Using Earth Pressure to Drill through Soft Grounds.

Where shield excavation is carried out in non stable soils, a loss in stability of the tunnel face is prevented by creating a support pressure. With Earth Pressure Balance Shields, the cohesive soil loosened by the cutting wheel (1) serves to support the tunnel face, unlike other shields which are dependent on a secondary support medium.The area of the shield in which the cutting wheel rotates is known as an excavation chamber (2) and is separated from the section of the shield under atmospheric pressure by the pressure bulkhead (3).

The soil is loosened by the cutters on the cutting wheel, falls through the openings of the cutting wheel into the excavation chamber and mixes with the plastic soil already there. Uncontrolled penetration of the soil from the tunnel face into the excavation chamber is prevented because the force of the thrust cylinders (4) is transmitted from the pressure bulkhead onto the soil. A state of equilibrium is reached when the soil in the excavation chamber cannot be compacted any further by the native earth and water pressure.

The excavated material is removed from the excavation chamber by an auger conveyor (5). The amount of material removed is controlled by the speed of the auger and the cross-section of the opening of the upper auger conveyor driver.The auger conveyor conveys the excavated material to the first of a series of conveyor belts. The excavated material is conveyed on these belts to the so-called reversible conveyor from which the transportation gantries in the backup areas are loaded when the conveyor belt is put into reverse.

The tunnels are normally lined with steel reinforced lining segments (7), which are positioned under atmospheric pressure conditions by means of erectors (6) in the area of the shield behind the pressure bulkhead and then temporarily bolted in place. Mortar is continuously forced into the remaining gap between the segments' outer side and the rock through injection openings in the tailskin or openings directly in the segments.

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