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Soft Ground and Hard Rock

Workers generally use two basic techniques to advance a tunnel. In the full face method, they excavate the entire diameter of the tunnel at the same time. This is most suitable for tunnels passing through strong ground or for building smaller tunnels. The second technique, shown in the diagram below, is the top heading and bench method. In this technique, workers dig a smaller tunnel known as a heading. 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.

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.

Marc Isambard Brunel, a French engineer, invented the first tunnel shield in 1825 to excavate the Thames Tunnel in London, England. 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 Nike Outlet had completed this process on one section, powerful screw jacks pushed the shield forward.

In 1874, 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 1874, 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. Soon, engineers in New York, Boston, Budapest and Paris had adopted the Greathead shield to build their own subways.

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 Nike Outlet Online 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, Nike Outlet Online 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 Nike Outlet 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.