White Sandstone Research Articles

Experimental Study on Acoustic Emission Features and Energy Dissipation Properties during Rock Shear-Slip Process.

The features of rock shear-slip fracturing are closely related to the stability of rock mass engineering. Granite, white sandstone, red sandstone, and yellow sandstone specimens were selected in this study. The loading phase of "shear failure > slow slip > fast slip" was set up to explore the correlation between fracture type, acoustic emission (AE) features, and energy dissipation during the rock fracturing process. The results show that there is a strong correlation between fracture type, energy dissipation, and AE features. The energy dissipation ratio of tension-shear (T-S) composite, shear, and tensile types is 10:100:1. The fracture types in the shear failure phase are mainly tensile and TS composite types. The differential mechanism of energy dissipation of different rocks during the shear-slip process is revealed from the physical property perspectives of mineral composition, particle size, and diagenetic mode. These results provide a necessary research basis for energy dissipation research in rock failure and offer an important scientific foundation for analyzing the fracture propagation problem in the shear-slip process. They also provide a research basis for further understanding the acoustic emission characteristics and crack type evolution during rock shear and slip processes, which helps to better understand the shear failure mechanism of natural joints and provides a reference for the identification of precursors of shear disasters in geotechnical engineering.

Acoustic emission characteristics and damage evolution of different rocks under uniaxial compression conditions

Due to the complexity of the strata, it is difficult to monitor and identify the disasters induced by rock fractures in the process of mining deep coal resources. This will seriously affect the safety and sustainable mining of coal. Therefore, it is necessary to understand the failure mechanisms and acoustic emission (AE) characteristics of different rocks. In this paper, uniaxial compression tests as well as simultaneous AE monitoring were carried out on four different rocks. The four rocks include yellow sandstone, white sandstone, marble and limestone. The mechanical properties, energy evolution and AE characteristics of different rocks were analysed. It is found that the AE response of rocks is closely related to the damage and fracture process. The more brittle the rock is, the less energy is dissipated before failure, and the less obvious the AE precursor is, and the RA-AF values can effectively characterise the failure modes of different rocks. Finally, the damage models were developed from the perspectives of AE energy and dissipated energy, respectively. The damage model based on dissipated energy can better reflect the stress and damage state of the rock, and the theoretical curves of stress–strain are in good agreement with the measured curves.

Understanding the evolution mechanism and classification criteria of tensile -shear cracks in rock failure process from acoustic emission (AE) characteristics

This study aimed to investigate the crack evolution and failure mechanism of rocks by conducting uniaxial compression tests (UCT) and Brazilian indirect tension tests (BITT) using four categories of rock specimens. Real-time acoustic emission (AE) signals were detected and recorded during rock failure processes by an AE monitoring system. The study focused on the accumulative AE hits curves, stress–strain curves, and other AE energy characteristics to divide the failure process into five stages, and determine the stress thresholds. Furthermore, the transition trend of rock crack types was analyzed based on RA-AF data. A novel classification criterion of crack types based on clustering and statistical theory was proposed, and the crack boundaries of the four rocks were determined. Additionally, the tension to shear ratios of the rocks in descending order were fine-grained granite, granite, yellow sandstone, and white sandstone. Finally, the crack development trend and failure characteristics of the rock were analyzed using the crack scale and crack dividing line. The study found that the failure mode of the rock could be predicted accurately by AE parameter analysis at Unstable Crack Growth (Stage-IV).

Failure precursor characteristics of different types of rocks under load: insights from critical slowing down of acoustic emission

In this study, uniaxial compression tests and simultaneous acoustic emission (AE) monitoring were carried out on four rocks (yellow sandstone, white sandstone, marble and limestone). The mechanical properties and AE energy evolution characteristics of different rocks were analysed. With the help of critical slowing down (CSD) theory, the AE precursor characteristics of their failure were investigated. It is pointed out that the AE during rock loading has a CSD phenomenon. A sudden change in the variance of one of the CSD indicators can be regarded as a precursor to failure, and it has the advantage of being more accurate and sensitive to failure than the autocorrelation coefficient. The stress level of a rock's failure precursor is closely related to its brittleness characteristics. The higher the brittleness of the rock, the more backward the failure precursor is, and the more difficult the early warning is. The study aims to provide new indicators and references for the monitoring and early warning of rockbursts and other disasters induced by rock fracture in deep underground engineering.

