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A Double Shield TBM 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.
In normal operation ("double shield mode"), the gripper shoes are energized, pushing against the tunnel walls to react the boring forces. 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.
Regardless of the operating mode, working crews remain protected within the shields. Robbins Double Shield TBMs are capable of safely excavating a wide range of geologic conditions on a project. Our Double Shield TBMs are manufactured to suit your project requirements, in diameters from 1.6 m to 15 m (5 to 50 ft).
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If the geology of your tunnel ranges from soft soils to weathered rock, then a Robbins Earth Pressure Balance Machine (EPBM) is the correct technical solution for your project. This is particularly true when your project is located in an urban environment and ground surface subsidence cannot be tolerated.
EPBMs are utilized in the construction of rail tunnels, metropolitan subway systems, highway tunnels, and other projects where the tunnel will be constructed either partly or completely in soft soil beneath the water table.
Robbins has integrated over 50 years of TBM design and jobsite experience into our EPBMs. Nobody has a wider range of experience designing innovative machinery to cope with difficult underground conditions. Our engineers will tailor your EPBM to handle the specific challenges of your project. From "Smooth Flow" cutterheads that reduce friction to super-reliable sealing systems and Variable Frequency Cutterhead Drives, Robbins has taken the traditional EPB design to the next level.
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A Robbins Main Beam TBM is complex in its design, yet relatively simple in concept. The front of the TBM is a rotating cutterhead that matches the diameter of the tunnel. The cutterhead holds disc cutters (ranging from 11" to 20" in diameter) which are positioned for optimal boring of the given rock type. 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. A unique 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.
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Robbins Single Shield TBMs protect workers from broken rock until the tunnel lining can be safely installed. The body of the machine is enclosed in a shield that is marginally smaller than the diameter of the tunnel. The flat, low-profile cutterhead minimizes disturbance of the face as it bores, and prevents large blocks from collapsing and causing excessive boring stresses. The front of the TBM is a rotating cutterhead that matches the diameter of the tunnel. As the cutterhead turns, a ring of hydraulic cylinders provides forward thrust through shoes that push against the tunnel lining. The cutterhead holds disc cutters (ranging from 11" to 20" in diameter) positioned for optimal boring. The transfer of high thrust through the rolling disc cutters creates fractures in the rock, causing chips to break away from the face. Boring and lining installation are performed sequentially. To steer, cylinders orient the articulated cutterhead in the required direction.
Robbins Single Shield TBMs are available in diameters from 1.6 to 15 m (5 to 50 ft). All Single Shields are equipped with high-speed segment erectors for rapid tunnel lining installation. To steer the machine, the cutterhead is articulated. The short shield length enables a small turning radius and minimizes exposure to squeezing ground forces that could potentially trap the machine.
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This type of boring machine is used for tunneling in stable to unstable rock for the excavation of traffic tunnels (road and rail), supply tunnels (water, gas and electricity) and other special applications.
The main elements in the construction of the shield tunnel boring machine are the shield which gives the machine its name, a construction like a steel tube in which all other components of the machine are embedded, the cutter head and its drive housing, the advance mechanism and the robot-like device for installation of the tunnel lining, the so-called erector. The tunnel lining, usually consisting of prefabricated concrete sections, is installed with the erector, protected by the rear shield mantle, the so-called tail shield.
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Earth pressure balance tunnel boring machines
Historically, EPBs were developed in order to do away with the mucking system and the reprocessing of boring slurry. To do this, the soil must ideally consist of fine elements (clay and silt) and natural water. This type of mix is rare, which is why foams were developed.
Basic principle
The earth pressure balance tunnel boring machine or EPB consists of a cutting chamber located behind the cutterhead. This chamber is used to mix the soil + water foam. It is maintained under pressure by the mucking system. The system consists of a screw conveyor whose speed is controlled so that constant pressure inside the chamber is guaranteed. The material exits the screw conveyor via a hatch that can be closed when the TBM is shut down. The material is then removed by a belt conveyor and/or train cars circulating in the tunnel.
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This type of boring machine is suitable for driving in stable rock and is used for the excavation of traffic tunnels (road and rail), supply tunnels (water, gas and electricity), pressure water utility tunnels for hydroelectric plants, tunnels for cable cars high in the mountains, tunnels for geological investigations and other special applications.
The main elements in the construction of the open tunnel boring machine are the cutter head with its associated drive housing, the support construction (the so-called gripper) and the advance mechanism.
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Tunneling systems for mixed ground
Dual mode - two process technologies in one
As the term 'dual mode' indicates, two process technologies are combined in these machines.
Shield machines
These machines are externally similar to shield machines and combine the functional features of a hard-rock shield tunnel boring machine and, optionally, of a slurry shield or a hydroshield machine (Bentonair, slurry shield). They are always used when tunnels have to be driven alternately in stable and unstable sections.
New style machines
Dual-mode tunnel boring machines are still a new type of machines and have been developed to meet the requirements of complex projects in strongly varying geologies. Where conventional tunneling techniques had to be used formerly, these machines now open the way to lower-risk and lower-cost tunneling in difficult geological conditions
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This type of boring machine is used for tunneling in stable to unstable rock for the excavation of traffic tunnels (road and rail), supply tunnels (water, gas and electricity) and other special applications.
The main elements in the construction of the shield tunnel boring machine are the shield which gives the machine its name, a construction like a steel tube in which all other components of the machine are embedded, the cutter head and its drive housing, the advance mechanism and the robot-like device for installation of the tunnel lining, the so-called erector. The tunnel lining, usually consisting of prefabricated concrete sections, is installed with the erector, protected by the rear shield mantle, the so-called tail shield.
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Slurry shield tunnel boring machines
Bentonite slurry, consisting of a specific clay and water mixture is universally used in boring and digging operations. The slurry has numerous properties: sealing of the cutting face, reduced friction on the TBM's structure, and transport of cuttings to the separation plant outside the tunnel.
The basic principle of this TBM is to maintain the tunnel's cutting face during the excavation phase by filling the working chamber, located behind the cutterhead, with slurry.
Slurry pressure is carefully controlled by a large air bubble which is maintained by a separately-controlled air supply system. This air bubble acts as a damper by absorbing sudden variations in mucking output.
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Double - or Telescopic - Shields unite the principles of the Gripper and the Single Shield TBM, enabling fast excavation even in varying rock formations.
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Herrenknecht Earth Pressure Balance (EPB) Shields are in their element in soft ground. Cohesive and incohesive soils with high clay or silt content and low water permeability provide the ideal conditions for EPB tunnelling.
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For use in solid rock, the principle of the gripper is both the most simple and most effective technique. The single gripper machine braces itself against the rock using two gripper plates. Then, hydraulic cylinders push the cutterhead into the tunnel face at high pressure, grinding it with cutter rings. Double gripper machines work according to the same principle, using four hydraulically operated gripper plates.
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Fluid-supported Mixshields are specialists for multi-layered, complex geologies, particularly those with a high water content, and are a patented trademark of Herrenknecht. This functioning principle is particularly well-adapted to heterogeneous soils: Thanks to the center-free drive with the floating bearing of the cutting wheel and the modular structure of the machine, the operating mode can be altered to suit the soil conditions in the tunnel.
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Single Shield TBMs are the right choice for brittle or soft rock. In shield tunnelling, the tunnel is lined with concrete segments. To tunnel forward, the hydraulic thrust cylinders of the Single Shield TBM push against the last installed lining ring.
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