Steel Pipe Application: Tunneling
Underground tunnels serve a wide range of functions. Small-diameter tunnels can carry utility lines and function as pipelines. Larger-diameter tunnels can serve as subways and passages for railways and cars. The method used to construct a tunnel is determined by several factors, including geology, cost and potential environmental/residential disruption.
Mining a Tunnel Using Steel Pipe
There are several different methods to mine a tunnel using steel pipe. These include the immersed tube method, the cut and cover method, the top down method and the boring machine method.
Immersed Tube Method
For this method of tunnel construction, workers prepare an underwater tunnel site by digging a trench at the bottom of the waterway. Steel sections of tunnel shell are constructed offsite, each of which may be several hundred feet long. The ends of the steel pipe sections are sealed, and the section is brought to the tunnel site, tied to anchors, and sunk. As each section sinks, it’s guided into place in the trench. The sections are connected to each other and a rubber seal is installed between each section, ensuring a watertight connection.
Cut and Cover Method
During the cut and cover method, workers dig a trench that’s large enough to contain the tunnel.
Tunnel walls are built before digging begins in order to keep the trench from collapsing during excavation, which is done by driving steel sheets into the ground. When the desired size is reached, a cage of reinforced steel pipe is lowered into the trench and secured with concrete. This process is repeated until the tunnel shell has been completed, after which it’s covered by replacing the excavated soil.
Top Down Method
Using the top down method, a parallel pair of walls are inserted into the earth along the tunnel's path by driving steel pipe piles. A trench is dug between the walls to a depth equal to the planned distance from the surface to the inside of the tunnel roof. The tunnel roof is formed between the walls by framing and pouring reinforced concrete into the bottom of the trench. After the tunnel roof is complete, it’s covered with a membrane, and excavated soil is piled above it. A front-end loader is used to dig out the soil between the walls and under the tunnel roof. When the desired depth has been reached, a concrete floor is poured to complete the tunnel shell.
Boring Machine Method
Using the boring machine method, tunnels are dug using a drill that slowly rotates and grinds away the rock and soil. Welded steel pipe is often needed for boring, since other materials are limited in how far they can be drilled. Unlike other materials, steel pipe can be manufactured specifically for the tunnel being dug, and the distances to which it can be drilled is only limited by the constraints of the drill rig. Specifications like steel pipe wall thickness, internal diameter, joint types, and linings and coatings can all be designed custom to suit any tunneling project.
Steel Pipe Tunnel Linings
When digging a tunnel, grout is often used to stabilize the soil or fill voids behind the tunnel lining. This grout may contain a mixture of materials, though the most common modern lining material is concrete reinforced by steel pipe, which can be cast in place or prefabricated in panels.
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