An Experimental Investigation of Micro- and Macrocracking Mechanisms in Rocks by Freeze‒Thaw Cycling

• Main Author: Maji, Vikram
• Format: Thesis
• Language: unknown
• Published: figshare 2020
• Subjects: Geophysics; FOS Earth and related environmental sciences; Ice; permafrost
• Online Access: https://dx.doi.org/10.6084/m9.figshare.12643244; https://figshare.com/articles/An_Experimental_Investigation_of_Micro-_and_Macrocracking_Mechanisms_in_Rocks_by_Freeze_Thaw_Cycling/12643244

The 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 0 o C 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.