Art in Driving in Underground; Tunnel Boring Machine (TBM) by Eng. H.W. Isuru Indunil
This is the start of a series of articles on Tunnel Boring Machines (TBM). This is the first article in which I wish to discuss the introduction of a modern TBM, its operation, and components.
Infrastructure development is a vast spreading industry in the world regardless of the region and countries. Since, most of the countries are trending to develop underground structures for the purpose of the hydropower, transportation, and railways . Typically underground excavation practices are either by conventional drill and blast or by the modern development of mechanized tunneling.
Mechanized Tunneling Method ( horizontal tunnel boring by Tunnel Boring Machines (TBM), vertical shaft boring by raise bore, directional drilling, roadheaders, etc.
Conventional Drilling and Blasting
This article will provide a basic introduction to TBMs.
What is TBM technology in underground space development?
Since man discovered explosives, the conventional method for underground access has been drilling and blasting (D&B). As Sri Lankans, we have heard about ancient underground shallow tunnels made by Kings, belonging to Rawana periods, King Walagamba period and so on. Today, the conventional D&B method is a well-developed technology adopted for construction of underground structures. However, the mechanized tunneling method is widely spreading and being utilized in many underground development projects around the world. Presently, the Sri Lankan government and technical experts are seeking the feasibility of using mechanized tunneling methods for the forthcoming underground development projects in Colombo and other suburbs under the Megapolis Development Plan.
Mechanized tunneling method offers advantages and disadvantages over the conventional D&B method as described below:
Advantageous
Disadvantages
Continuous operation (rock breaking, muck haulage, support installation, etc)
High plant costs (where new TBMs are to be employed, requiring upfront financing contracts)
Smooth excavation facilitating ventilation and hydraulics in water transfer tunnels
Long lead time for manufacturing and delivering of TBMs
Lower cost for ground support and tunnel lining due to better stability and less damage to the rock mass
Little flexibility on circular profile, curvature, and enlargements for full face rotary TBMs
Improved Personal safety
Higher learning times
Reduced number of access adits and shafts
Performance risks when unexpected variations in geological conditions are encountered.
Minimum impacts on adjacent structures (in terms of settlements of soils or blast vibrations, hence safer excavation is achieved in urban areas)
Limited flexibility - difficulty to change system once established
Less labor intensive and provides much safer working conditions
Less environmental pollutions (noise, dust, smoke etc)
The first use of TBMs in Sri Lanka, is at the Uma Oya Multipurpose Development Project. There are two Double Shield Hard rock TBMs, for development of the tailrace tunnel (already excavated) and the headrace tunnel (15.6 km long, currently being constructed). According to my knowledge, there is neither any systematic or formal study, publication or research paper has been done in Sri Lanka, about Mechanized Tunneling method or the TBM techniques.
3D animation TBM S574 "Martina" - Showing how the huge EPBM S574 "Martina" works.
TYPES OF TUNNEL BORING MACHINES
According to the ground conditions, TBMs can be categorized into two main types:
Soft Rock TBMs: Earth Pressure Balance (EPB) and slurry shield type TBMs
Hard rock TBMs: open type TBMs and shielded TBMs
There are two types of soft rock/ground TBMs in practice as mentioned below.
Earth Pressure Balance (EPB TBM) Type
EPB is used in the ground with less than 7 bar pressure . This type is very suitable for digging tunnels in unstable ground, where soil types such as clay, silt, sand or gravel are predominant (e.g., Thames Water Ring Main section of the London Underground and the Channel Tunnel).
With EPB shields , the excavated ground is used as the support medium. The force of the push cylinders is transferred via the pressure bulkhead onto the earth slurry in the excavation chamber. The front shield of the EPB TBM is filled with debris extracted by means of a screw conveyor. The release of the muck via the screw conveyor provides the pressure to maintain stability at the face. Foam injection is sometimes used to render the excavated material more homogeneous, thus facilitating its excavation. The excavated material is removed via the screw conveyor and transferred onto a belt conveyor from where it can be transported to the surface either by rail transport or a truck. It uses a combination of a disk cutter and cutting bits, both made of Tungsten Carbide.
The Channel Tunnel, the Thames Water Ring Main, sections of the London Underground, and most new metro tunnels completed in the last 20 years worldwide have excavated using this type of machine.
This type is suited for tunnel excavations in highly permeable unstable terrain, or under civilian structures sensitive to ground disturbances. This type is also used in places where there is significant groundwater .
For the purpose of removal of slurry material , bentonite will serve as support and transport medium. The excavated material is transported to the surface via centrifugal pumps. The recycled bentonite is then returned to the circuit. When digging in highly unstable or liquid terrain, the pressure exerted by the terrain is directly governed by the depth at which digging is performed. It is, therefore, necessary to balance the pressure exerted by the terrain. The front shield of the slurry TBM is filled with excavated material and bentonite slurry. The slurry acts as a transport medium for the excavated material and also provides stability at the face. When cutter changes or face examinations are required, then the slurry is drained, and compressed air is introduced through an air lock to provide support to the excavation. The pressure within this air bubble is subjected to fine control. Bentonite injection waterproofs the working face and improves its resistance.
Immediately behind the excavation precast concrete segments are installed to support the ground, and the annulus between the excavated profile and the segmental lining is filled with peagrave and grout to stabilize the segments.
Hard rock TBMs are designed for boring through solid rock and solid aggregate. Hard rock TBMs use disc cutter mounted on the machine cutter head. Powered disc cutters create compressive stress fractures in the rock, causing it to chip away. The fractured rock (tunnel muck) is transferred through openings in the cutter head to a conveyor on to muck cars for removal from the tunnel.
This type has no shields, leaving the area behind the cutter head open for rock support. There are two grippers on either side, which pushes against the side walls of the tunnel, to reach the machine’s forward thrust, while the machine is continuously steered. At the end of the stroke, the rear legs of the machine are lowered, the grippers and propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring begins again. The open-type, or main beam, TBM does not install concrete segments behind it as other machines do. Instead, the rock is supported in conventional ways using rock bolts, shotcrete, steel arches, steel straps and wire mesh and/or a cast in-situ concrete lining.
In shielded TBMs precast, concrete segments are typically used to support the unstable tunnel walls behind the machine.
Single Shield TBMs
In single shield TBMs, the excavation is done using the grippers to grip the tunnel walls. Then moving the TBM forward is achieved by pushing off against the concrete segments installed behind the cutter head.
Robbins Single Shield TBMs
Double Shield TBMs
Machine double shielded TBM can act in two modes. In the stable ground, the TBM can grip against the tunnel walls to advance forward. In unstable, fractured ground, the thrust is transferred to the Auxiliary thrust cylinders that push off against the tunnel segments behind the machine.
In this article, I provided a basic introduction about TBM, and discussed its operation, and different types of TBMs. In the next article, I expect to provide information on TBM onboard facility, pre-excavation techniques and measuring of TBM performances. In addition to reading this article, you can surf the Internet to get more information and details on TBMs.
Eng. David Lees (Chief Site Supervisor -Amberg Engineering) in UmaOya Multipurpose Development Project.
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