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Drill-and-blast Method

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The advancement of long tunnels through hard rock well before tunnel boring machines (TBMs) were invented relied entirely on the drill-and-blast method. Today, the drill-and-blast method is still widely practiced and used in building shorter tunnels through hard rock where the use of tunnel boring machines is not justified and too expensive. The drill-and blast-method is also used in combination with full face drilling with tunnel boring machines.[1]

In the drill-and-blast method, a drilling jumbo is used to drill a predetermined pattern of holes to a selected depth in the rock face of the proposed tunnel’s path. The drilled holes are then filled with explosives such as dynamite. The charges are then detonated, causing the rock to crack and break apart. The loosened debris or muck is then dislodged and hauled away. Other tools such as a pneumatic drill or hand tool are then used in smoothing out the surface of the blasted rock.

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[edit] History

Aside from manually digging tunnels, blasting and removing rock debris manually was undertaken by the drill-and-blast method in its most rudimentary form, which was the process by which most tunnels were advanced until the advent of tunnel boring machines. The first blasting agent was gunpowder, used to carve out a long canal tunnel measuring 515 feet (157 m) in length in France in 1681. Gunpowder was eventually replaced with nitrogylcerine (stabilized in a dynamite form) in tunnel blasting by 1850. Steam and compressed air were also used to power drills that bored holes in rock.[2]

By 1931, the first drilling jumbos comprised of 24 to 30 pneumatic drills rigged on a frame and welded to a truck bed were developed to advance tunnels that would reroute the Colorado River around the Hoover Dam construction site in the U.S.[3]

[edit] Process

Drill-and-blast tunnels may vary in length from 16.4 to 197 feet (5 to 60 m) and are used for underground utility, highway, railroad, mass transit tunnels, and underground mining.[4]

In smaller tunnels, drills are individually mounted on bars or columns with an adjustable clamp that permits movement. In a larger tunnel, drills are mounted onto a drilling jumbo, a type of portable carriage with one or multiple platforms that are outfitted with bars, columns, and/or booms to support simultaneous drilling in any number of patterns. The jumbo moves through the tunnel as excavation proceeds.[5]

The holes are then filled using a ramrod with a charge or explosive—usually dynamite or, in some circumstances, ammonium nitrate explosives. After the holes are loaded, they are detonated and the broken muck is removed from the tunnel using a mechanical mucker, a type of power shovel. Muck can also be transported out of the tunnel by the use of narrow gauge trucks pulled by locomotives, diesel trucks, a conveyor belt, or through a pipeline as a slurry mixture.[6] The most important principle associated with the drill-and-blast method is that the energy generated from the explosives must be allowed to be directed in the correct alignment. To carry this out properly, the geological condition of the rock bed, the angle, size, and spacing of the drill holes, and the energy factor have to be taken into consideration and precisely calculated.[7]

One of the drawbacks of the drill-and-blast method is that it poses some obvious safety risks for workers. The fumes and gases generated from detonating explosives are toxic. This is why the air always has to be circulated through some type of ventilation duct in the blast section following detonation. The charging of explosives also produces a high volume of dust. Other risks are undetonated explosives and the noise pollution generated from blasting. Blasting can also weaken or fracture the rock around the tunnel, thereby increasing the likelihood of uncontrolled rockslides or falling debris.[8] Additional temporary ground support in the form of rock bolting and shotcrete is often utilized once the tunnel is excavated.[9]

A switch to the use of pumpable emulsion explosives and non-electric initiation methods has revolutionized the practice of the drill-and-blast method. Pumpable emulsions are mixed onsite and are not actually converted into an explosive agent until pumped and loaded into the blasting holes. Such advancements in the method have touted it as a plausible excavation choice over tunnel boring machines in the construction of the Gotthard Tunnel in Switzerland. Still under construction, the tunnel’s 35.4-mile (57-km) path transcends approximately 1,640 feet (500 m) above sea level through less than favorable mountain terrain and rock conditions.[10] With over 124 miles (200 km) of tunnels to be advanced, the drill-and-blast method is being viewed as a likely option after a TBM machine named GOBI got stuck in a section of unstable, soft rock in July 2005.[11]

[edit] Equipment

[edit] References

  1. Excavation Methods. Norconsult, 2008-09-29.
  2. Tunnel. Answers.com, 2008-09-29.
  3. Tunnel. Answers.com, 2008-09-29.
  4. Drill and Blast Tunnels. Traylor Bros., Inc., 2008-09-29.
  5. Tunneling Techniques, 2008-09-29.
  6. Tunneling Techniques, 2008-09-29.
  7. Drill and Blast Tunnels. Traylor Bros., Inc., 2008-09-29.
  8. Tunneling Techniques, 2008-09-29.]
  9. Drill and Blast Tunnels. Traylor Bros., Inc., 2008-09-29.
  10. International Tunneling and Underground Space Association.
  11. The Gotthard Base Tunnel. Discovery Channel, 2008-09-29.