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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Appendix D - Tunnel Boring Machines
A Tunnel Boring Machine (TBM) is a complex system with a main body and other supporting elements to be made up of mechanisms for cutting, shoving, steering, gripping, shielding, exploratory drilling, ground control and support, lining erection, spoil (muck) removal, ventilation and power supply. Figure 6-11 shows a general classification of various types of tunnel boring machines for hard rock and soft ground.
Figure D-1 Classification of Tunnel Boring Machines (Figure 6-11)
This Appendix is intended to demonstrate the components and excavation sequences of common types of tunnel boring machines (TBM) applicable for hard rock and soft ground conditions. The Principal Investigators appreciate Karin Bäppler and Michael Haßler of Herrenknecht AG (Herrenknecht), and Lok Home of The Robbins Company (Robbins) for generously providing excellent illustrations, and photographs and information for large-diameter TBM applications.
D.2 Hard Rock TBM
As shown in Figure 6-11 above, tunnel boring machines (TBM) suitable for rock tunneling nowadays are full-face, rotational (types of cutter head) excavation machines and can be generally classified into two general categories: Gripper and Segment based on the machine reaction force. Three common types of hard rock TBMs are described hereafter:
D.2.1 Open Gripper Main Beam TBM
The open gripper-beam category of TBMs is suited for stable to friable rock with occasional fractured zones and controllable groundwater inflows. Figure D-2 (Robbins) illustrates a typical diagram of a modern open gripper main beam TBM and highlights the major components including:
Figure D-2 Typical Diagram for an Open Gripper Main Beam TBM (Robbins).
The front of the gripper TBM is a rotating cutterhead that matches the diameter of the tunnel (Figure D-3). The cutterhead holds disc cutters. As the cutterhead turns, hydraulic propel cylinders push the cutters into the rock. The transfer of this high thrust through the rolling disc cutters creates fractures in the rock causing chips to break away from the tunnel face (Figure 6-9). A floating gripper system pushes on the sidewalls and is locked in place while the propel cylinders extend, allowing the main beam to advance the TBM. The machine can be continuously steered while gripper shoes push on the sidewalls to react the machine's forward thrust. Buckets in the rotating cutterhead scoop up and deposit the muck on to a belt conveyor inside the main beam. The muck is then transferred to the rear of the machine for removal from the tunnel. At the end of a stroke the rear legs of the machine are lowered, the grippers and propel cylinders are retracted. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring begins again.
Figure D-3 Herrenknecht S-210 Gripper TBM (Herrenknecht)
Figure D-3 shows the front of the Herrenknecht S-210 Gripper TBM used in the construction for the Gotthard Base Tunnel, Switzerland. See Table D-1 for more data about the machine (Herrenknecht). Although uncommon, hard rock gripper TBMs with a diameter over 46' (145m) have been made, and this limit is constantly being challenged and extended for new mega projects.
D.2.2 Single Shield TBM
Notes: (1) Shield; (2) thrust cylinders; (3) segmental lining; (4) cutterhead; (5) muck bucket; and (6) conveyers
Figure D-4 Typical Diagram of Single Shield TBM (Herrenknecht)
As shown in Figure D-4, the Single Shield TBMs are fitted with an open shield (unpressurized face) to cope with more brittle rock formations or soft rock. The TBM is protected by the shield (1), and extended and driven forward by means of hydraulic thrust cylinders (2) on the last completed segment ring (3). The rotating cutterhead (4) is fitted with hard rock disk cutters, which roll across the tunnel face, cutting notches in it, and subsequently dislodging large chips of rock (Figuer 6-9). Muck bucket (5), which are positioned at some distance behind the disks, carry the dislodged rock pieces behind the cutterhead. The excavated material is brought to the surface by conveyers (6).
Figure D-5 illustrates a simplified cross section of Single Shield TBM.
Figure D-5 Typical Diagram for Single Shield TBM (Robbins)
Figure D-6 above shows the cutterhead of the Herrenknecht S-256 Single Shield TBM used in the construction of the Islisberg tunnel, Switzerland, which on completion will be the longest underground section of the western Zurich bypass, will be directing transit traffic to central Switzerland around the city. The diameter of the cutterhead is about 38' (11.8 m). See Table D-1 for more data about the machine (Herrenknecht).
