Construction of Two Road Underpasses at Ma On Shan, Hong Kong SAR

摘要

Two road underpasses each some 200m long, 16m wide and 12m high were constructed in tunnel during 2002-2003 to serve as slip roads as part of a new major trunk road (Road T7) in Ma On Shan, Hong Kong Special Administration Region. The tunnel excavation encountered weak, altered and metamorphosed sedimentary rock and strong granite rock. Power tools and drill & blast techniques were employed for the tunnel excavation. Due to the presence of heavily sheared stratified rocks and over extended lengths of the tunnel, the initial temporary support system adopted for the weak rock excavation comprised a combination of support canopy (forepoling), a lightweight steel set and fibrereinforced shotcrete. The initial temporary support design was re-assessed by numerical analysis (FLAC model) with a range of geological input parameters based on detailed face mapping and monitoring carried out during construction. This led to the subsequent omission of the invert strut of the steel set. The effects of blasting vibration on the closely spaced large span tunnels was monitored using seismographs and the results fed back into the design and procedures to optimise the production blasting. The vibration as detected at ground surface was monitored to be all within the allowable maximum peak particle velocity of 25mm/s. Permanent lining construction was facilitated with the use of movable lining shutters. The regular input from a specialist geotechnical advisor to the contractor helped improve the efficiency of the construction sequence and the excavation for the underpasses was completed one month ahead of programme.