions shown in Fig. 2-16 a and d are not recommended because the protruding part of the lattice beam is fouled by shotcrete and is extremely difficult to clean before the spraying of the next layer. Of the single-layer solutions, only c can be recommended, with reservations also b, as long as the layer is completed within one day before the surface of the shotcrete becomes dirty or dusty. If solution d is used, it is important that the lattice beams are not dirtied by the shotcrete.

Figure 2-16 Sprayed-in lattice beams with mesh reinforcement (the working steps shown with a star can be omitted when steel fibre shotcrete is used)

The working joint between the individual rounds should be positioned outside the lattice beam (Fig. 2-17 and Fig. 2-18). For single-layer solutions in the groundwater, it can be appropriate to provide a filter layer behind the shotcrete layer to enable grouting if these locations leak water.

Figure 2-17 Working joint in a permanent shotcrete lining with mesh reinforcement. Care should be taken that the sheets of mesh, which form the starter reinforcement for the next section, are not dirtied by the subsequent spraying of shotcrete

Figure 2-18 Working joint in a permanent shotcrete lining with steel fibre shotcrete. Care should be taken that the unreinforced shotcrete applied as a protective layer does not get into the area of the working joint

According to the profiles of the excavation and structure, it can be possible to prefabricate cross-sectional shapes. As with wall-type structural elements, the geometrical cross-section shapes are based on the intended excavation cross-section. The various profile shapes are easier to handle than full-wall profiles due to their lighter weight. Lattice beams, however, need similarly greater construction depths, which means that thin shotcrete layers can no longer be achieved.

2.6 Advance support measures

Local worsening of the rock mass properties, particularly the shortening of the stand-up time of the rock mass after excavation, may require special advance measures at the face. Advance support measures can also be used when low settlement is specified, for example when tunnelling below buildings or sensitive facilities. Depending on the project-specific local conditions, the following measures could be appropriate:

– Steel lagging sheets and plates.

– Spiles, tube spiles.

– Canopies, as pipe screen or jet grouting screen.

– Ground freezing.

The individual measures described here have been well known and used to a lesser or greater extent for many years and are increasingly and systematically being integrated into the tunnelling concept.

2.6.1 Steel lagging sheets and plates

Closed lagging with driven steel sheets, “Cologne support” (Fig. 2-19), is similar to classic tunnelling with frame sets and forepoling, except steel sheets are used instead of the former timber elements. The support arch corresponds to the cap piece of the frame set; it is only used for temporary support, as is the head support, which advances with the excavation. One variant of this method is the driving of the sheets over only one field, which however tends to break the spiles or sheets if they are not driven far enough into the unexcavated ground.

In non-cohesive soils, the soil tends to run out, and cavities tend to be created particularly at the sheet overlaps. Mortar filling behind the driven sheets has been found useful for the reduction of settlement (Fig. 2-20).

Figure 2-20 Mortar filling behind steel sheets (Stadtbahn Mülheim, contract 7, 1978) [90]

Structure	Examples	Purposes
Tunnels	Rail tunnels, Underground rail tunnels, road tunnels, canal tunnels	to provide transport routes
Small tunnels	Main structures Unpressurised tunnels, pressure tunnels, siphons Access tunnels	transport of drainage water, drinking water and service water all-year access to caverns with avalanche protection supply of fresh air to underground cavities
	Ventilation tunnels Auxiliary structures Grouting tunnels Pilot tunnels Viewing tunnels Adits	access for grouting works investigation of geological conditions surveying of underground structures to provide an additional starting point
Shafts	Main structures Inclined and vertical shafts	transport of drinking water and drainage pressure relief (surge) in hydropower stations supply of fresh air to underground cavities transport of personnel and material
	Auxiliary structures Mucking shaft Surveying shaft	transport of excavated material surveying
Pipelines	Sewers and drains Water supply pipes District heating pipes Gas supply pipes Oil pipelines Cable ducts	transport of goods, energy or news
Caverns	Industrial caverns Storage caverns Protection caverns	to house power station turbines or assembly halls storage of goods provision of underground shelters for the population in case of air raids or military bunkers
Chambers	Storage chambers Explosive chambers	storage of goods storage of explosives during the construction of a tunnel