The tunnel boring machine, often referred to as a “mole,” is a state-of-the-art piece of machinery utilised for digging tunnels with a circular cross-section through various types of soil and rock formations. These machines can burrow through hard rock, sand, and an array of other materials with ease, with tunnel diameters ranging from just a metre for micro-TBMs to an impressive 16 metres. For tunnels less than a metre in diameter, trenchless construction techniques or horizontal directional drilling are used instead.
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Tunnel boring machines: Types
Hard rock TBMs
In the excavation of hard rock, either shielded or open-type tunnel boring machines (TBMs) can be employed. The disc cutters on hard rock TBMs, situated within the cutter head, produce compressive stress fractures in the rock, breaking off chips from the tunnel face. The resulting debris, referred to as muck, is transferred through the cutter head to a conveyor belt and removed from the tunnel through a series of conveyors or muck cars.
Open-type TBMs lack a shield, leaving the area behind the cutter head open for rock support and using a gripper system that pushes against the tunnel walls for propulsion. The machine can be continuously steered while gripped but must be ungripped to steer, like the Wirth machine. At the end of a cycle, the rear legs are lowered, the grippers and propel cylinders are retracted, and the machine advances with the grippers re-engaging and the rear legs lifting for the next boring cycle.
The Main Beam TBM does not require concrete segments for support and instead uses methods like ring beams, rock bolts, shotcrete, steel straps, and wire mesh. In contrast, in fractured rock environments, shielded TBMs are utilised, installing concrete segments for the tunnel wall support.
The Double Shield TBM has two modes of operation, relying on gripping the tunnel walls for stability in solid ground and on thrust cylinders for propulsion in the unstable, fractured ground. The Single Shield TBM operates similarly but is limited to use in only fractured ground as it can only push against concrete segments.
Soft ground TBMs
In the realm of soft ground tunnelling, there are two primary types of TBMs: Earth Pressure Balance (EPB) and Slurry Shield (SS) machines. Similar to Single Shield TBMs, both types of machines advance forward by utilising thrust cylinders to push against concrete segments.
The EPB TBM is used in soft soil conditions with pressure levels of less than 7 bar, and its cutter head incorporates a combination of cutting bits made from tungsten carbide, carbide disc cutters, and/or hard rock disc cutters. The Earth Pressure Balance machine is designed to maintain a balance between the earth’s pressure and the machine’s pressure, allowing it to hold up the soft ground. To further stabilise the ground, operators and automated systems add substances like bentonite, polymers, and foam to the soil.
In situations where there is high water pressure and substantial amounts of groundwater, the Slurry Shield TBM is necessary. These machines provide a completely enclosed working environment by mixing the soil with bentonite slurry. The slurry must be expelled from the tunnel through slurry tubes and processed in large separation plants on the surface to separate the dirt from the slurry and recycle it back into the tunnel.
Despite the use of TBMs reducing the need for numerous workers at high pressures, a caisson system may still be required at the cutting head of Slurry Shield TBMs. Any workers entering this area for maintenance, inspection, or repair must be medically cleared as “fit to dive” and trained in lock operation procedures.
Tunnel boring machines: Advantages
As an alternative to the traditional drilling and blasting method in rock and manual excavation in soil, TBMs offer a number of advantages, including minimal disruption to the surrounding area and a smooth tunnel wall, ultimately reducing the cost of lining the tunnel. However, the upfront cost of building a TBM can be high, and transportation can prove challenging.
Despite these setbacks, as tunnels become longer in modern times, the cost of using a TBM compared to drilling and blasting becomes more favourable due to its increased efficiency and shorter project timeline. The largest TBM built to date, measuring 15.43 metres in diameter, was created by Herrenknecht AG for a project in Shanghai, China and specifically designed to bore through soft ground, including sand and clay.
Tunnel boring machines were first introduced in the 1950s and have since evolved into highly sophisticated machines capable of excavating tunnels through a variety of materials, including rock, earth, and even ice. They consist of a cutting head, a conveyor system, and a support structure, which helps to stabilise the tunnel and keep the machine in place. The cutting head rotates and cuts into the ground, breaking up the material and creating a hole that the conveyor system carries away. The support structure ensures that the tunnel remains stable and prevents cave-ins during the excavation process.
FAQs
What are the types of TBMs?
There are two main types of TBMs in hard rock: Shielded and Open-type. Slurry Shield and Earth Pressure Balance Machines (EPB) are the two major types for soft ground.
What is the working principle of TBMs?
In order to shatter rock chips off the tunnel face, hard rock TBMs use disc cutters to induce compressive stress fractures in the rock. Soft ground TBMs use thrust cylinders to push against concrete segments to advance forward, maintaining a balance between earth pressure and stability
How does a TBM transfer excavated material?
Through apertures in the cutter head, the excavated material—also known as muck—is transported to a belt conveyor, which transports it through the machine to a network of conveyors or muck cars for disposal from the tunnel.