Effects of excavations on freight tunnels at Block 37 in Chicago

Development of the Block 37 in downtown Chicago included construction of a multi-story building and two tunnel connections, the State and Dearborn Street Connections. During excavation of the Dearborn Connection, the freight tunnel underlying the excavation zone developed a 5.2-m-long longitudinal crack at its crown and walls. Two Three-dimensional finite element analyses were conducted to study the impact of the Block 37 building and Dearborn Connection excavations on the freight tunnel that cracked during excavation. The pre-construction axial forces, bending moments and internal stresses computed based on long term loads were used to define the loading condition of the freight tunnel before construction. The results of the two finite element analyses were used to compute the construction-induced axial forces, bending moments and the corresponding internal stresses imposed on the segment of the freight tunnel concrete liner that was cracked, explain the cracking of the tunnel and assess the validity of the pre-construction loads at the tunnel liner.