Self-stabilization of toppling and hillside creep in layered rocks

• Hillside creep of layered rocks has kinematic similarity to toppling.
• Toppling and hillside creep of layered rocks are best understood in relation to the toppling hinge surface.
• The toppling hinge surface can be observed directly or inferred from structural data.
• Toppling and hillside creep of layered rocks can self-stabilize when a mirror image configuration is formed.

Toppling and hillside creep in layered rocks share the characteristics of rotation of layers under gravity. It has been observed that some toppling processes tend toward self-stabilization. The potential for self-stabilization of these processes is greatly influenced by the degree of constraint provided by surrounding material. The degree of constraint is reduced by the toppling surface daylighting on the slope surface. Toppling slope failure typically involves a daylighting failure surface whereas hillside creep by toppling of layers can occur on a surface which may be parallel to the slope surface. The failure surface is termed the toppling hinge surface for both block and flexural toppling. A review of published field photographs and selected field examples, identified as hillside creep of layered rocks, indicates that it is common for the rotated layers to form a mirror image symmetry across the toppling hinge surface. The mirror image condition is a unique condition in which the strain parallel to the slope is equal in the initial and toppled orientations. Prior to reaching the mirror image condition, the strain parallel to the slope is less than the initial condition resulting in low confining stress across layer boundaries, thus facilitating motion. Rotation beyond the mirror image condition causes extensional strain parallel to the slope, which cannot be accommodated without additional disturbance such as freedom of movement of blocks at the slope toe and/or crest. Structural data collected in the Maitai Valley, South Island, New Zealand gives an example of the field observations associated with self-stabilized creep–toppling.