D.2.3 Double Shield TBM
A Double Shield TBM (Figure D-7) consists of a rotating cutterhead mounted to the cutterhead support, followed by three shields: a telescopic shield (a smaller diameter inner shield which slides within the larger outer shield), a gripper shield and a tail shield.
Figure D-7 Overview of a Double Shield TBM (Herrenknecht)
Figure D-8 Typical Diagram of a Double Shield TBM (Robbins).
In double shield mode, the gripper shoes are energized, pushing against the tunnel walls to react the boring forces just like the open gripper TBM. The main propel cylinders are then extended to push the cutterhead support and cutterhead forward. The rotating cutterhead cuts the rock. The telescopic shield extends as the machine advances keeping everything in the machine under cover and protected from the ground surrounding it.
The gripper shield remains stationary during boring. A segment erector is fixed to the gripper shield allowing pre-cast concrete tunnel lining segments to be erected while the machine is boring. The segments are erected within the safety of the tail shield. It is the Double Shield's ability to erect the tunnel lining simultaneously with boring that allows it to achieve such high performance rates. The completely enclosed shielded design provides the safe working environment.
If the ground becomes too weak to support the gripper shoe pressure, the machine thrust must be reacted another way. In this situation, the machine can be operated in "single shield mode." Auxiliary thrust cylinders are located in the gripper shield. In single shield mode they transfer the thrust from the gripper shield to the tunnel lining. Since the thrust is transferred to the tunnel lining, it is not possible to erect the lining simultaneously with boring. In the single shield mode, tunnel boring and tunnel lining erection are sequential operations.
Figure D-9 above shows the cutterhead (about 40' diameter) of the Herrenknecht S-376 Double Shield TBM which is being used for the construction of Brisbane North-South Bypass Tunnel See Table D-1 for more data about the machine (Herrenknecht).
D.3 Pressurized Face Soft Ground TBM
As shown in Figure 6-11 above, various types of tunnel boring machines (TBM) are suitable for soft ground tunneling in different conditions. Chapter 7 presents briefly the history and development of shield tunneling machines. Table 7-4 (reproduced below) lists various types of shield tunneling methods in soft ground.
Nowadays modern pressurized-face closed shield TBMs are predominantly utilized in large diameter soft ground tunneling. Section 7.3 describes the principles of the two common types: earth pressure balance (EPB) machines and slurry face machines (SFM) , and offers guidelines for selecting between EPB and SFM. This appendix presents the components of each type of TBM and describes the construction sequences.
D.3.1 Earth Pressure Balance Machine
As discussed in Section 7.3, earth pressure balance machines (EPB) (Figure D-10) are pressurized face shield machines specially designed for operation in soft ground especially where the ground is silty and has a high percentage of fines both of which will assist the formation of a plug in the screw conveyor and will control groundwater inflows.
Notes: (1) Cutterhead; (2) excavation chamber; (3) bulkhead; (4) thrust cylinders; (5) screw conveyor; (6) segment erector; and (7) Segmental Lining
Figure D-10 Overview of Earth Pressure Balance Machine (EPB)
The EPB machine continuously supports to the tunnel face by balancing the inside earth and water pressure against the thrust pressure of the machine. The working area inside the EPB machine is completely sealed against the fluid pressure of the ground outside the machine.
As shown in Figure D-10, the soil is excavated (loosened) by the cutterhead (1) serves to support the tunnel face. The area of the shield in which the cutterhead rotates is known as an excavation chamber (2) and is separated from the section of the shield under atmospheric pressure by the pressure bulkhead (3). The excavated soil falls through the openings of the cutterhead into the excavation chamber and mixes with the plastic soil already there. Uncontrolled penetration of the soil from the tunnel face into the excavation chamber is prevented because the force of the thrust cylinders (4) is transmitted from the pressure bulkhead onto the soil. A state of equilibrium is reached when the soil in the excavation chamber cannot be compacted any further by the native earth and water pressure.
