Surface Buildings Risk Classification and Evaluation of In-Situ Piles Effect in Mitigation the Settlements Caused By Excavation of Metro Tunnels in Urban Areas (Case Study: Tabriz Metro)

Fast, easy, safe and low cost transportation is the base of progress and economic development. In large cities the cost of land ownership and the improvement of urban structure and environmental problems reduce the possibility of rapid development of surface transportation lines. The need for the construction of underground lines of transport seems to be the right approach. Like all the construction projects, there will be social and technical consequences; perhaps the most important technical effect is the induced surface settlement due to excavation of tunnels and the construction of underground stations on surface buildings. In this numerical study, by using finite element software "plaxis8.6", the risk classification of surface buildings with various story and width are evaluated. Because of considerable effect of tunnel geometry and surrounding soil geotechnical properties on surface settlements, the region near to S4 station of Tabriz metro line 2 adopted as a case study. In this part the tunnel has a low overburden and the width of the street that tunnel route passed under it is low. Considering operational limits, the use of in-situ concrete piles in the vicinity of the surface buildings is choose as a mitigation technique. The length of the pile and their center to center space were matters of interest. Based on obtained results the risk classification of buildings reduce from 3 to 2 category due to construction of piles, which indicates good performance of this technique. Accurate examination of interaction between structure and soil can't be completely defined. Therefore, different methods of modeling the surface structures were applied. The soil behavior was modeled by the Mohr–Coulomb and hardening soil (HS) criterions. Also, with analyses more than 220 model, the effects of tunnel contraction ratio and street width on induced surface settlements and performance of mitigation technique were studied.