ABSTRACT:

Within the city or Stuttgart (Germany) a number or railway and highway tunnels have been constructed in horizontal layered mudstones. limestones and lime-sandstones of the Lower Jurassic (Black Jurassic/Lias a). One example is the Osterfeld tunnel atthe eastern highway ring in Stuttgart-Vaihingen, which undercrosses residential buildings and a railway line with low overburden. The stability of the construction stages and small deformations have been achieved. By an advancing vault excavation with a curved tempo- rary invert supported by shotcrete. For the same reasons the round lengths were kept short and an advancing pipe umbrella was installedin the area of the railway crossing where the overburden amounted to on I) 5–6 rn. As part of the design three-dimensional finite elementstability analyses have been carried out. The results of these analyses were a valuable tool for the design and compared well with theresults of monitoring.

1.
INTRODUCTION

The Osterfeld tunnel undercrosses residential areas as well as thetracks of the Gaubahn and the S-Bahn. Especially at the railwaycrossing the overburden is low. During the heading of the tunnelunderneath the railway lines the railway traffic could not be interrupted.In this area, therefore, special measures for the support ofthe tunnel were required. Special attention was required. becausethe road tunnel is located in mudstone layers of the Lias a. which are Subjectedto high horizontal stresses.

2.
STRUCTURE

The Osterfeld tunnel. Which has been excavated by means of theNATM is approx. 400 m long and pan of the cast an ring around Vaihingen.a suburb of the city of Stuttgart.

3.
GROUND AND GROUND-WATER CONDITIONS

During the design phase for the new by-pass road, an extensive program for the exploration of the subsoil and ground-water conditions was carried out. In this program besides the sinking of core drillings, the investigation of soil, roek and water samples, water level measurements. combined extensometer and inclinometermeasurements as well as in-situ stress measurements were carriedout.

4.
FUNDAMENTALS OF THE DESIGN

Because of the small overburden. the high horizontal stresses and the low shear strength of the bedding planes in the Lias a. in which the tunnel is located. special problems concerning the stability of the tunnel arise.

5.
STABILITY ANALYSIS FOR THE STAGES OF CONSTRUCTION

The stability analyses for the stages of construction were carried out by the finite clement method for vertical slabs (sec Wittke, 2000). In the Lias a additional to the stresses due to dead weight horizontal stresses ~crll varying from 0.5 to 1.5 MPa - depending on the location of the considered computation section - were simulated, Additional analyses without consideration of increased horizontal stresses were also carried out. In the two first steps of the analysis the in-situ stare of stress due to dead weight and also the additional horizontal stresses in the Lias a are calculated.

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