WHY THAWING PERMAFROST ROCKS CAN BECOME UNSTABLE

The destabilisation of permafrost rocks is commonly attributed to changes in ice-mechanical properties (Davies et al. 2001). Changes of intact rock strength and their mechanical relevance for friction and critical/subcritical fracture propagation have not been considered yet. Yet compressive and tensile strength reduces by up to 50% when intact rock thaws (Mellor, 1973). This might play a key role for the onset of larger instabilities in thawing permafrost rocks.

The presence of permafrost can increase shear stress due to altered hydrostatic pressure (i.e. perched water) and cryostatic pressure (i.e. ice segregation). The shear resistance of ice-filled fractures responds to processes acting individually, in succession or in combination: (i) friction/fracture along rock-rock contacts, (ii) friction/fracture along rock-ice contacts, (iii) fracture/deformation of ice in fractures and (iv) deformation of frozen fill-material.

Theoretically, we defined a failure criterion of an ice-filled rock fracture, based on a Mohr-Coulomb assumption, with cohesive rock bridges, contact of rough fracture surfaces, ductile creep of ice and with a representation of rock-ice “failure” mechanisms. This model helps to understand destabilisation in space and time.

Empirically, we conducted friction tests on homogeneous fine-grained limestones (Zugspitze, Germany). In a temperature-controlled shearing box, we repeatedly tested mechanical properties of sand-blasted surfaces between +5° and –7°C. To better define changes in fracture toughness we analysed P-wave velocities of 40 freezing rock samples from alpine/arctic permafrost sites, assuming that changes in P-wave velocity correspond to changes in Mode I fracture toughness (Chang 2001).

Models and experiments imply that thawing-related changes in rock-mechanical properties may significantly influence early stages of the destabilisation of larger thawing permafrost rocks irrespective of the presence of ice in the system. Only after the deformation accelerates to a certain velocity level (where significant strain is applied to ice-filled discontinuities) ice-mechanical properties outbalance the importance of rock-mechanical components.