ABSTRACT:

A limit-equilibrium slope stability analysis linked with historical precipitation data has been conducted as part of a comprehensive geotechnical assessment of a large, deep-seated gravitational slope deformation (DSGSD) in northwestern Italy. The active slope mass has a maximum depth of 260 m and a surface footprint of approximately 1700 m wide by 2400 m long. It displays creep movements in response to seasonally elevated ground water. The stochastic stability analysis indicates the probability of sliding can be significantly reduced by excavating a drainage adit system as a means to permanently maintain lower ground-water levels.

1. INTRODUCTION

The left-abutment mountain slope of the Beauregard Dam on the Dora di Valgrisenche river of northwestern Italy (Figure 1) has experienced an ongoing creep deformation since the early days of reservoir filling in the 1960's. This large, deep-seated gravitational slope deformation (DSGSD) has a maximum depth of 260 m and a surface footprint of approximately 1700 m wide by 2400 m long (up the slope). Historical creep displacements of the mass have been directly associated with elevated water levels in the reservoir and to elevated seasonal ground-water levels that result from spring snowmelt and subsequent water infiltration to the subsurface [1, 2]. Since the initial filling of the reservoir behind this 132-m high concrete arch-gravity dam during 1959-1968, the designed operational lake level of 1770 m above sea level has never been attained, because the actual level has been restricted to a maximum of 1715 m, during 1969 to 1998, and then subsequently to 1710 m due to instability concerns for the left-abutment hillslope. Water from the reservoir is diverted through an intake tunnel to the Avise hydro-electric power station on the Dora Baltea river about 13 km away. This entire system was operated by the Italian Electric Energy Company (ENEL) up until July of 2001, when it was acquired by Compagnia Valdostana delle Acque (CVA). In addition to extending the regular, continuous monitoring of the DSGSD and of the dam, beginning in 2002 CVA initiated a series of comprehensive geologic and geotechnical investigations of the left-abutment slope to develop a better understanding of the DSGSD and its interaction with the dam. The rock mass foundation and the left-abutment slope are comprised of gneiss and mica schist of the Gran San Bernardo Series. Investigators believe the slope contains a large, deep-seated ancient landslide that may be related to schist foliation patterns. Since January of 1997, the lake level generally has been maintained between 1700 and 1705 m a.s.l. (except for a temporary drawdown in 2005). Annual slope movements seem to coincide with seasonal snowmelt and subsequent seasonal elevated ground-water levels. Monitored slope displacements typically have been less than 5 mm in low-snow years, but with a tendency to increase in high-snow years. The cumulative displacement at one monitoring site (PR4 plumb line) has been 190 mm over 36 years from 1969 to 2005 (Figure 2). Recent insitu monitoring of the concrete dam, as well as seismic tomography and numerical modeling, have indicated that the structure experiences deformation and cracking in response to loading from the left-abutment slope movement [2].

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