Triaxial Unloading Tests Research Articles

Accumulation Mechanism of Marine Shale Gas Reservoir in Anticlines: A Case Study of the Southern Sichuan Basin and Xiuwu Basin in the Yangtze Region

The study of tectonics is one of the important aspects of shale gas preservation. It is vital for understanding how to determine the enrichment regularity of marine shale gas in anticlines. This paper focuses on typical shale blocks in the southern Sichuan Basin and shale in the Upper Ordovician and the Lower Silurian. In this study, triaxial unloading tests, permeability tests perpendicular and parallel to the stratification plane, FIB-HIM tests, and inclusion analyses are carried out with real drilling data. The enrichment regularity of marine shale gas in anticlines is studied by considering 2 aspects: the angle of the limbs and the burial depth. For anticlines with adjacent synclines, the migration regularity of shale gas is considered by 3 aspects: the dynamics, channels, and processes of migration. This study reveals that a limb angle greater than 120° reflects relatively good conditions for shale gas preservation, while limb angles lesser than 70° indicate relatively poor conditions. This study also suggests that during the process of uplift, large-scale concentrated fractures will form at a certain depth range and horizontal stress field, resulting in the large loss of shale gas. The regression equation of the fractured depth (H) and the horizontal stress (S) is presented as H=15.404S−754.41 (with a correlation coefficient R2=0.6834). The stratification plane and the organic pores form the migration channel of natural gas that is horizontal to the stratification plane in shale. Under the condition of both anticlines and contiguous synclines, shale gas escapes through fractures resulting from extrusion along the anticline and the uplift effect. In addition, driven by differences in the formation pressure coefficients, shale gas is capable of migrating in a short-distance stair-type style from synclines to the adjacent anticlines. Thus, if the drilling costs allow, the well locations should be placed in the more deeply buried synclines.

Strain Energy Dissipation and Damage Evolution of Frozen Migmatite Under Triaxial Unloading

The aim of this paper is to study the effect of unloading, due to excavation, on the damage evolution and energy dissipation of frozen rock slopes. Triaxial unloading tests were conducted on frozen migmatite from the Dagushan Iron Mine using acoustic emission location technology. When the confining pressure was unloaded, the axial stress was held constant. The purpose of this experiment is to establish the relationship between macroscopic energy dissipation and microscopic damage. Damage is small in hydrostatic pressure stage but increases abruptly during the failure stage, and increase rate is about 10 times that of loading tests for frozen migmatite and 2 times that for room temperature. In the deformation acceleration stage, both the damage and dissipation energy increase suddenly and the damage increment increases before the dissipation energy, i.e., the accumulation of damage eventually manifests itself as macroscopic energy dissipation. The dissipation energy can be expressed by a superposition of two exponential damage functions. Moreover, the percentage of dissipation energy and damage, in failure stage, is more than 60% for unloading tests. However, for loading tests the percentage of damage in the failure stage is less than 25%, and most of the damage occurs in the unstable deformation stage.

Microscopic Characteristics of Fractured Sandstone after Cyclic Freezing‐Thawing and Triaxial Unloading Tests

The understanding of tunneling rock failure characteristics under unloading conditions in the cold region is critical for the proper design of rock tunneling support and mining safety operation. Given this understanding is currently limited, this study aimed to investigate the characteristics of fractured sandstone samples in the microscale after cyclic freezing‐thawing and triaxial unloading tests. The samples were first subjected to different cycles of freezing and thawing, followed by the triaxial unloading test and scanning electron microscopy imaging. The peak strength and damage dilatancy stress were measured from the stress‐strain curves. The microcrack characteristics (number, length, and width) were obtained through the image analysis. The results show that the decrease in peak strength and damage dilatancy stress was more significant by the first 20 freezing‐thawing cycles when the pore pressure gradient is maximum compared to the later freezing‐thawing cycles. The mechanical properties also significantly deteriorated when the severe freezing‐thawing treatments were performed. The fracture section mainly had the morphology of honeycomb‐like microstructure, stripped microstructure, and flocculent microstructure. The cracking extent was mainly influenced by the freezing‐thawing rather than the triaxial unloading test, but the azimuthal angle of microcracks was significantly altered by the triaxial unloading. To properly design rock tunneling support and safe operation of mining in the cold region, both impact of cyclic freezing‐thawing and the excavation operation direction should be considered.

