CHINA - DALIANGSHAN NO 1 HIGHWAY TUNNEL | PROJECT
Above: A central pilot drive was adopted for the first time to investigate the geology of the tunnel alignment and to support the construction and operation of the twin-tube express highway tunnel
karstic network with a huge volume of water. The flow is being directed into an underground sump to be used for both firefighting and maintenance purposes during the operational life of the long expressway tunnel. With the long drive to reach the only intermediate
adit, a tunnel boring machine (TBM) had to be used for most of the pilot excavation. The distance would not support adequate ventilation for drill & blast excavation, although the tunnelling method was used from the opposite portal and was expected to complete approximately 5km of the pilot tunnel; the TBM would complete the other 10km. An open gripper TBM was selected to bore the
majority of the pilot tunnel to allow for observation of the geology as encountered rather than being closed off by a shielded TBM and precast concrete segmental lining.
PILOT TUNNEL Next, the project managers had to decide where to locate the pilot tunnel and how to justify its added cost. In addition to enabling more investigation of the geological conditions, the pilot would further assist construction of the main highway tubes by: ● Providing space for laterally drilling pre-excavation grouting (PEG) and drainage arrays into the alignment of the main tubes;
● Supporting the ventilation needs during drill & blast advance of sections of the main tubes (these, and cross passages, are also excavated by the method);
•● Reducing the vibration impact in low strength and fractured rock;
● Shortening excavation of the main tubes by providing access points to open additional drill & blast faces and mucking out routes; and,
● Providing additional access points for advancing the final waterproofing and in-situ concrete lining processes of both the pilot and the main expressway tubes.
In addition to these in-construction benefits, the pilot would have to have a purpose for the twin highway tunnel in operation. This came to mind readily for the project managers: the pilot would run between the two main tubes at an elevation slightly lower than their invert levels, and serve subsequent highway operations as: ● A central drainage facility; ● A facility for emergency ventilation purposes; ● A refuge for people in the event of an in-tunnel accident;
● An access route for emergency and maintenance vehicles; and,
● A bypass to isolate sections of the highway tubes for maintenance operations.
Above: Limestone with karstic features, mudstone predicted to exhibit squeezing conditions, and faulted and fractured basalt raised the geological risks expected on the tunnel project
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