Near to Jincheng city, the newly-completed Xianshen River Bridge forms a critical element of one of China’s major north-south expressway links. The dramatic structure is on the Jincheng-Jiyuan Expressway works which is a section of the Erenhot Guangzhou Artery Highway.

The bridge nestles in the centre of a deep gorge and is flanked by steep cliffs more than 400m high. Its abutments link straight in to the portals of the tunnels which flank it. At one end the 3.5km-long Paipan Tunnel and at the other the 4.3km-long Yuehuquan Tunnel cut through the sheer precipices and deep pathless mountains along the southern foot of Mount Taihang.

At the bottom of the 20m-wide Xianshen River valley, below the project site, a seasonal river rises which becomes a torrential flood in summer while remaining completely dry for the rest of the year. The bridge is situated in a deserted and remote area with no population, no roads and no man-made structures to speak of. The inaccessible craggy peaks on both sides of the project corridor are part of the Macaque preservation zone. The nearest human settlement is Luohe village in Jincheng City; a 5km walk from the valley bottom below the bridge site or from the cliff top some 200m above the level of the bridge.

The geological survey revealed that the underlying rock at the bridge site consists mainly of Dolomitic limestone. At the tunnel portal where the bridge abutment is located, the bare rock was full of crevices, which meant it could become hazardous during bridge construction, with collapses or falling rocks likely to occur under blasting or other kinds of shock.

The extremely complex condition of the terrain made it impossible to find a way to investigate the geology locally, in particular on the sides of the valley at the bridge abutment/tunnel portals. Hence aerial survey was the primary means of data collection used for the bridge design.

Based on the masterplan scheduling of the whole project, the period of time for the planning, investigation, design and construction period of 30km of highways, bridges and tunnels was estimated at just four years. The scheduled construction period is particularly tight due to the complex terrain at the project site.

A solution had to be found to allow the design and construction of the bridge and the tunnels to be synchronised, while taking into account the practicalities of project implementation, economic viability and environmental protection. In the early stage of the design, comparisons were made of the technical and economical benefits of several different types of structure - a single-tower concrete extradosed bridge; concrete-filled steel tubular arch bridge and single tower steel-concrete hybrid girder cable-stayed bridge.

The most significant benefit of the single tower concrete extradosed bridge was that it would be founded on a main pier in the valley bottom, hence the ground conditions of the pier construction site were known, the quality and feasibility of construction were guaranteed, and environmental impact would be less severe.

Both the concrete-filled steel tubular arch bridge and hybrid girder cable-stayed bridge would be susceptible to the geological conditions of the V-shaped gorge. More importantly, it would be almost impossible to begin any construction of the bridge until the tunnel broke through into the valley. As well as numerous technical uncertainties, these solutions would have higher construction costs as well as taking longer to build, hence virtually ruling out these two alternatives.

After intensive analysis and comparison, the single-tower PC extradosed scheme stood out as the best choice to meet the possibility of synchronously designing and building both the bridge and tunnels, and it was considered advantageous to the steep, narrow conditions under which construction and transportation would have to take place. A further benefit was that its structure could potentially be modified to change the span length once construction had been started, depending on the results of the detailed geotechnical survey that would be carried out at the portal/abutment once the tunnel had broken through. What’s more, the construction technology needed to build this particular structural type would be easier to source. Consequently, single tower concrete extradosed bridge is selected for the project.

The bridge has two modest spans, one of 131m and one of 136m, making a total bridge length of 267m. The tower is 161m height up to the deck, above which it rises a further 53m.

The main girder is a three-cell single box, with oblique webs and a cross-section which varies along its length. The box is 11m deep at the pier top and narrows to 4m depth at the cantilever tip. On the top of the box the width is 26m and at the bottom it varies from 10m to 14m; the deck also has a 5m overhang. In the process of cantilever cast-in-situ construction, each box segment is 4m long. As well as the starter segment, 30 pairs of girder segments were cast in site during cantilever construction. A 4.4m-long cast in situ segment was set at end from which construction started, and a 7.4m-long cast in situ segment was constructed at the other the end. At mid-span, there is a 2m-long closure segment.

To improve the aesthetics of structure, a solid rectangular cross-section with chamfered edges was adopted for the main tower above the deck, while the pier below the deck has an octagonal cross-section with a 1.5m-thick wall. Straight slope variation is applied to pier section from top to bottom.

The base of the pier had to be strengthened to resist loading from debris during the flood season, and several measures were adopted for this.

Internal stays were set up for the pier wall within the bottom 22m above the pedestal of the bearing platform; 15 stiffening ring beams were installed at intervals of 9m vertically within the hollow pier column in order to improve local stability. A simple stair access and lighting system was also incorporated within the pier for maintenance use.

In total the deck is supported by 22 stay cables, which are arranged in 11 pairs which run from the deck anchorage at one side of the tower, through the cable saddle in the tower, and down to the deck anchorage at the other side of the tower. Each cable consists of 55 strands; the longest cable is 245m and the shortest is 92m.

The anchorages in the deck are connected to every other girder segment along the centre line of the deck at a distance from 34.5m to 114.5m from the tower.

OVM250AT-55 stay-cable system was selected for the bridge; the main benefit this system offered was at the cable saddle on the tower, which consists of welded parallel steel guide pipes. Instead of the whole cable bundle going through a single saddle, each cable is threaded through a separate steel guide pipe, ensuring that the cables will not affect each other after being tensioned. This type of saddle is capable of improving the stress condition in the tower, distributing and transfering the loads evenly, and overcoming the partial stress concentration of a single pipe saddle.

The strands are distributed evenly in the slide-resistance anchorages, where mortar filling grips the strands, giving sufficient slide resistance capacity to overcome the unbalanced force on both sides of cables. In addition, the slide resistance force can be calculated which solves the problems of an uneven gripping force and poor slide resistance effect.

Meanwhile, epoxy-coated prestressing strands with several protective layers were also adopted to give corrosion-resistant stay cables. No grouting is needed for the saddle section, there is no need to remove the polyethylene coating from the unbonded strand, solving one of the problems of creating a corrosion-resistant saddle. The cable system has a fully waterproof structure, preventing water from penetrating into it. Two sets of dampers were installed at the two ends of the box girder to provide seismic resistance.

Before construction of the main structure could begin, a 5km-long concrete access road was built along the Xianshen River valley bottom to connect the bridge site to the supply area; it was specially designed to resist scouring in the summer floods. In order to secure the safety of the construction site, a dam was also built upstream, to intercept flooding and discharge it through a 500m-long spillway.

Groups of large-diameter piles were used for the foundations of the main tower. Once the piled foundation was complete, construction of the bridge was put on hold for five months while tunnel construction continued. The tower piers were then cast in situ using auto-lifting formwork in rises of 4.5m, each segment being finished in four to five days. Due to the tight restrictions on programme time, the construction of the tower continued throughout the winter period, using heat-retaining measures for the casting. In situ cantilever construction was used for the main girder construction, with each segment taking seven to nine days to complete. The bridge was finished last year, and opened to traffic in December.

Owner: Jincheng Highway Company

Consultant: CCCC Highway Consultants Company.

Main contractor: Hunan Road & Bridge Group Corporation

Subcontractor of stay cable & anchoring system: OVM Machinery Company

Sanhong Zhang is general manager of Jincheng Expressway Company, Zhengrong Li is chief project engineer of CCCC Highway Consultants Company, Shulong He is project manager of Xianshen River Bridge at Hunan Road & Bridge Group Corporation, and Wenxian Li is senior engineer of OVM Machinery Company.

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