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Theme lecture: Bored tunnelling in the urban environment Exposé sur le thème: T

Theme lecture: Bored tunnelling in the urban environment Exposé sur le thème: Tunnels forés dans un environnement urbain R. J. Mair - Geotechnical Consulting Group, London, UK R. N.Taylor - City University, London, UK ABSTRACT: The Report reviews the state-of-the-art and recent developments in geotechnical aspects of bored tunnelling in the urban environment. Various advances in tunnel construction techniques are highlighted, notably the increasing use of slurry and earth pressure balance shields. Methods of calculating the stability of tunnels are reviewed. In the context of tunnelling in the urban environment, particular emphasis is given in the Report to ground movements associated with tunnel construction, their modelling and prediction, and their effects on buildings. Reference is made to case histories of tunnels constructed in a wide variety of ground conditions. Ground loading acting on tunnel linings is discussed. The Report focuses on three areas in which there have been significant developments in ground treatment in recent years: face reinforcement, slurry and earth pressure balance shield technology, and compensation grouting. RESUME: Ce rapport examine l’état de l’art et les dévelopments récents concernant les aspects géotechniques des tunnels creusés en milieu urbain. Un certain nombre de progrès dans les techniques de construction sont illustrés, notamment l’augmentation de l’utilisation des boucliers à pression de boue et des boucliers à contrepression de terre. Les différentes méthodes de calcul de stabilité des tunnels sont examinées. Dans le contexte du creusement de tunnel en milieu urbain, une attention toute particulière est accordée dans ce rapport aux mouvements du sol associés à la construction du tunnel, leur modélisation, leur prédiction et leurs effets sur les bâtiments. Il est fait référence à plusieurs projets de tunnels construits dans une grande variété de type de terrain. Le chargement sur le soutènement du tunnel par le sol est traité. Le rapport se concentre sur trois domaines du traitement des sols dans lesquels des développements significatifs ont eu lieu au cours des dernières années: le renforcement du front, la technique des boucliers à pression de boue et des boucliers à contrepression de terre, et les injections de compensation. 1. INTRODUCTION The title of Plenary Session 4 of this Conference, Underground Work s in the Urban Environment, principally involves bored tunnel construction and deep excavations. This Report focuses only on bored tunnel construction, primarily because Professor Nussbaumer in his Report for Plenary Session 3 has addressed the subject of Retaining Structures and Excavated Slopes, and the associated Discussion Sessions have been concerned with the subject of deep excavations. It should be made clear that the term “bored” tunnel construction means any k ind of mined tunnel, as distinct from cut- and-cover tunnels. At the 7lh ICSMFE in Mexico in 1969, Professor Ralph Peck presented his seminal state-of-the-art paper on Deep Excavations and Tunnelling in Soft Ground. Since then there have been significant developments in the theory and practice of geotechnical engineering applied to underground construction, and there have been various major review papers devoted to the subject (Peck , 1969; Cording and Hansmire, 1975; Clough and Schmidt, 1981; Ward and Pender, 1981; O’Reilly and New, 1982; Schlosser et al, 1985; Attewell et al, 1986; Konda, 1987; Rank in, 1988; Uriel and Sagaseta, 1989; Clough and Leca, 1989; Cording, 1991; Fujita, 1989, 1994). There have also been various International Tunnelling Association Conferences (held annually) and National Tunnelling Conferences. In particular there have been two recent Symposia organized by ISSMFE Technical Committee 28: the first in New Delhi in 1994 and the second in London in 1996 (Fujita and Kusak abe, 1995; Mair and Taylor, 1996). The topics addressed in this Report are: • Advances in tunnel construction techniques • Principal design and construction requirements • Stability • Ground movements • Modelling and prediction of ground movements • Effects of ground movements on buildings • Ground loading on tunnel linings • Developments in ground treatment All of these topics are of particular importance in the context of tunnelling in the urban environment. 