Large span mined tunnels in soft ground: support design, instrumentation and observational approach
The northern 200m of the Liantang Lung Shan highway tunnels in Hong Kong comprises large span mined tunnels excavated through poor ground with low cover. Conventional staged tunnel excavation techniques, with heading (160m) and bench (115m), was adopted. An instrumented observational approach was developed to verify an optimised temporary support system for these challenging ground conditions. Ribs were instrumented and limit settlement and load values defined which if respected allowed the omission of top heading invert beam and increased bench advance lengths equating to a programme saving of 70 days compare to the original design. This intensive monitoring scheme enhanced control and safety of the works allowing a clearer understanding of load evolution through the various stages. This paper will present a comparison of the initial and optimised design together with the monitoring measures implemented.