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a result, the southern slope was found to be intersected by a ca. 200 m long fault zone
consisting of sheared rock (Figs. 25.6 and 25.7). On both sides of this fault zone the
rock was heavily jointed. Adjacent to the fault zone, an anticline consisting of sheared
clay slate was encountered (Figs. 25.6 and 25.7, see also Fig. 2.12). In addition, the
evaluation of the surface mapping yielded three discontinuity sets referred to as B, J1
and J2 (Fig. 25.7).
25.2.2 StabilityAnalyses
Sliding along joints of sets J1 and J2 dipping with some 30° towards the north was as-
sumed as a potential failure mechanism (Fig. 25.8). The friction angle of the joints was
then evaluated by back analysis of the slide, assuming limit equilibrium on the sliding
surfaces represented in Fig. 25.8. On the joints, no cohesion was assumed, and on the
sliding rock mass uplift and seepage pressure corresponding to the original groundwa-
ter level were applied. On this basis, the extent of the support measures required for
stabilizing the slope was evaluated (Rensing & Pierau 2006).
Figure 25.8
Assumed mechanism of failure (Rensing & Pierau 2006)
25.2.3 Installation of Tendons and Drainage
According to the results of the limit equilibrium analyses, a pre-stressing load of
620 kN/m was required to ensure the stability of the southern slope with a safety factor
of 1.2. This load was applied by tendons that were installed between the toe of the slope
and the lower berm (labeled blue in Fig. 25.9), along the fault zone (labeled red in Fig.
25.9) and along the three berms of the slope (labeled yellow in Fig. 25.9). A cross-sec-
tion through the southern slope is represented in Fig. 25.10, and Figs. 25.11 to 25.14
show photos of the excavation, drilling and installation of the tendons. Each tendon
was pre-stressed to the required service load after installation. Every third tendon was
tested for a load that amounts to 1.5 times of the service load (Fig. 25.15).
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