2.5 Rotational Failure


In rotational slips the shape of the failure surface in section may be a circular arc or a non-circular curve. In general, circular slips are associated with homogeneous soil conditions and non-circular slips with non-homogeneous conditions. Translational and compound slips occur where the form of the failure surface is influenced by the presence of an adjacent stratum of significantly different strength. Translational slips tend to occur where the adjacent stratum is at a relatively shallow depth below the surface of the slope: the failure surface tends to be plane and roughly parallel to the slope. Compound slips usually occurs where the adjacent stratum is at greater depth, the failure surface consisting of curved and plane sections.

The sliding of material along a curved surface called a rotational slide. These are of two types: circular and non-circular. While failures of this type do not necessarily occur along a purely circular arc, some form of curved failure surface is normally apparent. Circular shear failures are influenced by the size and the mechanical properties of the particles in the soil or the rock mass. Figure 13 illustrates a few typical modes of circular shear failure. This failure can occur in rock structures that exhibit no plane of weakness, and may not be associated with any underlying critical discontinuity.

Figure 14 shows a typical circular failure in a highly weathered rock slope above a highway. A circular failure occurs when the individual particles in soil or rock mass are very small as compared to the size of the slope. The broken rock in a fill tends to behave as soil and fail in a circular mode, when the slope dimension is substantially greater than the dimension of the rock fragments. Highly weathered rocks, and rocks with closely spaced, randomly oriented discontinuities such as rapidly cooled basalts also tend to fail in this manner. If soil conditions are not homogeneous or if geologic anomalies exist, slope failures may occur on non-circular shear surfaces (figure 15). For these conditions, non-circular failure surfaces should be analyzed.





Figure 13: Typical view of Circular  failure






Figure 14: Circular failure in highly weathered, Granite rock



Figure 15: Typical view of Non Circular  failure