Saturated Sandstone Research Articles

Fracture and deformation behaviors of saturated and dried single-edge notched beam sandstones under three-point bending based on DIC

It is of great significance to study the fracture and deformation characteristics of rock with different moisture conditions for the safety assessment of “fragile surface with cracks” in rock engineering such as dams and underground chambers affected by groundwater. Therefore, in this work, three-point bending tests were conducted on single-edge notched beam sandstone specimens with different moisture conditions (natural condition, dried condition, and saturated condition) under complex stress environment. Additionally, the digital image correlation (DIC) method was used to quantitatively research the fracture parameters and deformation behavior of specimens during pure mode I fracture and mixed mode I-II fracture. The results show that (1) the average peak load of dried sandstone is 42.18–116.08% higher than that of saturated sandstone. (2) By increasing the offset distance of the pre-notch (0–72 mm), the specimen is transformed from pure mode I fracture to mixed mode I-II fracture, with an increase in average peak load of 152.83–284.24%. (3) When the pure mode I fracture occurs in saturated sandstone, the fracture toughness is 66.74% of that natural sandstone, which is 46.28% of that dried sandstone. (4) In the mixed mode I-II fracture, the effective fracture toughness of sandstone with a consistent moisture condition is 1.05–1.70 times that of the pure mode I fracture. (5) The fracture toughness of saturated sandstone is most significantly affected by the loading mode. When the offset distance of pre-notch increases from 0 to 72 mm, the average effective fracture toughness increases from 4.324 MPa·mm0.5 to 7.357 MPa·mm0.5, increasing by 70.14%. Besides, according to the trend of the ap, which was calculated by the DIC method, the post-peak macroscopic crack propagation is divided into two stages: the post-peak stable propagation stage and the post-peak unstable propagation stage.

ANALYSIS OF POROSITY, PERMEABILITY, AND ANISOTROPY OF SANDSTONE IN FREEZE–THAW ENVIRONMENTS USING COMPUTED TOMOGRAPHY AND FRACTAL THEORY

An in-depth understanding of the deterioration characteristics of porous rock materials in freeze–thaw (F–T) environments is very important for rock mass engineering in cold regions. However, quantitative descriptions of key rock indicators such as porosity, permeability, and anisotropy are lacking. In this paper, computed tomography (CT) was used to study saturated intact sandstone, saturated fractured sandstone, and ice-filled fractured sandstone under various F–T cycles and stress states. Meso-structural parameters were obtained by reconstructing the three-dimensional fracture networks from CT images. Then, based on fractal geometry theory, the fractal dimension ([Formula: see text], tortuosity fractal dimension ([Formula: see text], and anisotropic two-dimensional fractal dimension ([Formula: see text] of the sandstone samples were analyzed quantitatively. The [Formula: see text] gradually increased during the F–T process, while [Formula: see text] gradually decreased. Compared with [Formula: see text], [Formula: see text] was found to describe changes in the absolute permeability of rocks under F–T cycling more accurately. Anisotropy in sandstone was enhanced by F–T cycling. After uniaxial compression, the [Formula: see text] value was the greatest in ice-filled fractured sandstone. In addition, the tree-like fracture structure produced by F–T cycling expanded the range of self-similarity, which enhanced the fractal characteristics of sandstone. However, due to the large frost heave pressure of ice-filled sandstone, fracture expansion accelerated in the later period of F–T cycling, which destroyed the self-similarity. These results assist in understanding the F–T characteristics of porous rock materials. The method described provides a new way to better evaluate and predict F–T-related engineering disasters.

