Effect of engineering geological characteristics of Tehran’s recent alluvia on ground settlement due to tunneling

Ground settlement due to the shallow tunneling in urban areas can have considerable implications for aboveground civil infrastructures. Engineering geological characteristics of the tunnel host ground including geotechnical parameters of surrounding soil, groundwater situation, and in situ stress condition are amongst the most important factors affecting settlement. In this research, ground settlement as a consequence of the excavation of the East-West lot of Tehran Metro line 7 (EWL7TM) has been investigated. This tunnel has been drilled into Tehran’s recent alluvia composed of fine-grained and coarse-grained soils. Findings indicate that the finegrained and coarse-grained soils do not have similar behavior during shallow tunnel excavation. In general, maximum settlements (Smax) occurred in the cohesion-less soil is greater than cohesive soil. In some sections of tunnel, measured settlements are lower than evaluated items, stemming from the lower volume loss (VL). Performance of TBM and localized cementation of Tehran alluvia in the considered area have been regarded as significant reasons of occurred discrepancy. In other sections of tunnel, measured settlements are greater than evaluated figures. According to relative thickness constancy of the overburden, this antithesis can be attributed to the existence of old and obsolete underground spaces.