A case study: A deep retaining system construction with pre-stressed anchors

Pre-stressed anchored walls are one of the most common deep excavation support systems in Istanbul. The risk associated with deep excavation works is high as failures of retaining wall or anchors could be catastrophic and may affect surrounding areas. Therefore, the design of anchored walls for deep basement construction works requires careful consideration of soil/rock-structure interaction. This is usually accomplished using the finite element method. However, in order to produce safe and economical design, proper understanding of the soil/rock behaviour is very important.In this study, applicability of pre-stressed anchors as stability increasing elements in deep excavations performed in highly weathered graywackes involving considerable discontinuous features is investigated with reference to field data obtained in a deep cut retaining system project. It was concluded that application of tension forces to already present discontinuous zones in jointed graywackes might have led to unexpected deformations in spite of the fact that the original project appeared to be over safe in many respects. It would be a better engineering approach to employ rock nails instead of pre-stressed anchors to increase overall shear strength of the rock mass and to avoid opening of the cracks and triggering of slides on residual sliding surfaces. One should be cautious to determine the weathering state of the rock mass and discontinuities during the site investigation stage.