Field Application for Evaluating Vertical Stress of Soil in Multilayered Ground

In the west coast and the coast of Busan, at several locations, an overconsolidated layer exists above the soft ground. For soft ground with an upper overconsolidated clay layer, significant errors exist between the design and the actual settlements in the field. For multilayered ground, although Boussinesq's theory is applied, significant errors still exist. In this study, ground settlements in the overconsolidated clay layer were predicted using the Burmister and Hirai method. Based on comparisons with field measurements, it was confirmed that the accuracy could be increased by more than 90%.

Tunneling Excavation Induced Settlements in Granular Soils:Case of Keçiören Subway (Ankara)

The need for underground transportation is increasing to reduce traffic within metropolitan cities. During excavation, ground loss within the soft soils and associated ground settlements might occur. Tunneling induced ground settlement has to be within tolerable limits. Otherwise, damage to ground level and infrastructures are experienced. In this study, reasons of ground settlement occurred during the excavation of twin subway tunnels at Keçiören (Ankara) are evaluated. In 2006, the first phase excavation caused ground settlement with no considerable damage. The second phase excavation started in 2012, both within the saturated sandy and gravelly soils of Çubuk Creek’s alluvium. At the time, several collapses and associated ground settlements were reported. Due, an investigation program started including resistivity surveying, borehole drilling and in-situ testing. Since ground settlements were known, back calculation was performed to obtain the settlement trough, inflection point and the change of contraction increment. Moreover, the stress conditions and external stresses exerted by existing structures are reviewed. Collapses with volume up to 80 m3 were observed nearby the buildings and free field during the second phase excavation which were determined through 2D resistivity data and boreholes. It was concluded that, the change of settlement at ground level and on the tunnel segments are related to contraction increment based on the numerical modelling of the twin tunnels. Possible reasons for collapse can be listed as TBM cutting head and shield defect, change in ground structure due to underground water level changes, and the impact of the previously excavated tunnel in 2006.