A case study on the importance of local geology on selection of cut-off wall construction procedure

Abstract

A cut-off wall was implemented for a shopping mall to be constructed in Eskisehir/Turkey. The subject site is located near Porsuk River on an alluvial plain. Local geology of the subject site consists of Quaternary aged alluvial deposits overlying Eosen and Neogene aged marls and limestones. In the soil investigation results, it was reported that the alluvial deposits consists of sandy gravely layers with occasional boulders. The groundwater level was around 4.0 m below ground surface. An excavation of 9.5 m depth for foundations and basements of the mall was deemed necessary. Considering the available soil investigation data, a retaining structure composed of diaphragm wall socketed into the marl layers and supported with one level of prestressed anchors was designed. The method of construction was selected therefore to implement the diaphragm wall by means of hydraulic grab equipment based on the reported subsoil conditions. According to construction schedule imposed, the site had to be divided into two pieces in order to proceed partial superstructure construction. For this purpose, a cut-off wall to be formed by jet grouting in the mid of the site was also envisaged. During the implementation of the diaphragm wall, the encountered local subsoil conditions indicated that there exist very hard conglomeratic levels below alluvial layers in various thickness and elevation which was not identified during soil investigations. Therefore, the penetration of diaphragm wall through this hard conglomeratic layer was very difficult and therefore, since the diaphragm wall could not be closed on foreseen schedule the designed jet grouting cut-off became absolute and as a result undesired delay in completion of excavation work was inevitable.