Shani Wallis, TunnelTalk
- Dignitaries gathered this week in Qingdao city, 700km south of Beijing in China, to open the world's longest sea-crossing bridge and its third longest sub-sea highway tunnel. The 41.5km long bridge and 7.8km long highway tunnel under Jiaozhou Bay inaugurated on Thursday (30 June, 2011) connect the historic centre of Qingdao with its economic and technological zones on Huangdao Island.
Record-setting bridge and Jiaozhou Bay tunnel crossings open
- The twin-tube six-lane highway,complete with central service tunnel, is China's second undersea highway tunnel and its completion follows that in 2010 of the 6km long Xiamen Xiang in the south-eastern Province of Fujian. Only the 10km long Trans-Tokyo Bay highway tunnel opened in Japan in 1997, and the 7.9km long Bømlafjord highway tunnel, opened in Norway in 2000, are longer.
- The new link was constructed within its 47-month schedule and for a total investment of 4.18 billion Yuan (approximately $US650 million). The tunnel is designed for 80km/hr road speeds and the two main tubes, at some 55m parallel, are connected every 750-1,000m with a vehicle cross passage and every 250-300m with pedestrian cross passages.
- The opening ceremony, which marked the end of what has been a deeply challenging sub-sea project, celebrated the achievements of a remarkably small owner's organisation of just 20 staff.
- "The project adopted the 'small owner, big society' concept of management," explained Jenny Yan Jinxiu, a director of the China Railway Southwest Research Institute in Chengdu. "Through the public bidding process, more than 30 big companies, including ours, were engaged for the research, design, independent third party supervision and construction of the project."
Undersea link in Qingdao city's highway system
- Further resources from the 'big society' were optimised by engaging renowned experts, scholars and academics from China and abroad to evaluate each plan and provide technical support. A number of large state-owned companies with a wealth of tunnelling experience were called in to address potential problems and solutions.
The new infrastructure is made up of two three-lane tubes and a central service tunnel
- Drill+blast of the two three-lane tubes and the 7m diameter service tunnel started in August 2007 from the Huangdaoside, and from the Qingdao side in September 2008. Final breakthrough was achieved in April 2010. Excavation was complicated by a relatively shallow 30m of rock overburden with a maximum 42m of water to mean sea level, a large excavated cross section for the two main tubes (up to 16m wide x 12m high) and identification of 18 geological faults, 13 of them in the 4km undersea section of the 7.8km tunnel.
- During construction pre-grouting was the key element for limiting groundwater ingress, with controlled blasting, a robust primary support regime, and continued monitoring of ground movement and convergence, ensuring safe excavation. "Application of advanced technology and a well-organised management system solved all technical and management difficulties," said Yan.
Each three-lane tunnel spans 28m
Three types of pre-grouting were developed to cope with the risk of water inflows. Full face, systematic pre-grouting was adopted for zones in which probe holes indicated high-volume, high-pressure inflows and weak surrounding-host rock. Part-face pre-grouting to stabilise poor geology in mixed-face cross-sections saved time and cost, while peripheral pre-grouting controlled medium volume and lower pressure inflows.
- To protect the tunnel from a corrosive environment of high chloride solution groundwater and high salt content mists at the portals, the 100-year design-life specifications required the use of C35 high performance shotcrete and C50 watertight concrete.
- Incorporating fly ash and silica fume additives increased the concrete's strength and thermal-crack resistance, decreased its permeability and prevented the alkali-aggregate reaction to greater extent. Raw materials and concrete were used in strict accordance with the requirements proposed by the concrete research team, to meet watertightness and corrosive protection performance of primary shotcrete and the in-situ final-lining concrete.
- Water inflow was controlled to a strict limit of 20 litres/min/100m of tunnel to minimise pumping costs. Through laboratory and on-site quality control testing of materials, application of new high-performance PVC waterproofing membranes and drainage designs and systems, and the arrangement of maintenance shafts and procedures to prevent clogging of the drainage systems, the amount of water inflow to date is less than the specification limits. Radial grouting was applied in zones where leakage exceeded the specification after shotcreting.
The Governor of Shangdong province, the Mayor of Qingdao, as well as other officials, attended the ceremony
Rock quality predictions
Comprehensive geological investigation methods were used during excavation to reduce risks associated with fracture faults and excessive water inflow. These included long-distance geophysical forecasting, short and medium-distance geophysical forecasting and systematic long borehole probe drilling.
- Long distance geophysical forecasting focused on identifying rock interfaces, faults and potential water or mud inflow, as well as the location and scale of such unfavourable geological bodies in front of the faces. TSP was used to predict faults and interfaces between soft and hard rock up to 150m ahead of the face, with high-resolution electrical equipment used to predict of water or mud inflows.
- Based on the results of long-distance forecasts, short or medium-distance predictions identified the type, location and scale of geological anomalies at 30m to 50m ahead the face. Preliminary predictions using digital geological drawings and detailed analysis were followed up with geological radar surveys in areas of concern. Predicted anomalies were then checked and confirmed by probe drilling and the deepening by up to 5m of the centre burn holes.
Interior of the new sub-sea highway link
- "These advanced techniques provided precise geological information, enabling us to reduce the blindness of tunnel construction and guarantee a safe and quick build," said Yan.
- An independent company was awarded the contract to check the stability of the excavation. Monitoring stations were installed at intervals based on the construction methods applied, geological information obtained from the 50m-further-advanced service tunnel excavation and the various grades applied to the quality of the surrounding rock.
- "The results of the monitoring and post analysis processes were used for informative planning, and for progressing the excavation, and will be used for the design of future tunnel projects," said Yan.
- Through application of the 'small owner team, big society' construction management concept and successful use of methods to predict geological conditions and monitor rock movements and stability, Yan confirmed Qingdao undersea highway tunnel was completed within its 47-month construction period and within schedule and to budget. This is a credit to all involved and to the systems deployed.
Links across the waters - TunnelTalk, Jan 2010
China's leading mega project status - TunnelTalk, July 2011
China's mega sea link moves forward - TunnelTalk, June 2011
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