This paper deals with the analysis of deformation behavior of a slow movement in a complex rock formation (Marly-Arenaceous Formation). The time history of displacements measured in the last 20 years shows that the ongoing deformation process is essentially a stationary creep phenomenon, also influenced by transient variations in pore pressure distribution. The movement was recognized after the discovery of intense cracking in the concrete lining of a hydraulic tunnel running across the slope, which will serve for water diversion from a rockfill dam, not yet in operation, located adjacent to the moving mass. Field investigations strongly suggest the ongoing movement takes place along a basal slip surface, formed by a tectonized clay rich interbed, with mobilized friction angle close to residual. The influence of reservoir level increase on the slope displacement and mobilized friction angle of the slip surface has been highlighted. The observed slope movement has been studied by employing viscoplastic model in 2D plane strain analyses (FLAC code). The steady state movement can be predicted reasonably using the model. Some creep analyses, carried out considering also the proposed stabilizing embankment at the toe of the slope have shown a substantial decrease in horizontal displacement velocity.
The slope movement presented in this study was recognized in 1985 after the construction of a hydraulic tunnel in right bank of Chiascio river valley (Italy) (Fig. 1). The movement affects large portion of a ridge which has a longitudinal direction approximately perpendicular to the river valley. The moving mass accounts for about 18Mm3 and is creeping at an average velocity of 13-17 mm/y.
A rockfill dam was built in 1984 just downstream of the moving mass to create a reservoir at the toe of the slope for water supply by a diversion tunnel located on the right bank. Up to now, the reservoir behind the dam has never been impounded. The tunnel departs at the toe of the slope perpendicular to the valley and runs downstream crossing the slope in the east part (Fig.1). So far, the diversion tunnel has been operated only once, in 1991-1992, not for regular water conveyance but for temporary deviation of the river flow.
The first sign of the ongoing movement was seen from a damaged section of the diversion tunnel located well inside the ridge, at 250m from the intake structure under an average overburden depth of 50 m. The damaged tunnel stretch was soon interpreted as the place where the tunnel axis intersects a deep-seated slip surface.