Shear Dilation Research Articles

Numerical study of the long-term performance of EGS based on discrete fracture network with consideration of fracture deformation

During heat extraction in fractured reservoir, the perturbation of pressure, stress, temperature could alter the fracture aperture, which feeds back to the long-term performance of EGS. In this work, a 3D THM coupled model was developed for the long-term performance based on the discrete fracture network. The apparent improvement of this model is both shear dilatancy and normal closure/opening are considered in a complete constitutive relationship under the THM coupled framework TOUGH2M-FLAC3D. The feasibility is verified and validated through comparing with analytical solution and previous numerical work. In application of the new approach in a fractured reservoir, it is found that the phenomenon of channel flow will concentrate the heat mining in partial fractures, and the fracture enhancement further reinforce the channel flow. The pressure effect on fracture transfers faster than the thermal effect. In the sensitivity analysis, the zero-stress aperture has a positive impact on heat extraction. Higher thermal expansion coefficient promotes the heat extraction as well. While, the dilation angle in this work has a neglectable impact on heat extraction performance. The thermal effect accounts for the most fracture variation under high thermal expansion coefficient. Therefore, the thermal effect should be considered especially in high temperature geothermal reservoir.

Laboratory hydraulic shearing of granitic fractures with surface roughness under stress states of EGS: Permeability changes and energy balance

To offer key parameters and constitutive laws required for field-scale multiphysics simulations that can accurately predict created fracture network structures in enhanced geothermal systems (EGS) and resulting energy extraction, we explore the full spectral of hydraulic shear process of granite fracture and revisits the linkage between hydraulic and mechanical properties during shear slips. Key results from our novel lab-experiments include the followings: (1) fracture permeability of granite increases due to the hydraulic shear slip even at an effective normal stress of over 50 MPa, (2) shear slip and stress drop are in proportion and increment of the fracture permeability increases with increasing total shear slip displacement, and (3) although hydraulic shear slip tends to make fracture surfaces slightly smoother, the factual characteristics of surface are maintained after slip. By combining our experimental results with the seismological analysis, we first explore energy balance during hydraulic shearing of the preexisting rock fracture and point out the crucial role of the elastic potential energy stored in the surrounding bulk rock masses. Subsequently, we derive a constitutive model related to permeability change of granite fracture during hydraulic shearing under the typical crustal stress of EGS and presume that the maximum change in fracture permeability due to shear dilation is ∼20-fold, although scale effect is not considered. Finally, via the high resolution 2-D mapping of the surface damages, we illuminate the formation of preferential flow paths within rock fracture during hydraulic shearing and its possible role on the improvement of fracture permeability. Thus, we successfully demonstrate both new and sophisticated insights in hydro-mechanical coupled processes during hydraulic shearing to improve the accuracy of fracture network designs in EGS technology.

Shear strength and failure criterion of geopolymer coral aggregate concrete under compression-shear loading

The utilization of geopolymer and coral aggregate in concrete preparation offers cost and time savings associated with the transportation of raw materials, as well as contributes to reducing carbon emissions and resisting the sulfate ions erosion in the marine environment. Concrete structures, including deep beams, corbels, and column joints, are typically subjected to compression-shear loading. For this purpose, a total of 54 cubic specimens are cast to test the compression-shear behavior of geopolymer coral aggregate concrete (GCAC) with different normal stress ratios (k=0,0.1,0.2,0.4,0.6,and0.8) and concrete strength grades (C20, C30, and C40). Subsequently, the failure patterns, load-shear displacement curves, and normal displacement-shear displacement curves of GCAC are obtained. The shear displacement, the shear strength, and its components are also analyzed. The results reveal that coral aggregate with high brittleness and low strength leads to more spalling powder and debris to occur as k increases, and is cut off on the shear surface. With an increase in k (k≥0.6), the descending phase of the load-shear displacement curve becomes steeper due to the higher severity of failure patterns. The shear dilation angle increases with the increase of the concrete strength grade, and first raises and then falls with the increase of k, reaching a maximum value at k=0.4, the same as the percentage of shear dilation strength. Furthermore, the increase in k has a significant negative impact on the percentage of cohesive strength, but not on that contact friction strength. Finally, based on the test results, the J2−I1fc model is determined to be the most suitable for GCAC among the three failure criterion models in this study.