EFFECT OF CALCINATION TEMPERATURE ON ELECTROMAGNETIC PROPERTIES OF SANDSTONE SATURATED WITH BFO FOR ENHANCED OIL RECOVERY (EOR) APPLICATION

In this paper, BFO NPs were synthesized to prepare nanofluid using the nitric-assisted sol-gel method followed by calcination at different temperatures 200, 350, 500, and 650°C. Sandstone samples were then saturated with as-prepared nanofluids for electromagnetic properties measurements. The effect of calcination temperature on the structural and morphological aspect of BFO NPs was studied using XRD and FESEM. The electromagnetic absorption of the saturated sandstones was measured using the vector network analyzer at X band frequency. The measured scattering parameters were validated with simulation using the finite element method. The simulation results showed that the maximum absorption from the simulation for each sample are 5.73, 5.93, 36.60, 33.19, 36.13dB for Sbrine, S200, S350, S500, S650, respectively. On the other hand, experimental results showed that the maximum absorption for Sbrine, S200, S350, S500, and S650 is 51.93, 11.63, 8.98, 7.79, 6.97dB, respectively. BFO NPs calcinated at 650°C have the best permittivity values of 10.08 and 3.27 and have the highest real permeability value of 1.22 compared to other samples. Conclusively, the BFO nanoparticle calcinated at 650°C shows the best potential for enhanced oil recovery, which can be selected for further study for EOR application.Keywords: Nanofluid, porous rock, calcination, microwave absorption, BiFeO3 NPs, sol-gel

Ionic Surface Dielectric Properties Distribution on Reservoir Sandstone

The petrophysical and dielectric properties for both carbonate and saturated sandstone with monovalent and divalent electrolyte they are accurately modeling of an isotropic dielectric properties has been the major research area in oil and gas industries for effective sweep efficiency. The reservoir petrophysical properties consist of cation and anion exchange capacity on a specific area and the sandstone porosity. The transportation of the ions is a charge carrier that mediates conduction in the pore fluids under the electrical double layers that exist between the minerals and the pore fluids interface. The dielectric anisotropic and the frequency-dependent behavior of reservoir sandstone with the minerals will be fully elucidated, it was revealed from the result obtained the effect of the anisotropic dielectric properties on the reservoir sandstone based on the influx of NaCl electrolyte modify the wettability of rock formation from oil-wet to water-wet at 9000 and 11000 ppm concentration with the aids of the electromagnetic field. The resistivity index of the reservoir sandstone reduces with the increase of electrolyte to the system.

Effect of liquid nitrogen freeze–thaw cycle on fracture toughness and energy release rate of saturated sandstone

To enhance the ability of tight sandstone gas extraction, liquid nitrogen (LN2) freeze–thaw cycle method can be employed to improve the physical and mechanical properties of the sandstone. Rock fracture behaviors and mechanical properties after fracturing play an important role to successfully implement reservoir stimulation. Since tight sandstone gas reservoirs are usually characterized by high water content, the influences of LN2 freeze–thaw cycle on the fracture toughness and energy-release rate of the saturated sandstone are studied in this paper. The three-point bending experiment is carried out on the notched semicircular bending sandstone samples subjected to different LN2 freeze–thaw cycle times under three different modes, that is, mode I, mixed mode (I/II), and mode II. The structure damage and facture morphology analyses are performed to explain the differences in fracturing behaviors. The energy release rate of the saturated sandstone subjected to the LN2 freeze–thaw cycle is corrected by the P-wave velocity. The experimental results indicate that the LN2 freeze–thaw cycle can promote the weakening of cementation between mineral particles and produce microcracks inside the saturated sandstone, further resulting in an increase in the roughness of the crack surface after loading failure. For all three modes, the fracture toughness and the energy-release rate gradually decrease with the increase in the LN2 freeze–thaw cycle times. For all the LN2 freeze–thaw cycles, mode I requires the minimum applied energy for the fracture propagation, while mode II requires maximum applied energy. In the three modes, the energy-release rate in mode I is the largest. Compared with mixed mode and mode II, the fracture toughness and fracture energy of mode I are more sensitive to the LN2 freeze–thaw cycle. The fracture energy depends on both the microstructure characteristics inside rocks and the mechanism of the fracture propagation. The results are helpful to better understand the initiation and propagation of cracks during the sandstone gas extraction.

