The efficient modern tunnel boring machine (TBM) that smoothly slices through clay, rock and shale is a result of many failed attempts at making a machine that is accurate, reliable and economical.

The Invention of the Tunnel Boring Machine

Though the first attempt was not even close to a TBM, it was the shield concept that is still used today that was born with the machine built by Sir Marc Isambard Brunel in 1825 to excavate under the River Thames for the Rotherhithe tunnel. The machine provided a shield under which the miners excavated and behind which the bricklayers made the tunnel lining.

The early 20th century saw London using variations of this concept to build underground railway tunnels called tubes. The "Mountain Slicer" invented by Henri-Joseph Maus in 1846 was designed to dig through the Alps between France and Italy to make the Frejus rail tunnel. The machine was about the size of a locomotive with about 100 percussion drills mounted at the front and driven mechanically.

However; the machine could not be used. Wilsons patented stone cutting machine, named after Charles Wilson, its inventor, was the first boring machine to be built in the United States. It was made of cast iron and used for the construction of the Hoosac tunnel for about 10-feet into the rock before it suffered an irreparable breakdown. The tunnel was later completed after many years using conventional tunneling methods of the time. There seems to be a pause in the attempts to use a machine for tunneling until about 100-years later when James S. Robbins made a successful attempt to build a machine that could dig through the very difficult Pierre shale at a speed of 160-feet in 24 hours. The rotating head technique used in the machine was what made a breakthrough by incorporating cutting wheels that gave rise to the modern TBM.

Tunnel Boring Machine Designs

The slurry shield machine (SS), also known as a closed-face machine, is used to excavate soils of varying hardness. The excavated soil is mixed with slurry to withstand the excavation by creating a positive face pressure. The soil to be removed is pumped out along with the slurry to a separation plant situated outside the tunnel where the slurry is separated from the muck for recirculation.

An earth pressure balance machine (EPB), is also a closed machine and is used for cohesive soils that are soft. The excavated ground is kept under pressure by controlled removal of the muck from the screw conveyor to create the positive face pressure required to keep the excavation stable. The spoil is later removed using conveyor belts or skips.

Hard rock machines are used for excavating rocks. It employs cutters or discs to crush the rocks and are specially designed to resist hard and abrasive material. The crushed rocks are removed using conveyors or skips.

Components of a Tunnel Boring Machine

A typical tunnel boring machine (TBM) consists of one or more shields and associated supporting machinery. The front end of the shield supports a cutting wheel behind which is a chamber that may be a closed machine i.e. under pressure or an open machine i.e. exposed to the exterior.

Depending on the size of excavation and strata encountered, the cutter head rotates at a speed of 1 to 10 rpm. A set of hydraulic jacks are positioned behind the chamber and are supported by the completed section of the tunnel. The rear portion of the TBM braces against the tunnel walls and uses it to push back against in order to advance the cutter head.

Support mechanisms that are located behind the shield include slurry pipelines, conveyor belts or skips, control rooms, transport rails for precast tunnel segments, electrical systems, ventilation mechanism, a dust removal system, etc. As the cutter head progresses, it excavates rock or soil and conveys it outside the tunnel system using conveyor belts and slurry pumped out though slurry pipelines respectively.

Hard Rock Tunnel Boring Machine

Hard rock tunnel boring machines use both shielded and open face machines. Disc cutters are mounted on cutter heads and brought to the face of the excavation. As the cutter head rotates, it performs a chipping action by creating compressive stress fractures in the rock just in front of the machine.

Open Face Tunnel Boring Machine

Open face tunnel boring machines have the area just behind the cutter head open for rock support and the machine advances by using a gripping system that uses the side walls of the tunnel to push against to create the forward thrust. When one stroke is completed, the rear legs, grippers and propel cylinders are lowered and retracted and repositioned for the next stroke. In this method, the excavated rock face is not supported by installing concrete segments but is held up by rock bolts, ring beams, wire mesh or shotcrete.

Shield or Closed Face Tunnel Boring Machine

Shield or closed face tunnel boring machines use both double shield and single shield technology. They are used in fractured rock and require using concrete segments to support the excavated unstable rock face.

When the tunnel walls are stable, the TBM can grip the side walls for advancing, but in case of fractured and unstable ground, thrust cylinders are used to shift the thrust and push against the installed segments instead of the weak tunnel walls.

Soft Soil Tunnel Boring Machine

Soft soil TBM’s are either the EPB or SS machine and work on the single shield technology that uses thrust cylinders to push against the installed concrete segments to advance. In soft ground with a pressure of less than 7 bars, EPB machines are used and employ a cutter head with a combination of disc cutters and cutting bits.

The earth pressure balance is maintained by controlling the rate of machine advance and muck removal to create a stable environment. To further stabilize the soft rock condition, bentonite, foam and polymers are injected into the ground as additives.

Slurry Shield Machines

Slurry shield machines are closed face machines and are used in soils known to have large amounts of groundwater and high water pressure. Excavated soil is mixed with bentonite slurry that is used to convey the muck out via a system of slurry tubes that are attached to slurry chambers. The slurry separation chambers separate the cuttings from the slurry and are recycled back into the tunnel.

Tunneling with a tunnel boring machine through cities and urban neighborhoods comes with the challenge of preventing subsidence or heaving so that the ground surface remains undisturbed. Maintaining soil pressure during and after tunneling is about the only way to achieve this.

Slurry shield (SS) and earth pressure earth pressure balance (EPB) methods are useful in such situations especially when the soil is of varied strata. Another challenge is the presence of pipelines, utility lines, and other tunnels that may be passing through that area. To avoid cross boring of any sort, the primary need is to obtain a detailed geotechnical report with necessary geo information that will help plan a route that will avoid conflict with other utility lines or prevent damage to existing infrastructure.