Maji, Vikram.pdf (19.87 MB)
An Experimental Investigation of Micro- and Macrocracking Mechanisms in Rocks by Freeze‒Thaw Cycling
thesis
posted on 2020-07-11, 08:33 authored by Vikram MajiThe fracture of rock during freezing and thawing poses a serious threat to rock slope stability
and represents an important geohazard in cold regions. However, mechanistic understanding of
microcracking processes, controls and rates, and the transition from micro- to macrocracking
during freeze‒thaw is limited. To investigate the mechanisms of cracking, two physical
modelling experiments supplemented by compressive tests were performed on specimens of
chalk and sandstone, monitoring and imaging micro- and macroscale deformation due to
freeze‒thaw cycling.
The microscale experiment repeatedly scanned two water-saturated specimens 20 mm in
diameter and 30 mm high, subject to downward freezing in a climate cabinet. Successive microcomputed tomography (μ-CT) images quantified the progressive development of structure and
strain during 20 freeze‒thaw cycles. The macroscale experiment imposed 12 bidirectional
(upward and downward) freezing cycles on three 300 mm cubic blocks over the course of 315
days, simulating an active layer above permafrost. Eight acoustic emission sensors recorded the
timing, location and energy released during microcracking events, while rock temperature,
surface heave and settlement, and subsurface strain were monitored continuously.
The microscale experiment generated different probability functions that correlate points,
clusters and linear movements of the progressive fracture phase extracted from scanned images
and showed dominantly vertical rather than horizontal microcrack growth. The macroscale
experiment brecciated a chalk block near modal depths of the 0oC isotherm during thaw, and
indicated high tensional activity and limited shearing. Ice segregation during thawing produced
more microcracking events than volumetric expansion produced during freezing. A statistical
model is proposed that distinguishes the mechanism of fracture propagation during freezing and
thawing.