Hysteresis in the ultrasonic parameters of saturated sandstone during freezing and thawing and correlations with unfrozen water content

Determining the mechanical properties of frozen rock is highly important in cold-area engineering. These properties are essentially correlated with the content of liquid water remaining in frozen rock. Therefore, accurate determination of unfrozen water content could allow rapid evaluation of mechanical properties of frozen rock. This paper investigates the hysteresis characteristics of ultrasonic waves applied to sandstone (in terms of the parameters of P-wave velocity, amplitude, dominant frequency and quality factor Q ) and their relationships with unfrozen water content during the freeze-thaw process. Their correlations are analysed in terms of their potential for use as indicators of freezing state and unfrozen water content. The results show that: (1) During a freeze-thaw cycle, the ultrasonic parameters and unfrozen water content of sandstone have significant hysteresis with changes in temperature. (2) There are three clear stages of change during freezing: supercooled stage (0 °C to −2 °C), rapid freezing stage (−2 °C to −3 °C), and stable freezing stage (−3 °C to −20 °C). The changes in unfrozen water content and ultrasonic parameters with freezing temperature are inverse. (3) During a single freeze-thaw cycle, the ultrasonic parameters of sandstone are significantly correlated with its unfrozen water content, and this correlation is affected by the pore structure. For sandstones with mesopores greater than 50%, there are inflection points in the curves of ultrasonic parameters vs. unfrozen water content at −3 °C during freezing and at −1 °C during thawing. It was found that thermal deformation of the mineral-grain skeleton and variations in the phase composition of pore water change the propagation path of ultrasonic waves. The inflection point in the curve of dominant frequency vs. temperature clearly marks the end of the rapid freezing stage of pore water, in which more than 70% of the pore water freezes. Consequently, the dominant frequency can be used as an index to conveniently estimate the unfrozen water content of frozen rock and, hence, its mechanical properties.

Coupling Effect of Strain Rate and Freeze-Thaw Temperature on Dynamic Mechanical Properties and Fractal Characteristic of Saturated Yellow Sandstone

Strain rate is not only an important influence factor for deformation property but also an important parameter for analyzing the dynamic mechanical behavior of rock material. In this study, the dynamic compressive mechanical properties of saturated yellow sandstone at four strain rates and six freeze-thaw temperatures are investigated by using the SHPB test system. The coupling effect of strain rate and freeze-thaw temperatures on the mechanical parameters of rock material are discussed in detail, and the relationship formula of peak strain and dissipated energy with strain rate and freeze-thaw temperature are also established. Finally, the fractal dimension characteristic of fracture specimens with the strain rate and temperature are analyzed by using the fractal dimension method. The research results indicate that (1) with the increase of strain rate, the increase speed of peak strain, peak strength, and dissipated energy at medium strain rate level was obviously higher than that at high strain rate level, indicating that the strain rate effect weakened at high strain rate. (2) Freeze-thaw temperature can improve the brittleness-ductile transformation rate of saturated specimens. (3) According to the strain rate sensitivity coefficient, at room temperature, the strain rate effects on peak strain and peak strength are weakest, while at -20°C ~ -30°C, they are most significant. In addition, the strain rate effect on dissipated energy is significant at room temperature, while weakest at -30°C. (4) The fractal dimension gradually increases with strain rate increasing or freeze-thaw temperature decreasing, indicating that the freeze-thaw environment has a positive function for increasing the damage and fracture degree of specimens for saturated specimen. Our research results can provide an extremely important theoretical basis for the dynamic disaster prevention and structural design of rock engineering in cold regions.

