The shear strength of rocks along discontinuities has the highestinfluence on rock slope stability; therefore, its accuratedetermination is crucial. This paper presents a potential solutionfor the 3D surface detection of the rock discontinuity surfacesby using photogrammetric methods (ShapeMetriX3D),from which one of the most important shear strength influencingparameters can be determined and analyzed. From thedetermination of the angle enclosed by the plane of the samplesurface and the shear plane in the direction of shear, besidesthe effect of the upslope/downslope shearing further analysiscan be carried out in case the examined rock sample containsbedding planes or foliation. The magnitude of the influence onthe shear strength for both cases is demonstrated on existingdirect shear strength test measurements that were carried outon over-consolidated Mont Terri Opalinus Claystones from theRock Laboratory of Mont Terri in Switzerland.

The evaluation of shear stress versus shear displacement curves is in the main focus of geotechnical engineering. Such curves, depending on the rock assessed, consist of a quasi-linear section, followed by a “kick” representing the peak shear strength, and a residual part, mostly parallel to the abscissa. The aim of the present study is to facilitate the future automatic detection of these crucial characteristics to take a step towards replacing their visual/analogue determination via modern statistical tools. Breakpoint detection methods (Cross-Entropy, Change Point Model) were applied to curves obtained from laboratory shear tests describing the shearing along discontinuities of nine Mont Terri Opalinus Claystone samples. Smooth and moderately rough claystone surfaces were studied. Results indicated that the end of the rising section and the kick observed on the shear strength curves was effectively approximated with the Change Point Model framework. An additional practical advantage of applying statistical tools such as breakpoint detection to shear strength determination is that it ensures the comparability of the obtained results.