Cunene River Bridge, Xangongo Winner

PROJECT AWARDS: WINNERS AND FINALISTS SAICE – MURRAY & ROBERTS STEEL CATEGORY: INTERNATIONAL PROJECTS XangongoCunene River Bridge, INTRODUCTION 1 The new Cunene River Bridge is an excellent feat of engineering achieved under extremely challenging conditions. The bridge links the major cities of Lubango and Ondjiva in south-western Angola, and replaces a host of temporary structures that were used to span the Cunene River after the last perma- nent structure had been destroyed early in the Angolan conflict. The strategic location of the old bridge as the main link between Angola and other southern African countries made it a prime target during the civil war. The Angolan Roads Agency (INEA) appointed Aurecon, together with its joint venture partners Gabeng and BKS, for the design and preparation of bid documents. This joint venture was also responsible for site supervision. CUNENE RIVER BRIDGE, XANGONGO The Angolan Government invested approximately US$49 million in the project. Construction on the 880 m long bridge WINNER – International Projects category began in January 2007, and its inaugura- tion took place on 14 September 2009. KEY PLAYERS 1 Construction of the new bridge over the Client Institudo de Estradas de Angola Cunene River at Xangongo in Angola – view Professional Team Aurecon in joint venture with BKS and Gabeng of completed concrete substructure Main contractor China Road and Bridges Corporation Civil Engineering | December 2010 89 2 3 Logistical and natural challenges on Hydrology and hydraulics N Reduced construction period required this project included landmines, floods The Cunene River at this location has for launching of structural steel box during the rainy seasons, the width of the a catchment area of 52 400 m2. The girders and no staging and formwork river, lack of skilled labour, difficulty in the maximum measured flood occurred required acquisition of building materials due to the in 1951. The peak flood was estimated Once the structural steel box girders distance from commercial centres, lack of to be 14 180m3/s and more than 50 years had been launched into position on the good aggregates, long supply lead times, later a higher flood has not been recorded. concrete substructure, construction of importation procedures and border delays. A free board of 2 m above the maximum the concrete deck could continue un- recorded flood level was provided. interrupted and was not affected by the PROJECT OVERVIEW The river is not very stable and the flooding river. The crossing site for the new bridge is main channels shift after each flood located 20 m downstream of the remains season. Therefore the bridge had to have Concrete precast decking of the original bridge structure over the an opening long enough to accommodate The concrete deck consists of 1 384 pre- Cunene River, northwest of the village of the different main channel positions. cast concrete elements with in situ cast Xangongo in the Cunene Province. The strips in-between. This method reduced bridge spans a perennial river channel of UNIQUE FEATURES construction time, as a large propor- approximately 100 m and then crosses It was important to the Client that the new tion of the deck could be manufactured the flood plain of the river for a distance bridge should be completed by the deadline. while the construction of the box girders of 780 m to the start of an embankment This had an influence on many aspects of and substructure was in progress. which traverses the remainder of the flood the design philosophy. The contractor’s con- Construction time of the concrete deck plain. The plain is regularly inundated struction sequencing and turnaround times was considerably reduced thanks to the during the rainy season, which starts played a significant part in the detailed speed of placing the precast elements, from approximately November each year design of the bridge works. In this region, and the reduced requirement for form- and lasts to the end of March of the fol- concrete aggregate is scarce and has to be work before fixing of reinforcement and lowing year. For a large part of this period, hauled from a long distance to the site. This placing of concrete in the connection the flood plain is often inaccessible due to led to the design of an economic solution in strips. Eventually a monolithic con- the high level of the river. the form of a composite deck. The special tinuum deck slab was provided to form features of the bridge include: the composite section. DESIGN APPROACH AND AESTHETICS From the start it was clear that the poor Composite construction Steel box girders foundation conditions would lead to a The choice of a composite construc- A comprehensive quality assurance re- bridge configuration with fairly long tion design utilising structural steel box gime was put in place to ensure that the spans and a deck as light as possible. It girders and a concrete deck had the fol- highest standards and quality of work- was