Weathering by segregation ice growth in microcracks at sustained subzero temperatures: Verification from an experimental study using acoustic emissions

Abstract

In a continuing effort to better understand the frost‐induced breakdown of rock, experiments were designed specifically to assess a theoretical model of crack propagation due to segregation ice growth in water‐saturated rocks with interconnected cracks (Walder and Hallet, 1985). A rectangular block of Berea Sandstone was frozen unidirectionally while the temperature and acoustic emissions, which reflect microfracture propagation events, were monitored. Acoustic emissions were counted and approximately located as a function of time and temperature while the rock sample was subjected to a fixed temperature gradient. The experimental results indicate considerable frost damage to sandstone due to ice growth in an open system with migration of water to freezing centres much as segregation ice grows in soils, as has been previously suggested. Freezing‐induced microfracture propagation events are not associated with the freezing temperature, which is about —0.2 ºC for Berea Sandstone; most of the fracture activity occurs at distinctly lower temperatures, between —3 ºC and —6 ºC, in accord with our theoretical predictions. Such microfracturing does not require freeze‐thaw cycling or even falling temperatures; temperatures were held constant but spatially nonuniform for the duration of most experiments. Through a series of experiments other aspects of the model are being tested and the influence of lithology is being examined. Diverse geomorphic implications of this model are discussed because it offers attractive alternative insights to those available with the conventional view of frost weathering. A plea is made to strive towards a more fundamental and unified view of frost weathering and related phenomena.

Evidence of frost-cracking inferred from acoustic emissions in a high-alpine rock-wall

“…Disjoining forces between the ice and the rock can then cause ice-filled cracks to widen as water is drawn in from the surrounding medium by a free-energy gradient. This phenomenon operates at temperatures below zero in a temperature range depending mostly on the pore size distribution, permeability and fracture mechanical properties of the rock (Hallet et al, 1991). This could explain why AE activity increases for temperatures several degrees below 0 1C.…”

“…The experiments of Hallet et al (1991), revealing sustained microfracturing activity throughout a 3-day period during which temperature and temperature gradients were held constant in the sample, demonstrated the ability of ice segregation to fracture rock. However, in the field, such conditions are never achieved and it may be difficult to robustly distinguish the role of volumetric expansion (as water turns into ice) from that of ice segregation.…”

“…Another possible mechanism related to ice formation is the cryo-suction or ice segregation (Hallet et al, 1991;Coussy, 2005). Ice in pores or cracks is surrounded by an unfrozen water film due to disjoining (intermolecular) forces.…”

“…The formation of ice within rock is likely to be an important driver of near-surface frost weathering (Hallet et al, 1991) and rock damage at the depth of several meters (Murton et al, 2006), and in steep terrain, this process may be crucial for the slow preconditioning of rock fall from warming permafrost areas (Matsuoka and Murton, 2008;Gruber and Haeberli, 2007). However, the transfer of corresponding theoretical insight and laboratory evidence to natural conditions characterized by strong spatial and temporal heterogeneity of the rock properties (e.g.…”

Geology, glacier retreat and permafrost degradation as controlling factors of slope instabilities in a high-mountain rock wall: the Monte Rosa east face

“…Additionally, the formerly ice covered rock is unprotected from mechanical and thermal erosion. The penetration of the freezing front into previously thawed or unfrozen material has the potential to intensify rock destruction through ice formation in cracks and fissures (Hallet et al, 1991;Haeberli et al, 1997;Matsuoka et al, 1998;Kneisel, 2003). Such ice formation, in turn, reduces the near-surface permeability of the rock walls involved and may cause increased hydraulic pressures inside the non-frozen fissured rock sections.…”