Time-Frequency Response of Acoustic Emission and Its Multi-Fractal Analysis for Rocks with Different Brittleness under Uniaxial Compression

The occurrence of rock burst hazards is closely related to the brittleness of rocks. Current research has paid less attention to the in-depth relationship between rock brittleness and acoustic emission (AE) signal characteristics and precursor information caused by rock fracture. Therefore, in order to further improve the accuracy of the AE monitoring of rockburst hazards, uniaxial compression tests were carried out and AE were monitored for rocks with different brittleness (yellow sandstone, white sandstone, marble, and limestone) in this paper. The relationship between the mechanical properties and the time-frequency characteristics of the AE was analyzed. In addition, the multi-fractal theory was introduced to further deconstruct and mine the AE signals, and the multi-fractal characteristics of AE from rocks with different brittleness were investigated. The results show that the stronger the brittleness of the rock, the higher the main frequency and main frequency amplitude of the AE. Brittleness is positively correlated with the multi-fractal parameter Δα (uniformity of data distribution) and negatively correlated with Δf (frequency difference between large and small data). In addition, the dynamics of Δα and Δf provide new indicators for AE monitoring of rock stability, and their abrupt changes can be regarded as precursors of failure. The weaker the brittleness of the rock, the earlier the failure precursor is and the more significant it is. This has potential engineering application value, which can help identify rockburst precursors and take timely protective measures to ensure engineering safety.

Proterozoic - basal Cambrian stratigraphy across Nares Strait: correlation between Inglefield Land and Bache Peninsula

In the Kane Basin region of Nares Strait (78° to 79°N) unmetamorphosed Proterozoic and basal Cambrian platform strata overlie the Precambrian Shield both in Inglefield Land in Greenland, and on Bache Peninsula in Ellesmere Island. The platform succession is composed of two shallow-water elastic sequences separated by an erosional disconformity. The lower formation, of multicoloured sandstone and siltstone with minor shale and stromatolitic dolomite, is assigned to the redefined Rensselaer Bay Formation; the upper, composed of red and overlying white sandstone with dolomitic sandstone and siltstone at the top, is assigned to the Dallas Bugt Formation. Basic sills from the Rensselaer Bay Formation have yielded isotopic ages as old as 1200 m.y., while the trace fossils Rusophycus and Skolithos suggest a basal Cambrian age for the Dallas Bugt Formation.
 The platform sequences of Inglefield Land and Bache Peninsula show striking similarity in lithology and thickness and the detail correlation is supported by the evidence from isotopic ages of basic sills and from trace fossils. In both areas the bipartite succession is overlain with apparent conformity by dolomites that pass upwards into limestones containing Early Cambrian macrofauna.
 The Rensselaer Bay Formation thickens southwards and passes into the much thicker sedimentary sequence of the Thule Basin, while the Dallas Bugt Formation and the overlying dolomites thicken northwards, indicating that the two formations were laid down in separate sedimentary basins. The close correlation of the platform successions and the corresponding configuration of the depositional slopes on both sides of the Kane Basin clearly indicate that the Proterozoic - Early Cambrian sedimentary pattern has not been radically disturbed by tectonism or by any appreciable transcurrent movement along Nares Strait.

U–Pb detrital zircon ages from Cretaceous - Paleogene white sandstone member in Western Tabei Uplift (China) as provenance indicators

