MULTIPLE TUNNEL INTERACTION

Abstract

An intricate pattern of four interweaving mass rapid transit (MRT) tunnels, each of 5.85 m diameter extrados, have been constructed through generally stiff to hard Old Alluvium overlain by very soft to soft clayey or loose granular sediments of the. Kallang formation and fill, as well as a pre-grouted buried channel of the latter sediments. A unique opportunity was thus presented to study a multiple tunnel interaction problem of unusual complexity. Hence a detailed field instrumentation programme was carried out at the site to measure ground and lining responses due to the tunnel excavations. To analyse the tunnel interaction problem, a finite element model of linear elastic, isoparametric quadrilateral and quadratic triangular elements was developed and systematically refined until site measurements were reasonably closely-represented by the model. Thus, bending moments and thrusts in linings as well as end-of-construction or undrained displacements in the intervening ground between tunnel excavations obtained from measurements were found to compare well with finite element predictions. Matching trends were also obtained between convergence measurements at linings and analytical prediction, such as the tendency of in situ tunnels to "bulge" towards an oncoming tunnel. Also, stress meter and total pressure measurements concurred on the general tendency for compressive stresses in tunnel linings to increase with greater tunnel interaction. Furthermore, it was found that, in accordance with known behaviour, lateral surface settlements due to single tunnel excavations fitted a normal distribution function well. Also, for twin horizontal as well as multiple tunnels, the overall settlement profile was biased towards earlier tunnel drives as a result of interaction with a current tunnel, corresponding settlements exceeding those obtained by superposition of individual tunnel effects alone. In addition, incremental longitudinal surface settlements including the effects of tunnel interaction were found to fit an error function distribution. For practical determination of bending moments between interacting tunnels following Peck's (1969b) approach, analytical as well as measurement results indicated a maximum radial deformation ratio of 0.1 per cent for half pillar width separation between tunnels decreasing linearly with pillar width to zero value at two diameters'separation. This result would provide the best economy of design of all published findings. Furthermore, both measurement and analytical results also indicated that the effect of multiple tunnel interaction was to generally increase bending moments and thrusts in linings but with a tendency for the latter to increase more significantly so that a greater margin of safety was ultimately achieved from the design point of view.