Development and practical applications of twin-screw shield method

Abstract

As Japan's population becomes more concentrated in large cities, the demand for urban facilities and diverse urban functions is resulting in greater congestion underground. Meeting this demand requires the development of a low-cost technology for constructing large-scale facilities at great depths below the surface. The current dominant technology for constructing tunnels in urban areas is the shield method, which includes both the slurry and mud-pressure methods. The latter method is seldom used at depths of 30 m or more below the surface because at such depths, the face of the tunnel is subject to high water pressures, which cause the muck and groundwater to spurt out from the screw conveyor of the shield-tunneling machine. The soil pressures at the tunnel face then become uncontrollable, causing the ground around the tunnels to deform. To solve this problem, Taisei Corp. and Ishikawajima-Harima Heavy Industries Co. Ltd. began researching screw conveyors in 1996 and recently developed the twin-screw (TS) shield method. In the twin-screw shield method, the muck is continuously and stably discharged by automatically controlling the rotational speed of the twin-screw conveyor. This maintains the desired soil pressure at the tunnel face at all depths, no matter how fast or slow the machine is advanced. The two screws are engaged and rotated in opposite directions, creating a mechanical seal zone in the conveyor that prevents muck and groundwater from spurting out from the conveyor. The benefits of this method include the ability: (1) to control the soil pressures at the tunnel face so that the surrounding ground does not deform, (2) to control the muck discharge volume by controlling the rotational speed of the twin-screw conveyor, and (3) to transport the muck from the discharge nozzle to the back of the machine. This method has been applied in two shield-tunneling projects, first for a utility conduit with the overburden of 22 m in cohesive soil, and then for a subway tunnel with the overburden of 8 m in sand and gravel. The success of projects proved the excellence of the method. For both projects, soil pressures at the tunnel face could be kept at a set value by the automatic control of the win-screw conveyor's rotational speed even when the advance speed of the shield tunneling machine was changed. As a result, deformation of the ground around the tunnels could be controlled. Further studies are under way for the measures against the wear of flights and casing of the twin-screw conveyor that is expected in the ground containing much gravel. The authors aim to make the twin-screw shield method into a standard mud pressure shield method by providing the twin-screw conveyor with greater durability through the studies. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.