## 2 Possible Failure Modes of the Slope  
The Dabenliu quarry slope is a typical high-steep bedding slope. As discussed above, there are three kinds of discontinuities in the rock masses of the slope: interlayer shear zones (bedding joints), small faults, and cross joints. The layers themselves are relatively hard, with high persistence, whereas there are numerous interlayer shear zones with a small spacing compared with the height of the slope. It seems that the slope is composed of a series of thin slabs obliquely superimposed together. After the slope is excavated, the dip of the strata is nearly equal with the angle of the excavated slope in the north-west front slope, as the excavation lines depict in Figure 3. However, the strata in the south-west side slope intersect obliquely with the slope strike. At the same time, there are a lot of small faults and cross joints within the rock strata. The failure of the slope may manifest in the following modes after excavation:  
(1) Slipping-buckling failure. The north-west front slope is a dip slope, with the dip angle of strata being almost parallel to the slope gradient. Even though layers never daylight at the slope toe, the exceptional persistence and weakness of the interlayer shear zones allow for the buckling of layers. In the initial stage after excavation, the middle and upper parts of the slope strata may slip slowly along the bedding joints because the strata are so steep and the shear strength of the bedding joints is so weak to the point whereby it cannot resist the huge sliding force proceeding downslope. The slipping deformation may be very small since the bedding joints at the slope toe are not daylighted; however, a small deformation is enough to cut off the cohesion between layers. Then, the huge self-weight of the middle and upper layers may transfer to the lower part with a finite width, causing the lower part to gradually buckle. The buckling occurs more easily when triggered by an earthquake or when cross joints exist. Because the strata are hard, thin, and highly persistent with a lot of cross joints and small faults, the failure will manifest as block extrusion and collapse. These modes are shown in Figure 4. Its occurrence may cause a large-scale rock avalanche. (2) Slipping along the interlayer. This failure mode happened in the south-west side of the slope on 29 June 2011. The crushed rock was composed of the thin-extremely thin slates located between the elevation of 1895 and 1930 m. The length of the crushed rock was about 3-5 m along the slope surface and 2-3 m in depth, and the volume was about 550 m\({}^{3}\), as shown in Figure 5.  
(3) Block failure. During slope excavation, unstable blocks can be formed under a certain number of unfavorable combinations of joints, interlayer shear zones, or small faults. This mode occurred in the south-west side of the slope on 5 July 2011. The crushed rock, located between the elevation of 1925 and 1985 m, was composed of the metamorphic quartz strata interlayered with the slates. Its morphology was akin to a tetrahedron block, and it had a volume of about 4100 m\({}^{3}\), as shown in Figure 6. The small fault F2 along with some interlayer shear zones formed the cutting faces of the block.