Bridging the Bays
With close to 20 years of preparation and construction, China's Hangzhou Bay Transoceanic Bridge-the world's longest transoceanic bridge-is scheduled for completion in 2008 and full operation in 2009. The notable transportation link in east China has been designed to be operational for more than a century and is projected to have a significant impact on the region's economic development. This significant bridge project has been called "the greatest construction project the Chinese government has ever undertaken." Due to the 36-km (22-mile) span and special position of the bridge, the world-class project is utilizing GPS technology to provide control for the construction process as well as to perform construction surveying.
Longest Span in the WorldWhen completed, the six-lane Hangzhou Bay Transoceanic Bridge will be the longest sea-crossing bridge in the world-and the first such structure to be constructed in China. Fifteen times as long as the Golden Gate Bridge and longer than both the famed Chesapeake Bay Bridge-Tunnel and Saudi Arabia's Baharian Causeway, the Hangzhou Bay Bridge is a two-way expressway with three lanes in each direction. It is the largest bridge project in China in terms of workload; the undertaking is estimated to require 767,000 tons of steel; 1,291,000 tons of cement; 11,600 tons of petroleum asphalt; 19,100 cubic meters of timber; 2,400,000 cubic meters of concrete and 7,000 foundation piers.
The highlight of the bridge's design is its unique shape. Elegantly shaped like an "S," the design has taken into account the aquatic environment of Hangzhou Bay as well as its aesthetic enjoyment to drivers and passengers. The project incorporates landscape design, borrowing aesthetic ideas reflected in the Su Causeway of West Lake, a classic scenic spot in Zhejiang province. Each of the two fairways of the Hangzhou Bay Bridge, north and south, includes an arch shape, which further adds to the project's charm.
Its Regional ImpactLocated in the Zhejiang Province on China's east coast, the bridge will be a shortcut in the coastal highway system, part of the country's highway system linking Tongjiang in the Heilongjiang Province (central east coast) with Sanya in the Hainan Province (southeast coast). The bridge crosses the Quiantang River at the Yangtze River Delta, one of China's most developed regions. The area contains 10 percent of the nation's total population and accounts for 22 percent of China's gross domestic product. The new bridge is expected to make the region's economy even more robust by strengthening linkages between the delta's economically robust and fast-growing cities including Shanghai. In fact, the bridge shortens the distance between Shanghai, the nation's business center, and Ningbo by 120 kilometers (75 miles), cutting its current four-hour travel time in half. The project is also anticipated to enhance Ningbo's importance in transportation, and help build Shanghai into an international shipping center, develop tourism and lessen the burden of nearby highways such as the Shanghai-Hangzhou-Ningbo Expressway.
Designed for a DecadeThe Hangzhou bridge project was in planning stages for almost a decade, much due to the extremely challenging sea environment, which is considered one of the world's most complicated. More than 600 experts from around the world carried out more than 100 technical studies on the bridge planning to ensure project success. The bridge includes a 32-km (20-mile) span over the Hangzhou Bay, a gulf on the East China Sea. Hangzhou Bay is widely known for its Qiantang River Tide, one of China's natural wonders and one of the three highest tides on Earth. The new bridge is expected to increase tourism to the area partly by increasing access to the Qiantang tides, which are already a major tourist draw. Millions of Chinese and international visitors have visited the China International Qiantang River Tide-Watching Festival since it began in 1994. Qiantang tides can create huge, deafening waves, reaching as high as 7.5 m (25 ft) and moving at speeds of 24 to 29 km (15 to 18 miles) per hour. The area also has difficult soil conditions, treacherous currents and typhoons. With these complex issues, building the Hangzhou Bay Transoceanic Bridge was expected to be a surveying and construction challenge.
Bridge contractor China Railway Bridge Bureau Group Co. Ltd. knew surveying and construction had to attain the highest standards in order to meet these challenges. The China Railway Bridge Bureau Group Co. Ltd. has constructed numerous projects including the first bridge over the Yangtze River, the Wuhan Yangtze River Bridge and the Shantou Bay Bridge. While conventional optical systems could be used for control near the coast as well as to position piers at a distance, the project requires highly accurate positioning 9-25 km (5.6-15.5 miles) off the coast. Conventional systems would either reduce the accuracy or require extra control stations, thereby demanding more work as well as potentially increasing the error accumulation. Therefore, engineering surveying became the primary technical challenge in building the 36-km-long (22-mile-long) bridge.
After carefully studying construction requirements, the Hangzhou Bay Transoceanic Bridge Control Center chose GPS technology. More specifically, the group selected GPS receivers capable of real-time kinematic centimeter accuracy in three dimensions. The Zhejiang Transportation Bureau held a special meeting on bridge standards in which they examined and adopted the "Special Technical Standards on the Hangzhou Bay Transoceanic Bridge." GPS surveying is an essential component of the standards.
GPS Meets Area's ChallengesBridge surveying requires high accuracy; more complicated bridges require even higher accuracy. A complex project such as the Hangzhou Bay Transoceanic Bridge would naturally place unique demands on GPS equipment. Initially, construction surveyors established a construction control network to create a coordinate system of the survey area as well as to enable precision surveying. They then established a Continuously Operating Reference Station (CORS) system based on the control network to provide continuous RTK GPS data. With this, construction surveyors were able to undertake the ongoing setting-out tasks. Finally, they carried out the as-built measurements to prove exactly where the structures were placed in the ground. In addition, after the bridge is in operation it will still require highly accurate surveying to monitor deformation (movement or change due to the environment or heavy usage).
"The bridge project could not be done without a unified GPS reference station system," says Xiao Genwang, chief director of the control center and deputy general manager of the 1st Engineering Co. Ltd. under the China Railway Bridge Bureau Group. "The stations can provide information for all builders, thus ensuring the coherence of data. In addition, the 12 bid sections could be under construction any time, which requires reference stations that operate at all times and under all weather conditions. As a result, the CORS system has become very important to us." Because of this reliance, the project control center established an RTK GPS CORS on each side of the bank to provide accurate real-time positioning for the sea operation.
During construction, the GPS reference station system must operate continuously, a very demanding requirement for GPS equipment. Approximately 65 Trimble 5700 dual-frequency waterproof RTK GPS receivers are employed at the sites where the bridge will span the Bay of Hangzhou. The receivers continuously track satellites, improve survey efficiency and ensure high accuracy in any type of weather. Three Trimble 5700 CORS GPS systems are used as base stations to broadcast the correction data required for precise measurements of the project. Additional Trimble 5700 GPS systems are located on piling barges to provide accurate real-time positioning of the piers and pre-fabricated sections of the bridge. The pre-fabricated sections are moved to the appropriate location on barges and then positioned using GPS for position and orientation.
The importance of the GPS equipment is highlighted in the positioning of piers, which are fundamental to the bridge. Any error in positioning could render a pier useless, resulting in irreversible damage to the project. Because of the rough waves and high tides of Hangzhou Bay, pier positioning has proved uniquely difficult. GPS technology has become crucial in responding to these challenges.
The Trimble 5700 RTK GPS systems are also utilized for surveying coastal topographic detail and hydrographic surveys of the seabed. Using equipment from the same manufacturer across the whole construction site has enabled all project surveying, mapping and construction to share a common geodetic reference system. In addition to GPS, the project utilizes Trimble DiNi 12 digital levels and Trimble Geomatics Office software. According to the China Railway Bridge company, the equipment helps improve efficiency while reducing costs, provides high accuracy and very close tolerances, and is easy to use and rugged enough for any weather condition. And with the GPS base systems providing correction data across the entire construction area, overall project accuracy is assured and working efficiency is improved.