Failure mechanism and deformation prediction of soft rock tunnels based on a combined finite–discrete element numerical method

The large deformation mechanisms and predictions of soft rock tunnels have always been important but difficult to solve problems in the field of geotechnical engineering. A combined finite–discrete element numerical simulation method (FDEM) was used to study large deformation mechanism, classification and prediction. The deformations or failure mechanisms of tunnel surrounding rock were revealed, and the failure modes and displacement value prediction of surrounding rock with different strength-stress ratios were also investigated. The following conclusions were obtained: (1) Critical hysteresis damping can be adopted to simulate the progressive large deformation process of soft rock tunnels, which can obtain the final deformation of unsupported tunnels and avoid a dynamic response. (2) Under concentrated tangential stress, the surrounding rock undergoes X-shaped conjugate shear fracture, and this type of fracture network continues to propagate toward the depth of the surrounding rock until the model reaches a stable state; the reduction in tunnel cross-section is mainly caused by the macroscopic movement and volume expansion of rock fragments, and the latter is due to the generation of a large number of macroscopic voids. (3) As the strength-stress ratio decreases, the deformation or failure modes of the surrounding rock can be divided into four categories: elastic–plastic deformation, closed fracturing, shear dilation and broken expansion. (4) Finally, a prediction equation for isotropic and homogeneous soft rock unsupported tunnel deformation with general size driven by hydrostatic in situ stress is obtained, which indicates that with an increasing strength-stress ratio, the surrounding rock displacement decreases as an exponential function, with a correlation coefficient of R2 = 0.997. In addition, the robustness and reliability of the prediction equation is verified.

Characteristics of Regional GPS Crustal Deformation before the 2021 Yunnan Yangbi Ms 6.4 Earthquake and Its Implications for Determining Potential Areas of Future Strong Earthquakes

The 2021 Yangbi Ms 6.4 earthquake in Yunnan, China, occurred in an area where the Global Positioning System (GPS) geodetic observations are particularly intensive. Based on a detailed retrospective analysis of the GPS observations of about 133 stations distributed in the proximately 400 km × 400 km region that contains the area affected by the earthquake., we obtain a high-resolution GPS velocity field and strain rate field and then derive the present-day slip rates of major faults in the region with the commonly used half-space elastic dislocation model and constraints from the GPS velocity field. Furthermore, by calculating the seismic moment accumulation and release and deficit rates in the main fault segments and combining with the distribution characteristics of small earthquakes, we evaluate the regional seismic risk. The results show that (1) there was a localized prominent strain accumulation rate around the seismogenic area of the impending Yangbi Ms 6.4 earthquake, although this was not the only area with a prominent strain rate in the whole region. (2) The seismogenic area of the earthquake was just located where the strain direction was deflected, which, together with the localized outstanding maximum shear strain and dilatation rates, provides us with important hints to determine the potential areas of future strong earthquakes. (3) Of all the seismogenic fault segments with relatively high potentials, judged using the elapsed time of historical earthquakes and effective strain accumulation rate, the middle section of the Weixi–Qiaohou fault has a higher earthquake risk than the southern section, the Midu–Binchuan section of the Chenghai fault has a higher risk than the Yongsheng section and the Jianchuan section of the Jianchuan–Qiaohou–Lijiang–Xiaojinhe fault has a higher risk than the Lijiang section.

Hybridized random forest with population-based optimization for predicting shear properties of rock fractures

Rock mass fractures play an essential role in controlling the shear behavior of natural rocks. The clear understanding of the fracture's shear parameters is of great significance in maintaining underground structure stability. A novel approach combining the random forest (RF) model and two population-based optimization algorithms respectively (the Sine Cosine Algorithm (SCA) and the whale optimization algorithm (WOA)) was proposed to predict the shear strength, peak shear displacement, and dilation angles of rock fractures. 84 direct shear data was divided into training set and test set for training and testing the proposed models. Coefficient of determination (R2), root mean square error (RMSE), and mean absolute error (MAE), three indexes were utilized to evaluate the model prediction accuracy. The prediction performance indicated that SCA-RF and WOA-RF models achieved high accuracy in predicting the shear strength and dilation angles of rock fractures. These two models also perform satisfactorily in predicting the peak shear displacement by optimizing the dataset division and fitness function. Comparing with Barton-Bandis and Asadollahi-Tonon empirical models, the proposed models have significant advantages in estimating the shear properties of rock fractures. It has important implications for the development of accurate and reliable models for predicting the shear behavior of natural rocks, that some data processing techniques were used based on the characteristics of the direct shear dataset.

