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Material-form relationships on talus slopes in southwestern British Columbia Evans, Stephen G.

Abstract

Talus slopes were investigated in a process-material-response framework. The work was concerned with clarifying concepts and terminology concerning slopes of granular materials and interpreting talus slope angles in the light of this clarification; verifying this interpretation in a field investigation; and seeking statistical relationships between talus slope angle and material properties. Field investigations were carried out in South West British Columbia. Slopes were investigated in the southern Coast Mountains and. in the Similkameen Valley. Theoretical concepts relating to slopes in granular material were discussed. Two angles of repose were distinguished; a peak angle of accumulation (α[sub c]) defined as the steepest angle attainable by a mass of granular material, and a lower angle, the angle of repose (α[sub r]) to which the material slides after failure. α[sub c] and α[sub r] were related to concepts of shear resistance and the angle of internal friction (0); α[sub c] was linked to 0 and α[sub r] was thought to correspond to the residual angle of internal friction for a given material. α[sub c] and α[sub r] were related through a regression equation of the form; α[sub c] = -3.29 + 1.273 (α[sub r]) These concepts were examined with reference to talus slope form and some of the contradictions in the literature were presented. The characteristic and limiting slope angles noted in review were found to correspond to α[sub r] and α[sub c] respectively for talus material. This correspondence gave rise to the supply induced transformation hypothesis which appeared to provide a suitable transformation model for rock-fall talus. The relationship between material properties and slope angle was examined using parametric multivariate statistics. Significant correlations, at the 99% level, were obtained between segment angle and size (inverse) and segment angle and sorting (direct). At the 95% level significant correlations were found between segment angle and sphericity (inverse) and Zingg's Flatness Ratio (direct). In multiple regression analysis only 37.11% of the variation in slope angle was accounted for by material properties (sorting and the variance in Zingg's Elongation Ratio) at the 95% level of significance. Shape factors contribute very little to the explained variance whilst fabric related variables contribute nothing. Implications of these results for talus slope development were discussed. Rockfall talus slopes subject to supply-induced transformation processes are thought to have a distinctive morphology which may be an explanation for the typical profile concavity noted on such slopes. Determinants on the frequency of talus slides were examined. The problem of the basal layer cannot be ignored in a consideration of talus slope development models.

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