In situ-determination of the effective shear strength of landslide-prone layers of the Upper Red Sandstone.

The paper describes how the common limit state design of a rock slope can be reliable improved when the necessary strength parameter values are determined by field tests. For the design of a down cutting in slide prone layers of red sandstone in the transitional zone of the overlying shell limestone, laboratory and field tests were implemented in order to determine the shear strength within the critical range. The typical limit state for sliding on predestinated weathered layers deliver far better results with direct in situ shear tests, than with triaxial test on undisturbed samples of rock. The basis for a long term design are the shear strength determined with both test types, which has to be reduced, as the amount of the cohesion as part of the resistance might be reduced due to the reload during excavation and the progressive weathering process. The specific geological site conditions as well as the implementation and evaluation of the in situ shear tests and the triaxial tests are presented in the paper. As a result, appropriate values for the long term shear strength are proposed.