隧道外文翻译

TBM TUNNELLING IN DIFFICULT GROUND

CONDITION

Giovanni Barla and Seb astiano Pelizza

ABSTRACT

This paper is to discuss TBM tunneling in difficult ground conditions, when problems are met which may reduce dramatically the average progress rates and practical consequences may be such as to pose serious questions on the use of mechanized TBM tunnelling versus drill blast and other so-called traditional excavation methods. Following a few remarks on rock TBM tunnelling in relation to the selection and dimensioning of the machine, the attention is posed on the limiting geological conditions which may be envisaged with respect to the use of TBM tunnelling and on the importance of geological and geotechnical investigations, in order to derive an appropriate understanding of the rock mass conditions along the line of the tunnel. The discussion is centered upon the relatively more important or difficult ground conditions including borability limits, instability of excavation walls, instability of excavation face, fault zones and squeezing. Whenever available to the authors and based on project experience, the point of view is illustrated by case examples, which give the opportunity to underline specific difficulties encountered and recommendations.

INTRODUCTION

TBM excavation represent a big investment in an unflexible but potentially very fast method of excavating and supporting a rock tunnel (Barton, 1996). When unfavorable conditions are encountered without warning, time schedule and practical consequences are often far greater in a TBM driven tunnel than in a drill and blast tunnel.

The unfavorable conditions can be produced by either a rock mass of very poor quality causing instability of the tunnel or a rock mass of very quality (i.e. strong and massive rock mass) determining very low penetration rates. However, it is to be observed that when using the full face mechanized excavation method, the influence of the rock mass quality on the machine performance has not an absolute value: the influence is in fact to be referred to both the TBM type used and the tunnel diameter.

Right from the beginning of its earliest applications, the use of full face mechanized excavation was to overcome the limits imposed by local geology, the

economic challenges and schedule competitions of the drill and blast method and other so-called traditional excavation methods. A prominent example is given by the recent (from 1995 to 2000) construction of the one tube 24.5 km long Laerdal Tunnel in Norway, the world’s longest road tunnel.

The 100m*m cross section tunnel is being excavation in a Precambrian gneiss, a very good and stable rock mass: the supports are on average only 7-8 rock bolts plus a 7cm thick shotcrete lining per meter of tunnel. The excavation is carried out by the drill and blast method, which been evaluated to be less expensive and more reliable than the ;use of alarge diameter TBM. The average progress rate is 4.8 – 5.0 km per year with two faces, against the 2.3-4.8 km per year, estimated for a large –diameter TBM (Kovari et al., 1993)

With this background in mind, this paper is intended to address the problem of TBM tunnelling in difficult ground conditions. Based on a few selected case examples, the discussion is centered upon the relatively more important or difficult ground conditions which can be listed as follows: borability limits; instability of excavation walls; instability of excavation face; faut zones; squeezing.

ROCK TBM TUNNELLING

The practically infinity number of combinations rock, soil and environmental conditions which may be encountered during tunnel excavation has determined a great difference in the types and characteristics of the available TBM’s . There are many different schemes for the classification of tunnelling machines. For the example the AITES/ITA Working Group No.14 (Mechnisation of Excavation ) is currently working on the definition of an internationally acceptable classification of TBM’s with the purpose of establishing terminology and “terminology” for the optimum choice of the machine (Table 1).

Rock tunnelling machine can be grouped in to three main categories (Table2): Unshielded TBM(i.e. Open TBM), Single Shielded TBM which is the way of creating new types of TBM’s that are suitable for application over a wider range of geological conditions, even though the distinction between TBM for rock and TBM for soft ground remains.

From the point of view of rock TBM dimensioning, it is necessary to point out