49 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 arterial 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. The detailing for the railway viaduct was carried out by TCE (Transport Consultant and Engineers), see previous page. Subway The subway will consist of U-shaped concrete troughs for the two access roads and a closed section with at least 2,2 m depth of water from the river Geeuw running over it. Where possible at the ends of the two access roads a change will be made to a sheet (plastic) construction that will be built above water level at a maximum excavation of approximately -7 m below NAP (Normal Amsterdam Water Level). This transition will be determined by the bursting criterion of the reasonably well sealing layer of loam in the subsoil, which occurs between approximately -20 m and -16 m NAP. Screw injection piles will provide the foundation for the concrete construction that supports the structural floor. An underwater concrete floor will be used in the deepest section of the tunnel due to this bursting criterion. This will also be retained by the screw injection piles. The concrete construction will consist of 10 sections, of which the middle two will comprise the closed tunnel section. A steel cover plate will be placed over the section joints of the closed section to protect it from being damage by ships’ anchors. The floor and wall thicknesses of the U troughs will vary with the depth from 0,50 to 1,10 metres. The closed section will have a floor and wall thickness of 0,80 m and a roof thickness of 0.65 metres. The sheet sections will use 1,5 mm HDPE. In addition to the top sheet constructions at the two ends of the access, sheet constructions will also be used on both sides of the four eastern concrete sections. This is intended to keep the concrete outer walls low and to create an open view. The entire subway will be constructed inside sheet piling walls; large sections in a temporary polder construction. Consideration was given to making (partial) use of permanent polder constructions, but pumping tests showed that the degree of sealing of the layer of loam was insufficient for this and that also relatively salty seepage water was to be expected. Water cistern Close to the deepest point of the aqueduct, a rainwater cistern will be constructed for both the concrete and the sheet constructions. The concrete cellar will be divided into a wastewater section, from which the water will be pumped to the sewage system, and a rainwater section from which the water will be pumped to the river Geeuw during heavy rain. There will be an overflow between the two cistern sections with a screen to catch floating materials to prevent any possible floating oil or waste reaching the Geeuw. Viaducts for road traffic Both viaducts over the western access to the aqueduct will consist of inverted prefab T sections with a cast-in-situ compression zone. Project strategy The detailing of the in-situ concrete structures included the modelling (dimensioning) and reinforcement of these sections in Allplan and subsequently making the working drawings, for which translation from 3D to 2D was required. Of the ten sections, the four most easterly sections are straight and the remaining six sections are curved, both in their vertical and horizontal axes. This made the modelling particularly difficult. A so-called bridge and tunnel modeller that could handle this type of twisted section was used for this. This module made it possible to make the model with great accuracy. This was advantageous when the detail drawings were made. However complex the shape, every cross-section made is precise and you do not have to worry about the model anymore. You just have to concentrate on the layout of the detail drawings. The quality of the final product is thus many times greater, because there are no mistakes in the cross-sections. Furthermore, immediate insight is provided into areas of conflict that arise during the modelling of the aqueduct. A solution can be found promptly, because conflicts arising in a detailed design can be quickly visualised. Modifications can now be made during the design process, while formerly in many cases these would not have been discovered until construction, leading to potential delays and extra costs. At the time of writing this article the project is still under construction. Reinforcement of the aqueduct is being done at the moment. This is being done per section in conformance with the contractor’s construction plans. The reinforcement for the water cistern, which is located at the deepest point, has already been placed. The working drawings for this part have been completed and are ready for the construction. Project technical data Project name: Construction of Geeuw aqueduct Sneek Principal: The Province of Friesland Location: Sneek Length: 239,24 m Width: int. 23,85 m Quantity of concrete: approx. 8750 m³ Quantity of steel reinforcement: approx. 1.170.000 kg Realisation Geeuwaquaduct Sneek
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