Three modes of mechanical rock-ice failure in permafrost rock slopes

In recent years, frequent rock slope failures in permafrost regions exceeding 1 Mio m³ have been documented on a regional (European alps) and global (Andes, Caucasus) scale. Yet, the fracture behaviour of ice-filled joints under high loads remains insufficiently understood, precluding a definitive assessment of modelling approaches. Strictly speaking, the brittle-ductile transition is not yet defined for these loads in the ice mechanical literature.

This study presents novel data to extend the Mohr-Coulomb failure criterion for rock-ice interfaces (Krautblatter et al. 2013, Mamot et al. 2018) for rock overburden exceeding 16 m. Consequently, we propose a governing law for the transition between ductile ice creep and brittle fracture, explicitly incorporating the effects of temperature, stress, and deformation rate in permafrost rock environments.

More than 100 shear experiments were conducted at high normal stresses, simulating rock overburden of up to 65 m (1600 kPa). The tests were performed at temperatures ranging from -0.5 °C to -4 °C, with strain rates consistently maintained in the range of 10⁻³ s⁻¹.

Extending the Mohr-Coulomb criterion to higher overburden pressures revealed, for the first time, consistent increases in friction angle and cohesion as temperatures decreased within the examined range. We define 3 novel sectors of mechanical behaviour:

(i) Ductile deformation

(ii) Single-brittle failure

(iii) Stick-slip failure

Ductile ice deformation occurs within the temperature range of -1 °C to -0.5 °C and exhibits marginal dependence on normal stress. Below -1 °C and at normal stresses below 800 kPa, the rock-ice interface undergoes single brittle fracture. At higher stress levels, ice healing mechanisms are activated, leading to periodic stick-slip fracture behaviour.

We integrated this temperature- and stress-dependent characterization of material behaviour into a three-phase failure model to enhance the rheological representation with respect to numerical modelling of high-magnitude failures.

This study refines the Mohr-Coulomb failure criterion for ice-filled rock fractures by incorporating high-load mechanisms and defining the brittle-ductile transition as a function of stress and temperature, providing valuable insights to improve mechanical models of large-scale permafrost rock slope instabilities.

 

Krautblatter, M.; Funk, D.; Günzel, F. K. (2013): Why permafrost rocks become unstable: a rock-ice-mechanical model in time and space. In: Earth Surf Processes Landf 38 (8), S. 876–887. DOI: 10.1002/esp.3374.

Mamot, P.; Weber, S.; Schröder, T.; Krautblatter, M. (2018): A temperature- and stress-controlled failure criterion for ice-filled permafrost rock joints. In: The Cryosphere 12 (10), S. 3333–3353. DOI: 10.5194/tc-12-3333-2018.