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

Abstract

Ground settlement due to the shallow tunneling in urban areas can have considerable implications for aboveground civilinfrastructures. 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, groundsettlement as a consequence of the excavation of the East-West lot of Tehran Metro line 7 (EWL7TM) has been investigated. Thistunnel has been drilled into Tehran’s recent alluvia composed of fine-grained and coarse-grained soils. Findings indicate that the finegrainedand 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 thanevaluated items, stemming from the lower volume loss (VL). Performance of TBM and localized cementation of Tehran alluvia in theconsidered area have been regarded as significant reasons of occurred discrepancy. In other sections of tunnel, measured settlementsare greater than evaluated figures. According to relative thickness constancy of the overburden, this antithesis can be attributed to theexistence of old and obsolete underground spaces.