Clayey soils might lead to swelling, settlement and strength issues when considered as foundation or fill material. The use of additives to stabilize soils has been a major concern in the improvement of engineering characteristics. Upper Pliocene-aged clays cause various geotechnical problems in the residential area of Ankara. The aim of this study is to investigate the effect of lime, gypsum and lime with gypsum mixtures on the swell percent, swell pressure and unconfined compression strength of highly plastic clays in the Batikent region to specify an appropriate mixture ratio for soil stabilization. The optimum water content and maximum dry unit weight of clay were determined using standard compaction tests. Lime, gypsum and lime with gypsum mixtures were re-compacted in order to evaluate their effect on the swell percent, swell pressure and unconfined compression strength over curing time. Swell percent and swell pressure reduce with increasing additive percent over time, while the strength increases. The lowest improvement ratio is determined for gypsum. The optimum mixture is concluded to be 6 % lime, which corresponds to a 99.55 % decrease for swell percent, 98.98 % decrease for swell pressure and 191.87 % increase for the unconfined compressive strength for 90 days.

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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.

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