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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
5 RESIDUAL RISK ASSESSMENT
The residual risk from a potential mass movement has to be
analysed independently from mitigation measures. This is due
to the fact that the mitigation measures are designed for a
certain defined load (e.g. design earthquake). However,
remaining residual risks may comprise events larger than the
design event and resulting slope instabilities. Thus, also the
impacts of such instabilities on both the safety of the dam and
reservoir (probability and consequences of overtopping) have to
be assessed.
For this both empirical approaches (Fritz et al. 2003, Heller
2007) and numerical models (e,g, Grilli and Watts 2005) may
be applied.
6 MONITORING MEASURES
Slope stability behaviour can be monitored a) pointwise, b)
linear and c) areally and can be measured in-situ and/or by
remote sensing methods (see Sect. 8 References). Point data can
be obtained through triangulation, levelling, GPS surveys, wire
extensometer, joint- or crackmeter, laser distance meter and
water level gauge measurements. Line data may be obtained
from inclinometer, extensometer and Trivec measurements
and/or from fibre optic sensing techniques. The deformation
field of a surface of a landslide can be obtained by
photogrammetry, terrestrial or satellite based radar
interferometry and terrestrial or airborne laser scanning. A
variety of these monitoring methods has already been
successfully applied to some well-documented reservoir slopes
(e.g. Leobacher and Liegler 1998, Tentschert 1998, Watson
2006, Zangerl et al. 2009, 2010).
7 CONCLUSION
Assessment of slope stability of potential reservoir sites requires
interdisciplinary knowhow, comprising intensive field
investigations and sound determination of soil characteristics.
Data thus obtained enable the assessment of slope stability (due
to changed boundary conditions) and the design of appropriate
mitigation measures.
For the design phase, possible scenarios of mitigation
measures are described and presented in a matrix form. These
measures serve as a basis during later design phases and during
execution when the stabilization measures are allocated to the
appropriate slopes in the reservoir according to the
corresponding refined geological model and boundary
conditions.
The scenarios 1 to 7 (Table 2) may be applied to different
slope conditions (Figure 1: A to D), i.e. areas with acceptable
slope stability, areas where near-surface stabilization measures
are required, and areas with shallow to deep seated mass
movements which have to be stabilized.
On the basis of available geological surface and subsurface
data (field survey, geophysics, drillings) representative
geological slope cross sections are established in order to
illustrate, evaluate and assess the current and future slope
stability in the individual regions. These geological sections and
geotechnical data are used to for stability calculations.
Due to the repeated water level fluctuations, some areas may
experience erosion and landslide processes (landslides, flow
processes), especially in areas with large granular or cohesive
soil layers. Stability studies including all load cases (including
earthquakes) have to be carried out. If the safety calculation
without stabilization measures returns an insufficient safety
factor, measures required to achieve the required level of safety
have to be determined. The goal of constructing mitigation
measures is that the slopes for the mentioned load combinations
(including earthquake load) remain stable.
For the monitoring of slope stability during the construction
and operating phase, instrumental measurements of selected
slope areas are required. These include both episodic campaigns
and permanent measurements, e.g. geodetic surveys, levelling
and inclinometer measurements.
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