65 Introduction Anyone driving from Heerenveen to the Afsluitdijk over the A7 will discover that Sneek is still a considerable hindrance - the A7 motorway is interrupted by a 4-kilometre stretch of the N7 bypass to the south of Sneek. This situation will last until mid-2008 when the last section of N7 will have been upgraded to the A7 motorway. The eastern half of this southern bypass section will be upgraded over the existing route. About midway along the existing N7 section, the river Geeuw, the railway line from Sneek to Stavoren, the exit road Sneek-IJlst and a dikeroad will be crossed by the route of the new A7 motorway. In this new situation there will be an aqueduct under the Geeuw, at which over the access road on the west side viaducts will be constructed for the railway, the Sneek-IJlst road and in addition a local road that runs over the Alde Himdyk. The western half of the new A7 branches away from the existing N7 from the aqueduct and goes round the business park De Hemmen for which there are expansion plans. Geeuw project The structural Geeuw project is divided into ten subway sections, one water cistern, two prefab traffic viaducts, one railway viaduct and one aqueduct with the connecting pieces and caps. All insitu concrete structures were detailed by Witteveen+Bos, see next page. The railway viaduct was detailed by TCE. Railway viaduct The Sneek-Stavoren line is a single-track, non-electrified railway. The railway viaduct over the entrance to the aqueduct is constructed as a pre-stressed trough beam. The beam was placed directly on the corners (without intermediate abutments) on four bored piles. The piles were placed at night when the line was out of service. These concrete piles are 1.2 metres in diameter and are approximately 36 metres deep. They are reinforced over the top 14 metres. The rail trough is completely separate from the aqueduct. Only the final transverse restraint of the trough is carried by the outer walls of the aqueduct. The railway trough was constructed raised next to the railway dike and was slid into place in September 2006, with the railway being out of service for less than a weekend. Following this, the construction of the aqueduct that runs underneath was begun. The aqueduct crosses the railway at an angle of approximately 67°. The bored piles under the railway trough are positioned at right angles to the railway line at each end of the trough viaduct. The spanning of the trough is approximately 33 metres, at a construction height of 2.15 metres. The floor screed is 0.65 metres thick. Project strategy The railway viaduct is a slender design with relatively thin trough walls. Thus the percentage of reinforcement in the walls and floors is high. A large amount of reinforcement comes together at the end of the trough beams. The distribution of forces from the prestressing anchors results in a quantity of split reinforcement. The encasement tube for the prestressing cables runs through the split reinforcement. The split reinforcement is crossed by reinforcement from the floor. This reinforcement functions as a tension rod between the two walls. In addition, the walls of the trough beam are reinforced by transverse braces, torsion braces, flank bars, suspended reinforcement for the floor and compression reinforcement. Compression reinforcement is required to meet the ultimate limiting condition for the compression zone height to prevent brittle failure of the structure. This quantity of reinforcing steel can lead to problems if the position of the bars is not carefully worked out. The use of Allplan enabled a number of conflict points to be solved immediately by making certain bars a different shape. Checking the complex reinforcement is now much easier for the structural engineer, because the reinforcement can be seen on a 3D animation screen. A three-dimensional image in combination with two-dimensional cross-sections gives a good picture of the actual situation. This manner of working has helped to avoid a large number of conflict points during construction. Project technical data Project name: Construction of Geeuw aqueduct Sneek Principal: Province of Friesland / ProRail Location: Sneek Length: 38,95 metres Span: 32,95 metres Width: 7,45 metres Height: 1,95 metres Quantity of concrete: approx. 310 m³ Quantity of steel reinforcement: approx. 53,000 kg Railway of ‘Geeuw’ aquaduct The project concerned a railway viaduct for the SneekStavoren line, this is a single-track, non-electrified railway. The railway viaduct is constructed as a pre-stressed trough beam. The beam was placed directly on the corners (without intermediate abutments) on four bored piles. The piles were placed at night when the line was out of service. These concrete piles are 1.2 metres in diameter and are approximately 36 metres deep. They are reinforced over the top 14 metres. The rail trough is completely separate from the aqueduct. Only the final transverse restraint of the trough is carried by the outer walls of the underlying aqueduct. The railway trough was constructed next to the railway dike and was slid into place in September 2006, with the railway being out of service for less than a weekend. Realisation Geeuwaquaduct Sneek - Spooraquaduct
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