Anchoring of a retaining structure with glass fibre strand anchors

Abstract

In Copenhagen, Denmark, a large construction pit has been designed to facilitate the excavation for the basement of a new district in the city called “Postgrunden”. The retaining wall is constructed by sheet piles installed into the Copenhagen Limestone with the use of pre-drilling and with ground anchors in two levels. The construction pit as well as the anchors are temporary, however, both parts are left in the ground after construction of the basement has been concluded. Most of the anchors were constructed as normal steel strand anchors, however, at one location glass fibre anchors were found to be the best solution due to restrictions concerning installation under third party property. Therefore 82 ground anchors with glass fibre strands were installed. These anchors were the first of their type used in production in Denmark. However, in Denmark there is no Eurocode or National Annex on composite materials at the time of the design and therefore the designers had no direct standard to design by. The design of the glass fibre anchors was therefore based on the index of safety method from Eurocode 0 along with a set of fracture tests made in a laboratory. The tests were used for a statistical analysis that along with a required index of safety gave the characteristic ultimate resistance of the strands and the elastic modulus. During installation the contractor had to consider the different material behaviour of the glass fibre strands compared to the traditional steel strands, e.g. the brittle shear tolerance, that dictates the assembly of the glass fibre strands to steel strand couplers to allow for testing and prestressing of the anchors. Further, the glass fibre anchors are not as heavy and stiff as the traditional steel anchors, which added an extra challenge during installation below the ground water table. Anchor testing was performed using traditional anchor testing equipment. The hollow plunger jacks were chosen in respect to the long piston stroke required for the extra elongation of glass fibre strands due to a low modulus of elasticity. All anchors were tested with either suitability or acceptance test to a factor of 1.43 or 1.30 above the design load, respectively.