Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 346

Because of these deficiencies the application of classification systems can lead to both
a too-conservative and an unsafe design. Examples of too-conservative designs are re-
ported in Einstein et al. (1779), Hoek & Brown (1980b), Kaiser et al. (1986), Bieniawski
(1988) and Trunk & Hönisch (1990). Cases for unsafe designs are commented in Pells
(1997), Riedmüller & Schubert (1999), Vallejo (2003), Zabrodsky (2003) and Pells &
Bertuzzi (2008).

12.6 Case History Road Tunnel Österfeld

The Österfeld tunnel, which was designed according to DGGT (1995) and ERTC9
(1997), serves as an example where the use of classification systems would have led to
an unsafe design and construction.

The approximately 400 m long road tunnel is part of the eastern by-pass around
Vaihingen, a suburb of the city of Stuttgart (Wittke et al. 2002). The tunnel crosses
below buildings and railway constructions with low overburden (Fig. 12.14). It is
almost completely located in claystone layers of the Black Jurassic (Lias

α ). Fig-

ure 12.15 shows a photograph of the Lias (Sections 16.7.2, 18.2, 19.2 and 20.3)

including the most important rock mechanical parameters, indicating a marked an-
isotropy of deformability (E 1 > E 2 , cf. Section 3.2.1). In-situ stress measurements
using triaxial cells (Section 16.2) carried out before construction showed that hori-
zontal stresses of

α

Δσ H in the order of 0.2 - 1.9 MPa exist in the ground in addition
to the stresses due to gravity (cf. Chapter 9). These stresses are differential stresses
due to pre-loading as a result of sedimentation and subsequent unloading caused
by erosion referred to as “diagenetic consolidation” (Sections 9.4, 16.7.2, 19.2 and
20.3) that the Lias

α had suffered in this area some 150 million years ago (Geyer &

Gwinner 1986).

Figure 12.14 Österfeld tunnel, longitudinal section (Wittke et al. 2002)