Design of TBM work shaft and TBM launching chamber

Abstract

The paper is focusing on the considerations in relation to the design and constructions of the large diameter work shaft and the TBM launching chamber for Contract T03 of the Singapore deep tunnel sewerage system (DTSS). This contract includes the design and construction of a 6.0 m finished ID main sewer tunnel, a number of access shafts, lateral connection tunnels as well as structures on the surface. The main sewer tunnel was constructed by means of an earth pressure balanced shield tunnel boring machine (EPB TBM), which was launched from a 7.6 m diameter launching chamber excavated at the bottom of the 12 m diameter, 32 m deep work shaft. The design and selection of construction methods employed for the TBM work shaft were significantly influenced by the proximity of the works to the existing test track of the Bishan MRT depot and a highway interchange with depressed road, which was concurrently under construction at the time of shaft construction. Owing to the presence of the MRT test track in the depot area, stringent performance requirements were established which influenced the selection of construction methods and required groundwater recharging and ground treatment in the vicinity of the shaft. The purpose of the jet grouting treatment was to increase the wall stiffness for reduction of wall displacements and reduction of permeability in the surrounding soil effectively cutting off seepage into the excavated shaft. A total number of 11 groundwater recharging wells were installed around the shaft and along the site boundary to the MRT test track area. In addition, the work shaft in its permanent condition will form the junction of three main sewer tunnels of the DTSS with a de-aeration chamber from an adjacent sewer drop shaft and a temporary back shunt tunnel for TBM operation. Therefore, the lower section of the shaft was designed considering significant amount of shaft lining space consumed by these five tunnel openings. The launch chamber at the bottom of the work shaft was excavated based on the principles of the NATM and supported by sprayed concrete lining. It is one of the main principles of the NATM to tolerate a certain safe level of ground displacements, thereby utilizing the contribution of the ground to the support the excavated cavity. However, given the stringent displacement limits of the MRT test track, minimization of displacements was one of the major objectives for the initial support design. The about 8.5 m diameter excavation was carried out in three separate stages (top heading, bench and invert), which followed each other at closest possible distance in order to minimize ground movements and close the ground support lining as close behind the excavation face as possible. Checking face stability conditions was an integral part of the design of excavation sequence. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.