Observed loading behavior during cross passage construction for Brisbane Airport Link project

Abstract

The analysis and design of cross passages for twin bored tunnel projects provides unique challenges when considering the 3D geometry, geotechnical behavior, and interaction between the internal tunnel structures and the ground. Current practice involves complicated modeling to approximate the cross passage behavior and facilitate design of the necessary support structures, often consisting of a combination of geotechnical solutions (e.g. grouting, ground freeze), excavation support (shotcrete, spiling, rock bolting), and structural solutions (internal props) to maintain the mainline tunnels and the opening space. However, little work has been done to validate these solutions with field data from construction projects. The Center for Underground Construction and Tunneling at the Colorado School of Mines has been provided with strain gauge field data for the Brisbane Airport Link, courtesy of Arup, one of the design firms on the project. A thorough evaluation of this data has been conducted to establish the development of forces in the mainline tunnel structures (segments and propping) throughout the cross passage excavation sequence. Results from the gauges have been compared to basic analytical and numerical solutions for validation. The observed behavior of the cross passages during excavation was established. Key mechanisms driving behavior include the effects of prop installation and jacking on the segmental lining, the unloading effect observed with geological excavation, the development of stresses due to soil arching, and the effects of locked-in lateral stresses around the tunnels. The influence of the two-layer heterogeneous geology of the project was determined to be a key factor in driving the majority of these behaviors. The analysis is concluded with a discussion of the importance of each mechanism to potential future design of cross passages and potential future developments for cross passage instrumentation schemes.