Underground connection of cut-and-cover part and shield tunnels under the condition of high pressure groundwater

Abstract

An underground railway station, which is typically a box-shape structure approximately 20 m wide and 200 m long including platforms, concourse, and passenger facilities, is generally built under a street by the cut-and-cover method. However, the street above Oi-machi Station of Tokyo Rinkai-Line is only 15 m wide and carries heavy traffic. It is unrealistic to construct the station under the street by the cut-and-cover method. Thus, it is planned to build the station structure under the adjacent station plaza by the cut-and-cover method and to build the 5 m wide platforms inside the shield tunnels, which are 10 m in diameter and built one above the other under the street as shown in figure given below. Then, the station structure and platforms are connected in the underground. The connections between the station structure and shield tunnels are 30 m long along the tunnel axis, and moreover, lower part is located in the gravel layer under the condition of 0.3 MPa of high pressure water. The biggest challenges of this project are to ensure that water is properly cut off by chemical grouting and to minimize the movement of shield tunnels and surrounding ground when building the connection structure. It is planned to conduct the primary grouting from the cut-and-cover part before the shields pass, then the secondary grouting from the inside of the tunnels. Grouting pressure and grouting volume are monitored and recorded at all grout holes, numbering more than 1000. If the grouting pressure does not reach the designed pressure, additional grouting is performed. Water leakage is measured to the nearest millilitre per minute to ensure water-cut off by chemical grouting. Three-dimensional elastic analysis is conducted to evaluate the movement of the shield tunnels and the surrounding ground to determine the construction plan. As a result, the following measures are taken to minimize the movement of the shield tunnels and the surrounding ground: (a) The 30 m long excavation area is divided into eight sections, two sections vertically and four sections horizontally. (b) The excavation size is minimized for installing the side girders between the cut-and-cover part and the shield tunnels. (c) The eight sections are excavated as symmetrically as practical. (d) Preloading is introduced to the reinforcing struts for preventing segment deformation. This paper introduces the construction sequence, problems encountered during construction and the counter-measures taken, including measurement of the tunnels and the surrounding ground movement and effectiveness of the chemical grouting. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.