Underground tunnels are becoming increasingly important in today’s system of mass transportation, which moves millions of commuters in cities on a daily basis. However, underground construction is dangerous, and many precautions are necessary. The following five challenges in tunnel construction should give an idea of the huge challenges posed by tunnel construction.
“Ground behavior” refers to how the material of the ground will behave when construction has started. The “ground”—the rock or soil—behaving differently than expected is the most significant problem involved in tunnel construction. The nature and composition of the ground has implications for how construction should be approached: with hand-mining, drill-and-blast, with shield or with tunnel-boring machines. Additionally, the presence of water can affect ground behavior.
One major problem related to ground behavior is stand-up time. A stand-up time problem arises when the ground cannot support itself for the time during which the tunnel is being constructed. This issue affects the type of ground support used, the manpower requirements of construction, production and schedule, equipment selection, and cost.
Another major problem related to ground behavior is in-situ stress, which refers to how some rock at certain depths is loaded with stress from other rock strata and from locked-in tectonic stress resulting from geologic development. When tunnels are excavated in stressed rock, local stress fields are disrupted and new sets of stresses are induced in surrounding rock, which can have disastrous effects.
Avoiding accidents related to unpredicted ground behavior is not a simple matter, and an approach of probability-based risk assessment should be taken in order to minimize the chances of an accident.
The existing conditions of the construction site are often sensitive to the construction process. Engineers should take care to know as much as possible about surface and underground structures, as well as possible hazardous materials stored in the ground.
Sometimes construction can expose gases previously trapped underground. These gases can be toxic and can cause significant construction problems. If and when gas is encountered, the gas’s properties must be evaluated and studied for its potential effect on personnel.
The worst industrial accident in American history occurred because of hazardous existing conditions during the construction of the Hawks Nest Tunnel. The tunnel was being built by Union Carbide through the Gauley Mountain in Fayette County, West Virginia. The tunnel was meant to redirect water from the New River to drop it down almost 200 feet to power turbines and create electricity. However, during construction, workers encountered 99.4 percent pure silica that they were instructed to drill without protection. As a result, more than 750 workers died.
Rock Face Fall-out
Rock face fall-out refers to when rocks fall from the construction face as a result of weakening ground and support conditions. According to studies, most accidents because of rock fall happen when workers approach a tunnel, cutting face in order to mount a steel arch support.
In order to prevent rock face fall-out, the technique of “shotcreting” is used. Shotcreting refers to spraying a newly excavated surface with a coat of concrete in order to give temporary support to the newly exposed rock face. It also helps stop ground surface deterioration and secures loose materials. If done remotely, this greatly minimizes the dangers of rock face fall-out.
Compressed-air techniques of tunnel construction are used mainly on driving earth tunnels through water-bearing soils or adjacent to bodies of water. The technique is the most reliable technique of tunnel construction when the tunnel is being built below the water table, especially when face instability and water flow are predicted to be acute.
Compressed-air techniques can pose risks to construction and personnel, and they are expensive. Personnel must be specially certified to work in the unique environment posed by compressed-air construction techniques. This raises both personnel and equipment costs since items needed such as air locks and compressors are expensive. Decompression, flooding, and fire are among the possible risks of compressed-air construction. Therefore, other kinds of techniques should be evaluated for their possible effectiveness before engineers decide to use compressed air.
Groundwater and Water Flow
The presence and movement of water can influence ground behavior, which poses risks to tunnel construction. Flowing water might carry materials into newly excavated openings, causing general instability in the mass of rock. Additionally, water can change the ground’s physical properties and behavior, making it unpredictable.
Moreover, the presence of water makes handling of material difficult and may necessitate a centralized pumping system, which takes time and money to install, raising general costs and disrupting the construction schedule.
Though necessary, underground construction is a dangerous and difficult process that requires preparation and many safety precautions.
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