Amey Consulting

The East Gasworks tunnel is a 480m long Victorian structure constructed in the 1850’s in the vicinity of King’s Cross station. The Regent’s Canal passes over the Tunnel at its southern end. The structure was constructed in brick except in areas where the tunnel passes underneath the Canal where the crown is constructed from cast iron girders. The Tunnel is to be reopened to rail traffic with the installation of a precast slab track system. The existing spoil within the tunnel needs to be removed to allow for the installation of a slab track system. Although a 3D Finite Element Analysis (FEA) was undertaken and revealed minor uplift of the invert of the tunnel, there were still concerns regarding the consequences that potential displacements could have on the stability of the Tunnel underneath the Canal and along the Tunnel. A monitoring system was installed at the crown and walls of the Tunnel to record the displacements during the construction. However, installation at the invert of the tunnel, where maximum displacements were expected, was not feasible. Back-analysis technology (Daarwin) was implemented to calibrate the stiffness parameters included in the FEA using readings taken in areas of the tunnel lining other than the invert. The movements of the tunnel lining were back-analysed as the layers of spoil were removed. Hence, the tunnel displacements generated during the excavation of a specific layer, were accurately predicted using adjusted stiffness parameters obtained during the excavation of a previous layer. Therefore, displacements were precisely estimated before the excavation reached the critical invert of the Tunnel and re-evaluation of the construction process was possible. Each phase proactively managed the risk associated with potential displacements of a subsequent phase. Geotechnical parameters are usually derived from limited geotechnical investigation data that may not be representative of the real behaviour of the materials. Given the age of the structure, the uncertainty surrounding the geotechnical parameters and the inability of measuring displacements at some areas of the tunnel, the combination of a monitoring system with back-analysis feedback was critical in managing the risks involved in the construction activities. The monitoring sensors located at the crown and walls of the Tunnel recorded lower displacements than those expected from the 3D FEA. The monitoring readings were back-analysed thereby obtaining more realistic stiffness parameters. Back-analysis revealed that stiffness values of the soil were higher than those utilized in the previous FEA. The new stiffness parameters were re-entered in the previous 3D FE model in order to obtain displacements at the invert of the tunnel, where no readings could be taken. Obtaining these parameters at an early construction stage allowed for anticipation of close-to-real displacements at more advanced stages. In addition to this, monitoring trigger levels could be adjusted as the removal of the spoil progressed. The summary of the benefits of this technology are: • Validation of assumptions of design. • Improvement of risk management during construction. • Optimization of time and cost during construction processes.

This category rewards initiatives that are boosting productivity, safety or quality in the tunnel fit-out process through new products, equipment or methods, and initiatives which are smart ways of maintaining or improving assets via routine maintenance. The innovative product, equipment or methodology must have been used on a specific project and be able to demonstrate a benefit to the overall project outcome, as well as delivering greater client satisfaction. This category recognises the inherent value of existing underground infrastructure and work delivered to ensure operational efficiency and maximise the life span of such assets.