Influence of intermediate principal stress ratio on strength and deformation characteristics of frozen sandy soil under different moisture contents

Frozen ground often contain non-uniform moisture content and are under true triaxial stress state and their practical engineering application are still explored. The present study is carried out to investigate the strength and deformation characteristics of frozen soil with different moisture contents under true triaxial stress state conditions. A series of true triaxial tests were conducted on frozen soil specimens having constant minimum principal stress (σ3) and constant intermediate principal stress ratio (b). The experimental test results showed that the stress-strain curves represent the strain-hardening characteristics under different test conditions. The intermediate principal stress ratio (b) has a dual effect of strengthening and weakening on the strength of frozen sandy soil. When 0 ≤ b ≤ 0.5, the strengthening effect was found to be dominant in the frozen soil specimen, whereas, the weakening effect was dominant when 0.5 < b ≤ 1. The strength of frozen soil under true triaxial stress state is 8.18% to 36.33% higher under different water content conditions, compared to that under generalized triaxial compression stress state. The difference in deformation between the direction of the intermediate principal stress and the minimum principal stress increased with the increment in the intermediate principal stress ratio value. The volumetric strain in the soil sample initially exhibited the characteristics of shear contraction and then shear dilation.

Visualized direct shear test of the interface between gravelly sand and concrete pipe

The shear stress–shear displacement relationship and shear strength parameters of the interface between the pipe and the surrounding soil are important for designing the jacking force. One most commonly used method to measure the shear strength of the pipe–soil interface is the direct shear test. This paper presents the results of a series of direct shear tests conducted in the laboratory on the pipe–soil interfaces for different moisture contents. Simultaneously, digital image correlation is used to supplement the research of the horizontal displacement field of the interface. The results show that the increase of normal stress will makes the interface more prone to strain hardening during the shearing process, and the influence of the change of specimen moisture content on the interface strain characteristics gradually weakens. The shearing process of the specimen mainly involved shearing contraction, and slight shear dilation occurs only when the moisture content is small. We proposed a unified model for describing the shear volumetric change caused by the combined action of moisture content and normal stress. The shear displacement field presents a layered distribution, which is related to the shear displacement corresponding to the shear stress.

Investigation on the change of shear strength of concrete with cold joint under the action of sulfate dry–wet cycles

Large-volume concrete structures in the saline areas of Western China often have cold joint structural surfaces due to the improper handling of construction joints, dividing them into old and new layers of concrete. In this sulfate-rich environment, the shear strength of concrete with cold joint and the evolution pattern of its composition with the sulfate dry–wet cycles are particularly important in assessing the quality of the interface between old and new concrete bond surfaces. Therefore, in this paper, direct shear tests and microscopic scans were performed on concrete specimens with cold joint at different pouring age intervals and numbers of sulfate dry–wet cycles. The mechanism of the effect of both on the shear strength and failure mode of concrete was analyzed. Results show that the sulfate dry–wet cycles of erosion and the age interval have significant effects on the shear strength of the cold with cold joint and their components. As the number of sulfate dry–wet cycles increased, the shear dilation of concrete specimens with cold joints under direct shear action was reduced, the internal friction angle initially increased and then decreased for specimens with a short pouring interval (0–0.25 day), and the reduction of the internal friction angle of the specimens with a long pouring age interval (0.25–28 days) was nonlinear. The shear dilation of the specimen increased, and the internal friction angle decreased nonlinearly with the increase of the pouring interval. In addition, the failure criterion of specimens transformed from the bilinear Mohr–Coulomb criterion before erosion to the linear Mohr–Coulomb criterion under the action of sulfate dry–wet cycles.

