Mechanical Properties Of Sandstone Research Articles

Experimental Investigation of the Mechanical Behavior in Unloading Conditions of Sandstone After High-Temperature Treatment

A detailed understanding of damage evolution in rock after high-temperature treatment in unloading conditions is extremely important in underground engineering applications, such as the disposal of highly radioactive nuclear waste, underground coal gasification, and post-disaster reconstruction. We have studied the effects of temperature (200, 400, 600 and 800 °C) and confining pressure (20, 30 and 40 MPa) on the mechanical properties of sandstone. Scanning electron microscopy studies revealed that at temperatures exceeding 400 °C, new cracks formed, and original cracks extended substantially. When the confining pressure was 20 MPa, a temperature increase from 400 to 800 °C resulted in a 75.2% increase in peak strain, a decrease in Young’s modulus and peak strength of 62.5 and 35.8 %, respectively, and transition of the failure mechanism from brittleness to ductility. In the triaxial compression tests, the specimen deformed in a more obvious ductile failure manner at higher confining pressure, whereas in the unloading confining pressure experiments, brittle failure was more obvious when the initial confining pressure was higher. We focused on the effects of temperature and initial confining pressure on peak effective loading stress and peak ductile deformation during unloading. At temperatures of >400 °C, the peak ductile deformation increased rapidly with increases in the high temperature treatment or initial confining pressure. The peak effective loading stress decreased sharply with increased temperature but barely changed when the initial confining pressure was varied.

Effect of modified ethylsilicate consolidants on the mechanical properties of sandstone

Silicon alkoxides, such as silicon ethoxide or its oligomers (generally called ethylsilicates), are excellent consolidants for the weathered silicate materials. Their drawbacks, the shrinkage and cracking, can be reduced by the formation of wider pores within the gels or by the modification with particles. The sol-gel process was found to depend on the properties of particles added, especially on their size and surface properties. While the hydrophobization of the particle surface decelerated the process, the adding of hydroxylated particles had only a slight effect on the polycondensation process. In order to suppress the formation of micropores and to support the formation of more advantageous, substantially wider mesopores, organic amines are preferable as catalysts. The role of primary amines with a longer alkyl chain seems more complex than that of mere catalysts. If particles were added, no substantial effect on the character of the porosity was observed. The mechanical properties of gels modified by embedding particles or by creating mesoporosity were substantially changed. Due to the embedding of particles both the hardness and Young’s modulus were increased. On the other hand, the formation of the mesoporosity within the gels had an opposite effect. The modification of consolidants by the creation of mesopores in the gels enabled to achieve an even consolidation of the stone characterized by moderate strength changes due to consolidating treatment. On the contrary, the variants with particles exhibited much higher strength increase in the surface part of stone specimens compared to inner or bottom stone areas, which is not suitable for a satisfactory consolidating intervention.

Physical and mechanical properties of sandstone containing a single fissure after exposure to high temperatures

In order to investigate the physical and mechanical properties of sandstone containing fissures after exposure to high temperatures, fissures with different angles α were prefabricated in the plate sandstone samples, and the processed samples were then heated at 5 different temperatures. Indoor uniaxial compression was conducted to analyze the change rules of physical properties of sandstone after exposure to high temperature, and the deformation, strength and failure characteristics of sandstone containing fissures. The results show that, with increasing temperature, the volume of sandstone increases gradually while the quality and density decrease gradually, and the color of sandstone remains basically unchanged while the brightness increases markedly when the temperature is higher than 585°C; the peak strength of sandstone containing fissures first decreases then increases when the temperature is between 25°C and 400°C. The peak strain of sandstone containing fissures increases gradually while the average modulus decreases gradually with increasing temperature, and the mechanical properties of sandstone show obvious deterioration after 400°C. The peak strain of sandstone containing fissures increases gradually while the average modulus decreases gradually with increasing temperature; with increasing angle α of the fissure, the evolution characteristics of the macro-mechanical parameters of sandstone are closely related to the their own mechanical properties. When the temperature is 800°C, the correlation between the peak strength and average modulus of sandstone and the angle α of the fissure is obviously weakened. The failure modes of sandstone containing fissures after high temperature exposure are of three different kinds including: tensile crack failure, tensile and shear cracks mixed failure, and shear crack failure. Tensile and shear crack mixed failure occur mainly at low temperatures and small angles; tensile crack failure occurs at high temperatures and large angles.

