Shield Tunneling Method
Definition - What does Shield Tunneling Method mean?
The shield tunneling method is a boring technique that consists of a shield (a protective metal cylinder) and trailing support mechanisms. During this trenchless method, a rotating cutting head is positioned at the front of the shield.
The shield acts as a temporary support structure that prevents the surrounding soil from becoming unstable. This is especially critical in large tunnels, where extreme ground pressures can result in cave-ins and other safety hazards.
The cutting head advances the boring machine through the soil for a short distance until the tunnel can be lined with a stable support structure (typically concrete lining segments). Once the permanent support structure is in place, the machine pushes itself off the lining segment via hydraulic jacks to drill further into the soil.
In addition to supporting the surrounding soil, the shield also houses several mechanisms, including soil removal systems, slurry pipelines, control rooms, and rails for transporting the lining segments.
The shield tunneling method may be done with drilling slurry (slurry tunnel boring machine) or without (earth pressure balance or EPB shield). Similar to other trenchless drilling methods, systems are in place during shield tunneling to remove the soil from the borehole to the ground surface.
Trenchlesspedia explains Shield Tunneling Method
Figure 1: Parts of a shield tunnel boring machine (source)
The shield tunneling method allows for continuous boring production while maintaining safe work conditions. The first documented method of this trenchless construction technique was in 1825, when British civil engineer, Marc Brunel, used a rectangular tunneling shield to excavate and construct the Thames Tunnel. In 1840, American inventor and patent lawyer, Alfred Beach, suggested that a circular shield may be more effective.
How Does Shield Tunnel Boring Work?
Shield tunnel boring can be broken down into two distinct phases: the tunneling phase and the ring building phase.
During the tunneling phase, the rotating cutting wheel at the front of the shield is pressed against the tunnel face via hydraulic cylinders. This rotation action grinds the soil, breaking it up into small fragments for removal.
Directly behind the shield and cutting wheel is a screw conveyor system (which looks similar to an auger drill) that transports the soil to the rear of the shield. At the same time, the hydraulic jack pushes against the outer tunnel segments to drive the cutting head forward.
When the shield and cutting head are advanced to a predetermined distance, the ring building phase begins. A mechanical transportation system in the tunnel boring machine installs the next lining segment directly under the shield. This new segment then acts as a support that allows the hydraulic jacks to further push the cutting head into the soil.
If necessary, water, bentonite, or foam can be applied to the tunnel face via an injection system at the front of the cutting wheel assembly.
For shield tunneling in unstable soils, the instability of the soil at the tunnel face is counteracted by creating a support pressure behind the cutting wheel. In the case of the EPB shield method, the soil removed from the tunnel face is used to support the tunnel face.
Figure 2: Section of the shield tunnel boring machine showing the balancing of earth pressure at the face (source)
The quantity of soil removed by the conveyor system and the quantity of soil removed from the tunneling process must be in a constant state of equilibrium to ensure that there is optimal counter pressure to support the tunnel face.