Institute for Geotechnical EngineeringTunnelling and Rock Engineering Group

Kalman Kovári 1 & Marco Ramoni 2 1 Consulting Engineer, Oberengstringen (Zurich), Switzerland

2 Institute for Geotechnical Engineering, ETH Zurich, Switzerland

"Urban tunnelling in soft ground using TBMs" Key Note Lecture

International conference and exhibition on tunnelling and trenchless technology "Tunnelling and trenchless technology in the 21st century"

Sheraton Subang Hotel & Towers, Subang Jaya – Selangor Darul Ehsan, Malaysia 7-9 March 2006

Reference: Kovári, K. & Ramoni, M. (2006): "Urban tunnelling in soft ground using TBMs"; Tunnelling and trenchless technology in the 21st century; International conference and exhibition on tunnelling and trenchless technology, Subang Jaya – Selangor Darul Ehsan; 17-31; The Institution of Engineers, Malaysia.

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Urban tunnelling in soft ground using TBMs Kalman Kovári 1 and Marco Ramoni 2 1 Consulting Engineer, Oberengstringen (Zurich), Switzerland 2 Institute for Geotechnical Engineering, ETH Zurich, Switzerland Abstract The increasing demand world-wide for shallow tunnels in urban areas even with com-plex geotechnical conditions and under highly built-up areas has enhanced the rapid technological development of the last decade. The excavation diameter has already ex-ceeded 15 m. This paper deals in a unifying manner with the key features of this par-ticular field of modern tunnelling: the elements of urban environment, typical ground conditions with the decisive role of ground water and the technology of face support. Particular attention is given to risk management discussing typical patterns of ground failure, the safety plan, the constructional measures and the monitoring of ground be-haviour and of surface structures. Recent case histories illustrate the various problems which can arise in difficult situations and the usefulness of systematic risk manage-ment. 1 The urban environment The need for urban tunnels is growing rapidly world-wide. Frequently, nowadays, low overburden is combined with a large diameter. Figure 1 shows the breakthrough of the recently completed Zimmerberg Railway Tunnel in the heart of the city of Zurich with a bore diameter of 12.3 m (Kovári and Bosshard, 2003). The EPB shield with the larg-est diameter of 15.2 m is presently in operation for a road tunnel in Madrid beneath a highly built-up area (Herrenknecht and Bäppler, 2005). Generally, structures of various types have to be under-tunnelled. These may be build-ings, roads, railroads, bridges, etc. The sensitivity of these structures to ground settle-ments as well as the potential damage due to ground collapse may vary within ex-tremely wide ranges. The presence of frequently hidden foreign objects in the ground is also one of the specific features of urban tunnelling. These include old wells, ground anchors, sheet-piles, abandoned utilities such as for gas and sewage, but also tree trunks, artificial fills, etc. In this specific field of tunnelling there are generally major constraints in the selection of the horizontal and vertical alignment. As a rule it is generally preferable to run the tunnel in public ground under main roads or streets. However, underpassing buildings,

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roads and other structures is often unavoidable. Also the serious restrictions to be over-come when selecting places of attack and planning material transport from and to the site are of great practical importance. Such restrictions also apply to sinking drill holes for ground exploration, for groundwater control or ground consolidation. There are two major risk scenarios in urban tunnelling as shown in Figure 2, i.e. col-lapse up to the ground surface and damage due to ground settlements. In urban areas damage to buildings and roads has a high visibility. Risk aversion is very pronounced, which may lead to strong opposition to further underground projects in towns or even elsewhere. But the loss of public confidence in the technology is also very damaging (Figure 3).

b)

a)

Figure 1 Breakthrough of the Zimmer-berg Railway Tunnel, May 7th 2001(Kovári and Bosshard, 2003)

Figure 2 Major risk scenarios in urban tunneling: a) Collapse up to the ground surface b) Damage due to ground set-tlements

sandclay

boulders

rock karst

filling

gravel

Figure 3 Collapse up to the ground sur-face in built-up area (Kovári andBosshard, 2003)

Figure 4 Typical ground conditions in urban tunnelling

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2 Ground conditions Ground conditions in urban tunnelling also have some important common features. In most cases near the ground surface recent weak geological formations are often en-countered. There are frequently changing conditions due to the presence of lenses, lay-ers, boulders, etc. and in rock open joints and in limestone karst formations. A water table situated above the tunnel or crossing the tunnel profile requires particular atten-tion (Figure 4). In soil the grain size distribution is one of the decisive factors when selecting the type of TBM shield to be applied. The preferred ranges of application for slurry and EPB shields are shown in the grain size distribution diagrams. Apart from the grain size distribution the shear strength of the material on a large scale is very important. In clay the undrained cohesion cu and in sand and gravel the drained cohesion c is of interest. Because of the very low stress level in the ground even small values of cohesion are generally decisive in investigating the stability of the working face. In any case, unfortunately we have no means of determining such low values of cohesion based on exploratory boreholes and laboratory tests. As a rule one assumes the extreme case of zero cohesion. In the presence of groundwater the permeability of the ground is of great practical im-portance. One has to know whether the permeability of the ground is low or high. If there is any relevant heterogeneity the ground profile showing the individual forma-tions with the various values of permeability should be investigated (Figure 5). The pie-

a) b)

permeabilitylow/high?

Figure 5 Practical interest of ground permeability: a) Magnitude b) Heterogeneity

b)excavated soil

c)compressed air

a)bentonite

Figure 6 Face support by closed shield tunnelling a) Slurry (slurry shield) b) Excavated soil (EPB shield) c) Compressed air

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zometric head with respect to the tunnel is an important factor for the stability of the working face as well as for a successful permeability reduction of the ground. Also the required air pressure during maintenance in the working chamber strongly depends on the piezometric head in the ground. For the sake of completeness we also mention the influence of the mineral content, which may determine the degree of wear of the tools or may even lead to damage of the cutter head. 3 TBM Technology The three ways of supporting the face are slurry, excavated soil and compressed air (Figure 6). The extraction of the muck is carried out in the first case through a pipe and in the two other cases by a screw conveyor. Slurry shields are generally provided with a rock crusher at the entrance of the sucking pipe. 3.1 Face support Any study of the stability of the tunnel face begins with the assumption of a simplified failure mechanism in the ground ahead of the face. The three-dimensional model shown

a)

b)

Figure 7 Assumed three-dimensionalfailure mechanism (Horn, 1961)

Figure 8 Face support by slurry shield a) Stabilizing effect of slurry b) Loss of effective support force due to suspension infiltration (Anagnostou and Kovári, 1994)

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in Figure 7 (Horn, 1961) is close to reality and is simple to handle. It consists of a wedge and a prism in a state of limit equilibrium. The model is basically an extension of Janssens's silo theory to three dimensions. The mobilized shear strength along the failure surfaces is one of the key factors in the statical calculations. 3.2 Slurry shield In Figure 8a the stabilizing effect of the slurry at the face is demonstrated. Due to the excess pressure p a so called filter cake is formed on the surface of the face having the function of a membrane. It prevents the infiltration of the slurry into the ground. The formation of the filter cake requires sufficient fines in the ground and, depending on the applied excess pressure p, a critical limit of the permeability of the soil. If the slurry is allowed to infiltrate into the ground its supporting effect rapidly dimin-ishes with increasing distance (Anagnostou and Kovári, 1994). The above shown wedge/prism model (Figure 7) allows the reduction of the support force to be calcu-lated. For a given set of parameters, if the penetration e of the slurry (Figure 8b) reaches half of the tunnel diameter D the value of the supporting force S falls to ap-proximately 40 % of its initial value S0. Appropriate conditioning of the suspension and reduction of excess pressure may prevent breaking through the filter cake or considera-bly reduce the infiltration of the slurry into the ground.

Figure 9 Face support by EPB shield:effective support pressure s' at the inter-face between muck and ground

Figure 10 Face support by EPB shield: influence of the piezometric head h in the working chamber on the effective support pressure s' (factor of safety 2.0)

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3.3 EPB shield Face stabilization with EPB shield (Anagnostou and Kovári, 1996) relies on the effec-tive grain to grain pressure (effective stress s') at the interface between muck and ground as well as on the control of the piezometric head in the working chamber (Fig-ure 9). In the case of a difference between the head in the chamber and the undisturbed water table seepage into the chamber will occur. In order to calculate the seepage forces di-rected towards the tunnel face a numerical flow analysis has to be carried out using the Finite Element Method. Such an analysis yields a three-dimensional steady-state poten-tial field. Darcy's law is assumed for modelling seepage flow. The destabilizing effect of the seepage forces acting on the wedge in Figure 7 is clearly seen from such calcula-tions. The considerable influence of the piezometric head h in the working chamber is illus-trated in Figure 10. For a tunnel with a diameter of 10 m, an overburden of 20 m and for the selected shear strength parameters (c and ) a decrease of the necessary effec-tive support pressure s' with increasing piezometric head h results. For a piezometric head h = 10 m the effective support pressure s' will be 200 kPa. For h = 30 m – i.e. in case of hydraulic equilibrium – practically no effective pressure is needed. The calcula-tions were carried out with a factor of safety of 2.0. In practice low values of the sup-port pressure s' are preferable – as shown in the diagram. 4 Risk management The greatest concern in urban tunnelling is a face collapse reaching up to the ground surface. In the case of a slurry shield the collapsed ground fills up the working chamber and the slurry rises to take its place. Figure 11 shows such a crater filled with slurry – fortunately not under urban conditions. The typical patterns of ground failure are shown schematically in Figure 12. Such types of collapse can also occur with the use of an EPB shield when inadvertently more material is extracted than the corresponding theo-retical excavated volume of the tunnel. The created opening (Figure 12b) – if it remains unfilled – may collapse after some time – possibly resulting in the formation of a crater at the ground surface.

b)a)

Figure 11 Collapse up to the ground surface in an undeveloped area; craterfilled with slurry (Kovári and Bosshard,2003)

Figure 12 Typical mechanisms of ground failure: a) Collapse up to the ground surface b) Instability of the working face

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In order to reduce the risk associated with such mechanisms, systematic risk management has to be carried out. Risk management requires a methodical and well-structured procedure. For this pur-pose the safety plan is the best instru-ment. Basically, it involves a visualisa-tion of the objects it deals with, whereby for a clear-cut system or sub-system the facts, assumptions, scientific knowledge, operational instructions, etc., are pre-sented in a plan – as shown schemati-cally in Figure 13. The first step is to define the tunnel sec-tion that should be subjected to a risk analysis by specifying the kilometre dis-tances Tm X and Tm Y. The system can involve a whole construction lot or just a part of it, as for example when passing under a building, bridge foundation or

important traffic arteries. The data to be represented consists of the topography, the structures at the ground surface, the geology, the groundwater conditions, foreign ob-jects located within the design tunnel profile, exploratory boreholes and specific as-pects of the construction method. As shown in Figure 13, apart from the category of undesired events (risk scenarios), all possible triggering mechanisms and finally the planned countermeasures with the associated separate documents are presented. The safety plan has to be constantly updated according to the actual state of knowledge; therefore it is valid only for a given time period. The two main types of measure to reduce risk are constructional and monitoring. De-pending on the nature of the specific problems under consideration additional measures can also be applied. 4.1 Constructional measures One of the most important constructional measures to reduce risk consists of grouting operations. They aim at increasing strength and stiffness and/or reducing the permeabil-ity of the ground. A typical example is the grouted body in the roof area of the tunnel. If the slurry or EPB shield over a long stretch has to underpass a highly urbanised area with difficult geotechnical conditions, stopping the TBM for maintenance purposes is generally inevitable. In such cases it is advantageous to prepare at predetermined loca-tions and in well in advance one or more stations. These can provide safe conditions even without compressed air for quick maintenance work in the chamber. Such stations may also be important for avoiding a time delay in the completion of the tunnel. In some geological formations loosened blocks between the cutter head and the face may present a major problem. The cutter head tools may be damaged and the face over-excavated leading to local instability. Even minimum grouting operations to keep the blocks in place during cutting by the disks generally provide a satisfactory solution. In

Figure 13 Structure of a safety plan inurban tunnelling

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Figure 14 such blocks are schematically represented and also a photo is shown with a boulder blocked between the disc cutter and the face of the tunnel. Apart from damag-ing the tools such blocks may cause major delays due to the necessary work in the chamber – frequently under compressed air. In some situations only constructional measures involving underground structures al-low an adequate reduction of risk. An example is the umbrella over the roof of a large diameter tunnel made by pipe jacking a series of tubes filled with reinforced concrete. Such a solution is mandatory if with such a small overburden the execution of a grouted body in the roof becomes impossible. 4.2 Monitoring Systematic monitoring of ground deformations including surface settlements, the be-haviour of nearby urban structures, the groundwater table and machine performance form an integral part of the safety plan. In Figure 15 the most frequently monitored physical quantities are represented schematically. The results of field measurements can be used to assess structural behaviour with re-spect to safety and/or serviceability requirements. In such cases, the determination of acceptable behaviour should be evaluated in combination with other observations in or-der to decide whether corrective measures are necessary or not. Such procedures can obviously be applied only for the case of ductile structural behaviour. Decision-making based upon measurements is impossible when the structural behaviour is brittle (e.g. tunnel face instabilities), since the prediction of deformation values close to collapse is highly unreliable. Possible design optimization or corrective measures during construction necessitate ap-propriate contractual regulations. Depending on the predicted mechanism it must be established whether point-wise measurement is adequate or whether measurements along profiles or over surfaces are necessary. For the definite location of instruments the zones of primary concern and critical areas where additional instrumentation may be required to obtain meaningful results should be identified. The layout and spacing between instrumentation arrays de-

Figure 14 Boulder blocked between cutter head and tunnel face (Kovári and Bosshard, 2003)

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pends on factors such a stratigraphy, level of detail and degree of redundancy required. The latter is recommended in order to check the proper working of the monitoring sys-tem. The plan of data acquisition includes details of frequency of readings, data transmission and data storage. Readings may be taken at intervals or continuously (in real time), de-pending on a specific construction stages or time events. Regarding the management of the monitoring programme some guidelines need to be mentioned. In contract documentation, responsibilities for installation and commission-ing, calibration, provision of baseline data, monitoring, information flow, data interpre-tation and reporting must be clearly defined. It is often advantageous to appoint an in-dependent monitoring contractor who carries out the monitoring work and, on a real-time basis, delivers the results to all parties involved in the construction of the tunnel project (client, designer, site agent if present, contractor, etc.). The evaluation of the monitoring results should also be done on a real-time basis by an appropriate experi-enced party (either the client with his consultant or the designer of the detailed stage design of the tunnel). The contract conditions should provide the necessary empower-ment to require immediate stabilizing measures according to the results of monitoring. 4.3 Visualization of risk management In Figure 16 a visual representation is given of how different measures can produce a reduction of the risk for a failure of the working face and its possible propagation up to

Figure 15 Most frequently monitoredphysical quantities

Figure 16 Reducing the risk of collapse up to the ground surface

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the ground surface. According to its definition, risk has two components: probability of occurrence w and amount of damage D. In a quantitative appraisal the product of these two factors defines the risk: R = w x D. In the graph the case is discussed of an urban tunnel with the possibility of face failure and a pos-sible triggered collapse up to the ground surface. The probability of occurrence of these undesired events is assumed to be the same. But, as seen from the graph, the associated amounts of damage are different. In order to reduce the prob-ability of both events the support of the working face might be improved by a better conditioning of the slurry or the muck in the chamber (1). If the corre-sponding decrease in risk is insufficient,

one can also create a grouted body in the roof of the tunnel resulting in a greatly re-duced probability of ground failure up to the surface (2). If a further risk reduction is required the possible amount of damage can be reduced by closing roads or/and evacu-ating buildings (3). The corresponding point in the graph indicates the accepted risk. 5 Grouted body After giving a general overview of urban tunnelling in soft ground using TBMs we turn now to the discussion of key aspects of grouted bodies. We have seen that the grouted body in many cases forms an integral part of a TBM drive with EPB and slurry shields. Firstly, the issue of planning and execution will be discussed followed by design con-siderations. The shape of the grouted body in the cross-section of the tunnel may be as shown in Figure 17. In a ground with a sufficient average cohesion (considered on the scale of

a) b)

Figure 18 Statical action of grouted body: a) Transverse direction b) Longitudinal di-rection

Figure 17 Possible forms of the groutedbody

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the cross-section of the tunnel) but with locally and erratically occurring inclu-sions of cohesionless materials, minimal grouting in the roof area is generally suf-ficient (Figure 17, case 1). In the ab-sence of ground water in many cases such a measure allows one to work in an open mode with an EPB shield or with low excess pressure with a slurry ma-chine. Such a consolidated body will have neither a well defined shape nor a prescribed minimum cohesion. There-fore, such measures are not a suitable object of statical calculations. In the other cases (Figure 17, cases 2-5) a well defined shape and size of the grouted body with clearly defined shear strength parameters is aimed at. The most impor-tant and most frequently applied type of body is represented by cases 2 and 3. For practical reasons case 2 is generally preferred. Grouted bodies according to cases 4 and 5 are extremely work-intensive and costly. In addition, from

the statical point of view they can hardly be justified. Case 6 in Figure 17 shows the above mentioned stations for the planned maintenance work in the chamber. For practical reasons systematic grouting operations are generally carried out from out-side the TBM. Drilling and grouting from the ground surface does not require special

p (bar)

c (kPa)

Figure 20 Effect of grouted body on the necessary support pressure p acting at the face for a given set of parameters including factor of safety (SF) in function of the cohesion c of the untreated ground

Figure 19 Relationship between geo-metrical parameters (b, d, h), load q and uniaxial strength c of the grouted body

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measures against groundwater pressure as in other cases (from the tunnel, an auxiliary gallery, a pit, etc.). The most important factors in the plan-ning and execution of grouting opera-tions are the arrangement of the drill holes with the distance between the valves along it as well the composition of the grout, the applied pressure and the grouted volume per valve. Depending on the nature of the ground and the required mechanical properties (strength and/or permeability) in many cases several suc-cessive grouting operations from the same valve must be carried out. In order to achieve the minimum re-quired shear strength and/or permeabil-ity as well as a satisfactory homogeneity of the grouted body it is necessary to apply advanced methods of data acquisi-tion and data processing. The visualiza-tion of the absorbed grout quantity within the planned grouted body is of particular interest. In this way the results

of data monitoring may reveal areas with a need for a further grouting campaign until the required density of grout is achieved. 5.1 Statical considerations A grouted body above the tunnel (Figure 18) acts statically both in the lateral and longi-tudinal directions. Within the slab virtual arches can be assumed provided the material is stressed to its limit state. Thus, under a uniformly distributed vertical load the state of stress in such an arch is defined by the uniaxial strength of the grouted body. For an arch under these conditions simple relations exist between the geometrical parameters b, d and h (Figure 19) as well as the magnitude of the load q and the uniaxial strength c of the body. The formula can easily be obtained by assuming that the bending mo-ments are zero at the supports and in the centre of the arch. It shows that the load bear-ing capacity q* is proportional to the height h, thickness d and uniaxial strength c of the body and inversely proportional to the square of the span b. The factor of safety SF can be defined as the ratio of the maximum possible load q* to the effective load qeff. 5.2 Support pressure / grouted body Consider now the effect of the grouted body on the necessary support pressure p (Fig-ure 20) acting on the working face for a given set of parameters in function of the cohe-sion c of the untreated ground. As can be seen from the graph in Figure 20, for a cohe-sionless soil the presence of the grouted body reduces the necessary support pressure p by a factor 2. This is due to the fact that in the case of the grouted body there is no ver-tical load q acting on the wedge.