Unloading Creep Characteristics of Frozen Clay Subjected to Long‐Term High‐Pressure K0 Consolidation before Freezing

Artificial ground freezing has been widely applied in the construction of vertical shafts in deep and thick alluvia. As an important factor, the in situ creep behavior of deep, frozen soil affects the mechanical properties of frozen walls and the safety and stability of shaft linings. Acquiring creep characteristics and deep soil parameters by methods that ignore the engineering and geological situations is currently inadvisable. A series of triaxial unloading tests were conducted with frozen clay subjected to long‐term high‐pressure K0 consolidation before freezing to research the unloading creep characteristics, creep strength, and other parameters of the clay, and the results indicated the following: (1) The creep behaviors of frozen clay are affected by the consolidation time and consolidation stress. Long‐term high‐pressure K0 consolidation reduces the creep strain and creep rate of frozen clay. (2) The decrease in the ice and the unfrozen water contents of frozen clay caused by the prolongation of consolidation time result in an increase in the long‐term strength and instantaneous strength. Consolidation time has an obvious effect on long‐term strength and weakens the creep property of frozen clay. Consolidation stress significantly affects the instantaneous strength. (3) The deformation resistance capability of frozen clay is enhanced by compaction; thus, E1, η1, and η2 increase with prolonged consolidation, and the nonlinearity of the accelerated creep increases.

Review of unloading tests of dynamic rock failure in compression

Dynamic rock failure at the walls of an opening, so-called rock burst or strain burst is one of the major types of rock failure capable of causing human and financial loss. Initially the rock at the place of the future excavation is in a polyaxial stress state. When excavating an opening, the rock elements at the excavation boundary are overloaded in the tangential (with respect to excavation wall) direction and unloaded in the radial direction. This situation can be reproduced by the modified true triaxial test in which only one surface of prismatic rock sample is unloaded. This paper reviews the tests under true triaxial unloading condition and the corresponding failure modes and types (static or dynamic). In these tests the violent (dynamic) ejection of rock fragments and arc-shape fractures near the free face are often observed. However these tests are affected by friction between the sample and the loading platens. We analyse the end friction effect and show that unlike the conventional end friction effect produced in uniaxial and biaxial loading tests, the true triaxial unloading test induces additional shear stress needed to prevent sliding of the rock sample from the loading platens. This additional shear stress is shown to considerably affect the geometry of fractures.

Influence of the Elastic Dilatation of Mining-Induced Unloading Rock Mass on the Development of Bed Separation

Understanding how mining-induced strata movement, fractures, bed separation, and ground subsidence evolve is an area of great importance for the underground coal mining industry, particularly for disaster control and sustainable mining. Based on the rules of mining-induced strata movement and stress evolution, accumulative dilatation of mining-induced unloading rock mass is first proposed in this paper. Triaxial unloading tests and theoretical calculation were used to investigate the influence of elastic dilatation of mining-induced unloading rock mass on the development of bed separation in the context of district No. 102 where a layer of super-thick igneous sill exists in the Haizi colliery. It is shown that the elastic dilatation coefficient of mining-induced unloading hard rocks and coal were 0.9~1.0‰ and 2.63‰ respectively under the axial load of 16 MPa, which increased to 1.30~1.59‰ and 4.88‰ when the axial load was 32 MPa. After successively excavating working faces No. 1022 and No. 1024, the elastic dilatation of unloading rock mass was 157.9 mm, which represented approximately 6.3% of the mining height, indicating the elastic dilatation of mining-induced unloading rock mass has a moderate influence on the development of bed separation. Drill hole detection results after grouting, showed that only 0.33 m of the total grouting filling thickness (1.67 m) was located in the fracture zone and bending zone, which verified the result from previous drill hole detection that only small bed separation developed beneath the igneous sill. Therefore, it was concluded that the influences of elastic dilatation of mining-induced unloading rock mass and bulking of caved rock mass jointly contributed to the small bed separation space beneath the igneous sill. Since the accurate calculation of the unloading dilatation of rock mass is the fundamental basis for quantitative calculation of bed separation and surface subsidence, this paper is expected to be a meaningful beginning point and could provide a useful reference for future, related research.

