Trial excavation for cut and cover tunnel construction in glacial till - a case study from Dublin

Abstract

The Dublin Port Tunnel links the M50 “C” ring road around Dublin to the port area. The central part of the project comprises twin bored tubes, with shallower lengths of cut and cover tunnels at either end. Advantage was taken of the shallower alignment and competent ground towards the north to address tight land-take constraints, using steep 12m high slopes, with diverted Motorway traffic at the crest. The basic design required soil nails over the full slope height. However Dublin Boulder Clay experience suggested an observational approach whereby nails could be omitted unless required by adverse geology or unsatisfactory monitored performance. To substantiate this novel design, a trial excavation was undertaken with detailed logging, fabric studies, sampling and laboratory testing. The 11m deep, 75° cut was surcharged to model traffic loading and instrumented to measure movements and pore pressures. No nails were installed and different face protection types were used. Finite element analysis was employed in the design of the trial cut, associated lab testing and field instrumentation. Back-analyses of observed behaviour allowed careful calibration of FE models for use in the main excavation design. The paper describes the planning, design and execution of the trial; summarises monitoring results; discusses lessons learned on construction methods, necessary modifications to the numerical model, and the methodology for successful monitoring.