The impact of changes in active-layer thickness on the depth of pervasive macrofracturing (brecciation) in frost-susceptible bedrock is unclear but important to understanding its physical properties and geohazard potential. Here we report results from a laboratory experiment to test the hypothesis that active-layer deepening drives an increase in the depth of brecciation. The experiment simulated active-layer deepening in 300 mm cubic blocks of limestone (chalk) and sandstone. Temperature, surface heave and strain at depth were measured during 16 freeze-thaw cycles. Macrocracks photographed at intervals were digitally analyzed to visualize crack growth and to quantify crack inclination and length. In chalk, an upper horizon of macrocracks developed first at about 100 mm depth in a shallow active layer during cycles 1-8, followed by a lower horizon at about 175-225 mm depth in a deeper active layer during cycles 9-16. The longest cracks (>35 mm) were most common at inclinations of 0-30 from the horizontal, and numerous cracks <5 to 15 mm long developed at inclinations of 40-50 , with some longer vertical to subvertical cracks linking the two brecciated horizons. Overall, the observations support the hypothesis that a thickening active layer drives deeper rock fracture by ice segregation. K E Y W O R D Sactive layer, freeze-thaw, ice segregation, limestone, rock fracture, sandstone, strain
Abstract-Understanding the failure mode and crack propagation in rock can provide useful information for safe and economic design of various structures in rock. Many researchers have developed theoretical criteria for rock failure with crack growth based on experimental observations. Numerically using cohesive zone model for brittle material with an assumption of some plasticity is found to be a good approach to predict the crack growth in rocks. The cohesive zone model is popularly used for fracture simulation in brittle materials, uses traction-separation law. The tractionseparation law represents the material damage zone in front of the crack tip where the material elements are pulled apart. The extended finite element method (XFEM) enhanced the capability of the classical finite element method capturing the crack propagation problems. The important feature of XFEM is that, it can extend the crack without any remeshing which makes it suitable for fracture process analysis. The present paper combines XFEM approach with cohesive zone model (CZM) to analyze the crack growth for rock using ABAQUS. The results of the analysis are compared with the experiments carried out in the laboratory and with available literatures on crack growth in rocks. The present paper demonstrates different crack types (tensile/shear) that gets initiated from the pre-existing flaw with respect to the applied loading. The numerical model using ABAQUS shows a good agreement with the theoretical and experimental results while predicting the crack propagation.
A major problem in studies of rock fracture by frost is the paucity of direct observations in space and time of the initiation and growth of microcracks and their transition to macrocracks. Such observations are essential to understand the location, timing and controls of rock fracture by freeze–thaw. The aim of the present work is to image and elucidate the early stages of rock fracture by applying imaging and statistical methods to a frost‐weathering experiment using intact specimens of a limestone (chalk) and sandstone. First, micro‐computed tomography (μ‐CT) is used to visualize rock fracture in three dimensions over the course of 20 freeze–thaw cycles and to estimate transverse strain using a pixel‐based approach. Second, probabilistic correlation functions are applied to quantify the progressive expansion of the fracture phase and associated damage to rock specimens. The method of μ‐CT is demonstrated for visualizing the growth and coalescence of microcracks and their transition to macrocracks. Fracture proceeded faster and to a greater extent in chalk relative to sandstone, and the macrocracks in chalk were mostly concentric and vertical. Both fracture development and positive transverse strain (dilation) accelerated after cycle 15, suggesting that a threshold has been exceeded, after which macrocracks were evident. Of three probabilistic correlation functions applied to the μ‐CT results, the modified lineal‐path function – which measures the continuous connectivity of the fracture phase in a specific direction – reveals that damage was more extensive in the chalk than the sandstone. It also allows a novel approach to define and quantify three zones of microcracking during freeze–thaw cycling of anisotropic rock: (1) the zone of inherent flaws; (2) the zone of active microcracking; and (3) the zone of weak influence during microcracking. The broader significance of this work is that it provides a new approach to investigate mechanistically how frost action damages rock. © 2019 John Wiley & Sons, Ltd.
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