Experimental study of the effect of thermal damage on resistivity and mechanical properties of sandstone

Experimental study of the effect of thermal damage on resistivity and mechanical properties of sandstone

Mechanism of formation of wiggly compaction bands in porous sandstone: 2. Numerical simulation using discrete element method

AbstractWiggly compaction bands in porous aeolian sandstone vary from chevron shape to wavy shape to nearly straight. In some outcrops these variations occur along a single band. A bonded close‐packed discrete element model is used to investigate what mechanical properties control the formation of wiggly compaction bands (CBs). To simulate the volumetric yielding failure of porous sandstone, a discrete element shrinks when the force state of one of its bonds reaches the yielding cap defined by the failure force and the aspect ratio (k) of the yielding ellipse. A Matlab code “MatDEM3D” has been developed on the basis of this enhanced discrete element method. Mechanical parameters of elements are chosen according to the elastic properties and the strengths of porous sandstone. In numerical simulations, the failure angle between the band segment and maximum principle stress decreases from 90° to approximately 45° as k increases from 0.5 to 2, and compaction bands vary from straight to chevron shape. With increasing strain, subsequent compaction occurs inside or beside compacted elements, which leads to further compaction and thickening of bands. The simulations indicate that a greater yielding stress promotes chevron CBs, and a greater cement strength promotes straight CBs. Combined with the microscopic analysis introduced in the companion paper, we conclude that the shape of wiggly CBs is controlled by the mechanical properties of sandstone, including the aspect ratio of the yielding ellipse, the critical yielding stress, and the cement strength, which are determined primarily by petrophysical attributes, e.g., grain sorting, porosity, and cementation.

Influence of grain size, humidity and state of stress on the mechanical properties of sandstones

Influence of grain size, humidity and state of stress on the mechanical properties of sandstones

Influence of wet–dry, freeze–thaw, and heat–cool cycles on the physical and mechanical properties of Upper Red sandstones in central Iran

The effects of wet–dry, freeze–thaw, and heat–cool cycles on the physical and mechanical properties of Upper Red Formation sandstones located southwest of Qom, central Iran, were determined. Five different types of sandstones were selected, and freeze–thaw, wet–dry, and heat–cool cyclic tests were performed. The freeze–thaw test was carried out for 30 cycles, and the P-wave velocities, porosities, and uniaxial compressive strengths of the specimens were determined after every five cycles. Heat–cool and wet–dry cycles were repeated for 40 cycles. Based on the strength deterioration ratio, it was found that freeze–thaw cycles degrade the physical and mechanical properties of sandstones more strongly than heat–cool and wet–dry cycles do. The results also show that the presence of zeolite cement has a significantly effect on sandstone resistance to freeze–thaw cycles. Further, it was found that sandstone strength as well as petrographic properties such as grain size and grain contacts do not have the expected effect on sample deterioration during freeze–thaw cycles. Finally, it was concluded that pore size plays an important role in sandstone resistance to freeze–thaw cycles.

Experimental Investigation on Mechanical Properties of Sandstone in Three Gorges Reservoir Area under Unloading Condition

Test results show that unloading failure is more brittle than loading failure, and rock samples suffer more damage under unloading failure, especially in high stress. Unloading failure leads to deterioration of deformation modulus, internal friction angle, and cohesion, whose values changes with the changing of stress state. Unloading stress-strain curves of sandstone samples could be divided into four stages, loading stage, unloading stage, brittle stress drop stage, ideal plastic stage, and well fitted by polynomials. There is springback deformation in the second stage, which decreases with increase of initial confining pressure. These test results could offer theory support for analysis of deformation and stability of actual projects.

