Publication:  World Tunnelling (2013)
Authors: Carlos Herranz, Ángel Silvestre, Alberto González and Ángel Del Amo

Summary: Upon completion in 2016, the SR99 Tunnel will be the largest-diameter tunnel in the world at 17.5m. It will be routed 2.15 miles (3.46 km) beneath downtown Seattle in Washington State, US, and tunnelling is to be carried out using an earth pressure balance (EPB) tunnel boring machine (TBM).


This paper discusses the process undertaken to assess tunnel-induced settlement, evaluate potential damage evaluation and design mitigation measures. The latter includes a structural system in the southern stretch with low overburden (the South End Settlement Mitigation Plan), micropile walls and fibre-reinforcement polymer (FRP) deck strengthening of the AWV. Compensation grouting was also considered to protect four buildings, and although it was not the option finally chosen, it is described in this paper. The evaluations included three-dimensional multi-staged FLAC3D models and step-by-step plastic SAP2000 structural models. A variable volume loss was considered along the alignment based on previous experiences with EPBs and a Small-Strain soil constitutive model was used in the FLAC3D models


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