Infrared radiation test on the influence of water content on sandstone damage evolution

The rock mechanics uniaxial compression testing system and a thermal imaging camera were used to research the influence of water on sandstone damage evolution and the concurrent infrared radiation (IR) characteristics on sandstone samples with different water contents. The field of view of the thermal imaging camera divided the sandstone surface into several small areas of equal area. When one of the small areas of sandstone ruptured (that was when a damage point was generated in the sandstone) the IR information from that area underwent abrupt changes. Based on this monitoring, a quantitative index of infrared radiation, the IR count (IRC), was adopted and investigated as the indicator for damage evolution of rocks that also linked damage intensity with the corresponding IR temperature. The research results showed that when the sandstone stress experienced a stress drop, the IRC will simultaneously undergo a sudden increase indicating that the IRC was controlled by the stress. However, this stress control on IRC decreased with increasing water content due to the softening effect of water on the sandstone. Among them, the control effect of stress on IRC reaches 100% for dry sandstone, which is reduced to 75% for sandstone with a water content of 0.5% and 66.7% for sandstone with a water content of 1.5%. The control effect of stress of saturated sandstone on IRC is 57.1%. For sandstones with other water contents, but at the same damage evolution stage, the average IRC (IRC¯) for sandstones showed an upward trend as the water content increased. Compared with other damage evolution stages, the IRC¯ of the water-bearing sandstone had the most significant difference in the failure stage, indicating that water had the most significant influence on the damage evolution of the sandstone in the failure stage. This IRC was generated at the location of a new damage point on the sandstone surface. Therefore, based on the instant of time node when the sandstone IRC suddenly increased, the IRC spatial distribution map at that corresponding time was drawn to accurately locate the sandstone damage and fracture area. The characteristics of the damage evolution process of sandstones with different water contents were the same over the time before the sudden increase in IRC, the damage points formed were disorderly distributed, and the number of damage points was less. The difference was that as the water content increased, the number of sandstone damage points became larger and more concentrated.

Fracture evolution and failure characteristics of sandstone under freeze-thaw cycling by computed tomography

Long-term freeze-thaw (F-T) action causes the expansion of the fracture network in rock mass with ice-filled joints, triggering more serious engineering geological disasters. This study explores F-T damage effects and loading failure in double-fractured white sandstones with different saturation types (saturated and ice-filled). Saturated intact, saturated fractured, and ice-filled fractured sandstones were subjected to F-T cycle tests and uniaxial compression tests. Three-dimensional images of the specimens under various F-T cycles and stress states were obtained using computed tomography. The evolution in fracture morphology was evaluated visually, with pore network model parameters used for quantitative characterisation. Sandstone fractures evolved according to four main processes: frost-heave cracking, progressive expansion of damage, rock bridge penetration under load, and shear failure. The proportion of connected pores was used to estimate the influences of F-T cycling on pore-structure connectivity and fracture expansion in different specimen layers. During F-T cycling, pore characteristics vary according to size: micropores (equivalent radius < 100 μm) account for >60% of pores and increase in number more than pores of other sizes. Mesopores (100–500 μm) gradually decrease in number but their volume fraction remains >70%. Macropores (500–1000 μm) have the greatest increase in volume fraction. Early F-T cycling induces the connection of pore throats, while later cycling accelerates their expansion. Hydrostatic pressure increases are the leading cause of damage in ice-filled fractured sandstone, which exhibits greater variations in porosity, heterogeneity coefficient, and pore-throat parameters under F-T cycling than saturated fractured sandstone. The results provide insight into fracture evolution and failure characteristics in fractured rock masses with different saturation types (saturated and ice-filled) in cold regions.

