Earth Pressure Balance Shield

Published: October 30, 2017 | Last updated: July 5, 2023

What Does Earth Pressure Balance Shield Mean?

An earth pressure balance shield (EPBS) is the front shield for an earth pressure balance tunnel boring machine (EPBTBM). While the terms are distinct, they are sometimes used interchangeably in common parlance. EPB technology uses mechanical pressure at the face of the tunnel to counterbalance earth pressures. This negates the need for compressed air to control subsidence of the tunnel face. In that sense, it has revolutionized soft-ground tunneling and become a prevalent tunneling method in high-traffic areas.

The acronym of earth pressure balance shield is often used in one of two ways: “EPB Shield” or “EPBS.”


Trenchlesspedia Explains Earth Pressure Balance Shield

The earth pressure balance shield is a mechanical device that uses a full face to support the ground in front. The selection of an EPBS for a particular job is dependent on the type of soil being bored. An earth pressure balanced machine (EPBM) is the first choice in homogenous, cohesive soils. With EPB, the extracted soil itself acts as a supporting medium for the machine. Conditioning agents are injected into the soil to make it more suitable to the task. The balance of the shield is controlled by the careful coordination of the advancement speed and the speed of the excavating screw.

Tunneling shields are nothing new. The first use of the technology was by Marc Isambard Brunel in 1818. He used the method to create a tunnel under the River Thames. An advancement known as the Greathead Shield was used in the construction of the London Underground beginning in 1884. The earth pressure tunnel boring machine (EPTBM), like other tunnel boring machines (TBMs), uses a rotating cutting wheel and an automated soil extraction system. The forward EPB shield is reminiscent of the Greathead design.


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