Micro-macroscopic mechanical behavior of frozen sand based on a large-scale direct shear test

The mechanical properties of frozen soil are of utmost importance for infrastructure development in cold regions, and the microscopic characteristics play a vital role in revealing the mechanism of strength and deformation evolution. In this study, the mechanical behaviors of frozen sand at various negative temperatures are investigated by a self-developed temperature-controlled large-scale direct shear apparatus. The shear stress-shear displacement curve, dilation curve and shear strength characteristics at different negative temperatures and normal pressures are analyzed. According to the test results, as the cooling temperature decreases, the cohesion and internal friction angle of frozen sandy sample both increase significantly, and the shear dilatation tends to be larger. The brittle behavior of frozen sand becomes more obvious at lower temperatures. Then, a discrete element method (DEM) model is established to investigate the microscopic mechanical properties of frozen sand in the direct shear test. The particle movement, force chain evolution, and shear surface development during the shearing process are analyzed. The simulations of the DEM model can well explain the microscopic mechanism of the macroscopic strength and deformation law of frozen sandy soil. It is hoped that the research results can provide a reference for the DEM application in frozen soil mechanics.

Fiber reinforcement effectiveness in two different sand specimens

Fiber Reinforced Soils (FRS) are mixtures of discrete fibers with the soil to create a composite with improved mechanical properties compared to unreinforced material that depends on several soil and fiber properties. Therefore, comparative studies are needed to better understand their influence on FRS mechanical response. This paper analyzes the results of a comprehensive triaxial testing program performed on specimens of two different sands at the same relative density focusing on how the grain size distribution affects the composite behavior in terms of shear strength and dilatancy. It is shown that the grain size curve’s uniformity coefficient (Cu) is one of the critical variables controlling FRS’s dilatancy. Dune sand specimens (Cu = 1.79) presented dilatancy even for confining stresses as high as 300 kPa. The shear gains due to reinforcement were controlled by fiber length (L) and percentage (Pf), and size and shape of soil particles. River sand specimens with L = 51 mm and 1% fiber addition (dry mass) presented increments of 47.7 kPa in soil cohesion and a 5.2o increase in the soil friction angle compared to unreinforced material.

Numerical simulations on shear behaviour of rock joint network under constant normal stiffness conditions.

In this study, the numerical direct shear tests were conducted to investigate the shear mechanical properties of joint networks under constant normal stiffness (CNS) boundary conditions. The influence of random joint number on shear stress (τ), dilation (normal displacement, δv) and normal stress (σn) of rock mass were studied quantitatively with fixed main slip surface. At the same time, the internal stress evolution process and failure process were analyzed. The results reveal that the number of random joints (γ) has little effect on the shear and normal stresses. The normal displacement of the sample generally decreases as the number of random joints increases. In addition, the normal displacement of the specimen is absorbed by the random joints when the number of random joints in the specimen increases to a certain level: when γ is greater than 6 and the shear displacement (μ) reaching 10 mm, the specimen exhibits shear contraction. Therefore, the internal random joints mainly control the failure mode and dilatancy performance of the specimen, while the main joint of the rock controls the shear stress of the specimen.

Effect of gravel on rock failure in glutenite reservoirs under different confining pressures

Due to the existence of gravel, glutenite is heterogeneous and different from fine-grained rocks such as sandstone and shale in structure. To fully understand the effect of gravel on failure mode in glutenite, we performed triaxial compression tests on different glutenites. The results indicate that failure modes of glutenite are complex due to the existence of gravel. Under different confining pressures, three failure modes were observed. The first failure mode, a tensile failure under uniaxial compression, produces multiple tortuous longitudinal cracks. In this failure mode, the interaction between gravels provides the lateral tensile stress for rock splitting. The second failure mode occurs under low and medium confining pressure and produces a crack band composed of micro-cracks around gravels. This failure mode conforms to the Mohr-Coulomb criterion and is generated by shear failure. In this failure mode, shear dilatancy and shear compaction may occur under different confining pressures to produce different crack band types. In the second failure mode, gravel-induced stress concentration produces masses of initial micro-cracks for shear cracking, and gravels deflect the fracture surfaces. As a result, the fracture is characterized by crack bands that are far broader than in fine-grained rocks. The third failure mode requires high confining pressure and produces disconnected cracks around gravels without apparent crack bands. In this failure mode, the gravel rarely breaks, indicating that the formation of these fractures is related to the deformation of the matrix. The third failure mode requires lower confining pressure in glutenite with weak cement and matrix support. Generally, unlike fine-grained rocks, the failure mode of glutenite is not only controlled by confining pressure but also by the gravel. The failure of glutenite is characterized by producing cracks around gravels. These cracks are produced by different mechanisms and distributed in different manners under different confining pressures to form different fracture patterns. Therefore, understanding the rock microstructure and formation stress state is essential in guiding glutenite reservoir development.