Research on Microscopic Evolution Laws of Sandstone Deformation and Failure Based on the Particle Discrete Element Method

The fracture of sandstone is closely related to the condition of internal microcracks and the fabric of micrograin. The macroscopic mechanical property depends on its microscopic structures. However, it is difficult to obtain the law of the microcrack growth under loading by experiments. A series of microscopic sandstone models were established with particle flow code 3D (PFC3D) and based on the triaxial experiment results on sandstones. The experimental and numerical simulations of natural and saturated sandstones under different confining pressures were implemented. We analyzed the evolution of rock deformation and the rock fracture development from a microscopic view. Results show that although the sandstones are under different confining pressures, the law of microcrack growth is the same. That is, the number of the microcracks increases slowly in the initial stage and then increases exponentially. The number of shear cracks is more than the tensile cracks, and the proportion of the shear cracks increases with the increase of confining pressure. The cracking strength of natural and saturated sandstones is 26% and 27% of the peak strength, respectively. Under low confining pressure, the total number of cracks in the saturated sample is 20% more than that of the natural sample and the strongly scattered chain is barely seen. With the increase of the confining pressure, the effect of water on the total number of cracks is reduced and the distribution of the strong chain is even more uniform. In other words, it is the confining pressure that mainly affects the distribution of the force chain, irrespective of the state of the rock, natural or saturated. The research results reveal that the control mechanism of shear crack friction under the different stress states of a rock slope in the reservoir area provides a basis for evaluating the stability of rock mass and predicting the occurrence of geological disasters.

Effect of Loading Rate and Water on the Dynamic Fracture Behavior of Sandstone under Impact Loadings

The moisture content of rocks in nature often changes with the natural hydrological environment, and generally natural rock may be classified into three types, that is, dry, natural, and saturated rock. The corresponding responses to dynamic loads may have considerable difference, and therefore, impact tests in this paper were conducted on single cleavage triangle sandstone specimens with three types of conditions, respectively, under various loading rates. The ingredient of sandstone was determined by X-ray diffraction and optical microscopic image. The crack propagation parameters were recorded with the aid of a crack propagation gauge. The dynamic stress intensity factors were calculated using ABAQUS code. The results show that the fracture toughness of all the three types of specimens increase with loading rate, which could be expressed by power functions. Compared with dry and natural sandstone, the crack propagation speed of saturated sandstone specimens is more sensitive to the loading rate. Both the rate effect and the water effect, including water-weakening effect and water-enhancing effect, play a significant part in rock dynamic fracture behavior.

The effect of viscoelasticity and microplasticity on the P- wave attenuation in dry and water-saturated sandstone: An experimental study

Seismic studies use a viscoelastic model of a standard inelastic body that does not take into account the effect of microplasticity of the rocks. The modern approach introduces the new components, which indeed improves the efficiency of the seismic methods. The aim of this study is to determine the combined effect of viscoelasticity and microplasticity on the longitudinal wave's attenuation. The high-resolution measurements performed on dry and water-saturated sandstone by a pulse frequency of 1 MHz within the strain ranges of ~ (0.3–2.0) × 10-6 under 10 MPa confining pressure. Within the range of 0.5–2.5 MHz at both states of the dry and water-saturated sample, the frequency dependence of P- wave attenuation Qp−1(f) forms a peak due to the viscoelasticity. Microplasticity effect manifests as a non-linear dependence of the wave attenuation Qp−1(ε) on the strain amplitude. By increasing the strain amplitude the attenuation peak in the saturated sandstone shifts towards the higher frequencies and Qp−1- values. The indicators of microplastic deformation on the wave profile are the stress plateau and the stress drop feature. Within the range of 1.5–2.5 MHz frequencies the microplasticity is increased on the attenuation spectrum. The significant difference between dry and water-saturated sandstone can be observed as a change in peak and value of the attenuation. Considering microplasticity in the models and introducing the number of stress plateaus into a wave profile shows a significant effect of this factor on the wave attenuation. Unconventional behavior of the wave attenuation is explained by a joint action of viscoelastic and micro-plastic mechanism. The results of this study can contribute in improvement of the methods of geological interpretation of the seismic and acoustic data.

