Design and construction of a jacked tunnel below a live railway line in Australia

This paper presents a case study of a 53 m long jacked box tunnel of 7.75 m width and 4.56 m height underneath a live multiple track railway in Gladstone, Queensland, Australia. Two design considerations for minimizing ground settlement were 1) use of sixteen canopy tubes above the jacking box roof; and 2) installation of soil nails for the tunnel face stability. In addition a shielding system was developed to ensure that excavation would be within the shield without being beyond the unsupported ground. A crucial part of the design and construction was settlement limits which required that the exposed tunnel face also to be at a batter angle of no steeper than 45 degrees to the horizontal during construction. The jacking platform design was involved in three concept developments to provide a minimum horizontal jacking load of 60,000 kN. The selected design option using a piled raft was found to be suitable for the proposed box jacking operation requirements. The ground settlement predictions were carried out using both empirical and numerical methods by taking account of the adopted ground support and jacking systems. A comprehensive instrumentation and monitoring plan together with contingency measures have also been employed to verify the settlement predictions. The monitored results indicate that the design and construction methodology was a success.