Underground rock structures are frequently subjected to water erosion and dynamic disturbance simultaneously. Herein, in order to study the coupled effects of water and high strain rate on the ...mechanical behavior and microstructures of sandstone, we conducted a series of dynamic unconfined compressive tests on oven-dried and water-saturated sandstone core samples using a split Hopkinson pressure bar. Test results revealed that, three macroscopic final patterns, namely unbroken, axial split and pulverization, were observed. At given strain rates, the presence of water weakens the dynamic peak stress and the dissipated energy density (the ratio of energy dissipation to residual axial strain) of rock sample but enhances the elastic modulus regardless of the failure pattern. Additionally, the saturated sample owns a higher rate dependence of dynamic strength compared to the dry one under the explored range of strain rate (43.9–156.7 s−1), which indicates that water-weakening on rock strength gradually attenuates with the increase of strain rate. Interestingly, thin sections analysis microscopically showed that the failure of dry samples is characterized by the intra-granular fracturing in larger quartz grains while that of saturated samples by the inter-granular fracturing. The underlying mechanisms were interpreted with two micro-mechanical damage models.
•Dynamic compression tests are performed on dry and saturated rocks.•Water presence weakens the peak stress and the dissipated energy density but enhances the elastic modulus.•The rate dependence in strength of saturated samples is greater than that of dry samples.•The micro-damage mechanisms of dry and saturated samples under dynamic loading are different.
Rock failure phenomena, such as rockburst, slabbing (or spalling) and zonal disintegration, related to deep underground excavation of hard rocks are frequently reported and pose a great threat to ...deep mining. Currently, the explanation for these failure phenomena using existing dynamic or static rock mechanics theory is not straightforward. In this study, new theory and testing method for deep underground rock mass under coupled static-dynamic loading are introduced. Two types of coupled loading modes, i.e. “critical static stress t slight disturbance” and “elastic static stress t impact disturbance”, are proposed, and associated test devices are developed. Rockburst phenomena of hard rocks under coupled static-dynamic loading are successfully reproduced in the laboratory, and the rockburst mechanism and related criteria are demonstrated. The results of true triaxial unloading compression tests on granite and red sandstone indicate that the unloading can induce slabbing when the confining pressure exceeds a certain threshold, and the slabbing failure strength is lower than the shear failure strength according to the conventional Mohr-Column criterion. Numerical results indicate that the rock unloading failure response under different in situ stresses and unloading rates can be characterized by an equivalent strain energy density. In addition, we present a new microseismic source location method without premeasuring the sound wave velocity in rock mass, which can efficiently and accurately locate the rock failure in hard rock mines. Also, a new idea for deep hard rock mining using a non-explosive continuous mining method is briefly introduced.
Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection ...is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.
In underground rock engineering, rock structures at great depth are commonly subjected to water infusion, static geo-stress as well as the dynamic disturbances. It is thus essential to understand the ...influence of water saturation on the dynamic behavior of rock under static pre-stress. In this paper, coupled static-dynamic loading tests were conducted on oven-dried and water-saturated sandstone specimens with 0–62.5 MPa axial pre-stresses. Test results show that, for each pre-stress, the dynamic strength of both dry and saturated specimens increases with the increase in strain rate. The rate dependence of saturated specimens is greater. Under a similar loading condition (i.e., same pre-stress and similar incident energy), compared with dry rocks, saturated ones possess lower dynamic strength, consume less energy before failure, dissipate more energy during post-peak stage, and produce a larger proportion of finer particles. Moreover, the comparison of failure patterns between dry and saturated specimens under different pre-stresses suggests that the critical energy of each failure pattern for saturated specimen is lower than that for dry ones. This would be responsible for the more frequent occurrence of many dynamic geo-hazards at depth under hydrated condition.
To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on ...sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.
