INCREASES IN SLOPE STABILITY OF ROCK/SOIL MIXTURES DUE TO TORTUOSITY OF FAILURE SURFACES AROUND ROCK BLOCKS

Engineering geologists and geotechnical engineers commonly encounter weak, heterogeneous and geologically complex mixtures of strong blocks of rock embedded in soil-like matrices. The fabric is ubiquitous in a broad spectrum of geological mixtures ranging between melanges, fault breccias, lahar deposits, weathered rocks, glacial tills, and colluvium. Melanges and other rock/soil mixtures possess considerable spatial, lithological, and mechanical variability: characterization, design and construction in these materials are daunting tasks. Accordingly, practitioners often make the simplifying assumption that the mechanical behavior of rock/soil mixtures is adequately represented by the properties of the weak matrix materials and that there is no need to consider the contribution of blocks. However it has been well demonstrated that blocks do influence the mechanical behavior of melanges and other rock/soil mixtures.

Failure surfaces generated within the weaker matrix are forced to negotiate the boundaries of stronger blocks, creating tortuous failure paths. As block proportions increase, tortuosity increases and in turn, so does the length and shear resistance of the shear surfaces. By the same token, well-graded distributions of blocks tend to increase tortuosity more than uniform distributions. Using conventional analytical methods (such as PCSTABL), it is shown that the slope stability of rock/soil mixtures (as indicated by the Factor of Safety) increases with tortuosity due to the variations block size distributions and volumetric block proportions.