Anisotropic Mechanical Properties of Sandstone Under Unloading Confining Pressure at High Temperatures

High geothermal temperatures appear to be unfavorable for tunnel constructions in layered sandstone rocks with large overburden. The main objective of our research was to investigate the anisotropic mechanical characteristics of layered sandstones at various levels of geothermal temperature, to find answers to the following research questions: which temperature level is critical and which angle of orientation of the bedding is most unfavorable under unloading stress conditions in tunnel excavation. We carried out conventional triaxial unloading tests at five different temperatures from 20 to 150 $${^{\circ }}\hbox {C}$$ . The obtained results show there is a critical temperature value between 120 and 150 $${^{\circ }}\hbox {C}$$ , at which the strength properties of sandstone strongly decrease and the Poisson’s ratio strongly increases, as compared to those at lower temperatures. From 20 to 150 $${^{\circ }}\hbox {C}$$ , the sandstone failure mechanism changes from shear failure to tension failure. The elastic modulus of the sandstone decreases, the Poisson’s ratio slightly increases, and the peak strength decreases first and then increases, with increasing bedding angles. The bedding angle of 30 $${^{\circ }}$$ is the most unfavorable angle for the mechanical characteristics of sandstone. The ratios of $$\sigma _{ci}/\sigma _{f }$$ and $$\sigma _{cd}/\sigma _{f }$$ are not affected by the bedding angle, and these ratios are almost stable when the temperature increases from 20 to 120 $${^{\circ }}\hbox {C}$$ . However, they increase at 150 $${^{\circ }}\hbox {C}$$ . The tested layered sandstone is a low anisotropy rock, but from 20 to 150 $${^{\circ }}\hbox {C}$$ the anisotropy slightly increases.

Damage Evaluation for Rock Burst Proneness of Deep Hard Rock under Triaxial Cyclic Loading

High stress and strong excavation disturbance are the main causes of dynamic disasters, rock burst in deep hard rocks, and are more frequent and violent than those in shallow, which seriously restricts the deep mining. Given rock burst encountered in deep mining of Lingnan metal mines, the optimized triaxial cyclic loading and unloading tests are designed to characterize the performance of rock failure and to evaluate the rock burst proneness. The correlation between elastic energy index and damage evolution is built, and rock burst proneness in each status is analyzed; furthermore, the dissipation energy in the failure process of deep rocks is explicated. In this paper, the law that the elastic energy index via damage increases is drawn. In terms of the dynamic disaster conditions in the deep rock, the identification approach for the damage zone of the rock burst is established.

A new energy index for evaluating the tendency of rockburst and its engineering application

Based on energy-transfer analysis of triaxial unloading tests under four different controlling conditions, the rock burst energy release rate (RBERR)-an energy index combining the local energy release rate (LERR) and the limit energy storage rate (LESR), was proposed. LERR refers to the values of the elastic strain energy release of an element after brittle failure, which indicates that RBERR>0 can be used to determine whether the element belongs to an excavation damage zone. RBERR was implemented to simulate the evolution process of rock damage and failure with advancing excavation face. The results predicted using RBERR were consistent with those obtained from an actual rockburst. In addition, a synthesis method based on microseismic (MS) monitoring information and RBERR simulation analysis was implemented for the Huainan coal mine. A study on the spatial- temporal evolutional laws of MS events built relationships between MS monitoring information and excavation process and; consequently, the potential danger areas of rockburst were determined. RBERR was used to analyze the location and intensity of rockburst in potential danger areas, which could be valuable in the design of rockburst- resistant measures in deep- buried construction.