Evolutions of the unfrozen water content of saturated sandstones during freezing process and the freeze-induced damage characteristics

The precise determination of unfrozen water content is of great significance to understand the freezing process of pore water and reveal the freeze-induced damage mechanism for frozen rock mass. In this study, low field nuclear magnetic resonance (LF-NMR) tests were conducted on five types of frozen sandstone samples. The changes in the T2 distribution and unfrozen water content (including unfrozen bound water content and unfrozen movable water content) were obtained and then analyzed. The experimental results indicate that there is little unfrozen movable water exists in the sandstone sample at the temperature of -5 °C and lower, whereas the amount of the unfrozen bound water is slowly decreased during the freezing process. The pore size distribution largely dominates the speed of water-ice phase transition and the total ice volume in the sandstone samples. A larger equivalent average pore radius (rm) leads to a higher speed of water-ice phase transition. The freeze-induced strains of the frozen sandstones were theoretically calculated based on a frost heave model. It is indicated that the freezing strain increases rapidly in the temperature range of 0 °C to -10 °C, while tends to be stable with further decreasing the temperature. Owing to the different pore size distributions, the movable water induces more substantial freezing strain than the bound water in terms of the Wuding sandstone and Linyi sandstone having an rm larger than 1.6 μs. However, it is exactly inverted for the other three sandstones including the Tengchong sandstone, Luzhou sandstone and Zigong sandstone with much smaller rm. The study provides a reasonable method for quantifying the unfrozen water content and freeze-induced strain in frozen rock, as well as important guidance for the design and construction of rock engineering in cold regions.

Linking the mechanical properties of frozen sandstone to phase composition of pore water measured by LF-NMR at subzero temperatures

Water-bearing porous rocks can be greatly strengthened by freezing; its microscopic mechanisms lie in how subzero temperature modifies pore structure but have not yet been clearly defined. In this study, we link the mechanical properties of frozen sandstone to phase composition of pore water (i.e., relative amount of unfrozen water and ice). Both the strength (uniaxial compressive strength and tensile strength) and the phase composition of pore water (tested through the nuclear magnetic resonance (NMR) method) of frozen sandstone at nine subzero temperatures were measured. The results showed the following: (1) subzero temperature strengthened saturated sandstone significantly, both the strengths increased in a three-stage mode and each stage showed an approximately linear trend as temperature decreased from 0 to – 20 °C; and (2) change of phase composition with freezing displayed two distinct stages: the rapid decreasing stage of unfrozen water content (0 to – 4 °C), when bulk water and capillary water froze almost completely, and the slow decreasing stage (− 4 to – 20 °C), when adsorbed water froze partially. Based on direct observations on microscopic pore structure of sandstone and detailed discussion on the mechanical interactions between water, ice, and pore wall, phase composition may modify the mechanical properties of frozen rocks through the following effects: the supporting effect of ice under compression, the crack-filling effect of ice, the cementing effect of unfrozen water film under tension or under shearing load, and the frost damage effect. The former three are strengthening effects, while the latter one is weakening effect.

Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

AbstractThe transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where the temperature is likely 35°C–200°C. To test the importance of such elevated temperature on the strength and deformability of sandstone, a series of constant strain rate, triaxial deformation experiments were performed on three different water saturated sandstones at either ambient temperature or 150°C. For each sandstone, an effective pressure range was used which spanned both the brittle and ductile deformation regimes, up to a maximum of 120 MPa. In the brittle regime, we observed a temperature‐dependent lowering of the yield stress of between 8% and 17%. Within the ductile regime, we observed an even greater reduction in the yield stress of between 9% and 37%. A further notable observation is that the transition from dilatant, brittle behavior to compactive, ductile behavior tends to occur at a lower effective pressure at elevated temperature. The weakening observed at elevated temperature can be explained by a reduction in fracture toughness, which is shown mathematically to cause greater weakening in the ductile regime than in the brittle regime. The apparent reduction in fracture toughness at elevated temperature is potentially driven by a combination of a reduction in surface energy and, to a minor extent, an increase in subcritical crack growth rate.

