Abstract
The essential requirements of a successful tunnel boring machine (TBM) tunnelling operation are complete characterization of the ground and thorough identification of geological and geotechnical risks. This paper predicts the geological and geotechnical risks of Tabriz Urban Railway Line 2 (TURL2) in Iran. The railway consists of a 9.5-m-diameter, 22-km-long tunnel and 20 stations. The tunnel passes through a soft alluvial sedimentary combination of clay (CL), silt (ML), sand (SM, SP) and gravel (GP). The tunnel alignment has been divided into three parts subject to the geological and geotechnical conditions that might be encountered during construction. The investigated geological–geotechnical risks of the soft ground along the TURL2 are oversize grains, soil abrasiveness, liquefaction potential, differential permeability and clogging. The results show that all of the investigated geological–geotechnical risks are present in the TURL2 route and that improvement of the tunnel route soils is essential. For example, to reduce the liquefaction potential, it is necessary to decrease the water table level or else improve the soft soils by injecting cement grout. In addition, in order to reduce clogging of the tunnel route soils, the use of foam as a soil conditioner is appropriate.
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