The Western Tabei Uplift is one of the largest first-level structural units in the Tarim Basin with great potential for oil and gas resources. Previous studies have established sedimentary provenance of the upper and lower strata in the unconformity of the Cretaceous Bashijiqike Formation and Paleogene Kumugeliemu Group in the Western Tabei Uplift. However, little attention has been paid to the light-colored, white-ish sandstone at the interface of this unconformity, whose sedimentary provenance remains to be investigated. Based on morphology, U–Pb age and trace element analyses of detrital zircon, our study reveals five age peaks in detrital zircon age distribution, ∼300 Ma, ∼420 Ma, ∼820 Ma, ∼1.8–1.9 Ga, ∼2.4 Ga. Potential sources of white sandstone include the Quruqtagh region to the east (E-source), the South Tianshan to the north (N-source), the west of East Tianshan and the north part of Quruqtagh to the north-east (NE-source). Their detrital zircon U–Pb age information is collected and compared with those from the white sandstone. The detrital zircon age comparison demonstrates that the E-source is the primary sedimentary source area for the white sandstone, the N-source may also provide some additional sediments. The detrital zircon age pattern also indicates that the origin of the NE-source is probably supplied by both the E-source and N-source together. Our finding could shed new light on the provenance and sedimentary model of Cretaceous-Paleogene white sandstone member in the Western Tabei Uplift and provide a theoretical basis for further exploration of lithological oil reservoirs in this section.

Effect of Water Content on the Impact Propensity of White Sandstone

Impact ground pressure is one of the most common dynamic disasters induced by mining activities, and water content is an important factor affecting such dynamic disasters. In this paper, uniaxial compression test, cyclic loading and unloading test, and acoustic emission test were conducted on white sandstone using RMT-150B rock mechanics test system and DS5 acoustic emission test system. The influence law of water content was analyzed on the strength characteristics, energy change characteristics, and impact propensity of white sandstone. The results showed that (1) the internal structure of the sandstone gets softened with the increase of the water content. The cohesive effect within the rock also begins to weaken, which in turn reduces the stiffness of the material and enhances its plasticity. The ability of the rock to resist elastic deformation becomes weaker, resulting in lower compressive strength and elastic modulus when the rock is subjected to external forces, making it more prone to deform and fail. The decrease in compressive strength of the water-saturated rock is 33.3%, and the decrease in its elastic modulus is 28.1% compared to the dry rock. (2) As the water content increases, the cohesion of the rock decreases and the internal structure of the rock fails more easily, which ultimately makes the energy needed for rock destruction lower. As a result, the total energy, elastic energy, and dissipative energy of the rock are reduced. The accumulated AE energy also decreases with the increase of the water content, indicating that rocks with higher water content gather less elastic energy before damage and accumulate less energy when deformation damage occurs. (3) The impact energy index and elastic energy index are negatively correlated with the water content. The impact energy index is reduced by 28.6%, and the elastic energy index is reduced by 20.9% for the saturated rock compared to the dry rock. The elastic energy index and impact energy index both decrease with the increase of rock water content, indicating that the less elastic energy is stored before the destruction of the rock and no excess energy is transformed into energy in rock crushing when the rock breaks, and therefore, the impact propensity of the rock is smaller. The results of the study can provide a theoretical basis for underground construction as well as rock fracture destabilization.

Vanadium mineralization at Los Chihuidos sediment-hosted Cu–V deposit, Neuquén Basin, Argentina: An approach to vanadium ore forming processes

Vanadium mineralization at Los Chihuidos deposit of the Neuquén Basin is linked to the development of a redox front system related to the inflow of hydrocarbons into the red sandstone of the Huincul Formation. Interaction of hydrocarbons with oxidized red beds and connate water generated redox reactions where hematite was dissolved due to iron reduction resulting in the discoloration of the red strata. At the contact between oxidized red sandstone and reduced white sandstone, precipitation of specific mineral phases resulted in the V ore with minor amounts of Cu. With the implementation of the redox interface, abundant V-montmorillonite and V-hematite precipitated at the more oxidizing conditions and Cu-V-corrensite-type at the more reducing conditions of the redox front. As the redox front advanced with fluids constantly migrating into the reservoir, more reducing conditions were stablished, promoting chloritization and minor illitization with V-Cu incorporation and continuous upgrading of the ore. Main ore mineralogy consists of clay minerals including V-bearing montmorillonite, Cu-V-corrensite-type, V-di-trioctahedral chlorite and Cu-tri-trioctahedral chlorite with minor V-illite–smectite mixed-layer minerals and associated secondary V-hematite. Chloritization over illitization was favored due to high amounts of Fe and Mg in detrital clasts and in connate fluids and by low K availability related to low amounts of detrital K-feldspar. The spatial transition of V and /or Cu bearing clay minerals observed through the mineralized redox front at Los Chihuidos deposit (kaolinite → smectite → illite/smectite → corrensite-type → di- trioctahedral -chlorite → tri- trioctahedral -chlorite) and the related variation of V–Cu concentrations in bulk rock are indicative of increasing pH and decreasing Eh of resident solutions from red to white sandstones during the hypogene mineralization process. Late influx of Cu-rich oxidized basinal brines precipitated main copper ore with Cu-sulfides in the white sandstone up to the contact with the redox front in contact with hydrocarbons. During uplift and exhumation, percolation of meteoric water promoted remobilization of V and Cu and the precipitation of oxidized V–Cu ore.