Experimental study of ultra-high backpressure influence on the monotonic mechanical characteristic of silty sand

In this paper, a series of monotonic triaxial tests were carried out at a relatively wide range of backpressure conditions (from 100 kPa to 29 MPa) to investigate its influence on the shear properties of silt sand.Backpressure effect on the shear strength, excess pore pressure, and volumetric strain behavior were analyzed,and the underlying influencing mechanism was preliminarily discussed. According to test results, the application of higher backpressure conditions on quasi-saturated samples resulted in a significant buildup of negative excess pore water pressure, which led to an increase in shear strength. The trend became less pronounced after the backpressure exceeded 5 MPa. Nevertheless, within this range, the shear strength increased by approximately 40 kPa for every 100 kPa increase in backpressure. Furthermore, the consolidated-drained (CD) test results revealed that the value of backpressure (at least within 29 MPa) had no detectable influence on the shear properties of saturated samples. It was indirectly proved that the undrained shear strength was influenced by the change of effective stress caused by excess pore water pressure in the shear dilation stage of quasi-saturated samples. The applicability of the effective stress principle to predict soil strength even in ultra-high backpressure conditions was confirmed.

Stress-state dependency of the strain ratio of quasi-elastic granular soils under cyclic loading

Poisson’s ratio, besides its transverse–axial strain ratio definition, also plays a significant role in traditional elastic theory as an elastic constant. However, this elastic role of Poisson’s ratio faces a challenge when applied to granular soils. A so-called “strain ratio” is therefore introduced and measured in this paper for granular soils under quasi-elastic state. With the same definition of the transverse–axial strain ratio as Poisson’s ratio, the strain ratio is investigated under cyclic triaxial tests for its stress-state dependency. The results show that Poisson’s ratio is consistent with strain ratio only in the initial state of shear, and there is a non-negligible deviation in subsequent shear. The conversion relationship of elastic constants does not hold for the strain ratio with increasing deviator stress. A special case is analyzed when the strain ratio is greater than 0.5 and the specimen is subject to a quasi-elastic shear dilatancy. Discrete element simulations are conducted to clarify the microscopic mechanism of quasi-elastic dilatancy by tracking the evolution of the contact number of particles and the trajectory of the contact. This study provides new insights in understanding the elastic behavior of granular soils.

Energy equation and stress–dilatancy relationship for crushable granular materials incorporating particle breakage

The ROWE’s stress–dilatancy relationship is widely employed to characterise the shear dilatancy behaviour of granular materials. However, particle breakage energy dissipation is excluded from the ROWE’s energy balance equation. In this study, three crushable granular soils with significantly different strengths were subjected to triaxial testing, and the dissipation of energy during the tests was explored. For all three materials, the incremental particle breakage energy dissipation increases and then decreases. It was assumed that at the critical state, particle breakage ended and only frictional energy dissipation occurred. To more accurately characterise the energy dissipation process, an exponential expression of particle breakage energy dissipation is added to the ROWE’s stress–dilatancy relationship. The proposed stress–dilatancy relationship is consistent with the shear dilatancy behaviour of crushable granular materials, and the modified energy equation can more precisely describe the energy dissipation process. The hypothesised stress–dilatancy relationship is applicable to three distinct types of crushable granular materials with varying strengths, including sand-gravel material, landslide dam material and calcareous sand.

Constitutive modeling of a laumontite-rich tight rock and the application to poromechanical analysis of deeply drilled wells

Geological formations containing laumontite-rich rock are usually treated as problematic for geo-energy production projects because the presence of laumontite mineral can promote complex mechanical behaviors. Previous laboratory results indicate that rock formations with a higher laumontite content display severe stress sensitivity in poromechanical responses. With an increase in confining pressure, there is a transition from dilation to compression regime and the resulting localization styles range from shear dilation to compaction bands. In this study, we conduct finite element modeling of constitutive behaviors of rocks retrieved from the tight glutenite reservoir formation using a thermodynamic-consistent plasticity model. The shear dilation to compaction transition is well characterized. Poromechanical analysis is also conducted to analyze the plastic zone development around a borehole drilled in an over pressured reservoir. The simulated stress-paths of key points around the borehole are used to demonstrate the plastic strain development processes. The impact of in-situ stress on the wellbore stability is highlighted, and a comparison with the results from using the traditional plastic constitutive model is conducted.

