Abstract
The thermomechanical behavior of coal and sandstone during excavation using the freezing method is a new challenge for coal mining and geotechnical engineering. In this paper, the influence of temperature on the mechanical characteristics and fracture patterns of two types of saturated rocks (coal and sandstone) were investigated. A series of laboratory tests, including the Brazilian tensile strength (BTS), uniaxial compressive strength (UCS), and triaxial compressive strength (TCS), were conducted at temperatures of 20, −4, −10, and −15 °C. The results indicated a significant increase in their strength when the temperature was reduced from 20 to −15 °C, especially near the phase-transition point. Then, a theoretical model was proposed to predict rock strength change with temperature, based on the phase-transition theory. To evaluate this model, the predicted results were compared with experimental data, where a good correlation was identified. In addition, four failure patterns were observed in indirect tensile tests (i.e., layer activation, central fracture, noncentral fracture, and central and layer activation), and three types of failure modes in compression tests (i.e., axial splitting, shearing along a single plane, multiple fracturing). The evolution of the rock damage was divided into four stages: Crack closure, fracture initiation, critical energy release, and rupture. These results could be applied to evaluate and predict the mechanical behavior of saturated coal and sandstone during excavation using the freezing method.
Highlights
In recent years, since coal mining was extended to deep and thick alluvium layers, the freezing method has been considered an effective way to resist the high initial stress in these circumstances [1,2,3,4].The freezing method is an excavation technique using artificial refrigeration to temporarily consolidate the wall rock in an aquifer
Experimental investigations on the mechanical properties of geotechnical materials were conducted under freezing conditions [9,10,11,12]
+1 where the parameter β is related to the phase transition process, which reflects the changing rate of ice crystal content, and through known data sets of σc∗ and σc−res, β can be obtained by fitting with least-squares [37]; and σc−res is the increment between compression strengths at saturated and residual water contents
Summary
Since coal mining was extended to deep and thick alluvium layers, the freezing method has been considered an effective way to resist the high initial stress in these circumstances [1,2,3,4]. Materials 2019, 12, 992 system and physical indices (e.g., point-load strength, block punch index, Schmidt rebound hardness, effective porosity, density) were used to estimate rock compression or tensile strengths [10,13,14]. Quantitative analyses of the mechanical properties of freezing coal and sandstone are limited, and the effect of unfrozen water content is frequently ignored. To better understand the effects of temperature on the mechanical and fracture characteristics of water-bearing sandstone and coal, Brazilian tensile tests, and uniaxial and triaxial compressive tests were performed under various temperature conditions. A predicted relationship between strength and temperature was proposed Their failure patterns in the tensile and compressive tests considering freezing states were analyzed
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