Evaluation of excavation induced surface settlement of Kirazlı and Ikitelli twin metro line

The control of ground settlement induced by construction of metro tunnels in metropolitans and their effects on buildings are of great importance. Surface settlement induced by tunnelling is dependent on many factors such as soil and groundwater conditions, the thickness of the cover over the tunnel, the size and shape of the excavation section and construction techniques. Within the scope of the thesis the subsidence which was observed during the asynchronous tunnel excavations which are parallel ,and close to each other, evaluated. The thesis focus on modelling and prediction ground subsidence associated with construction of Kirazlı-İkitelli twin tunnels bored by TBM mechines. The focal point of the study is the modeling of surface settlements due to the shallow twin metro tunnels built between Kirazlı and İkitelli and is the comparison of calculated and mesured settlement. The tunnels was excavated between 2008 and 2010 with four Tunnel Boring Machine (EPB-TBM). The excavation diameter of tunnels is 6.50 m. The investigated section of metro tunnels is 2.2 km. long and excavation was carried out in Miocene aged Güngören Formation. The metro tunnels were opened in the Güngören Formation with grayish green color, clay, sand and silt level, and upward marl intercalation. The excavations made with the TBM machine are at a depth of 25 m. to 41 m. and are driven below the groundwater level. Generally, more groundwater has been found in areas where sand levels and lenses are located. In the study, the research data determined in the feasibility study of the line were taken as basis. The engineering geology of the twin tunnel route was prepared by taking into consideration the exploration drilling data on the line and the formation and conditions encountered during the opening of the tunnels. Coordinates of the measuring points pre-installed on existing building facades have been measured and monitored periodically, depending on the time, to monitor and control the surface subsidance on the line. The periodic readings obtained for specific monitoring sections, characteristics of the geological environment, progresses, cover thickness and hydrogeological conditions were evaluated together, and the effects of tunnelling on soil disturbance were examined. The measurements of surface settlements were analyzed to highlight the impacts of contruction two side by side tunnels. The surface subsidance obtained from the monitoring data were evaluated by Gauss function and 3D finite element analysis results were compared. Since most of the investigated tunnel passes through sandy clay-sand levels, it is assumed that the drained conditions are valid for the analyzes and Mohr-Coulomb liner elastic model is used for the soil parameters. The settlement formed after the opening of the tunnel advanced from the front were compared with the settlement formed from the second tunnel passing from the same section. The influence of the surrounding soil on the settlement magnitude during tunnel boring process was analysed by using phased tunneling. It has been recognised that the settlement trough calculated in cross-sections, using finite element method is similar to the settlement trough observed in field measurements. In the analysis section, tunnel excavation at 31.25 m. covering thickness causes a maximum settlement of 13 mm. It was determined that the amount of settlement measured on the second tube after the passage of the first tube exceeded this value (13 mm) due to the change in natural stress condition The computed surface settlements by finite element analysis are compared with the field measurements. The comparison indicates that the surface displacement profile computed by the FEA has the same trend as the surface displacement profile obtained from the monitoring data. Driving of the first tube resulted in a settlement of 13 mm, this value reached 30 mm after the second tube was excavated. The maximum settlement computed by finite element analysis is 25 mm. A good correlation coefficient (R2 = 0,99) was determined between the internal friction angle and the k parameter which gives the width of the settlement trough. For the Güngören Formation k parameter can be determined by 1-tan.