Jiaxing Qiao's research while affiliated with Shaoxing University and other places

To explore the mechanical properties and damage characteristics of basalt under high temperature and high pressure, triaxial compression tests are conducted on thermal damage rock samples, and the evolution process from progressive damage to macroscopic failure of rock is tracked and quantified by CT image reconstruction and acoustic emission technology. The results show that: (1) the ability of basalt to resist load and deformation changes from slight enhancement to rapid deterioration with the increase of temperature, and 600 ℃ is its threshold. The failure mode of rock gradually changes from brittleness to plasticity with the increase of temperature, and its post-peak stress gradually presents certain plastic flow characteristics. (2) With the increase of temperature, the initial damage variable D0 of basalt presents a stage feature of changing from low-speed development to rapid growth. When the temperature exceeds 400 ℃, the total damage variable D will have obvious precursor characteristic of small increase before the drastic change, and this characteristic will become more obvious with the increase of temperature. (3) The main mineral types of basalt hardly change with the increasing temperature, while the temperature has a certain influence on the proportion of some components. The proportion of anorthite and enstatite shows the changing trend of first decreasing and then increasing, while the evolution of andesine is opposite, and the proportion of other mineral components is basically unchanged. (4) The main reason for the improvement of mechanical properties of basalt within the temperature threshold is that the expansion of mineral particles and the constraint of confining pressure promote the closure of internal primary cracks and increase the compactness. However, when the heat treatment temperature of basalt exceeds the threshold value, the structural deterioration caused by heat treatment is gradually prominent, and the defects generated play a leading role in the mechanical properties of rock.

To explore the thermal damage deterioration characteristics of basalt, the evolution of physical parameters, mechanical properties and failure modes was investigated. Based on computed tomography image reconstruction techniques, the spatial distribution and morphological characteristics of the pores of basalt were explored. The results indicate that thermal damage leads to the phase transition of basalt mineral grains and uncoordinated expansion and deformation, increasing the thermal deterioration of rock specimens. The temperature of 800 °C is the threshold for rapidly deteriorated physical properties of basalt, which has deformation characterized by the transition from ductility to brittleness. With the increase in temperature, basalt specimens transit from shear failure to tensile-shear combined failure, and then to tensile splitting failure. Meanwhile, irregular block-shaped collapse is transformed to strip-shaped rock fragments spalling. The crack width based on CT technology and three-dimensional (3D) image reconstruction of the crack volume can quantify the structural deterioration characteristics of basalt induced by thermal damage. When the temperature increases: 25 °C → 600 °C → 1000 °C, the corresponding porosity of the rock changes from 6.86% → 7.04% → 18.02%, exhibiting an evolution from low-speed development to high-speed growth. The thermal damage sensitivity of different lithologies at high temperatures differs, among which, the thermal damage sensitivity of marble is the highest, followed by granite and then basalt, the sensitivity of sandstone is the lowest.

Understanding the effects of thermal fatigue damage on the failure mechanisms of rocks is a key concern in underground engineering. The effects of high temperature on the physical–mechanical behaviors and the failure mechanism of basalt under uniaxial compression are investigated with a combination of acoustic emission (AE), computed tomography (CT), and scanning electron microscope (SEM). The high temperature heavily affects the physical–mechanical properties of basalt but has no effect on the mineral compositions. The evolution characteristics of inter‐event time function F (τ) and cumulative AE energy can be employed to characterize the fracture process of thermally damaged basalt. The damage mechanisms of thermal fatigue are attributed to the occurrence of intergranular cracks, intragranular cracks, and transgranular cracks and irregular holes within basalt. The failure mechanisms of basalt change from shear fracture to mixed tensile–shear fracture and finally to tensile fracture based on the statistical characteristics of low and high dominant frequencies.

The tailings pond is a dangerous source of man-made debris flow with high potential energy. The oxidative acidification of tailings may cause the instability of the pond and induce serious safety accidents. The influence of oxidation and acidification degree on macro mechanical properties of tailings is discussed from the aspects of mineral composition and microstructure. The results show that as the degree of oxidation and acidification of tailings sand increases, the overall structural performance and load-bearing capacity decrease, and its cohesion ( c ) and internal friction angle ( φ ) show a decreasing trend. In fact, the engineering properties of tailings with different oxidation and acidification degrees are dominated by the physicochemical composition and structural characteristics. On the one hand, as the degree of oxidation increases, acidic substance will neutralize with CaCO 3 and CaMg(CO 3 ) 2 , resulting in the loss of cemented substance and the decrease of cementation force between tailing sand particles as well as the gradual destruction of the integrity of tailing sand. On the other hand, the increase of oxidation and acidification degree of tailing sand leads to a gradual reduction of outline (2 D ) fractal dimension and gray surface (3 D ) fractal dimension of surface laminated structure as well as the obvious reduction of laminated structure and its roughness of tailings sand.

Freeze-thaw damage of rocks is one of significant natural causes for geo-hazards such as collapse and rockfall in alpine areas. To explore damage evolution in granite in a freeze-thaw environment, specimens of granite in Nyingchi Prefecture, Tibet, China were collected as research objects. Saturated and dry rock specimens were subjected to cyclic freeze-thaw tests of 0, 36, 72, and 144 cycles and freeze-thaw damage of the rock was analyzed by combining computed tomography (CT) scanning and three-dimensional (3D) visualization. Results show that the peak stress of granite decreases to different extents with the increasing number of freeze-thaw cycles; compared with dry rock specimens, saturated granite deteriorates more significantly and shows obviously different stress–strain curves under loading. The moisture condition exerts significant influences on the degree of freeze-thaw damage to granite: after 144 freeze-thaw cycles, the mass loss rates of dry and saturated rock specimens are 0.06% and 0.44% and their loss rates of uniaxial compressive strength (UCS) reach 4.08% and 26.2%, respectively. Under freeze-thaw cycles, the frost heave of pore water causes initiation and development of micro-defects and new micro-cracks mainly develop along relatively weak areas such as inherent defects (pores and fractures) and boundaries between different mineral crystals, resulting in intergranular and transgranular cracking. For dry rock specimens, the non-uniform contraction and expansion of minerals therein are main causes for deterioration inside the rock. The freeze-thaw damage of rocks is calculated using the improved elastic modulus loss method. The damage development trend conforms to the strength deterioration trend and the pore development in the rock, reflecting the evolution of freeze-thaw damage to granite.