Construction of two road underpasses at Ma On Shan, Hong Kong SAR

Abstract

Two road underpasses each some 200 m long, 16 m wide and 12 m 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 alignments are curved horizontally with gentle vertical gradients and encounter weak, altered and metamorphosed sedimentary rock and strong granite rock. Power tools and drill and blast techniques were employed for the tunnel excavation. Due to the presence of heavily sheared stratified rocks and an inclined principal joint set system, the influence of groundwater seepage on tunnel stability was critical. The initial temporary support system adopted for the weak rock excavation comprised a combination of support canopy (forepoling), a lightweight steel set and fibre-reinforced shotcrete. Detailed rock mapping and monitoring of excavated rock faces were undertaken to optimise temporary tunnel supports. 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 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 associated with production blasting. The vibration as detected at ground surface was monitored to be all within the allowable maximum peak particle velocity of 25 mm/s. Permanent lining construction was facilitated with the use of movable lining shutters and the underpass construction was able to be completed one month ahead of programme. Prior to and during construction there was significant interaction between the contractor and a geotechnical specialist acting as an advisor to the tunneling contractor. The full-time input of the specialist advisor has proven to be worthwhile and cost effective leading to optimization in temporary support systems in poor quality rock and blasting patterns in more competent rock. The specialist geotechnical advisor was engaged in regular and detailed mapping of exposed rock faces, collection and interpretation of monitoring data and high level analyses of construction options. This led to an efficient and productive communication and problem solving relationship with the engineer's staff on site and in their design office. The regular input from the advisor made a significant contribution to the completion of the tunnel excavation works one month ahead of programme, despite a delayed start. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.