Drilling and blasting
Encyclopedia
Before the advent of tunnel boring machine
s, drilling and blasting was the only economical way of excavating long tunnel
s through hard rock, where digging is not possible. Even today, the method is still used in the construction of tunnels, such as in the construction of the Lötschberg Base Tunnel
. The decision whether to construct a tunnel using a TBM or using a drill and blast method includes a number of factors such as:
Tunnel length is a key issue that needs to be addressed because large TBMs for a rock tunnel have a high capital cost, but because they are usually quicker than a drill and blast tunnel the price per metre of tunnel is lower. This means that shorter tunnels tend to be less economical to construct with a TBM and are therefore usually constructed by drill and blast. Managing ground conditions can also have a significant effect on the choice with different methods suited to different hazards in the ground.
(such as with the Blue Ridge Tunnel
in the United States, built in the 1850s), it was not until more powerful (and safer) explosives, such as dynamite
(patented 1867), as well as powered drill
s were developed, that its potential was fully realised.
Drilling and blasting was successfully used to construct tunnels throughout the world, notably the St. Gotthard Tunnel, the Jungfraubahn
and even the longest road tunnel in the world, Lærdalstunnelen
, are constructed using this method.
The positions and depths of the holes (and the amount of explosive each hole receives) are determined by a carefully constructed pattern, which, together with the correct timing of the individual explosions, will guarantee that the tunnel will have an approximately circular cross-section.
Typically a rock support system would include a number of these support methods, each intended to undertake a specific role in the rock support such as the combination of rock bolting and shotcrete.
Tunnel boring machine
A tunnel boring machine also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre to almost 16 metres to date...
s, drilling and blasting was the only economical way of excavating long tunnel
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
s through hard rock, where digging is not possible. Even today, the method is still used in the construction of tunnels, such as in the construction of the Lötschberg Base Tunnel
Lötschberg Base Tunnel
The Lötschberg Base Tunnel is a railway tunnel on the BLS Lötschbergbahn's Lötschberg Line cutting through the Alps of Switzerland some below the existing Lötschberg Tunnel. It is currently the world's longest land tunnel and accommodates passenger and freight trains. It runs between Frutigen,...
. The decision whether to construct a tunnel using a TBM or using a drill and blast method includes a number of factors such as:
- Tunnel length
- Managing the risks of variations in ground quality
- Required speed of construction
- The required shape of the tunnel
Tunnel length is a key issue that needs to be addressed because large TBMs for a rock tunnel have a high capital cost, but because they are usually quicker than a drill and blast tunnel the price per metre of tunnel is lower. This means that shorter tunnels tend to be less economical to construct with a TBM and are therefore usually constructed by drill and blast. Managing ground conditions can also have a significant effect on the choice with different methods suited to different hazards in the ground.
History
While drilling and blasting saw limited use in pre-industrial times using gunpowderGunpowder
Gunpowder, also known since in the late 19th century as black powder, was the first chemical explosive and the only one known until the mid 1800s. It is a mixture of sulfur, charcoal, and potassium nitrate - with the sulfur and charcoal acting as fuels, while the saltpeter works as an oxidizer...
(such as with the Blue Ridge Tunnel
Blue Ridge Tunnel
The Blue Ridge Tunnel, also known as the Crozet Tunnel, was the longest of four tunnels built on the Blue Ridge Railroad to cross the Blue Ridge Mountains at Rockfish Gap near Afton Mountain in central Virginia...
in the United States, built in the 1850s), it was not until more powerful (and safer) explosives, such as dynamite
Dynamite
Dynamite is an explosive material based on nitroglycerin, initially using diatomaceous earth , or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp. Dynamites using organic materials such as sawdust are less stable and such use has been generally discontinued...
(patented 1867), as well as powered drill
Drill
A drill or drill motor is a tool fitted with a cutting tool attachment or driving tool attachment, usually a drill bit or driver bit, used for drilling holes in various materials or fastening various materials together with the use of fasteners. The attachment is gripped by a chuck at one end of...
s were developed, that its potential was fully realised.
Drilling and blasting was successfully used to construct tunnels throughout the world, notably the St. Gotthard Tunnel, the Jungfraubahn
Jungfraubahn
The Jungfraubahn is an gauge rack railway electrified at 3-phase 1,125 volts 50 Hertz, which runs 9 kilometres from Kleine Scheidegg to the highest railway station in Europe at Jungfraujoch...
and even the longest road tunnel in the world, Lærdalstunnelen
Lærdalstunnelen
Lærdal Tunnel is a long road tunnel connecting Lærdal and Aurland in Sogn og Fjordane, Norway. It is the longest road tunnel in the world, and carries two lanes of European Route E16, the main road between Oslo and Bergen. In 1975, the Parliament of Norway decided that the main road between Oslo...
, are constructed using this method.
Procedure
As the name suggests, drilling and blasting works as follows:- A number of holes are drilled into the rock, which are then filled with explosives.
- Detonating the explosive causes the rock to collapse.
- Rubble is removed and the new tunnel surface is reinforced.
- Repeating these steps will eventually create a tunnel.
The positions and depths of the holes (and the amount of explosive each hole receives) are determined by a carefully constructed pattern, which, together with the correct timing of the individual explosions, will guarantee that the tunnel will have an approximately circular cross-section.
Rock support
As the tunnel is incrementally excavated the roof and sides of the tunnel need to be supported to stop the rock falling into the excavation. The philosophy and methods for rock support vary widely but typical rock support systems can include:- Rock boltsRock boltA rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be tunnels or rock cuts. It transfers load from the unstable exterior, to the confined interior of the rock mass....
or rock dowels - ShotcreteShotcreteShotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....
- Ribs or mining arches and lagging
- Cable bolts
- In-situ concrete
Typically a rock support system would include a number of these support methods, each intended to undertake a specific role in the rock support such as the combination of rock bolting and shotcrete.