Evolution of residual strain and strain energy in rocks under various types of uniaxial cyclic loading-unloading

In the process of roadway excavation, the rock mass around the roadway is often subjected to cyclic loads, and the rock mechanical properties and strain energy evolution under cyclic loads are obviously different from those under compression, so it is urgent to carry out research on rock mechanical properties and strain energy evolution under cyclic loads. This study aims to investigate the evolution of residual strain and strain energy in rocks under uniaxial cyclic loading-unloading experiments (UCLUE). Four types of rocks, namely coal, white sandstone, red sandstone, and granite, were subjected to uniaxial compression experiments (UCE) and various uniaxial cyclic loading-unloading experiments (UEACLUE). The findings are as follows: analysis of UEACLUE revealed a gradual decrease in residual strain with an increasing number of cycles, leading to its eventual disappearance. However, if the cyclic loading-unloading (CLU) was continued beyond this point, the rocks displayed a reappearance of residual strain. The number of cycles required to eliminate residual strain was found to be inversely proportional to the peak strength of the rocks, while directly proportional to the upper limit value of UCLUE. Among the different stages of the uniaxial cyclic loading and unloading test, the plastic stage of white sandstone exhibited the largest disparity in dissipated strain energy, followed by the plastic stage of red sandstone, with coal displaying the smallest difference. Analysis of dissipated strain energy in the four types of uniaxial cyclic loading and unloading tests revealed differences of 0.00348 mJ▪mm−3, 0.03488 mJ▪mm−3, 0.02763 mJ▪mm−3, and 0.01619 mJ▪mm−3 in the plastic stage for the respective rock types. Furthermore, examination of the input strain energy density (ISED) and dissipated strain energy density (DSED) during the CLU process showed a linear relationship between these variables. Additionally, the investigation of ISED and DSED in other types of UCLUE demonstrated adherence to the cyclic-linear dissipation law (CLDL). The study of mechanical properties and strain energy evolution under CLU is of positive significance for the development of rock fatigue damage and rock damage mechanics.

Subcritical crack growth and fracture behavior of three rocks containing quartz under wet and dry conditions

Understanding the subcritical crack growth behavior of rocks under water-rock interaction is essential to evaluate the stability of deep resource extraction and the integrity of nuclear waste disposal or CO2 sequestration. Using the double-torsion technique, we performed relaxation tests on three rocks with different quartz contents, namely, quartz amphibolite, white sandstone and red sandstone, in air environment and in introduced water conditions to systematically investigate the effect of water on subcritical crack growth behavior of three rocks. The subcritical fracture behavior of the rock responded very rapidly to water: when water entered the crack tip, 1) the crack velocity increased with decreasing of stress intensity factor; 2) the slope of the load decay curves increased abruptly; and 3) the strain values at the bottom monitoring points extracted from three-dimensional digital image correlation increased immediately. Compared with the air environment, the subcritical crack growth indexes of quartz amphibolite, white sandstone and red sandstone in fluid conditions were reduced by 27.08 %, 80.81 % and 86.14 %, respectively, and the crack velocities were increased by one order of magnitude. The Ka values of quartz amphibolite, white sandstone and red sandstone were less sensitive to water than the subcritical crack growth indexes, decreasing by 9.43 %, 17.35 % and 37.33 %, respectively. Data from these tests reveal that the subcritical fracture properties of the three rocks were degraded despite the small amount of water flowing through the sample, proving that the long-term effects of water on actual engineering cannot be ignored.

