Construction of shaft tunnel by slurry pipe jacking through sedimentary ground under high water pressure

Abstract

Pipe jacking technology has been developed rapidly in recent years to satisfy demand for distances of more than 1 km or curves of less than 10 m radius. A shaft tunnel driven by pipe jacking, another type of development, was adopted in order to provide ventilation to the Imaigawa underground reservoir tunnel under the national highway in Yokohama, Japan, which has been temporarily in service since 2001. Downward excavation of the 2000 mm ID ventilation tunnel starts from ground surface and reaches to the crown of the existing 10,800 mm ID reservoir tunnel, which is located 87.8 m deep, through the stiff clay layer. Depression angle of the tunnel is set as 75.6 degrees since the ventilation building is constructed in the existing park along the highway. Pore water pressure at the arrival is 0.66 MPa. This project is one of the largest in this type of tunnels in Japan. This paper focuses on equipment and facilities adopted on the project and construction methodology of the inclined pipe jacking. Specially-designed equipment and facilities were planned and employed to overcome anticipated difficulties; namely shield machine, jacks and reaction frame, lift-preventer, inclined elevator, and survey system. Slurry shield machine was specially designed considering the high water pressure, stiff clay excavation, accurate directional control, connection work to the existing reservoir, and simple dismantling. It shall be noted that the machine can not access the existing reservoir tunnel and the dismantled motors of the machine had to be removed through the tunnel driven. The thrust jacks and reaction frame were designed to facilitate easier setting of pipes and ground anchors were used against the reaction force of up to 8000 kN. To prevent the lift due to buoyancy to the shield machine and pipes during excavation, which was significant when the thrust jacks are released, the lift-preventer with power casing jack and ground anchors were developed. Inclined elevator, which is used for pipe connection and maintenance, was designed in such a way that it moves to give space to the pipe setting. Since accuracy was vital for the tunnel drive, survey system with laser theodolite and non-prism laser distancemeter (with automatic target recognition), which enables constant monitoring, was adopted. Relatively complicated construction sequence was developed bearing smooth operation and safety in mind. It was however completed successfully due to the well-prepared sequence. This method is considered effective for inclined shaft construction, such as ventilation shafts or access tunnels. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.