Effects of Excavation at Block 37 on Adjacent Freight Tunnels: Performance and Analysis.

摘要

The freight tunnels in Chicago were constructed approximately 50 feet below ground surface in early 1900s. The tunnels were hand-excavated and supported with steel ribs and wood laggings during excavation. The final liner was formed by placing unreinforced concrete against the wood lagging. Currently, these tunnels are used to house sensitive utilities. Any future deep excavations or tunnels in downtown Chicago will cause changes in stresses in the freight tunnels, and any impact on the tunnels can only be investigated by knowing the existing stresses. This thesis provides a sound understanding of the existing internal forces and stresses in the final liner of freight tunnels, which was developed by conducting numerical modeling and analytical investigation, so that the impact of future constructions on these tunnels can be properly evaluated.;The axial forces, bending moments and internal normal stresses at the final liner of typical freight tunnels in Chicago were calculated using finite element analyses and represent the most realistic existing stresses in the tunnels at their present condition. The analyses addressed the long term loads acting on the tunnels as a result of consolidation and seepage forces (due to leakage of water through tunnel joints), construction of the initial support and final liner, and the amount of ground stress release during excavation. The amount of ground stress release was calculated utilizing finite element modeling. Construction of the freight tunnels according to the information available in the literature and the field data were incorporated in the numerical analyses.;Development of the Block 37 in downtown Chicago included construction of a building and two tunnel connections, the State and Dearborn Connections. Three-dimensional finite element analyses were conducted to study the impact of these activities on the freight tunnels. The existing axial forces and bending moments, which were computed based on finite element analyses, were used to define the pre-construction condition of the tunnels in the analyses. During excavation of the Dearborn Connection, the freight tunnel underlying the excavation zone developed a 17-foot-long longitudinal crack at its crown and walls. The results of the analyses explained the cracking of the freight tunnel and confirmed the applicability of the calculated pre-construction axial forces, bending moments and internal normal stresses.