Design of Junctions between the Main Tunnel and Connection Tunnels for Tideway Central Contract
Tideway project is a new 25km long combined sewage storage and transfer system beneath the river Thames in central London currently under design and construction. Three design and build contracts have been awarded. The contract for the central section, the biggest and most challenging one, consists of 12.7km of tunnel and 8 sites with deep inlet shafts and ancillary structures.A particularly challenging design aspect of the tunnel section is its connections to the shafts. There are 6 smaller Connection Tunnels between the Main Tunnel and the inlet shafts. In addition to variable geology and groundwater conditions the most significant design challenge has been to develop a solution that confines all construction activity to within the Connection Tunnel prior to break-in. This is to allow the critical path Main Tunnel excavation to continue unimpeded. This paper presents the design process and key considerations behind some of the main design decisions. It provides an introduction to the Tideway project and specific site constraints on the construction of the junctions. It then presents the design process of the junction, including the evaluation of construction options, development of construction sequence and mitigation of risk. The numerical modelling analyses, including extensive parametric studies and calibrations, are described. The analyses show that construction of the junctions from the Connection Tunnel is feasible and the resulting Main Tunnel lining forces and deformations can be controlled.Tideway, Tunnel Junctions, Calibration, Numerical AnalysisLondon, a major global city, is still relying on a combined sewerage system which was designed by Sir Joseph Bazalgette and constructed in 1860s. This system that captures both rainwater runoff and the sewage has a design capacity for four million people. However, as London’s population grew to eight million by 2012 the sewerage system could not cope with the volume of the combined sewage overflows (CSOs), leading to increasingly frequent overflows to the river Thames. These discharges have to be reduced to improve the ecology of the river Thames and comply with the EU’s Waste Water Treatment Directive (EU 1991). The overall Tideway scheme is a significant new combined sewage storage and transfer system that will help protect the River Thames by tackling the problem of overflows from the capital’s ageing sewers. The Main Tunnel, a major component of the system, will run from Acton in West London to Abbey Mills Pumping Station in East London.