SUBCRITICAL STRESS CONTROLS ON ROCK STABILITY

Stress in rocks can affect their damage, fracture, and erosion processes. It is a common notion that increased stress leads to increased crack propagation and fracture, which in turn causes increased damage and erosion rates. As rocks are uplifted and exhumed from the crust, the conditions change from a critically stressed and confined state to low confinement, and lower stress magnitudes, and the direction of the principle stress can even revert (Zang and Stephansson, 2010; Leith et al., 2014). At the Earth’s surface, the stresses that act on rocks are typically sub-critical. Topographic perturbation can cause stress concentrations and create barriers to stress transmission (Miller and Dunne, 1996). Yet, due to reduced confinement, fractures can open, exposing fresh rock to be weathered (St. Clair et al., 2015). Weathering depends on temperature and environmental conditions of the rock, which are driven by and interact with atmospheric processes. The exposure to atmosphere, hydrosphere and biosphere results in lower, ambient temperature ranges than in deep crustal rock. In addition, water is ubiquitous available. Its effect fluctuates over time with avaialbility, and may include phase changes (freeze, evaporation) as well as chemical and mechanical effects due to biota and human activities (Anderson, 2019).

The rheology of rocks under these conditions are less explored. Near-surface stress fields and conditions have been investigated on short time-scales in the context of natural hazards and infrastructure. The importance of rock physics and fracture mechanics near the Earth’s surface have recently been more and more recognized on timescales of e.g. landscape evolution, weathering, erosion and long-term waste storage. The acting mechanisms for progressive rock failure at and near the Earth’s surface are subcritical both in rate and magnitude.

In the subcritical range, applied stress not only leads to cracking and fatigue, but also to toughening, frictional interlocking and strengthening. The observed mechanism depends on the mode, magnitude and orientation of the stresses. We use three sets of laboratory experiments to explore mechanisms controlled by the subcritical stress and provide interpretations and explanations.