An experimental study on the erosion of sandstone by self-excited oscillation cavitation waterjet in submerged environment

Self-excited oscillation cavitation waterjet (SOCW) has good prospects for application in deep sea mineral extraction of its cavitation characteristics. This study conducted the erosion experiments of three types of sandstone, red sandstone, white sandstone, and black sandstone by SOCW under a submerged environment. The erosion characteristics of three types of sandstones under the impact of SOCW at various erosion times, standoff distance, and pressure were compared. It was found that, a) Red sandstone with larger porosity is more likely to produce a large damage area and has the best rock breaking effect. Taking the standard working condition of this experiment as an example (t = 50s, Pj = 16 MPa, S/De = 4), the erosion depth of red sandstone can reach 1.9 times that of white sandstone and 5.5 times that of black sandstone; the erosion area can reach 19.6 times that of white sandstone and 6.2 times that of black sandstone; the erosion quality can reach 5.2 times that of white sandstone and 7.4 times that of black sandstone. b) With the increase of erosion time and erosion pressure, the erosion index of sandstone shows an increasing trend. However, in this study, there is an optimal standoff distance S/De = 4 to maximize the erosion depth of the sandstone. Again, there is an optimal standoff distance to maximize the erosion hole area, S/De = 10 for the red sandstone and white sandstone, and S/De = 6 for the black sandstone. c) The special phenomenon of the pressure difference formed after the collapse of the SOCW bubble in the submerged environment, which acts on the moisture content fine pore space inside the sandstone, the fine pore zone, was observed by scanning electron microscope (SEM). The microscopic characterization of the water wedge effect was observed by computed tomography (CT), and the trend of crack development is shown by the binarization processed images of the sandstone internal damage section. This result can be a theoretical basis to guide the working parameters of nozzles in deep sea mining, especially for the selection of standoff distance.

Determination of Rock Tensile Strength Based on Arc Loading Splitting Test considering Modulus Effect

Tensile strength is an important control parameter for the design and stability analysis in rock engineering. In order to reveal the influence of elastic modulus on tensile strength test results, Brazilian splitting tests were carried out on specimens with different strengths, such as mudstone, white sandstone, green sandstone, red sandstone, and marble. Longitudinal and transverse strains were monitored to analyze the influence of elastic modulus on the maximum contact angle and peak contact load between head and disc. A numerical model of Brazilian splitting tests was built, and the contact load between the head and rock disc were detailed revealed. Based on the experimental and numerical results, a modified formula to calculate tensile strength considering the modulus effect was proposed theoretically. Results show that the maximum pressure between the head and disc increases and the contact angle decreases with the increasing of elastic modulus, which show great influence on the splitting failure; the distribution of contact load can be divided into three types: parabolic I, sinusoidal, and parabolic II, which are suitable for soft rock, medium-hard rock, and hard rock, respectively; the new models can improve the calculation accuracy of rock tensile strength.

Lithostratigraphy of a long, fossiliferous Oligocene sequence: Revisiting Saint Jacques, Nei Mongol, China

