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Earth Balance Pressure Method

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Construction Processes

Earth balance pressure (EPB) tunnel shields or tunnel boring machines (TBMs) are used in excavating and advancing tunnels through any type of soft ground or soil condition, particularly below the water table.[1] Used in North America in only in the last three decades, the earth balance pressure method has actually revolutionized soft-ground tunneling as the technique has become widely applied in constructing shallow, soft-ground, close-to-the-surface tunnels in heavily congested urban areas. Today, earth balance pressure machines are commonly used in the excavation and driving of rail and highway tunnels, metropolitan subway systems, and other civil works projects that require tunneling in a soft soil, below the water table.[2]

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[edit] History

Earth pressure balance shields originated in Japan in the 1970s. The first reported use of an EPB tunnel boring machine was in 1974 for driving an 11-foot (3.35-m) diameter, 1.2-mile (1.9 km) long aqueduct tunnel.[3]

In the U.S., the first time an EPB machine was deployed was in the excavation of a 3,002-foot (915-m) long tunnel in 1981[4] as part of the San Francisco Clean Water Program.[5]

Historically, the method has been comparable to, and direct competition with, the use of slurry walls as a form of mechanized tunneling in ground that needed to be stabilized near the tunnel face and whereby excess spoil needed to transported to the surface.

[edit] Process

The underlying principle of the EPB method is that the excavated soil or muck itself is used to provide continuous support to the tunnel face by balancing earth pressure against the forward pressure of the machine.[6]

As the shield advances at the face, the cutterhead on the TBM rotates through the earth. The excavated soil is then mixed together with a special foam material that actually alters its viscosity or thickness and transforms it into a flowing material.[7] The use of a foaming agent to break down muck into a liquefied form provides some obvious benefits. This muck is then stored and controlled in a pressurized chamber located inside the cutterhead, and is used to apply support and balance pressure to the tunnel face during the excavation process. The foam acts as a lubricant that conditions the soil to a suitable fluidity, in effect reducing the risk of clogging in the pressurized chamber head or muck storage area.[8]

A screw conveyor then removes excess fluidized muck in controlled volumes from behind the cutterhead and in front of the “pressure bulkhead.”[9] Synchronizing the screw conveyor with the actual speed of the tunnel boring machine, and equalizing the actual volume of soil traveling into and out of the machine establishes earth pressure balance during excavation, thereby also reducing the risk of surface or ground settlement.[10] The performance of the EPB machine, however, largely depends on the actual properties of the excavated muck. The soil may range from coarse sands, to types of gravel, to stiff clays.

The EPB TBM also has the unique capability of placing a continuous ring of segmented liners from within the tail shield of the machine inside the tunnel as it advances. These concrete segments provide critical additional reinforcement and support and accomplish all tunnel construction in one-pass.[11]

There are many advantages to be drawn from using the earth pressure balance method on soft soil conditions. For example, the method eliminates a need for the use of compressed air. As little as 25 years ago, compressed air would have been used to advance a long tunnel.[12] Working in these conditions was very dangerous for workers. However, recent technological developments in tunnel boring machines such as the EPB have led to a safer, less hazardous working environment. There is also no chemical contamination to contend with. Equally, EPB tunnel boring machines are built to work well in small confined spaces and are easier to operate and control than conventional open-faced shields.[13]

[edit] Equipment Used

[edit] References

  1. Earth Pressure Balance Machines. Robbins, 2008-09-29.
  2. Earth Pressure Balance Machines. Robbins, 2008-09-29.
  3. Chou, Shao-Fan. Earth Pressure Balance Method, 2008-09-29.
  4. Abu-Farsakh, Murad Y. Computational model for the siumlation of the shield tunneling process in cohesive soils. Wiley: InterScience, 2008-09-29.
  5. Romero, Victor S.; Hansmire, William H. New Methods for Building Protection from Settlement due to Underground Transit Construction. Jacobssf.com, 2008-09-29.
  6. Earth Pressure Balance Tunnel Boring Machine. YDSS Servicing, February, 2007. (accessed: 2008-09-29)
  7. Earth Pressure Balance Tunnel Boring Machine. YDSS Servicing, February, 2007. (accessed: 2008-09-29)
  8. Earth Pressure Balance Tunnel Boring Machine. YDSS Servicing, February, 2007. (accessed: 2008-09-29)
  9. Earth Pressure Balance Machines. Robbins, 2008-09-29.
  10. Tunnel Boring Machine (TBM) Tunnels. Traylor Bros., Inc., 2008-09-29.
  11. Earth Pressure Balance Machines. Robbins, 2008-09-29.
  12. http://www.toronto.ca/ttc/coupler/0598/tunnel.htm
  13. Chou, Shao-Fan. Earth Pressure Balance Method, 2008-09-29.