Influence of temperature on the brittle failure of granite in deep tunnels determined from triaxial unloading tests

High ground temperatures lead to the risk of rockburst in hard rock tunnels located in high-stress environments. However, existing rockburst studies have not considered the influence of temperature. Therefore, the laboratory experiments were conducted to measure the brittleness of granite samples under different temperature scenarios, and the tendency of rockbursts were evaluated at different temperatures in this paper. The brittle failure mechanism of granite was explored by using uniaxial and triaxial rock tests under the influence of thermo-mechanical coupling. The results indicated that the post-peak deformation of the rock samples changes from ductile to brittle with an increase in temperature to 100 °C. The frequency of shear failure occurred in this test increases with increasing temperature, and the brittle failure of the surrounding rock is enhanced with an increase in temperature. The results indicated that rockbursts occurring in deep granite tunnels at high ground temperatures will be more intense than those occurring at room temperature. The results of this study can be applied to predict the intensity of rockbursts before the tunnels are excavated.

Study on strength and deformation characteristics of sandy mudstone based on triaxial unloading test

To achieve the strength and deformation characteristics of sandy mudstone under the unloading condition, a triaxial unloading test was performed on a group of sandy mudstone taken from the diversion tunnel of a hydropower station. The results show that the stress-strain curve slope of sandy mudstone increases suddenly at the moment of unloading confining pressure, and there is no significant decline in the whole test. There is usually only a whole shear failure surface in the samples damaged under low initial unloading confining pressure while those damaged under high one develop tensile fracture. The introduction of semilog method helps to solve the problem that it’s difficult to get the peak intensity of sandy mudstone directly based on the variation characteristics of the stress-strain curve. From the initial stage of unloading to the damage of the samples, the reduced percentage of sandy mudstone deformation modulus is positively correlated with initial unloading confining pressure. The decrease rate of deformation modulus and increase rate of axial strain of sandy mudstone are both mutate under destructive confining pressure. The research results can provide an useful reference to research on the mechanical properties of similar soft rocks.

Experimental Study on the Dilatancy and Fracturing Behavior of Soft Rock Under Unloading Conditions

To reveal the deformation mechanism during tunneling in deep soft ground, triaxial unloading confining pressure tests and triaxial unloading creep tests were carried out on sandy mudstone specimens to study the dilatancy and fracturing behavior of soft rock. In the triaxial unloading confining pressure tests, the stress path and different unloading rates were considered to reflect the unloading characteristics of the excavation methods. The unloading rate effects and the rock damage evolution law are studied. The following conclusions are obtained from the results. Firstly, when the unloading rate is smooth, the peak strengths and deviatoric stress–strain curves under the unloading condition are close to those under the conventional loading condition. Secondly, the post-peak brittle characteristics are more apparent with the increasing unloading rates. Thirdly, the soft rock undergoes five deformation and failure regimes of elasticity, pre-peak unloading damage–dilatancy, post-peak brittle drop, linear strain softening and residual perfect plasticity under quasi-static smooth unloading of mechanized excavation which is mainly focused on in this study. Fourthly, the damage evolution law at the pre-peak damage–dilatancy stage follows an exponential function. Fifthly, during the post-peak stages, multistage microfractures are initiated, propagated and finally coalesced forming a shear-fragmentation band with a certain thickness, accompanied by significant volumetric dilatancy. In the triaxial unloading creep tests, multistep unloading of the confining pressure was applied, while the axial pressure was kept constant. The results show that when the deviatoric stress is larger and the experienced creep time is longer, the unloading effect and creep characteristics become more apparent accompanied with obvious lateral dilatancy, eventually leading to significant creep–dilatancy. The progressive failure with time is caused by the damage accumulating with time-dependent crack expansion, which can be called as ‘time-dependent damage and fracturing’. The reasons for the above evolution process are presented, then the deformation mechanism of soft rock is revealed. The soft rock deformation mainly consists of two parts. One part is the pre-peak damage–dilatancy and post-peak fracture–bulking produced at the excavation unloading instant. The other part is creep–dilatancy caused by time-dependent damage and fracturing in a period of time after excavation. The above-mentioned results of damage, dilatancy and fractures evolution process are in good agreement with the in situ monitoring results and previous studies about the surrounding rock convergence, fracturing and EDZ (excavation damaged zone) development.