The excavated material is removed from the excavation chamber by a screw conveyor (5). The amount of material removed is controlled by the speed of the screw and the cross-section of the opening of the upper screw conveyor driver. The pressure in the excavation chamber is controlled by balancing the rate of advance of the machine and the rate of extraction of the excavated material by the screw conveyor. The screw conveyor conveys the excavated material to the first of a series of conveyor belts. The excavated material is conveyed on these belts to the so-called reversible conveyor from which the transportation gantries in the backup areas are loaded when the conveyor belt is put into reverse.
The tunnels are normally lined with reinforced precast lining segments (7), which are positioned under atmospheric pressure conditions by means of erectors (6) in the area of the shield behind the pressure bulkhead and then temporarily bolted in place. Grout is continuously injected into the remaining gap between the segments' outer side and the surrounding medium injection openings in the tailskin or openings directly in the segments.
Manual or automatic operation of the EPB system is possible through the integrated PLC and computercontrol systems.
As discussed above, the EPB machines support the tunnel face with pressure from the excavated (and remolded) soil within the excavation chamber and crew conveyor. Therefore, EPB machines perform more effectively when the soil immediately ahead of the cutterhead and in the excavation chamber forms a plastic plug, which prevents water inflow and ensures face support. This is accomplished by conditioning the soils ahead of the cutterhead with foams and.or polymers. O'Carroll 2005 lists the benefits of soil conditioning for the EPB machine operation including:
Figure D-11 shows the front of the Herrenknecht S-300 EPB TBM used in the construction of the M30-By-Pass Sur Tunel Norte project in Madrid, Spain. The diameter of the cutterhead is almost 50' (15.2m). See Table D-1 for more data about the machine (Herrenknecht).
Figure D-11 The EPB Machine for the M30-By-Pass Sur Tunel Norte project in Madrid.
D.3.2 Slurry Face Machine
As discussed in Section 7.3, slurry face machine (SFM) are pressurized face shield machines specially designed for tunneling in soft ground especially where the ground is loose waterbearing granular soils that are easily separated from the slurry at the separation plant. The SFM provides stability at the face hydraulically by bentonite slurry kept under pressure to counteract the native earth and groundwater pressure, and to prevent an uncontrolled penetration of soil or a loss of stability at the tunnel face.
Figure D-12 shows typical diagrams of Herrenknecht's mixshield machine which employs the slurry face support principle. At the mixshield machine face the soil is loosened by the cutterhead (1) rotating in the bentonite suspension. The soil then mixes with the bentonite suspension. The area of the shield in which the cutterhead rotates is known as the excavation chamber (2) and is separated by the pressure bulkhead (3) from the section of the shield under atmospheric pressure.
The bentonite suspension supplied by the feed line (4) is applied in the excavation chamber via an air cushion (5) at a pressure equaling the native soil and water pressure, thus preventing an uncontrolled penetration of the soil or a loss of stability at the tunnel face. For this reason the excavation chamber behind the cutting wheel is separated from the pressure bulkhead by a so-called submerged wall (6). The area of the submerged wall and pressure bulkhead is known as the pressure/working chamber. Note that unlike the typical slurry shield machines, in the mixshield machines. the support pressure in the excavation chamber is not directly controlled by suspension pressure but by a compressible air cushion between the pressure bulkhead and the submerged wall.
The loosened soil mixed with the suspension is pumped through the feeding circuit to the separation plant outside the tunnel. In order to prevent blockages to the feeding circuit and to ensure trouble-free operation of the discharge pumps, a sieve of largish stones and clumps of soil is placed in front of the suction pipe to block the access to the suction channel.
Notes: (1) Cutterhead; (2) excavation chamber; (3) bulkhead; (4) slurry feed line; (5) air cushion; (6) wall; (7) Segmental Lining; and (8) segment erector
Figure D-12 Overview of Slurry Face Machine (SFM) (Herrenknecht's Mixshield Machines)
Figure D-13 shows the Herrenknecht S-317 Mixshield TBM used in the construction of the Shanghai Changjiang Under River Tunnel Project in China. The diameter of the cutterhead is over 50' (15.4 m). See Table D-1 for more data about the machine (Herrenknecht).
Figure D-13 Photograph of Herrenknecht S-317 Mixshield TBM