The Unstable Rock Slope Kassen - A Hazard, Consequence and Stability Assessment of the Rock Slope
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NGU is currently in the process of identifying, mapping and classifying all potential, unstable rock slopes in Norway. In this context, a hazard and preliminary consequence assessment have been performed in this master thesis for the unstable rock slope Kassen, located in Kviteseid municipality in Telemark county. The study site is located at a north facing slope in the south-eastern end of the 27 long Bandak lake orientated WNW-ESE. This master thesis is a continuation of the authors specialization project, completed December 2015.Based on delimiting cracks observed in the field and on high resolution digital terrain models, seven smaller potential failure scenarios have been defined within the Kassen unstable area. The hazard and preliminary consequence assessment have been applied to all these scenarios, resulting in four medium hazard class scenarios and three low hazard class scenarios. The preliminary consequence assessment involves a volume estimation and run-out analysis of all scenarios, and the resulting volumes range from 0.11 2.09 million m3. All medium hazard class scenarios have computed run-out lengths which can reach Bandak Lake, and the susceptibility for this is especially high for scenario A, B and G due to steep terrain downslopes with average dip angles of 30-40°. Run-up heights from potential displacement waves for the medium hazard class scenarios were roughly estimated based on empirical relations developed by NGU. Only a displacement wave from scenario G (the biggest scenario) can cause devastating effects for smaller communities located at the shoreline at Bandak Lake. Based on a preliminary risk matrix, it is suggested that the Kassen site shall be periodically monitored in the future, continuing with annual dGNSS measurements. Stability analyses have been performed at two sites at Kassen which differ in rock mass quality, assumed failure mode and size. Input parameters for the analyses are obtained through geotechnical field measurements and laboratory tests. Both analyses are motivated by historic events.The first event is a rock avalanche of uncertain age, identified by deposits seen on bathymetric maps in Bandak Lake beneath the Kassen slope. The volume of the deposits are estimated to be 3.37 million m3. A back-analysis is performed using the software RS2 with the shear strength reduction technique for a detailed study of the parameters and trigger factors that affected the slope stability. It is demonstrated that this post-glacial rock avalanche occurred due to high groundwater pressure and some amount of strain softening. It is further assumed that the Kassen site have been active prior to the last glacial maximum, and consequently the debuttressing effect after the ice- retreat can have reactivated the slope and also contributed to the failure. For todays topography, with present geological and hydrogeological conditions the slope is stable with an Factor of Safety (FS) of 1.78.The second event is of relatively newer time and is registered in the National landslide database provided by NVE as a rockslide. The slide occurred from Skipet, a prominent cliff situated in the lower, western part of Kassen, in 1985. In the field, two potential unstable blocks were detected at Skipet, and the Limit Equilibrium Method is applied to find the Factor of Safety against planar sliding for these blocks. For dry conditions, the calculated FS for the upper and lower block are respectively 1.57 and 1.74. The lower block was still stable even when incorporating a full water pressure over the entire length of an open tension crack at the back of the block. The block shape test show that the blocks also are stable against toppling.