A Theoretical Explanation for Rock Core Disking in Triaxial Unloading Test by Considering Local Tensile Stress

Rock is a typical inhomogeneous material with a large number of flaws in different scales; the stress field of the rock in its elastic state consists of two parts: the elastic stress, which distributes uniformly in the entire region; and an additional stress, which only exists around the flaws. Theoretical expressions of the additional stress and local stress are derived based on the Maxwell model. Core disking which takes place under the condition that the axial stress is rapidly reduced while the confining pressure is kept unchanged is explained with a new method. Unloading duration’s effect on core disking is analyzed. A new criterion for core disking is presented based on attributing the core disking to the result of the exceedance of local tensile stress over the tensile strength. Based on our theoretical analysis and the conclusions from published resources, core disking is most likely to occur if the maximum principal stress is more than five to six times the tensile strength.

Experimental study on creep deformation and long-term strength of unloading-fractured marble

In order to take account of the unloading fractures in rocks due to the excavation, unloading-fractured marble specimens were obtained by the triaxial unloading tests. Then, creep tests on the unloading-fractured marble specimens under different stress levels and confining pressures were performed. Based on the creep tests, the creep deformation characteristics were studied and the results show that the creep curves exhibit the three stages of the idealised creep curve. Special attentions were paid on that the creep deformation and creep rate in lateral direction are far greater than that in axial direction and the lateral creep reaches the accelerated stage earlier than the axial creep. There are stress thresholds correspond to the inflection point of stress–strain relationship in lateral and axial directions, respectively, and the lateral threshold is less than the axial one. The creep failure is mainly caused by the strong expansion in lateral direction. Moreover, both the creep rate and deformation have positive relationships with the axial stress but decrease as confining pressure increases. The failure strain increases with increasing confining pressure. Then, the long-term strengths determined by the axial and lateral isochronous stress–strain curves, respectively, are discussed. Through the comparative analysis, the lateral creep reflects the creep characteristics of unloading-fractured marble more clearly and the long-term strength determined by the lateral isochronous stress–strain curves is safer for engineering applications.

Failure properties of rocks in true triaxial unloading compressive test

Slabbing failure often occurs in the surround rock near a deep underground excavation. The mechanism of slabbing failure is still unclear. In order to reveal the influence of the intermediate principal stress (σ2) on slabbing failure, true triaxial unloading compressive test was carried out based on the stress path of the underground engineering excavation, i.e., unloading the minimum principal stress (σ3), keeping σ2, increasing the maximum principal stress (σ1). The initiation and the propagation of slabbing fracture in rock specimens were identified by examining the acoustic emission (AE) and the infrared radiation characterization. The test results show that the failure modes of the granite and red sandstone specimens are changed from shear to slabbing with the increase of σ2. The AE characteristic of rock specimen under low σ2 is swarm type which is the main shock type under high σ2. The infrared radiation properties of rock specimen under different σ2 are also different. The temperature change area is just along the shear fracture such as the uniaxial compression. With the increase of σ2, the temperature change area is planar of rock specimen which proofs that the failure mode of rock specimen turns into slabbing.

True Triaxial Strength and Failure Modes of Cubic Rock Specimens with Unloading the Minor Principal Stress

True triaxial tests have been carried out on granite, sandstone and cement mortar using cubic specimens with the process of unloading the minor principal stress. The strengths and failure modes of the three rock materials are studied in the processes of unloading σ 3 and loading σ 1 by the newly developed true triaxial test system under different σ 2, aiming to study the mechanical responses of the rock in underground excavation at depth. It shows that the rock strength increases with the raising of the intermediate principal stress σ 2 when σ 3 is unloaded to zero. The true triaxial strength criterion by the power-law relationship can be used to fit the testing data. The “best-fitting” material parameters A and n (A > 1.4 and n < 1.0) are almost located in the same range as expected by Al-Ajmi and Zimmerman (Int J Rock Mech Min Sci 563 42(3):431–439, 2005). It indicates that the end effect caused by the height-to-width ratio of the cubic specimens will not significantly affect the testing results under true triaxial tests. Both the strength and failure modes of cubic rock specimens under true triaxial unloading condition are affected by the intermediate principal stress. When σ 2 increases to a critical value for the strong and hard rocks (R4, R5 and R6), the rock failure mode may change from shear to slabbing. However, for medium strong and weak rocks (R3 and R2), even with a relatively high intermediate principal stress, they tend to fail in shear after a large amount of plastic deformation. The maximum extension strain criterion Stacey (Int J Rock Mech Min Sci Geomech Abstr 651 18(6):469–474, 1981) can be used to explain the change of failure mode from shear to slabbing for strong and hard rocks under true triaxial unloading test condition.

