Engineering geological characterizing of alluvial sediments along the Line A of Qom metro project, Iran

The A Line of the Qom metro project is planned as a 14.7km long underground transit system with 14 stations in Qom city, Iran. The tunnel will be 9.40m in excavated diameter and has been divided into two sections, one designated for mining by mechanized system using an EPB machine and the other by conventional methods. Construction of the deign build project began in November 2011 and conventional tunnelling section is currently under construction. To determine hydrogeological, engineering geological and geotechnical characteristics of soil layers a site investigation program were performed along the project. In two phases of geotechnical studies a total of 68 boreholes and test pits with a total length of 2150m were advanced to the depth of at least 1.5D (proposed tunnel diameter) below the proposed track level at an average borehole spacing of 200m. In these studies required samples for doing laboratory tests were taken and field tests like SPT, pressuremeter, permeability, dawnhole, in-situ density, in-situ shear and Plate load tests were conducted. According to statistical analyses on results, the encountered soil materials have been divided into four different groups with different behaviours in mechanized and convectional tunnelling. Results of this study also reveals that four geological risks of 1) Tunnel collapse in water bearing zones of conventional section, 2) Sticky behaviour of fine grained soils; 3) Abrasiveness of coarse grained soils and 4) Existence of coarse grained soils with low fines content, have more important influences on EPB tunneling in this special project. In conventional section of the tunnel, occurrence of tunnel collapse due to existence of water bearing soils can be a serious problem. Results of these studies have been directly used for preliminary and permanent lining design, TBM Selection, determination of TBM technical specifications and recommendations for soil conditioning and also for structural design of different stations.