For a hundred years the Saint Jacques area has been known to produce rich Oligocene vertebrate fossils, yet only a handful of previous studies have focused on this area. Since 2010, we have conducted 12 field expeditions to Saint Jacques, and here we report findings from our paleontological excavations and stratigraphical investigations. Twenty-two fossiliferous blocks across the area are recognized and a chronostratigraphic framework has been established to aid fossil collection. Fossil-mammal materials have been recovered in situ from 1635 localities and additionally from surface sediments. Fossiliferous blocks in the area are correlated by lithological similarity and lateral tracing. Lithologically, the area is mainly composed of reddish silty mudstone and muddy siltstone, with three distinctive layers of grayish white sandstone. The measured composite stratigraphic column spans 239 meters and are divided into 12 lithostratigraphic units. Contrary to previous knowledge that Saint Jacques contains two Oligocene mammalian assemblages, our preliminary biostratigraphic analysis of small mammals shows that the area documents successive faunal transition from the Eocene to possibly the early Miocene. The hyracodontid perissodactyl Ardynia, the ctenodactyloid rodent Gobiomys, and the basal Glires Gomphos from the bottom litho-units imply the presence of the Eocene–Oligocene boundary, while small mammal assemblage of the top units is similar to Miocene faunas in northern China and Mongolia. Thus, rock strata in Saint Jacques likely span the Eocene through the early Miocene, bracketing an entire Oligocene sequence within. In sum, our re-exploration of Saint Jacques has greatly expanded the chronostratigraphic and taxonomic coverage of the mammalian fossil collection from this area. This long, successive Oligocene sequence makes an important record for studying the Eocene–Oligocene Transition. Further study in this area will contribute to a range of paleontological and paleoenvironmental questions.

Experimental Evaluation of Conjugate Flaws on Rock Dynamic Fracturing

The fracture behaviors of rocks under dynamic loading are significantly affected by flaws. Understanding regarding this fundamental mechanism of flaw-induced dynamic fracturing could aid in reducing dynamic geohazards in deep rock engineering. In this study, a series of dynamic loading experiments are conducted on conjugate flawed white sandstone specimens to study the effect of the geometric configuration of flaws on dynamic fracturing. The results show that the geometry configuration of flaws and the loading conditions both strongly affect cracking and failure behaviors. Two types of shear cracks and three types of tensile cracks are observed, four coalescence patterns are identified, and the global failure modes of rock are usually coupled with two or more coalescence patterns. The inhibiting and enhancing mechanism of flaws in regards to potential shear fracture are obtained. These two failure mechanisms depend on the angular relationship between the flaws and the potential shear strain field. The “guiding effect” of the flaws results in the deviation and deformation of shear cracks. Moreover, it is found that the loading condition dominates the fracture tendency of rock macroscopically, while the geometric setting of flaws significantly affects the fracture behavior and failure mode locally.

Comparative study of temperature effects on white sandstone and rockburst-like materials.

Rockburst physical model test, as one of the important means to study deep tunnel engineering, reflects the main influencing factors of rockburst into the model test through similar theory, so as to reveal the formation mechanism, influencing factors and evolution law of different types of rockburst in deep tunnels. In order to study the mechanical properties of white sandstone in deeply buried tunnels at high ground temperatures, materials suitable for conducting rockburst physical and mechanical tests were developed on the basis of the Daqian Shi Ling tunnel project, and similar material ratios were preferentially selected on the basis of white sandstone. Judged by the rock burst propensity, similar materials with low strength and high brittle characteristics, can better simulate the characteristics of white sandstone, and all show a strong propensity to rock burst, is the ideal rock burst similar materials. Uniaxial compressive tests were conducted on similar materials and the original rock at different temperatures, and comparative analysis was performed. Through the study of stress, displacement and modulus of elasticity, it was concluded that the compressive strength of similar materials gradually increased with temperature in the range of 20-100°C, and the vertical displacement at peak strength decreased with increasing temperature. The damage forms of white sandstone and similar materials at different temperatures were comparatively analyzed, and it was obtained that the damage forms of white sandstone and similar materials were basically the same, with a few specimens showing tensile and shear damage, and most specimens showing the form of combined tensile and shear damage. The study of rock burst similar materials and the development of the failure characteristics of rock burst under the action of thermal coupling are of great significance to the mechanism of rock burst generation and prediction.

