Dynamics of rock arches

Rock arches are dynamic natural features that bend, sag, sway and shake in response to different environmental forcings. We use ambient vibration seismic measurements to study the dynamics of natural rock arches, some of which are prominent landmarks. Our goal is to understand how arches respond to their environment (e.g. thermal stresses), and ultimately discern changes in internal strength over time. Resonance characteristics are a function of the arch’s overall mass and stiffness, and monitoring over time can provide indicret evidence of internal change. We simulatenously monitor in-situ deformation and rock temeprature at select sites. Our research will lead to new understanding of the life-cycle of natural arches, their response to external loads, and failure processes leading to their eventual collapse.

Paraglacial rock slope mechanics

Cycles of glacial erosion and retreat generate rapid changes in rock wall mechanical, thermal, and hydrological boundary conditions that drive cycles of in-situ stress change and rock fracture. We explore coupled thermo-hydro-mechanical damage cycles in forcing long-term progressive failure and conditioning paraglacial slope instabilities. We posit a model of THM stress changes where each mechanism is tied to the changing ice extents and therefore resulting rock mass damage is predictable in both space and time. Our work is based on investigations at the Aletsch glacier compiling landslide distributions, rock mass strength properties, ground deformation, temperature, and sub-glacial water pressure measurements, together with reconstructed Holocene ice extents.

Prehistoric rock avalanches

Catastrophic rock avalanches leave a dramatic and long-lasting impact on steep desert and alpine landscapes. However, due to a lack of accurately dated deposits, precise conclusions regarding the temporal occurrence of these events remain elusive. We analyze a number of case histories in North America and Europe in order to add to the growing data set of accurately dated and mapped prehistoric rock avalanches. Cosmogenic surface exposure dating has in some cases revised the age estimate from previous interpretations by thousands of years. We complete our analyses by modeling rock avalanche runout over the reconstructed terrain, and commenting on failure mechanisms with respect to retreating glaciers, paleoseismicity, and changing Holocene climate.

Remote seismic monitoring of alpine slope failures

Large rock slope failures generate significant and observable seismicity, with equivalent local earthquake magnitudes around 2-3, which are easily detected by instruments of a typical national or regional network. We analyze these rockslide seismic signals with three goals: 1. Automated detection and classification using Hidden Markov Models, a technique borrowed from speech recognition systems, to deliver information on timing and basic event type; 2. Estimates of key rockslide event properties, such as volume and runout distance, based on multivariate correlation analysis from a large database of past events; and 3. Accurate estimation of event location, which is otherwise poor using traditional earthquake locating techniques due to the emergent nature of rockslide seismic signals.

Seismic response of deep-seated slope instabilities

Site effects are rarely considered for analysis of earthquake triggering of rock slope failures, however our measurements show that fracturing of the rock mass within active slope instabilities can amplify shaking by factors of eight and higher. At the Randa instability in Switzerland, we measured significant spectral amplification and polarization within the unstable rock mass, which we relate through numerical modeling to the presence of compliant tension fractures. Slope deformation results in preferential fracture opening, creating meso-scale anisotropy in rock mass moduli. We are currently monitoring spectral resonance characteristics at another active slope instability in southern Switzerland with the goal of detecting temporal changes associated with internal progressive failure.

Rock slope fracturing and failure processes

We study progressive failure processes in hard-rock slopes driven by thermo- and hydro-mechanical interactions. In addition we analyze exhumation-induced stresses and their interaction with tectonic stress fields in creating regions of critical in-situ stresses in typical valley profiles that generate predictable distributions of bedrock damage and fracture. Our research relies heavily on field investigations and monitoring at the slowly moving rock slope instability above the village of Randa in southern Switzerland. We have been able to show that around 5 million m3 of crystalline rock, which is unstable today, is being driven solely by annual temperature changes in near-surface bedrock. Such deep thermo-mechanical effects are rarely considered relevant for large slope instabilities.