Abstract. Instability and failure of high mountain rock slopes have
significantly increased since the 1990s coincident with climatic warming and
are expected to rise further. Most of the observed failures in
permafrost-affected rock walls are likely triggered by the mechanical
destabilisation of warming bedrock permafrost including ice-filled joints.
The failure of ice-filled rock joints has only been observed in a small
number of experiments, often using concrete as a rock analogue. Here, we
present a systematic study of the brittle shear failure of ice and rock–ice
interfaces, simulating the accelerating phase of rock slope failure. For
this, we performed 141 shearing experiments with rock–ice–rock “sandwich”'
samples at constant strain rates (10−3 s−1) provoking ice
fracturing, under normal stress conditions ranging from 100 to 800 kPa,
representing 4–30 m of rock overburden, and at temperatures from −10 to
−0.5 ∘C, typical for recent observed rock slope failures in alpine
permafrost. To create close to natural but reproducible conditions, limestone
sample surfaces were ground to international rock mechanical standard
roughness. Acoustic emission (AE) was successfully applied to describe the
fracturing behaviour, anticipating rock–ice failure as all failures are
predated by an AE hit increase with peaks immediately prior to failure. We
demonstrate that both the warming and unloading (i.e. reduced overburden) of
ice-filled rock joints lead to a significant drop in shear resistance. With a
temperature increase from −10 to −0.5 ∘C, the shear stress at
failure reduces by 64 %–78 % for normal stresses of 100–400 kPa.
At a given temperature, the shear resistance of rock–ice interfaces decreases
with decreasing normal stress. This can lead to a self-enforced rock slope
failure propagation: as soon as a first slab has detached, further slabs
become unstable through progressive thermal propagation and possibly even
faster by unloading. Here, we introduce a new Mohr–Coulomb failure criterion
for ice-filled rock joints that is valid for joint surfaces, which we assume
similar for all rock types, and which applies to temperatures from −8 to
−0.5 ∘C and normal stresses from 100 to 400 kPa. It contains
temperature-dependent friction and cohesion, which decrease by
12 % ∘C−1 and 10 % ∘C−1 respectively due
to warming and it applies to temperature and stress conditions of more than
90 % of the recently documented accelerating failure phases in permafrost
rock walls.
Analysis of Changes of Forces and Normal Stress in Extradosed Tendons of the MS-3B Bridge Located on the Ring Road in Ostróda