Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench. One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.
Notice that the diagram shows tunneling taking place from both sides. Tunnels through mountains or underwater are usually worked from the two opposite ends, or faces , of the passage.
In long tunnels, vertical shafts may be dug at intervals to excavate from more than two points. Now let's look more specifically at how tunnels are excavated in each of the four primary environments: soft ground, hard rock, soft rock and underwater. Soft Ground Earth Workers dig soft-ground tunnels through clay, silt, sand, gravel or mud.
In this type of tunnel, stand-up time -- how long the ground will safely stand by itself at the point of excavation -- is of paramount importance. Because stand-up time is generally short when tunneling through soft ground, cave-ins are a constant threat. To prevent this from happening, engineers use a special piece of equipment called a shield.
A shield is an iron or steel cylinder literally pushed into the soft soil. It carves a perfectly round hole and supports the surrounding earth while workers remove debris and install a permanent lining made of cast iron or precast concrete. When the workers complete a section, jacks push the shield forward and they repeat the process. Brunel's shield comprised 12 connected frames, protected on the top and sides by heavy plates called staves.
He divided each frame into three workspaces, or cells , where diggers could work safely. A wall of short timbers, or breasting boards , separated each cell from the face of the tunnel. A digger would remove a breasting board, carve out three or four inches of clay and replace the board. When all of the diggers in all of the cells had completed this process on one section, powerful screw jacks pushed the shield forward.
In , Peter M. Barlow and James Henry Greathead improved on Brunel's design by constructing a circular shield lined with cast-iron segments. They first used the newly-designed shield to excavate a second tunnel under the Thames for pedestrian traffic.
Then, in , the shield was used to help excavate the London Underground, the world's first subway. Greathead further refined the shield design by adding compressed air pressure inside the tunnel.
When air pressure inside the tunnel exceeded water pressure outside, the water stayed out. Hard Rock Tunneling through hard rock almost always involves blasting. Workers use a scaffold, called a jumbo , to place explosives quickly and safely. The jumbo moves to the face of the tunnel, and drills mounted to the jumbo make several holes in the rock. The depth of the holes can vary depending on the type of rock, but a typical hole is about 10 feet deep and only a few inches in diameter.
Next, workers pack explosives into the holes, evacuate the tunnel and detonate the charges. After vacuuming out the noxious fumes created during the explosion, workers can enter and begin carrying out the debris, known as muck , using carts.
Then they repeat the process, which advances the tunnel slowly through the rock. Fire-setting is an alternative to blasting. In this technique, the tunnel wall is heated with fire , and then cooled with water.
The rapid expansion and contraction caused by the sudden temperature change causes large chunks of rock to break off. The Cloaca Maxima, one of Rome's oldest sewer tunnels, was built using this technique. The stand-up time for solid, very hard rock may measure in centuries.
In this environment, extra support for the tunnel roof and walls may not be required. However, most tunnels pass through rock that contains breaks or pockets of fractured rock, so engineers must add additional support in the form of bolts, sprayed concrete or rings of steel beams.
In most cases, they add a permanent concrete lining. Tunneling through soft rock and tunneling underground require different approaches. Blasting in soft, firm rock such as shale or limestone is difficult to control. Instead, engineers use tunnel-boring machines TBMs , or moles , to create the tunnel. TBMs are enormous, multimillion-dollar pieces of equipment with a circular plate on one end.
The circular plate is covered with disk cutters -- chisel-shaped cutting teeth, steel disks or a combination of the two. As the circular plate slowly rotates, the disk cutters slice into the rock, which falls through spaces in the cutting head onto a conveyor system. The conveyor system carries the muck to the rear of the machine. Hydraulic cylinders attached to the spine of the TBM propel it forward a few feet at a time. TBMs don't just bore the tunnels -- they also provide support.
As the machine excavates, two drills just behind the cutters bore into the rock. Then workers pump grout into the holes and attach bolts to hold everything in place until the permanent lining can be installed.
The TBM accomplishes this with a massive erector arm that raises segments of the tunnel lining into place. Department of Energy terminal storage facility. Underwater Tunnels built across the bottoms of rivers, bays and other bodies of water use the cut-and-cover method , which involves immersing a tube in a trench and covering it with material to keep the tube in place. Construction begins by dredging a trench in the riverbed or ocean floor. Long, prefabricated tube sections, made of steel or concrete and sealed to keep out water, are floated to the site and sunk in the prepared trench.
Then divers connect the sections and remove the seals. Any excess water is pumped out, and the entire tunnel is covered with backfill. The tunnel connecting England and France -- known as the Channel Tunnel, the Euro Tunnel or Chunnel -- runs beneath the English Channel through 32 miles of soft, chalky earth.
Although it's one of the longest tunnels in the world, it took just three years to excavate, thanks to state-of-the-art TBMs. Eleven of these massive machines chewed through the seabed that lay beneath the Channel.
Why so many? Because the Chunnel actually consists of three parallel tubes, two that carry trains and one that acts as a service tunnel. Two TBMs placed on opposite ends of the tunnel dug each of these tubes.
The remaining five TBMs worked inland, creating the portion of the tunnel that lay between the portals and their respective coasts.
Unless the tunnel is short, control of the environment is essential to provide safe working conditions and to ensure the safety of passengers after the tunnel is operational. One of the most important concerns is ventilation -- a problem magnified by waste gases produced by trains and automobiles. Clifford Holland addressed the problem of ventilation when he designed the tunnel that bears his name.
His solution was to add two additional layers above and below the main traffic tunnel. The upper layer clears exhaust fumes, while the lower layer pumps in fresh air. Four large ventilation towers, two on each side of the Hudson River, house the fans that move the air in and out.
Eighty-four fans, each 80 feet in diameter, can change the air completely every 90 seconds. Now that we've looked at some of the general principles of tunnel building, let's consider an ongoing tunnel project that continues to make headlines, both for its potential and for its problems.
The Central Artery is a major highway system running through the heart of downtown Boston, and the project that bears its name is considered by many to be one of the most complex -- and expensive -- engineering feats in American history. The "Big Dig" is actually several different projects in one, including a brand-new bridge and several tunnels. One key tunnel, completed in , is the Ted Williams Tunnel. Another key tunnel is located below the Fort Point Channel, a narrow body of water used long ago by the British as a toll collection point for ships.
Before we look at some of the techniques used in the construction of these Big Dig tunnels, let's review why Boston officials decided to undertake such a massive civil-engineering project in the first place.
Detecting Curved Underground Tunnels using Partial Radon Transforms Abstract: The Radon transform RT is known to be effective in detecting lines in noisy images, but it is not capable of detecting curves unless the curve parametrization is given.
In this paper, partial Radon transforms PRT are investigated as a tool to detect curved features such as underground tunnels in ground penetrating radar GPR images. The algorithm applies the Radon transform to small batches of the total image and updates the tunnel position parameters as new batches are used. Missing data, as well as finding the ends of tunnels can be handled with the proposed algorithm. Blackwall Tunnel Greenwich Guide. Top Europe stories now:. Mass resignations rock Turkey.
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