[attachment=15502]
CONTEXT
A Tunnel Boring Machine (TBM) also known as a "mole” is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to almost 16 metres to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.
Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional 'hand mining' in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less—this is because tunneling with TBMs is much more efficient and results in a shorter project.
The largest diameter TBM, at 15.43 m, was built by Herrenknecht AG for a recent project in Shanghai, China. The machine was built to bore through soft ground including sand and clay. The largest diameter hard rock TBM, at 14.4 m, was manufactured by The Robbins Company for Canada's Niagara Tunnel Project. The machine was used to bore a hydroelectric tunnel beneath Niagara Falls; the machine has been named "Big Becky" in reference to the Sir Adam Beck hydroelectric dams to which it is tunneling to provide an additional hydroelectric tunnel.
HISTORY
The first successful tunneling shield was developed by Sir Marc Isambard Brunel to excavate the Thames Tunnel in 1825. However, this was only the invention of the shield concept and did not involve the construction of a complete tunnel boring machine, the digging still having to be accomplished by the then standard excavation methods.
The first boring machine reported to have been built was Henri-Joseph Maus' Mountain Slicer. Commissioned by the King of Sardinia in 1845 to dig the Frejus Rail Tunnel between France and Italy through the Alps, Maus had it built in 1846 in an arms factory near Turin. It consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel. The Revolutions of 1848 affected the funding, and the tunnel was not completed until 10 years later, by using innovative but less expensive methods such as pneumatic drills.
In the U.S, the first boring machine to have been built was used in 1853 during the construction of the Hoosac Tunnel. Made of cast iron, it was known as Wilson's Patented Stone-Cutting Machine after inventor Charles Wilson. It drilled 10 feet into the rock before braking down. The tunnel was eventually completed more than 20 years later, and as with the Frejus Rail Tunnel, by using less ambitious methods
In the early 1950s, F.K. Mitry won a dam diversion contract for the Oahe Dam in Pierre, South Dakota, and consulted with James S. Robbins, founder of The Robbins Company, to dig through what was the most difficult shale to excavate at that time, the Pierre Shale. Robbins built a machine that was able to cut 160 feet in 24 hours in the shale, ten times faster than any other method at that time.
The breakthrough that made TBMs efficient and reliable was the invention of the rotating head mounted with disc cutters. Initially, Robbin’s TBM used steel picks rotating in a circular motion to dig the excavation front, but he quickly discovered that these picks, no matter how strong they were, had to be changed frequently as they broke or tore off. By replacing the picks with longer lasting disc cutters, this problem was significantly reduced. The design was first utilized successfully at the Humber River Sewer Tunnel in 1956 (Foley, 2009). Since then, all successful hard rock TBMs have utilized rotating cutting wheels with circular disc cutters.
GEOTECHNOLOGY
The geological and hydraulic information about the soil forms the basis for planning and executing tunneling and pipe jacking projects. Consequently, the expert evaluation of geological reports is of great importance because it enables soil conditions to be fully taken into account in mechanical design.
Soil investigations provide the basic data for selecting tunneling methods and cutting wheel tools that will cut through the native ground efficiently and economically. The classical fields of application of tunnel face support systems (slurry or earth pressure support) are defined by the types of soil or their permeability coefficients. Innovations based on additives (foam, polymers, tensides) in the field of chemical process engineering have extended the classical fields of application of the respective types of processes still further, resulting in a greater degree of flexibility of the machines used. It is absolutely essential that all those involved in the project have sufficient knowledge of the interactions between soil, additive and machine.
The most important ground or rock-mechanical parameters for designing a tunnel boring machine are:
• The grading curve
• Ground water penetration
• The consistency limits
• The rock/clay mineralogy
• The quality of the rock
• The hardness of the rock
Apart from the choice of process, particular attention must also be paid to the design of the cutting wheel. Here too the soil parameters have a great influence on the choice of design since the cutter rings used on hard rock, just like scraper tools used principally on cohesive loose soil, are exposed to abrasion and clogging by the soil.
Herrenknecht AG fulfills these requirements by making project-specific adaptations to the process and excavation techniques.
In order to achieve optimum tunneling, even in tunnel faces supported by slurry pressure, an extensive knowledge of geology is also necessary to prepare the support medium, bentonite, by means of a separation plant.
The tunnel boring machine concept devised by Herrenknecht AG therefore incorporates the expert implementation of hydro geological reports on which the mechanical design is based. At Herrenknecht, engineering technicians, construction engineers and process engineers collaborate in order to fully meet both the heterogeneous requirements of the soil and the requirements of the customer.