Dynamic mode I fracture properties of saturated white sandstone under ambient sub-zero temperatures

Dynamic fracture properties of frozen rocks are essential for better designs of rock engineering infrastructures in cold regions. Thus, a dynamic-cryogenic testing system combined with a high-speed camera is adopted to conduct dynamic tests under ambient room and sub-zero temperatures (20 °C, −10 °C, −20 °C, and −30 °C). A porous white sandstone (WS) is used to fabricate notched semi-circle bend (NSCB) specimens for mode I dynamic fracture tests. The percentage of ice and unfrozen water in rock pores under different temperatures is quantified using the nuclear magnetic resonance (NMR) method. For comparison purpose, dry specimens are also tested under the same ambient temperatures. The results show that the dynamic fracture toughness (DFT) for two sets of specimens all show obvious rate dependency. Except for −30 °C, DFTs of dry specimens are marginally influenced by the sub-zero temperatures. On the contrary at a given loading rate, DFT of frozen saturated specimens first drops to the lowest value at −10 °C before further increasing with the dropping ambient temperature. Compared with dry specimens, saturated specimens show lower DFTs and crack propagation velocities that are quantified with the digital image correlation (DIC) method. The experimental results are explained invoking the rock weakening and enhancing mechanisms of the ice and unfrozen water mixture in rock pores. These results are useful to blasting designs in cold areas.

Experimental study on the influence of prefabricated fissure size on the directional propagation law of rock type-I crack

To investigate the influence of prefabricated fissure size on the directional propagation law of rock type-I cracks, a simple device for simulating rock crack directional propagation was developed. Subsequently, loading tests for white sandstone samples with different prefabricated fissure lengths and widths were performed. The evolution laws of the deformation field and acoustic emission (AE) were analyzed during the entire crack propagation process. Finally, the influence of fissure size on the crack directional propagation mechanism was elucidated. The obtained results indicate that: The initial crack angle was mainly influenced by the prefabricated fissure width. The initial crack angle increased with the increase in the prefabricated fissure width. The crack directional propagation process can be divided into four stages: local strain-band initiation, local strain-band development, crack initiation, and crack coalescence. The crack maximum relative strain increases with an increase in the prefabricated fissure length but decreases with an increase in the prefabricated fissure width. This phenomenon indicates that increasing the prefabricated fissure length is advantageous to crack propagation at the fissure tip. The AE evolution process includes quiet, slowly increasing, booming, and decreasing periods. When the prefabricated fissure length increased, the durations of the quiet and slowly increasing periods decreased, but the durations of booming and decreasing periods increased. However, the durations of quiet, slowly increasing, and booming periods decreased as the prefabricated fissure width increased. In other words, increasing the prefabricated fissure length can enhance the stability of crack directional propagation, but increasing the prefabricated fissure width leads to the instability of main crack propagation path. In underground engineering practice, increasing the prefabricated fissure length and decreasing the prefabricated fissure width may be excellent approaches to realizing the crack directional propagation of rock or coal fracturing.

Experimental study on uniaxial creep characteristics of sandstone with pre-peak unloading damage

Deep resource extraction has been affected by the complex geological environment of "three highs and one disturbance" for a long time. The surrounding rocks experience strong unloading stress disturbance during the underground resource extraction. The creep characteristics of the perimeter rocks are of great theoretical and practical value. Firstly, the triaxial pre-peak loading and unloading tests on the prepared samples are conducted on the intact rock samples using the TAW-200 rock mechanics test system. Then the rock samples with different degrees of pre-peak unloading damage were prepared under the condition of pre-peak yield. Finally, the uniaxial creep tests were carried out to study the uniaxial creep mechanical properties. The results show that the wave velocities of the damaged rock samples are reduced to different degrees compared with those of the intact rock samples, the creep of the pre-peak unloading damaged rock has a time-dependent damage effect, and the study results are similar to those of the conventional uniaxial creep test of the white sandstone. The instantaneous strain at all stress levels of the damaged rock samples increased gradually with the stress level, which is consistent with the non-linear functional relationship; Based on the time-dependent damage effect, the two-parameter Weibull distribution function was introduced into the West Plains viscoelasticity model, and a creep damage model for white sandstone was proposed. The improved Nishihara model can simulate the uniaxial creep characteristics of each damaged rock sample at various stress levels.

Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

AbstractExtensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron‐oxide grain coatings. Reduced fluids such as hydrocarbons, CO2, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen‐saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K‐feldspar, calcite cement, fine‐grained illite, TiO2‐minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe2O3 concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron‐oxide concretions. Metals such as Zn, Pb, and rare‐earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon‐bearing fluids caused iron‐oxide and K‐feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines.