Geotechnical Engineering and Innovative Support System for Shallow Urban Subway Caverns in Rock, in Confined Built Environment

The Ottawa Light Rail Transit project involves the construction of a new transit facility that will comprise approximately 12.5 km of new electrified Light Rail Transit between Tunney’s Pasture and Blair Road in the City of Ottawa. The mined tunnel runs from the West Portal at STA 100+617 to the East Portal at STA 103+149 for 2532 m. There are three mined stations along the tunnel route, namely the Lyon Station, the Parliament Station and the Rideau Station. The paper focuses on the geotechnical engineering and design of the excavation and the temporary support of Lyon and Parliament stations. Both stations have a horseshoe section (in order to utilize the cross section in an optimum way) and they are excavated in shale and limestone with a minimal cover to the soil layer. In the vicinity of the stations exist some of the city’s tallest buildings (reaching or exceeding 100 m of height) with deep basements, which are finally connected with the stations. In order to minimize the impact to the existing buildings, an innovative construction sequence together with a tension tie system has been designed. Specifically, this paper presents the geotechnical conditions at the area of the two stations, the proposed construction methodology and the design and impact assessment considerations including a description of the implemented Finite Element Models. The design calculations for each station were based on a set of 2D models at critical sections and a set of 3D models incorporating all the critical elements of the interaction, i.e. non-linear soil/rock behavior, interfaces between the building and the soil/rock, building stiffness, staged excavation and tension ties. The models deliver calculations of the convergence, displacements and settlements due to the excavation of the cavern and the internal forces and dimensioning of all the structural elements (shotcrete shell, tension ties and buildings). The assessment of the adjacent structures has considered all the available structural data and was based on the imposed displacements as well as the additional internal forces and pressures developed on the building walls and slabs due to the cavern excavation.