found that a 50 m span length and lowing advantages: manship was maintained. The contractor a composite steel/concrete deck type of- N Reduced weight of the structure which engaged a special team of quality control fered the most economical solution. in turn reduced the size of foundations inspectors from the China National The design concept, using precast slab required Construction Steel Quality Supervision deck and manufactured steel girder sec- N Structural steel substructure could be and Test Centre to conduct quality con- tions, also meant that the new bridge could manufactured in parallel with the control testing both in the factory and on be completed in the shortest time possible. struction of the concrete substructure site. All welds were radiographically and 90 Civil Engineering | December 2010 4 2 Girders being transported in Angola 5 3 Launching in progress 4 Precast concrete deck panels on girders 5 Completed bridge open to traffic ultrasonically tested in the factory and on site to ensure the integrity of each weld. Since the girders would spend a long time at sea while being shipped from China to Angola, a comprehensive corrosion protection coating was specified for the structural steel girders. The use of steel box girders ensured that the biggest part of the bridge could be fabricated in workshop conditions with limited on-site welding. This ensured the quality of workmanship, while the testing of welding was better controlled. Furthermore, the relative light-weight steel construction ensured that the main structural element of the bridge could be launched from one side. The partially precast concrete deck slab with in situ infill panels was advanta- geous toward the overall construction in the shortest time possible. End tipping alignment of all girder sections. The posi- programme. The manufacturing of pre- was the chosen method for the piling tion of each girder section was noted and cast panels started at the same time as works of the bridge abutments. The same marked to ensure proper fit-up and align- the construction of the steel box girder. method was also used for the piers in the ment when assembling and welding the Only after the construction of the in situ secondary channels of the river. girders on site. Upon arrival in Angola, concrete infill, did the structure behave the girder sections were trucked 600 km in a composite manner. Deck construction to site – a challenging logistical under- The construction of 142 box girder taking in its own right. An assembly and Unusual construction aspects sections began in China while, at the welding yard was set up on site at one Foundation same time, 1 384 precast concrete deck end of the bridge to weld the consecu- Using a temporary fill, a working platform slabs were being constructed on site in tive girders together. All site welds were on the flood plain was created so that Xangongo village, Angola. Before ship- ultrasonically and radiographically ex- piling work would not come to a halt ping the girder sections to Namibe Port amined on site to ensure the integrity of during the five months of the year when in south-western Angola, the girders each weld. The sections were then incre- the flood plain was inundated. This ena- were pre-assembled in the fabrica- mentally launched using hydraulic jacks bled the 110 friction piles to be constructed tor’s yard in China to ensure fit-up and from the one end of the bridge. Civil Engineering | December 2010 91 Concrete deck slabs N Lack of skilled local la- The bridge deck consists of precast bour in the region. concrete panels and in situ concrete N Difficulty in acquisition of aggregates. strips designed to act in a composite N Complexity of construction due to the fashion with the structural steel box sophistication of the bridge design. girders. The advantage of constructing N Long lines of supply from China the deck using precast panels was that and South Africa for the pro- the casting of the panels could com- curement of cement, rebar and mence independently and ahead of other building materials. the fabrication and launching of the N Extremely poor condition of roads structural steel substructure of the which made the 600 km transport bridge. Since this reduced the time from port to bridge site very dif- required to complete the bridge, the ficult and time-consuming. contractor commenced the fabrication N Communication difficulties – the of the precast panels well in advance. majority of the contractor’s expatriate The 1 384 precast panels were finished staff did not speak English and all in good time, i.e. before the launching of communication therefore had to take the box girders had been completed. place via an interpreter, who was not a At the end of February 2009, the technical person; site meetings often contractor placed the first precast took place in four languages (English, concrete deck panels on the box girder Portuguese, Chinese and Afrikaans).

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