Image courtesy OCRWM/DOE http://www.ymp.gov/index.shtml

At its most basic, a tunnel is a horizontal tube hollowed through soil or stone. Erosion and other forces of nature can form tunnels. But man-made tunnels -- tunnels created by the process of excavation – present some of the most complex challenges in the field of civil engineering. Governments have honored tunnel engineers as heroes because their creations can be true technological marvels. That's not to say, of course, that some tunnel projects haven't encountered major complications – just think about the "Big Dig" in Boston, Massachusetts. But these hurdles haven't stopped engineers from dreaming up even bigger and bolder ideas, such as building a Transatlantic Tunnel to connect New York with London.

In this article, we'll look at how tunnels are built. There are many different ways to excavate a tunnel, including manual labor, explosives, rapid heating and cooling, tunneling machinery or a combination of these methods. But we can't quite dig into these topics just yet, there's just a little bit of physics to discuss, first.

Tunnel Physics
Tunnel engineers, like bridge engineers, must be concerned with an area of physics known as statics. Statics describes how four forces interact to produce equilibrium on structures such as tunnels and bridges. Tension expands, or pulls on, material. Compression shortens or squeezes material. Shearing causes parts of a material to slide past one another in opposite directions. And torsion twists material. The tunnel must oppose these forces with strong materials, such as masonry, steel, iron and concrete.

In order to remain static, tunnels must be able to withstand the loads placed on them. Dead load refers to the weight of the structure itself, while live load refers to the weight of the vehicles and people that move through the tunnel.

Building Tunnels
How a tunnel is built depends heavily on the material through which it must pass. Tunneling through soft ground, for instance, requires very different techniques than tunneling through hard rock or soft rock, such as shale, chalk or sandstone. Tunneling underwater, the most challenging of all environments, demands a unique approach that would be impossible or impractical to implement above ground. That's why planning is so important to a successful tunnel project. Engineers conduct a thorough geologic analysis to determine the type of material they will be tunneling through and assess the relative risks of different locations. They consider many factors, but some of the most important include soil and rock types, beds and zones (including faults and shear zones), groundwater (including flow pattern and pressure) and special hazards such as heat, gas or fault lines.

Image courtesy OCRWM/DOE http://www.ymp.gov/index.shtml In October 1998, miners completed the 1.7-mile cross-drift tunnel built for scientific studies near the potential repository area.

Often, a single tunnel will pass through more than one type of material or encounter multiple hazards. Good planning allows engineers to plan for these variations right from the beginning, decreasing the likelihood of an unexpected delay in the middle of the project. Once engineers have analyzed the material that the tunnel will pass through and have developed an overall excavation plan, construction can begin. The tunnel engineers' term for building a tunnel is driving, and advancing the passageway can be a long, tedious process that requires blasting, boring and digging by hand.

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 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.

Digging Big in Boston After great toil and turmoil, the construction on the Big Dig is complete. Originally slated to be $2.6 billion, the final price tag was an incredible $14.8 billion. But the payoff for Boston commuters should be worth the investment. The old elevated Central Artery had just six lanes and was designed to carry 75,000 vehicles a day. The new underground expressway has eight to 10 lanes and will carry about 245,000 vehicles a day by 2010.

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 super important. 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 pre-cast concrete. When the workers complete a section, jacks push the shield forward and they repeat the process.

Image courtesy AlpTransit Gotthard Ltd. http://www.alptransit.ch/pages/e/ Erstfeld - tunnel boring machine

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.

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.

Image courtesy OCRWM/DOE http://www.ymp.gov/index.shtml View of the rear of the tunnel boring machine showing the laser guidance system in operation.

Some tunnels -- like the tunnel connecting England and France (a.k.a. the Chunnel) -- require some pretty hefty equipment known as tunnel-boring machines (TBMs). The Chunnel, which is one of the longest tunnels in the world, took just three years to excavate, thanks to state-of-the-art TBMs.

Tunnel Boring
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.

As their tools improve, engineers continue to build longer and bigger tunnels. The next generation of tunnel-boring machines will be able to cut 1,600 tons of muck per hour. Engineers are also experimenting with other rock-cutting methods that take advantage of high-pressure water jets, lasers or ultrasonics.

Transatlantic Tunnel The proposed Transatlantic Tunnel would connect New York with London. The 3,100-mile-long tunnel would house a magnetically levitated train traveling 5,000 miles per hour. The estimated trip time is 54 minutes -- almost seven hours shorter than an average transatlantic flight.