Laboratory Observation and Micromechanics-Based Modelling of Sandstone on Different Scales

The mechanical properties of sandstone are, to a large extent, controlled by its microstructure. When sandstone is loaded, the stress conditions and stress history can influence the sandstone in terms of the deformation parameters, strength parameters, failure modes, as well as acoustic properties and other petrophysical parameters. In this paper, we show how we may use a discrete element model to compute the mechanical behaviour based on the microstructure of the rock, as obtained from micro-computed tomography. The model is calibrated with triaxial test data obtained with three different sandstones. The key element in the model is a contact law, attempting to capture deformation and failure at the level of the grain scale. A micromechanics-based core-scale model was also suggested using the same contact law but without explicitly mimicking the rock microstructure. The simulation results from both the microscale model and the macroscale model were in reasonably good agreement with the laboratory measurements on sandstone specimens.

Mechanical Properties of Sandstone under Loading and Unloading Conditions

The mechanical properties of rock mass under unloading conditions are essentially different from that under loading conditions. Triaxial compression tests and unloading confining pressure tests are conducted, and test results show that unloading failure is more brittle, and rock samples suffer more damage under unloading failure. The larger the initial confining pressure is, the easier of unloading failure is occurred. The increasing or decreasing values of rock deformation modulus under unloading conditions are within 10% of rock triaxial compressive strength. Unloading failure leads to deterioration of rock deformation modulus, which decreases gradually with confining pressure decreasing, and the decrease rates get bigger and bigger with unloading ratio of confining pressure increasing. Deformation modulus is only 24-34% of that under loading condition when rock strength goes down to residual strength.

Investigation of Geotechnical Parameters of Sandstone in Banda, Kogi State, Nigeria Using Regression Model

The study investigated geotechnical parameters of sandstone in Banda, Kogi State, Nigeria using regression model. To achieve the set objectives, in‐ situ tests were carried out by Schmidt hammer and laboratory tests were conducted on the sandstone samples collected from the study area. Five rock samples were prepared for the determination of density and porosity. The results obtained from the Schmidt hammer tests and those of density determined from laboratory tests were used to estimate the uniaxial compressive strength of the investigated rock. Point load index and tensile strength were determined from the laboratory tests and the relationship established by Brook, (1993) respectively. The results of the analyses reveal that the average density of sandstones is 2.08 g/cm3, the average porosity of the tested sandstones is 13.0% while the average uniaxial compressive strength value is 26.3 MPa. The average point load index and that of the tensile strength value are 1.17 MPa and 1.76 MPa respectively. The determined density, porosity, uniaxial compressive strength, point load index and tensile strength were analyzed statistically. From the statistical analysis mathematical models were established. The regression coefficients obtained from the analysis show that there are strong relationships between the physical and mechanical properties of sandstone.

Influence of number of injections on mechanical properties of sandstone cemented with microbe cement

Microbe cement, as a new green binder, relies on microbial-induced calcium carbonate precipitation and could consolidate loose particles to a whole body (bio-sandstone). The mechanical properties of sandstone cemented by microbe cement for different numbers of injections were investigated. The results show that the compressive strength of bio-sandstones increases with injection number: the average compressive strength of bio-sandstones could be up to 6·1 MPa after 14 injections of microbe cement. In addition, the rate of increase of compressive strength decreases with an increase in the number of injections. The microstructure of bio-sandstones with different numbers of injections was analysed by means of scanning electron microscopy. The results indicate that the microstructure of the bio-sandstones became increasingly dense with an increase in microbe cement injections.

Laboratory Investigation of the Effects of Temperature on the Mechanical Properties of Sandstone

An experimental study has been carried out as part of a wider programme of research in order to examine some of the physico-mechanical properties of the Jiaozuo Sandstone after exposure to extremely high temperatures. The mechanical properties of the rock under examination are discussed before the results of the thermo-mechanical response of the sandstone are presented and analysed. The range of temperature to which the sandstone has been exposed is 20–1,200 °C. The physical properties considered include the shape, volume, mass and density changes and the velocity of longitudinal and transverse elastic waves through the samples, before and after exposure to high temperature. The mechanical properties considered include the stress–strain response, the uniaxial compressive strength, the modulus of elasticity and the Poisson’s ratio. The results are analysed and discussed and possible mechanisms for the observed thermo-mechanical response, are postulated.

