ABSTRACT
At a junction, tunnels leave from a ‘parent’ tunnel/shaft and typically the connection is perpendicular. Sprayed concrete linings offer many advantages for support of such complex tunnel junction geometries. While the stress distribution in the main tunnel or shaft is reasonably well understood – at least conceptually – very few studies have considered the loads in the ‘child’/adit tunnel. These loads are critical for determining an optimum excavation sequence and lining thickness based on the effective stresses in the adit tunnel. Of critical importance is the longitudinal behavior of these tunnels which is often neglected in the design process. While some simplified analytical calculation methods can be used to assess the impact of the complex stresses in the junction on the ‘parent’ tunnel, there is no equivalent design tool for the adit. In this study, advanced three-dimensional numerical simulations investigate the structural-ground interaction during the sequential excavation of an adit off a vertical shaft. Real case study data from the Heathrow Airport Terminal 5 project is used to validate the results of the numerical analysis. The results highlight the key influences at an underground junction and thereby can provide a design basis to improve the efficiency of design at junctions and associated tunnels.
