2249
Soil slope stability of hydropower reservoirs - from geological site investigation
to design of mitigation measures
La stabilité des talus de réservoirs hydroélectriques - de l'investigation géologique du site
à la conception de mesures d'atténuation
Saurer E., Prager C., Marcher T.
ILF Consulting Engineers, Rum/Innsbruck, Austria
ABSTRACT: Many reservoir slopes are affected by different types of instabilities during the first impoundment, due to reservoir level
fluctuations and due to earthquakes. These potential failure processes are caused by complex hydro-mechanically coupled processes,
which in most cases cannot be fully assessed during the design stage due to a limited knowledge of the soil behaviour and properties.
Thus, for the site selection and the design of power storage plants, especially for the risk assessment and mitigation of potentially
unstable soil slopes, a systematic methodology for the selection of the appropriate stability measures is presented. Based on case
studies from both engineering projects and literature, a methodological approach and a decision matrix for the design of appropriate
mitigation measures depending on the failure mode and size of the potentially instable slope are proposed.
RÉSUMÉ : Un grand nombre de talus à proximité des réservoirs sont affectés par différents types d'instabilités en raison des
fluctuations des niveaux de réservoir et en raison de tremblements de terre. Ces procès potentiels de défaillance sont causés par des
transformations complexes hydromécaniques, qui dans la plupart des cas, ne peuvent être entièrement évalués au cours de la phase de
conception en raison d'une connaissance limitée sur le comportement et des propriétés des sols. Ainsi, pour la sélection des sites et la
conception des installations de stockage d'énergie, il est essentiel de tenir compte des expériences antérieures et d'appliquer ces
connaissances à la conception de nouvelles installations pour créer une stratégie d'évaluation des risques et d'atténuation. Ici une
méthode pour la sélection des mesures de la stabilité appropriées en fonction du mode de défaillance est fournie. Basé sur des études
de projets et de la littérature une méthodologie systématique et une matrice de décision pour la conception de mesures d'atténuation
appropriées en fonction du mode de défaillance et la taille de la pente potentiellement instables sont proposées.
KEYWORDS: soil slope stabilization, mitigation measures, decision matrix
1 INTRODUCTION AND FRAMEWORK
1.1
Reservoir slope instabilities (causes, triggers)
Concerning site selection studies for a water storage reservoir,
particular focus has to be placed on the assessment of terrain
stability, especially in the dam area but also along the reservoir
slopes.
In general, failures are related to changes of stability
parameters of natural slopes or artificial cuts. Related to
reservoirs, the disturbance may be caused by water level
changes (due to first impoundment and/or recurring water level
fluctuations during operation) which may cause catastrophic
(uncontrollable) slope failures of all sizes. According to Riemer
1995, the causes and triggers of soil displacements include, e.g.
- change of consistency due to saturation (fine soils)
- change of effective stresses (pore pressure)
- groundwater flow (steady and transient flow)
- external erosion, e.g. caused by waves
- internal erosion (suffosion, subrosion)
At the design stage of a reservoir, these processes have to be
considered in order to assess the slope stability. Commonly,
slope failures are only treated when they affect the
serviceability (storage volume loss, operational restrictions, land
use along shoreline, water quality) or failure of a reservoir and
its surroundings.
1.2
Slope stability assessment and risk management
Extensive literature reviews of landslides and reservoirs as well
as detailed case studies have been carried out by many authors
(e.g. in Riemer 1995). In particular deep-seated rock slides have
been monitored and investigated intensively, and monitoring
results have been reported over the last decades (Leobacher and
Liegler 1998, Tentschert 1998, Jäckli 1996, Watson et al. 2006,
Barla et al. 2010).
Moreover, process based investigation methods and
monitoring measures have been developed (Keusen 1998,
Zangerl et al. 1999, Leobacher and Blauhut 2010).
The issue of slope stability, in particular for dam safety,
during rapid drawdown and earthquake loading and has been
studied widely (e.g. ICOLD 1980, Casagrande 1937, Sherard
1963, Alonso and Pinyol 2009). Besides the stability
assessment,(e.g. according to Casagrande 1937), it is expected
that engineers are able to make a statement not only on stability
but also on the consequences of potential mass movements.
Following the stability assessment, a risk management plan may
be produced. Whereas several research projects on landslide
risk management have been undertaken (e.g. ICG 2009 and
2011), only few data concerning slope stability in the vicinity of
water reservoirs are publically available..
Here, an overview of a systematic approach to assess the
slope stability before and after reservoir impoundment with
particular consideration of earthquake and rapid drawdown
conditions and a proposal for a customized toolbox for slope
stabilization in cohesive and granular soils are provided.
2 SLOPE STABILITY ASSESSMENT
A possible flowchart for the slope stability assessment is
provided in Figure 1. The individual steps are described in the
subchapters below.