Relationship between the deformation characteristics and microscopic properties of sandstone explored by the bonded-particle model

Previous research shows that the mechanical properties of sandstone are strongly influenced by porosity and grain area ratio (GAR). This study explores how these microscopic factors affect the elasto-plastic deformation of sandstone, and adopts a modified bonded-particle model to study the deformation behavior of sandstone under biaxial loading. Compared with the natural sandstone, the analysis model can effectively capture the elasto-plastic behavior of sandstone including non-linear elasticity and the variations of plastic flow. Furthermore, the simulated results of biaxial compressive tests show that the mechanical behavior of sandstone with different porosities and GARs possesses the following characteristics: The cohesion is significantly influenced by both GAR and porosity. Greater GAR and porosity values lead to a lesser cohesion. However, the influence of GAR on the friction angle is insignificant, and a lower porosity leads to a higher friction angle. For elastic deformation, the elastic shear and bulk modulus increase with decreasing porosity and increasing GAR. For plastic deformation, the influence of porosity and GAR on the variations of plastic angle β under different shear stress ratios seems insignificant. On the other hand, the plastic strain trajectory ξ increases with increasing porosity, but the influence of GAR is insignificant.

The Effect of Cementation on the Mechanical Properties of Sandstones

Summary The variability in mechanical properties measured on sands from the Jauf and Unayzah formations of Saudi Arabia is observed to be dependent upon cementation. Understanding the role of cementation in controlling the mechanical properties can improve the design of hydraulic-fracture treatments and, hence, improve reservoir performance. Strength measurements from triaxial-testing data and examination of core in thin sections were used to relate the detailed microstructure and cementation to the variation of mechanical properties. Strength and elastic moduli were determined for 65 samples cored from five different wells. Forty-seven samples were analyzed in thin sections and point counted to determine and quantify cementation. Cements in these two formations have variable composition and habits; both affect the mechanical properties and strength. It is not sufficient to know that cements exist; it is also necessary to know where the cement occurs. Pure quartz overgrowths play a major role in increasing strength, while clay coatings play a minor role. Simple linear correlations were found relating cement concentrations to strength.

Chemical weathering mechanisms and their effects on engineering properties of soft sandstone and conglomerate cemented by zeolite in a mountainous area

Sandstone and conglomerate cemented by zeolite crystals in a humid mountainous area in Japan are weathered, and also weakened or strengthened, by chemical interaction with percolating water that was originally in equilibrium with the atmosphere. Near the ground surface, carbonic acid generated by the dissolution of carbon dioxide from the atmosphere and biological activity in the soil dissolves zeolite, and thereby weakens the rocks. At the oxidation front, the base of the oxidized zone, oxygen from the atmosphere reacts with pyrite in the rock to form sulfuric acid and iron oxide and/or hydroxide. The iron oxide and/or hydroxide precipitates on zeolite surfaces, and strengthens the rocks by cementing the grains to each other. Beneath the oxidation front and extending to the dissolution front, sulfuric acid migrates downward, dissolves zeolite, and thus weakens the rocks. This weathering mechanism and the resulting changes in mechanical properties of rocks generally are to be expected in marine sandstone and conglomerate with small amounts of clay in the matrix because the most common cementing mineral is carbonate which is also easily dissolved by acid, and further because pyrite is a common rock-forming mineral in marine sedimentary rocks. The effects of chemical weathering on the mechanical properties of sandstone and conglomerate are different from those of mudstone, because the latter contains much larger amounts of clay minerals. The cementation of constituent grains by iron oxide and/or hydroxide is inhibited by clay minerals, because clay minerals have high specific surface areas, and also because large amounts of iron can be incorporated in the minerals.

Petrographic and mechanical properties of sandstones, as influenced by depth

In a homogeneous massive medium-grained sandstone (Donbass), as the depth increases from 450 to 1430 m, the bearing increases by 60%, with the accompanying increase in volume weight, decrease in porosity (almost by 50%), decrease in volume of cementing substance, and other concurrent changes. Characteristically, the type of cementation changes progressively from basal to crystalline (almost entirely quartz at 1430 m). – V.P. Sokoloff.