2 ADVANCES IN TUNNEL CONSTRUCTION TECHNIQUES In recent years there have been considerable advances in techniques of bored tunnel construction in soft ground. It is convenient to highlight some of these in terms of open face and closed face tunnelling. Open face tunnelling covers all cases where there is easy access to the tunnel face, in contrast to closed face tunnelling. (a) Open Face Tunnelling There is increasing use of sprayed concrete in soft ground tunnelling to form linings, particularly for tunnels of shorter lengths and of non-circular cross-section. These are usually temporary, but may be the sole means of support for significant periods (sometimes more than a year) before the permanent lining is installed. Recent developments have involved composite sprayed concrete linings, in which sprayed concrete is used for both the temporary and permanent linings (e.g. Wittk e, 1995; Grose and Eddie, 1996; Negro et al, 1996). Non-circular cross-sections and divided faces are often adopted when using sprayed concrete, as shown in Figure 1, and this allows considerable flexibility in terms of modifying the construction sequence in response to observations. The use of sprayed concrete to form linings is sometimes referred to as the New Austrian Tunnelling Method (NATM). Ground treatment is more easily undertak en from within tunnels with open faces. Advances have been made in reinforcement of the 2353 Figure 1. Example of divided tunnel face using sprayed concrete linings (Institution of Civil Engineers, 1996) soil ahead of the face to improve stability and to control ground movements (Figure 2a). Improvements in jet grouting techniques are being made to form “umbrella arches” (Figure 2b) as a pre lining in difficult ground conditions. An extension of the concept of the umbrella-arch is the pre-‘ vault, developed in France and Italy, which is sometimes referred to as the mechanical pre-cutting method (Cazenave and Le Goer, 1996). This involves the cutting of overlapping slots around the tunnel periphery in advance of the excavation, and filling them by means of sprayed concrete (Figure 2c). The technique is often used in conjunction with face reinforcement and other forms of ground treatment. (b) Closed Face Tunnelling Considerable advances have been made over the last decade in the use of sophisticated closed face tunnelling machines which operate on the principle of a pressurized face. These machines are used in unstable ground conditions where the face requires support at all times; this principally applies to permeable ground below the water table (i.e. mainly sands or mixtures of sands, silts and clays) or soft clays. The slurry shield machine, illustrated in Figure 3, is most commonly used in water bearing granular soils. The face is supported by a pressurized bentonite or polymer based slurry, which is circulated so that it and the excavated soil are removed to a separation plant. Earth pressure balance (EPB) machines are being used more F (a) Face reinforcement (b) Jet grouting “umbrella arches” (c) Pre-vault (pre-cutting) Figure 2. Ground treatment and pre-lining techniques (after Schlosser and Guilloux, 1995) universally for all types of unstable ground; the principle is shown in Figure 4. By controlling the entry of soil and water through the cutter face by means of earth pressure balance doors, and by conditioning the spoil so that it can easily be removed through a screw conveyor, it is possible to control the pressure of the excavated soil in the chamber to balance the earth and water pressures in the ground. Recent developments have centred around the injection of special slurries, foams and other materials in EPB machines to improve the properties of the excavated soil and facilitate the proper control of the pressure in the chamber. This is essential for the control of face stability and for minimising ground movements. In the case of slurry shields, more is now understood about the factors affecting the efficiency of the slurry support of the tunnel face. These are discussed further in Section 4. Cutter driving ■ moter Tail seal Screw conveyor driving motor Belt conveyor Gate jack Bulkhead \ Erector Segments Cutter face Cutter frame Shield jacks Figure 4. Principle of the earth pressure balance machine (Fujita, 1989) 3. PRINCIPAL DESIGN REQUIREMENTS AND CONSTRUCTION As presented by Peck (1969), and developed by Ward and Pender (1981), the three most important requirements for the successful design and construction of a tunnel can be summarised as follows: (i) Stability The choice of excavation and construction technique must be suited to the ground conditions so that it is feasible to build the tunnel safely. Of prime importance is the stability of the opening prior to installation of the lining. Cutter driving Erector driving motor \ motor Tail seal Slurry supply Slurry return Erector / Agitator bnieia \ Cutter face Segments Figure 3. Principle of the slurry shield machine (Fujita, 1989) 2354 (ii) Ground movements and their effects Of particular relevance to the urban environment, construction of the tunnel should not cause unacceptable damage to surrounding or overlying structures and services. Prior to construction the ground movements should be predicted and their effects on the structures and services assessed. uploads/Ingenierie_Lourd/ bored-tunnelling-in-the-urban-environment-theme-lecture-by-mair-amp-taylor.pdf