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Editorials: Area 51
By Dennis Balthaser
TBMs (Tunnel Boring Machines)
My earliest memories of tunnels are the tunnels of the Pennsylvania Turnpike. It was many years ago, and the turnpike spanned the full width of Pennsylvania for a total of 360 miles, from the Ohio State line to New Jersey. The toll road superhighway, or turnpike, was the first of many to be built across the country.
The unique thing about the Pennsylvania turnpike, was the fact that seven tunnels were constructed through the mountains of Pennsylvania. When originally constructed, the tunnels were only 2 lanes (one lane in each direction.) Years later as traffic volume increased, several tunnels were expanded to two tunnels providing 2 lane access in each direction, while other tunnels on the turnpike were eventually by-passed. Building and maintaining the turnpike would have been quite an accomplishment, considering it officially entered service on October 1, 1940.
I have been interested in tunnels for years, and primarily how they are constructed and for what use. Many large cities around the world now use tunnels below ground to avoid the congestion that’s above, for various purposes, including vehicle traffic and rail systems. Other underground uses include hydroelectric projects, sewers, aqueducts, nuclear waste storage, mining, as well as many other applications. In previous articles I’ve written, I’ve mentioned that the government and military are big users of TBMs for building underground facilities away from public view.
In the early years of tunnel building, the primary method used to construct tunnels was blasting, which was not only extremely dangerous, but slow and hard work for those involved. With the introduction of tunnel boring machines (TBMs), drilling through the earth became a whole new way of creating these voids underground. As time passed, more companies began manufacturing these machines, and the size of TBMs also increased tremendously. One of the largest TBMs known today has a cutting head 63.3ft in diameter. The overall length of these monstrous machines can also reach 3-400 feet in length or more, including a conveyor system at the back for removing excavated material. The conveyor belt is comprised of layered rubber with steel wire reinforcing for strength. This is no toy; it’s a serious cutting machine capable of chewing up tons of dirt and rock as it slowly digs its way through the earth, and probably unknown to anyone above ground (depending on the depth below ground) where it’s working.
When contemplating constructing a tunnel, one of the first considerations has to be what type material will it be digging in. There are basically three types of Tunnel Boring Machines. “Open Hard Rock TBMs” are used in hard stable rock that depends on the integrity of the rock to support itself once the machine has passed through. The just carved walls do not need additional support. “Shielded Hard Rock TBMs” operate in rock that is not stable and has a high degree of fracturing, making dangerous cave-ins likely. Shielded Hard Rock TBMs have a system of automated machines that place reinforced concrete segments on the tunnel wall as reinforcement. The TBM moves forward by pushing off on the concrete segments just placed. Finally, “Soft Ground TBMs” are designed for tunneling through softer types of earth like clays, sandstone, and loose or wet ground. Sophisticated controls are used to maintain a balance between forward movement and excavated material to keep the tunnel stable. Concrete segments are also used on the just dug wall for stability.
If there is no horizontal access on the surface to the project, the TBMs must be lowered into the earth through vertical shafts and assembled below ground. To accomplish that, “Raise and Shaft Boring TBMs” are used for tunneling vertically. These types of TBMs are also used to dig ventilation shafts, access shafts, and maintenance shafts as well as common mine and drainage shafts. Although they are similar in design to the other TBMs mentioned, they are made to tunnel vertically instead of horizontally. If the tunneling project is entirely underground, due to the expense of removing the TBMs, a side shaft is dug and the TBM equipment is not brought back up to the surface but remains underground.
TBMs are like a giant drill. The front section is called the shield. At the front of the shield is the cutter head, and the cutter head resembles a giant circular cheese grater. Shield sizes vary from 3 to 60 feet in diameter. Because every project is different based on the materials the machine will encounter during tunneling, each TBM will have its own features, dimensions and other specifications. Surprisingly the crew on board the TBMs is usually only 6-10 operators and crew. Many others employees are needed for other related work in completing the tunnel.
When the TBM cutting head is pressed against the surface to be cut, the cutting beads will strike against the rock face, breaking it into smaller pieces. That force applied can vary depending on the equipment, but 26 tons is about the average.
In September 1972 a patent No. 3,693,731 was issued to inventors at Los Alamos National Laboratory for a tunnel boring machine that can use an electrical or “nuclear” heat source. The heat source would melt the rock, at which time the molten material would be disposed against cracks in the rock wall it just formed and would form a vitreous wall lining of the tunnel. That patent was issued 45 years ago, so what research progress in tunnel boring equipment might have been developed since then that we’re not yet aware of?
I understand the need for tunnels in large cities, due to available above ground space being limited, for solving transportation problems or utility needs. However, over the years in other articles I have written, I have questioned some of the underground governmental and military facilities that we are gradually hearing about.
With my civil engineering background, I will continue to be interested in knowing what is going on under our feet and how it is being accomplished.
Dennis G. Balthaser
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TBMs (Tunnel Boring Machines)
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