To investigate the effects of inclusions filled in holes, including inclusion strength, stiffness and shape, on the mechanical properties and fracture evolution of rock, a series of uniaxial ...compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine. Digital image correlation (DIC) and acoustic emission (AE) techniques were applied jointly to record and analyze the deformation and crack process of sandstone. The results indicate that both the inclusion type and shape are important factors affecting the strength and deformation properties of sandstone. The relatively rigid inclusion not only significantly improves the mechanical properties of pre-holed specimens, but also can provide a better support for structural integrity of rock after failure. Observations show that both DIC and AE techniques are capable of revealing the fracture evolution of specimens, and there is a good consistency between the mechanical behavior and surface strain fields and internal AE signals. Based on the strain localization characteristics, the crack sequences and relative stress levels for each group of specimens are summarized, especially the crack development inside the inclusion is first observed. The crack behavior and crack type around inclusions are greatly influenced by the inclusion type and shape due to dramatic changes in stress distribution around inclusions. After rock failure, the crack extent around inclusions can be characterized by the cumulative AE counts, and the final failure modes of specimens under different filling conditions can be categorized as tensile failure, shear failure and mixed tensile/shear failure.
A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic ...characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio (
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τ
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σ
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) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials.
Organoids are
self-assembling, organ-like, three-dimensional cellular structures that stably retain key characteristics of the respective organs. Organoids can be generated from healthy or ...pathological tissues derived from patients. Cancer organoid culture platforms have several advantages, including conservation of the cellular composition that captures the heterogeneity and pharmacotypic signatures of the parental tumor. This platform has provided new opportunities to fill the gap between cancer research and clinical outcomes. Clinical trials have been performed using patient-derived organoids (PDO) as a tool for personalized medical decisions to predict patients' responses to therapeutic regimens and potentially improve treatment outcomes. Living organoid biobanks encompassing several cancer types have been established, providing a representative collection of well-characterized models that will facilitate drug development. In this review, we highlight recent developments in the generation of organoid cultures and PDO biobanks, in preclinical drug discovery, and methods to design a functional organoid-on-a-chip combined with microfluidic. In addition, we discuss the advantages as well as limitations of human organoids in patient-specific therapy and highlight possible future directions.
To understand combined effects of water saturation and loading rate on the fracture behavior of rock materials, dynamic notched semi-circular bending (NSCB) tests were conducted on dry and saturated ...sandstone specimens under a wide range of loading rates using a modified split Hopkinson pressure bar (SHPB) setup. Test results revealed that, the dynamic fracture initiation, propagation toughness and crack propagation velocity of saturated specimen were apparently lower than that of dry ones at the same loading rate. The above parameters increased with the increase of loading rate. Compared with the dry specimen, the saturated specimen owned a higher rate dependency of the dynamic fracture initiation, propagation toughness and a lower rate dependency of crack propagation velocity. Moreover, dual effects of water on the fracture behavior under different loading rates were discussed. It is believed that the different rate dependencies of fracture behaviors between dry and saturated specimens was governed by the combined weakening and enhancing effects of water. A micro-mechanical model was further developed to explain the experimental results based on the duality of water and linear elastic fracture mechanics (LEFM).
Water inrush hazard is a major threat to safe mining and tunnel construction. During the water inrush process, nonlinear properties of rock mass can change gradually due to the erosion of flowing ...water. As a special rock mass, red sandstone subjected to water is prone to be disintegrated, resulting in the severe erosion. To study effects of erosion seepage on hydraulic properties of broken red sandstones, a series of seepage-induced particle erosion experiments were carried out. Forchheimer coefficient was used as an index to investigate the influencing mechanism of erosion on the nonlinear hydraulic properties. Experimental results show that, as erosion seepage processes, porosity and permeability of all samples increase, while non-Darcy factor of that decreases. Base on the variation of Forchheimer coefficient, the erosion seepage process can be divided into four stages, including the growth period, stationary period, decline period and stagnation period. Dual impacts of the erosion process on the Forchheimer coefficient were observed. On the one hand, the flow velocity is increased during the seepage process, and Forchheimer coefficient is promoted. On the other hand, the porosity of the rock mass is increased by the mass loss, and the growth of Forchheimer coefficient of the fluid is inhibited due to the erosion. The large particle rearrangement in the early stages is the main reason for the sharp rise of nonlinear hydraulic properties, and the particle loss can induce the decrease of nonlinear hydraulic properties. Prevention measures in the first two periods are more effective for mitigating water inrush hazard. Last but not least, with the increase of initial porosity and water pressure and the decrease of Talbot index, there are more rising stages of nonlinear hydraulic properties, and the greater difference between the mean value and the actual value.
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