Changing Characteristics of Sandstone Pore Size under Cyclic Loading

The size and distribution of pores in rocks are closely related to their physical and mechanical properties. It is important to study the structure and distribution of pore size inside the rock to assess the risk of damage to a given rock volume. These characteristics were studied under different pressures, pore diameters, and pore throat size distribution laws using a UTM5540 electronic universal testing machine, magnetic resonance imaging scanning, and low field nuclear magnetic resonance spectroscopy with cyclic loading on yellow sandstone. We found the following. (1) Under 0–10 MPa load, the peaks of the sandstone T 2 spectrum move left as load increases, and the porosity of the sandstone decreases. The peak area of the middle relaxation spectrum increases as pressure increases from 10 to 20 MPa, and a peak for the long relaxation time spectrum appears. (2) Under 0–10 MPa load, the spectral peak associated with a large pore moves left and decreases in area as pressure increases. Under 10–20 MPa load, the large-pore spectral peak moves right and increases in area as pressure increases. (3) Under the applied 0–10 MPa load, the porosity of water-saturated sandstone gradually decreases, and the sandstone NMR images darken with increasing load. The porosity of saturated sandstone gradually increases under 10–20 MPa pressure, and its NMR image brightens. (4) The number of small pore throats increases with increasing load, but the number of large- and medium-sized pore throats decreases. From 0 to 15 MPa, crack (&gt;1 micron) abundance decreases, and fractures are generated by compaction under a 20 MPa load. The pore interconnectivity is enhanced, as are the number and size of pores in the sandstone. With continuing increasing pressure, the numbers of pores and penetration of cracks increase, which damages the sandstone.

Attenuation and dispersion phenomena of shear waves in anelastic and elastic porous strips

PurposeThis paper aims to study the attenuation and dispersion phenomena of shear waves in anelastic and elastic porous strips. Numerical investigations are performed for the phase and damped velocity profiles of the wave. For numerical computation purposes, water-saturated limestone and kerosene oil saturated sandstone for the first and second porous strips, respectively. Some other peculiarities have been observed and discussed.Design/methodology/approachDispersion and attenuation characteristic of the shear wave propagations have been studied in an inhomogeneous poro-anelastic strip of finite thickness, which is clamped between an inhomogeneous poroelastic strip of finite thickness and an elastic half-space. Both the strips are initially stressed and the half-space is self-weighted. Analytical methods are used to calculate the interior deformations of the model with the involvement of special functions. The determination of the frequency equation, which includes the Bessel’s and Whittaker functions, has been obtained using the prescribed boundary conditions.FindingsImpacts of attenuation coefficient, dissipation factor, inhomogeneities, initial stresses, Biot’s gravity, porosity and thickness ratio parameters on the velocity profile of the wave have been demonstrated through the graphical visuals. These parameters are playing an important role and working as a catalyst in affecting the propagation behaviour of the wave.Originality/valueInclusion of the concept of doubly layered initially stressed inhomogeneous porous structure of elastic and anelastic medium bedded over a self-weighted half-space medium brings a novelty to the existing literature related to the study of shear wave. It may be helpful to geologists, seismologists and structural engineers in the development of theoretical and practical studies.

