Analysis of the performance of retaining systems in deep excavations in greywackes

Abstract

Due to the economic growth, the need for high-rise buildings and shopping malls with multiple basement levels increased in Istanbul and construction of retaining structures to support deep excavations reaching 25 to 40 meters became compulsory. The encountered subsoil formation in the deep excavations is soft rock greywacke locally known as Trakya formation, lithologically alternating sandstone, siltstone and claystones with various degrees of weathering and fracturing. This thesis covers an investigation about the retaining systems widely used in deep excavations in Istanbul. Since Istanbul is in a seismically very active region and obviously at risk of being hit by a major earthquake, flexible retaining walls should be preferred in deep excavations carried out in the city. Therefore special emphasis is given in soil nailing as a flexible retaining system. Six major case studies of deep soil nailing walls having total surface area of 63,000 m2 are thoroughly examined and the results of the performance of walls with different heights are compiled. As a result the values of performance ratio for soil nailed walls together with nail length and density in typical greywacke formation of Istanbul are developed based on these extensive case studies as a guideline for future applications. Finite element back analysis is utilized to verify stiffness parameters of greywacke formation resulting the measured lateral displacements and to come across a relation between the actual lateral displacements and the stiffness parameters, as well. Additionally, a significant case study is presented in this study for the utilization of various retaining systems for different subsoil and groundwater conditions encountered within a given site. Displacement data and experience obtained from this case study is provided as a valuable source of data and example for future applications in similar conditions within the city.