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Making Of Tunnels before the Advent of Tunnel Boring Machines
When excavation was not possible, drilling and blasting were the way that long tunnels were made through rock. This system has been replaced with tunnel boring machines, although the high cost of the machines and their limited availability still leads to tunnels being built in rock through the drilling and blasting method. In the drill and blast method, once the earth layer removal has exposed the rock, the rock face is drilled with a number of holes of about a meter or more in depth. The depth depends on the nature of the rock and the drilling equipment being used. These holes are then filled with explosive and are all blasted in a predetermined sequence which maximizes the yield of the broken rock. The broken rock is then removed and the new rock face that is created is again drilled and blasted. The entire process is intermittent and leads to lot of idle time, which can increase costs. Clearing of rock is either done by trucks or tippers, or through conveyor systems that are temporarily installed. The use of explosives also adds an element of danger to the operations and there are very strict rules on the explosive process, the storing and use of the explosive, and the expertise needed for such operations.
As the tunnel proceeds into the rock, additional problems may be faced in supporting the sides of the tunnel, and dealing with underground water and falling rock. Different systems are used for supporting the rock surfaces such as rock bolts, ribs, or arches made of steel or precast concrete, or in-situ concrete. This is a problem that has also to be faced for tunnels where tunnel boring machines are used and is a separate subject by itself.
Image source: Wikimedia :Tunnel Boring Machine
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The History of Tunnel Boring Machines
Tunnel boring machines first started as tunneling shields first used in the Thames Tunnel in 1825. Digging was still done in the traditional way after the shield had pulverized the rock. An actual boring machine using a number of percussion drills was made in 1845 for a tunnel between France and Italy. Machines were made in the United States even in those days but rarely survived to be granted permanent status as a reliable method of tunnel excavation.
The 1950s brought in a dam diversion project that required extensive tunneling work through shale. This is where James S. Robbins, who was the founder of the Robbins Company, entered the picture and made his first machine that was able to cut to a depth of 160 feet in a full day and night of work. His first machines used steel picks rotating in a circle, but these broke frequently and required replacement. This is when James Robbins came up with the idea of a rotating head mounted with disc cutters, an idea that persists to this day with refinements coming only in the propelling, thrusting, and rotating mechanisms, developments more to do with the stronger and harder materials now available. Cutting rates are now as much as 4000 feet in a month, and diameters for tunnels can be as high as 60 feet.
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How Tunnel Boring Machines Work
The head of a tunnel boring machine which has the rotating head and discs will press against the rock at the sides and exert a force of up to 26 tons on the rock, which allows the discs to chip at the solid rock. This falls on to buckets that take it to the conveyor belt that has been placed behind the machine and which conveys the rock to the tunnel face for further removal. The length of the conveyor requires constant lengthening or shifting to keep pace with the progress of the rotating head. The machine then moves forward with the help of grippers that hold on to the wall with the force of hydraulic rams that propel the machine forward. Each stroke moves the head about 6 feet where it is then anchored again.
Meanwhile immediately behind the machine, separate machinery is used to drill into the side rock at set spacing, so that rock bolts or dowels can be used to secure the rock. Steel mesh is also installed at this time to save the crew from falling rock.
The finishing of the rock face with concrete or other lining to create a permanent structure starts after about 200 feet of the rock has been drilled into.
Such machines are also used for softer material like clay and earth. The cutters and motive force for the rotating head may be less powerful, and here the tunneling engineer has to worry more about lining the tunnel as collapses of earth are frequent. The preferred method is to have precast liners or prefabricated steel elements which can be used to form a shuttering material for making a permanent concrete lining.
Image Source: Wikimedia : Tunnel Boring Machine
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A Little Bit about James Robbins
James Robbins was the founder of the Robbins Company, which was formed in 1951 and started with the design of tunneling and mining equipment. Its first model of a tunnel boring machine was based on a requirement for a dam diversion project.
The company was a privately held company and was taken over by his son Richard in 1959. It then acquired Moran Engineering in 1980 before itself being taken over by Atlas Copco in 1993. The tunnel boring business of Atlas was taken over by Boretec to form the Robbins Company as it exists today.