Conversion of strain energy in Triaxial Unloading Tests on Marble

Axisymmetric triaxial compression loading–unloading tests are conducted on twenty-seven marble specimens with initial confining pressures of 20, 30 and 40MPa and unloading rates of 0.1, 1.0 and 10MPa/s. It is found that magnitude of initial confining pressure and unloading rate significantly influence rock failure modes and strain energy conversion (accumulation, dissipation and release) during unloading. The failure mode of rock specimen is gradually changed from shear to tensile with increasing unloading rate. The pre-peak conversion rate of strain energy is increased with increasing unloading rate. This increase trend is enhanced by initial confining pressure. The post-peak conversion rate of strain energy has the similar increasing pattern of the pre-peak one, though it is several to ten times greater. Much strain energy is released after peak strength from the tested specimen and it may account for the occurrence of flying fragments. The higher the unloading rate and/or the initial confining pressure, the more severe the “flying fragment” phenomenon. The characteristics of strain energy accumulation, dissipation and release are investigated in three stages, i.e., elastic compression, pre-peak unloading, and post-peak fracturing. The rule of strain energy conversion for each stage is derived, and triaxial unloading tests and conventional triaxial compression are compared in terms of strain energy and its conversion.

Deformation and failure mechanism of phyllite under the effects of THM coupling and unloading

Although the study of TM (Thermo -Mechanics), HM (Hydraulic — Mechanics) and THM (Thermo — Hydraulic — Mechanics) coupling under a loading test have been under development, rock failure analysis under THM coupling and unloading is an emerging topic. Based on a high temperature triaxial unloading seep test for phyllite, this paper discusses the deformation and failure mechanism of phyllites under the “H↔M, T→H, T→M” incomplete coupling model with unloading conditions. The results indicate that the elastic modulus and initial permeability decrease and the Poisson’s ratio increases with increasing temperature; the elastic modulus decreases and the Poisson’s ratio and initial permeability increase with increasing water pressure. During the unloading process, rock penetrability is small at the initial elastic deformation phase, but the penetrability increases near the end of the elastic deformation phase; mechanisms involving temperature and water pressure affect penetrability differently. Phyllite failure occurs from the initial thermal damage of the rock materials, splitting and softening (which is caused by pore water pressure), and the pressure difference which is formed from the loading axial pressure and unloading confining pressure. The phyllite failure mechanism is a transtensional (tension — shearing) failure.

Research of Rock Mass Unloading Constitutive Model and its Engineering Application

Based on the results of triaxial unloading tests，the stress-strain characteristics and failure characteristics of rock mass have been discussed. Considering unloading degree as an important parameter，an elastic-brittle-plastic mechanical model has been established according to the unloading stress condition，which considers that unloading yield function varies linearly between Griffith with Hoek-Brown criterion after rock mass receives plastic deformation caused by unloading. Taking jinshajiang excavation slope of a hydropower station as engineering background, the three-dimensional continua fast Lagrangian analysis software FLAC3D is employed in the slope stability analysis under the excavation, and the distribution characteristics of the slop deformation is studied.

Study on Rock Unloading Failure and its Effects on Rock Burst

The mechanical parameters and failure characteristics were obtained based on the results of triaxial unloading tests of limestone. Results show that: The evolution of volumetric strain is determined by lateral strain under unloading condition. Failure modes of rock can be classified into three types. Deformation modulus decreases and Poisson ratio increases in the process of unloading. Mohr-Coulomb criterion can not describe rock unloading failure strength property. Rock burst occur during tunnel excavation are closely related to the rock unloading failure characteristic.