Question 4 of 11

Question 1.4 (6 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1.4.1 Circular failure occurs in weak, closely-jointed, weathered rock or soil-like material with no strongly dominant structural fabric, where the rock behaves effectively isotropically; the failure surface approximates a circular (or log-spiral) arc found by a Bishop/Fellenius-type slip-circle search rather than following any single discontinuity — this is the mechanism analysed with the Swedish circle method later in Question 4.

1.4.2 Planar (plane) failure occurs when a single, persistent, unfavourably-oriented discontinuity (bedding, a fault, a foliation plane) daylights on the slope face, dips less steeply than the face but more steeply than its own friction angle, and strikes within roughly 20° of the slope face strike; the block above that single plane slides as a rigid body, exactly the geometry analysed in Question 4.2.

1.4.3 Toppling occurs where steeply-dipping discontinuities dip INTO the slope (near-vertical, dipping back toward the rock mass) and are closely spaced, so that thin slabs or columns of rock rotate forward and overturn about a pivot near their base under their own overturning moment, rather than sliding — common in steeply-foliated or closely-jointed metamorphic rock.

1.4.4 Wedge failure occurs when two discontinuity sets intersect to form a line of intersection that daylights on the face and plunges more steeply than its own friction angle but less steeply than the face; the wedge-shaped block bounded by the two planes slides along their line of intersection — a full 3-D, two-plane analogue of the simpler planar case.

1.4.5 Ravelling (rubbleizing) is progressive, small-scale, grain-by-grain or block-by-block loosening and rolling/sliding of individual rock fragments down the face, typically in closely-fractured, weathered, freeze-thaw-affected or poorly-blasted rock with no single controlling discontinuity or arc; it is a surficial, ongoing maintenance/safety issue (rockfall catch berms, scaling) rather than a single discrete large-volume slide.