SF (-)

c (kPa)

Figure 21 Factor of safety SF for thestability of the face as a function of thecohesion c of the untreated ground for a selected value of the support pressure p acting on the face

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In Figure 21, for the same set of parameters but for a selected value of the support pres-sure p the factor of safety SF is given as a function of the ground cohesion c. Thanks to the grouted body even an extremely small value of the cohesion c suffices to obtain for the factor of safety SF the value 1.5. In the case of an EPB machine above the water ta-ble and a small value of ground cohesion and with no important objects to be under-passed an open mode of operation can be employed. 5.3 Advantages of grouted body The major factors favouring the application of a grouted body are: - high safety against collapse; - reduced ground settlements; - safety during work in the chamber; - keeping to time schedule. In a given case the decision must firstly take into consideration the risks involved and possibly the importance of the time schedule to be fulfilled. One of the major results which has emerged from recent tunnelling experience is that in some types of ground – despite advanced techniques of conditioning – only a grouted body makes TBM drives possible, for example in sandy gravel with almost no fines. It must also be kept in mind that with a grouted body any work necessary in the chamber can be carried out more quickly. From the application of a grouted body also various operational benefits may result. First of all the support pressure can be considerably reduced. As a result little or no in-filtration of the suspension into the ground is expected when using a slurry shield. In the case of an EPB shield less conditioning is needed and tool wear and the required torque are also reduced. Furthermore, better handling of muck is permitted. 6 Conclusions The main features permitting safe and economic tunnelling in soft ground under urban conditions using TBMs with slurry or EPB type of face support can be summarised as follows: First of all, an efficient TBM technology even for large diameter tunnels and complex geotechnical conditions is nowadays available. Improved methods of condi-tioning the suspension or the excavated ground in the EPB chamber are available to the engineer. The same is true regarding advanced grouting technology. The design proce-dure including statical calculations for the determination of the necessary support pres-sure or the shape, size and quality of a grouted body has a high reliability. Finally the advantages of a systematic risk management must be mentioned. References Anagnostou, G. and Kovári, K. (1994): "The face stability of slurry-shield-driven tun-

nels". Tunnelling and Underground Space Technology 9 (1994) No. 2, 165-174, Elsevier Science Ltd. Oxford.

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Anagnostou, G. and Kovári, K. (1996): "Face stability conditions with Earth-Pressure-Balanced shields". Tunnelling and Underground Space Technology 11 (1996) No. 2, 165-173, Elsevier Science Ltd. Oxford.

Herrenknecht, M. and Bäppler, K. (2005): "Risikobeherrschung hinsichtlich Durch-messergrösse sowie Sicherheits- und Logistiklösungen bei maschinellen Tunnel-vortrieben in städtischen Bereichen". Tunnel – Neue Wege – Neue Chancen, STUVA-Tagung '05 Leipzig, Forschung+Praxis 41, 65-69, Bauverlag BV GmbH Gütersloh.

Horn, M. (1961): "Alagutak homlokbiztositására ható vizszintes földnyomásvizsgálat néhány eredménye". Az országos mélyépitóipari konferencia elöadásai, Közleke-dési Dokumentációs Vállalat Budapest (in Hungarian). See also "Horizontaler Erddruck auf senkrechte Abschlussflächen von Tunneln". Landeskonferenz der ungarischen Tiefbauindustrie Budapest (German translation, STUVA Düssel-dorf).

Kovári, K. and Bosshard, M. (2003): "Risks in tunnelling: Analysis and procedures re-lating to the Zimmerberg Base Tunnel". Tunnel 6/2003, 10-31, Bertelsmann Fachzeitschriften GmbH Gütersloh.

Address of the Authors 1 Prof. Dr. Kalman Kovári, Consulting Engineer, Fabrikstrasse 4, CH-8120 Obereng-

stringen 2 dipl. civ. eng. ETH/SIA Marco Ramoni, ETH Zurich, Institute for Geotechnical En-

gineering, Tunnelling and Rock Engineering Research Group, HIL D14.3, POB 133, Wolfgang-Pauli-Strasse 15, CH-8093 Zurich