CHINA - DALIANGSHAN NO 1 HIGHWAY TUNNEL | PROJECT
PILOTING TOWARDS UNDERGROUND
HIGHWAY SUCCESS
Minimising construction risk in difficult geology ahead of excavations for the main twin tubes of the Daliangshan No 1 Highway Tunnel project, in Sichuan province, China, led to the developer to opt for a central pilot tunnel excavated mostly by TBM, a rarity for the sector. The project is the focus of much research. Shani Wallis reports
Below:
Two-thirds of the new 152km-long Lexi Expressway runs underground through twin tunnels
IMAGES COURTESY OF SICHUAN LEXI EXPRESSWAY CO
Prediction of bad ground ahead of a 15km-long, twin-tube four-lane dual carriageway highway tunnel through the Daliangshan mountainous region of Sichuan, in China, was giving the Project Director great cause for concern. The risk of encountering extreme geological conditions was high, ranging from: high water ingress and karstic voids in the predominantly limestone deposits; squeezing ground in reaches of 5MPa-20MPa mudstone; and, major fault features in basalt layers. Up to 93% of the long tunnel excavation runs through poor conditions of Rock Classes 4 and 5. Running blind into any of these conditions while advancing the 105m2
open face drill & blast headings
would put the workers, the equipment, and the entire project in jeopardy in terms of safety, increased costs and whole project delays. The solution for Project Director Lan Fu’an was to
promote a completely new and revolutionary approach to highway tunnel design, excavation, and operation. For the first time in highway design, this long expressway would start with excavation of a central, full-length
pilot between the main traffic tubes. This approach would change the design and excavation sequence of the project and of the fire-life safety, maintenance, and traffic management in operation. The central pilot would allow for investigation of the
geology and preparation for excavation of the two main tubes, and provide space to treat zones of poor ground both ahead of the pilot tunnel face and laterally into the alignment of the two main tubes. Fortunately for Project Director Lan, and after months
of discussion and convincing, the proposal to construct a 15km-long full-length pilot tunnel for early geological investigation was approved. This was a relief for the safety of workers as well as reducing risks associated with the overall project. While central pilot tunnels are well known for long
twin-tube railway links, this is believed to be the first full-length pilot tunnel for a highway project - and it took some convincing for the proposal and its added cost to be approved. As part of the decision-making process, the option to include the pilot raised the Daliangshan No 1 Highway Tunnel project to be a provincial government priority in Sichuan, and also an important national research project; this would bring research programmes and new design developments to the highway link project.
PROJECT DESCRIPTION The Daliangshan No 1 Highway Tunnel is the longest of several on the new Lexi Expressway where approximately two-thirds of the 152km-long route runs underground through twin tubes. In the mountainous region, most of the remainder runs on high-level viaducts that bridge narrow valleys. The full 33.5 billion Yuan RMB project (approx US$4.6
billion) will reduce the current journey time between the towns of Maibian and Zhaoieu from 5 hrs to 1.5 hrs. The investment is aimed to improve the quality of life, economic prosperity and connectivity of the citizens who live and survive in the remote valleys, towns, and rural areas of the Daliangshan Mountain region.
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