A State-Dependent Hypoplastic Bounding Surface Model for Gassy Sand

As a kind of partially saturated soil often containing dissolved gas with a high degree of saturation (S &gt; 85%), gassy sand sediment widely exists in the marine environment all over the world. Due to the effect of gas dissolution and exsolution on the pore fluid compressibility, its stress–strain and pore pressure responses are quite different from those of common saturated and unsaturated soils when subjected to undrained loading. Since almost all the gas bubbles are occluded in the pore water of offshore gassy sand, the matric suction may be neglected, and an undrained constitutive model for gassy sand is developed based on the existing hypoplastic bounding surface model for saturated sand. Both Boyle’s and Henry’s laws are employed in the model to characterize the equilibrium behavior of the gas compressibility and solubility, and then the equivalent compressibility coefficient of the pore fluid is obtained. To avoid unrealistic volumetric expansion, the concepts of the critical state and state-dependent dilatancy stress ratio are incorporated to describe its ultimate shear strength and dilatancy characteristics, respectively. Finally, the triaxial undrained test results on gas-charged sand from Hangzhou Bay are analyzed at three initial saturation degrees of 85%, 90%, and 100%, and two effective confining pressures of 50 kPa and 200 kPa. Moreover, carbon dioxide (CO2) was selected in the test, and the samples were loose with a relative density of 30%. It is noted that a good agreement is achieved between the simulation results and the experimental data, including the influence of gas content and confining pressure on the shear dilatancy and the mean effective stress increase at the beginning of the effective stress path, among others.

Experiment and particle flow simulation of unloading induced shearing properties for jointed sandstone materials

Shear tests on sandstone containing non-connected joints under the unloading normal stress but fixed shear stress conditions were conducted with various initial normal stress and unloading rate. During the unloading process, continuous shear dilation could be observed. For initial normal stress = 5–20 MPa, the sliding initiation normal displacement decreased by 17.14–66.32 % but the sliding initiation shear displacement increased by 21.91–98.19 %. The sliding initiation normal stress declined by 7.18–15.57 % for unloading rate = 0.002–0.01 MPa/s but increased by a factor of 3.18–3.53 with the initial normal stress. Shear sliding of the jointed samples was more prone to occur at a smaller initial normal stress with a larger unloading rate. From the numerical simulation results via PFC2D, during the unloading process, cracking was first initiated from central part of the rock bridge and propagated gradually to coalesce with cracks developed from the joint tips. The penetrating fracture surfaces led to shear sliding of the jointed samples, and shear wear occurred on the fractures as shear displacement continued increasing. For unloading rate = 0.002–20 MPa/s, quantity of cracks corresponding to shear displacement of 1.2 mm decreased by 12.18–36.34 %. Cracking degree was more serious for the samples at a low unloading rate due to progressively developed cracks near the main failure planes before shear sliding and more accumulated cracks in the shearing wear stage for fractures under a larger normal stress.

Experimental study on unloading induced shear performances of 3D saw-tooth rock fractures

A fractal model governing saw-tooth fractures was first introduced to replicate sandstone samples containing an inclined 3D penetrating rough fracture surface with various joint roughness coefficients (JRC). In conventional triaxial compression, the peak strength for fractured samples increased with both confining pressure and JRC. During the unloading confining pressure process, the normal stress of fractures declined but the shear stress increased, resulting in shear sliding of fractures. The shear displacement of fractures exponentially increased, and the positive normal displacement decreased gradually to negative values under coupling effects of shear contraction caused by normal stress and shear dilation due to climbing effects of fractures. Transition from quasi-static to dynamic sliding of the fractures was identified. The sliding resistance duration increased with confining pressure but decreased with JRC. After pre-peak unloading, the fracture surfaces presented a more significant surface wear response and JRC values decreased by 1.70%–59.20% due to more remarkable asperity degradation compared with those after conventional triaxial compression. The theoretical model for shear strength of fractures was established through improving the Ladanyi & Archambault model by introducing the relations between normal stress and surface wear ratios of fractures, which agreed well with the experimental results.