ABSTRACT:

The paper addresses problems associated with tunneling in poor ground under high overburden. Prediction of amount of convergence, estimation of creeping effects, and selection of appropriate support are extremely challenging tasks for the tunneling engineer. While conventional excavation methods offer a certain flexibility to cope with heterogeneous conditions, TBM´s still have to be improved to successfully master fault zones in Alpine tunnels. Emphasis in this paper is put to recent developments in short term prediction of rock mass behavior, prediction of tunnel closure, and ductile supports.

INTRODUCTION

When tunneling in squeezing rock a number of problems have to be dealt with. The main problems are: instability at the face, failing of supports due to high loads, underprofiles caused by excessive deformation, extreme heterogeneity in the rock mass itself, ground water, and long term creeping processes and swelling. Due to the geological complexity of fault zones different failure modes of the rock mass have to be expected at different locations. Thus an universally applicable tunneling method suitable for all types of squeezing rock cannot exist. Methods applied have to be adjusted to the local geological situation and stress field. Due to the higher flexibility up to now conventional tunneling methods are still preferred to TBM operation in squeezing ground conditions.

CONVENTIONAL TUNNELING METHODS
Full face versus sequential excavation

With conventional tunneling (drill & blast, roadheader, excavator) full face and sequential excavation methods are used. Strong regional preferences for one or the other of the methods can be observed. While for example in Italy full face excavation is preferred, in Austria and many other countries sequential excavation methods are applied for bigger tunnels. Another one is the early closure of the lining, providing a high support resistance close to the face.

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