Rock Bridge Failure Caused by the Aysèn 2007 Earthquake (Patagonia, Chile)

Abstract

Most studies related to earthquake triggering of rock slope failures are statistical investigations of the relationships between earthquake source properties to the spatial distribution of various landslide types, or strongly simplified dynamic stability analyses. Only very few investigators studied the detailed structural and kinematic properties of earthquake triggered rock slope failures. In 2011, field mapping and ground-based photogrammetry was used to characterize in detail a stepped translational rock slope failure with a volume of about 250,000 m3 triggered by the Aysen Fjord Mw 6.2 earthquake of April 21, 2007 (Southern Chile). The orientations of discontinuities and release planes were measured in metric 3D images using the software ShapeMetriX3D. Kinematic analyses were applied using Markland methods with Hocking refinement to study possible failure mechanisms. Strength properties of pre-existing fractures, rock and rock mass were assessed both from field work and laboratory tests on granodioritic and granitic samples. A detailed stability analysis was performed with simplified limit equilibrium methods and a two-dimensional numerical FE simulation using the code Phase2. The investigated 125 m high slope collapsed directly into the fjord along two very persistent discontinuity sets (F1: 214/80, F2: 278/50). A kinematic analysis for an initial slope orientation of 250/52° leads to the conclusion that planar sliding along the westward-orientated discontinuity set (F2) is preferred to wedge sliding along the intersection line of both discontinuity sets. This exposed rupture surface is composed of persistent pre-existing fractures with a basic friction angle of 30°–33° stepping through a few very fresh rock bridges of a few meters height and a few non-systematic pre-existing cross-joints. These steeply dipping failed rock bridges extend laterally over substantial portions of the F2 release planes. Stability and stress analysis were carried out for dry, stable and earthquake triggered conditions. The computed conditions for slope failure are compatible with the simplified slope geometry and rock mechanical data, including the observed rock bridges. Stress and failure analysis with multi-stage numerical simulations show yielded pre-existing F2 fracture surfaces, small pre-failure deformations (mm) and high maximum principle stresses of 6 MPa at the foot of the slope and of about 12 MPa at the rock bridges. During horizontal earthquake loading these rock bridges fail in tension under stress conditions equivalent to the tensile strength magnitudes determined from laboratory testing of samples taken at this rockslide location. Failure of rock bridges is supposed to increase the load at the foot of the slope, the confinement fails and the slope collapses catastrophically as observed during the April 2007 Aysen earthquake.