: This paper studies the failure mechanism of block-flexural toppling by using physical modelling test. Base friction apparatus was used in this study. The specimens were made by modelling material with the mixture of plaster, fine sand and water. Columnar and cross joints were created with different angles, spacing and slope height with a fixed slope angle of 70 ° and the size of toppling block of b/t=2 (height/base width). Based on the experimental studies, the following conclusions had been drawn. (1) The mechanism of block flexure toppling failure can be explained by using the initial stress concept. (2) The failure of block-flexure toppling can be classified into three modes: toppling, sliding and a combination of flexure toppling and sliding. The mode of failures depends on the angle, the spacing, the friction of columnar and cross joints, and the height of slope. Criteria for different modes of failure are proposed. (3) An empirical equation is proposed for determining the reduction of the friction angle with increased spacing of the joints. (4) A regime classification of block flexure toppling failure is developed.
1 INTRODUCTION
Toppling failures of slopes have been observed in a wide range of rock masses and can occur in both natural and excavated slope (Muller 1968, Freitas & Wattters 1973, Evans et al. 1981, Ishida et al. 1987). Toppling slope consists of several sets of regularly spaced fractures which strike parallel or almost parallel to the slope and dip into the slope face. The mode of toppling failure involves overturning or rotation from the existing joint plane. There are four principals type of toppling failure, namely: flexural toppling, block toppling, block flexure toppling and secondary toppling (Goodman & Bray 1976, Evans 1981).