Triaxial Creep Behavior of Red Sandstone in Freeze-Thaw Environments

To investigate the time-dependent mechanical properties of rock masses in cold regions under the effects of freeze-thaw cycling and long-term loading, triaxial multilevel loading and unloading creep tests were performed on saturated red sandstone samples subjected to different numbers of freeze-thaw cycles. The effects of freeze-thaw cycles and confining pressure on the creep properties, long-term strength, and creep failure mode of the rock were analyzed. The effect of freeze-thaw cycles on the microstructure of the rock was analyzed using scanning electron microscopy. The results showed that as the number of freeze-thaw cycles increased, the rock particle boundaries became more distinct, and more pores formed. The effect of freeze-thaw cycles on the creep deformation of red sandstone was related to the loading stress level. At low stress levels, the rock viscoelastic strain increased gradually and almost linearly with an increasing number of freeze-thaw cycles; in contrast, at high stress levels, the rock viscoelastic strain increased nonlinearly. The viscoplastic strain increased almost linearly with increasing freeze-thaw cycles. The fourth loading stress level (70% σ c ) corresponded to the transition of the creep deformation of the red sandstone. When the confining pressure was low, a higher stress level caused the confining pressure to have a more significant effect on the creep strain. However, as the confining pressure continued to increase, the effect of the confining pressure on the creep strain eventually disappeared. The long-term strength of the red sandstone decreased approximately linearly with an increase in the number of freeze-thaw cycles. When the number of freeze-thaw cycles and the confining pressure were high, the rock samples formed a transverse shear plane and were more fragmented than those without a transverse shear plane. These results provide a reference for construction in rock mass engineering and long-term stability analysis in cold regions.

Creep damage properties of sandstone under dry-wet cycles

Rock creep properties can be used to predict the long-term stability in rock engineering. In reservoir bank slopes, sandstones which are frequently used in the bank slope undergoing long-term effects of dry-wet (DW) cycles due to periodic water inundation and drainage may gradually accumulate creep deformation, resulting in rock structure’s damage or even geological hazards such as landslides. To fully investigate the effect of DW cycles on the creep damage properties of sandstone, triaxial creep tests were conducted on saturated sandstone with different DW cycles by using a triaxial rheometer apparatus. The experimental results show that both the instantaneous strain and the stabilized strain increase with the DW cycles. In addition, using the Burgers model, four kinds of functions including an exponentially decreasing function, a linearly decreasing function, a linearly increasing function and an exponentially increasing function were proposed to express the relationships between the shear modulus, viscoelastic parameters of the Burgers model and the deviatoric stress under different DW cycles. Through comparative analysis, it is found that the theoretical curves generated using proposed four kinds of functions are in good agreement with the experimental data. Furthermore, macromorphological and microstructural observations were performed on specimens after various triaxial rheological tests. For samples with small number of DW cycles, approximately X-shaped fracture surfaces were observed in shear failure zones, whereas several shear fractures including obvious axial and horizontal tensile cracks, and flaws were found for samples with relatively large DW cycles due to long-term propagation and evolution of micro-fissures and micro-pores. Furthermore, as the DW cycles increases, the variation in micro-structure of samples after creep failure was summarized into three stages, namely, a stage with good and dense structure, a stage with pore and fissure propagation, and a stage with extensive increase of pores, fissures and loose particles. It is concluded that the combination effect of permeation of water molecules through pores and fissures within sandstone, and the propagation of preexisting pores and fissures owing to the dissolution of mineral particles leads to further deterioration of the mechanical properties of sandstone as the number of DW cycles increases. This study provides a fundamental basis for evaluating the long-term stability of reservoir bank slopes under cyclic fluctuations of water level.

Microwave fracturing of water-bearing sandstones: Heating characteristics and bursting

Microwave treatment has been considered as a promising rock breakage method. The weakening mechanism of vaporizing expansion of pore water in sandstone under microwave irradiation was studied in this paper. The effect of water in the heating process was studied by comparing the heating characteristics of dry and saturated sandstones, which was further explained by the mercury intrusion porosimetry tests. Moreover, the influencing factors including rock strength, permeability, saturation degree and microwave power on rock bursting were investigated by comparing the bursting time of water-bearing sandstone samples under microwave irradiation. The results show that water plays a more important role in the heating process of sandstones with higher porosity and lower microwave sensitivity. The heating curves of saturated sandstones can be divided into 3 stages according to the heating rate, which is related to the interaction of water heating and steam escape during microwave irradiation. In addition, it is found that the favorable conditions of water-bearing sandstone bursting under microwave irradiation are low rock strength, low permeability, high saturation degree and high microwave power. This law can be explained by the interaction of rock properties and the generation, increase, diffusion, and escape of steam in rock.