Increase Of Confining Pressure Research Articles

Experimental Investigation of the Fracture Performance of Hard Rock with a Single Borehole by Using a Hydraulic Splitter

The continuous expansion of urban areas is severely restricted by limited land resources. A promising and universal solution to this problem is the development of underground spaces. However, during the excavation of underground rock tunnels, mechanical tools undergo severe wearing due to the strengthened mechanical properties of the rock. Engineering applications have demonstrated that hydraulic splitters can effectively fracture hard rocks. Therefore, in this study, the effect of the rock compressive strength, confining pressure, and borehole margin on the rock fracture performance was experimentally investigated. The hydraulic splitter can be effectively applied to fracture hard rock. The peak fracture pressure increases with the increase in the rock compressive strength, confining pressure, and borehole margin. Moreover, the peak fracture pressure under a bidirectional unequal stress confining pressure is lower than that under bidirectional equal stress, and the difference between these pressure values gradually decreases as the confining pressure increases. The peak fracture pressure with the free surface is lower than that without the free surface, which demonstrates that the presence of the free surface can alleviate the difficulty in realizing rock fracturing.

Research on triaxial compression failure characteristics and meso-simulation of brittle gypsum material

The laboratory tests and Particle Flow Code numerical simulation of triaxial compression are conducted to analyze fractal dimensions of fracture surface, strength properties, energy dissipation, damage laws and crack evolution. Under low confining pressure, the fracture surface of the brittle specimen is rough and the angle relative to the maximum principal stress is small. The specimen eventually exhibits local tensile-shear failure mode. With the increase of confining pressure, the strength of the specimen increases and yield platform for the stress–strain curve is more obvious. The main fracture surface becomes increasingly flatter and the fracture angle increases with the increase of confining pressure. The specimen exhibits a shear failure mode macroscopically. With the increase of confining pressure, the trend of transgranular shear failure becomes obvious and the micro-fracture surface has a smaller fractal dimension. In the triaxial compression test, energy storage, dissipation and release occur. When the storage limit is reached, the internal elastic strain energy releases sharply and the damage variable increases rapidly. As the confining pressure increases, the number of tensile and shear cracks is accumulated in an ‘S’ shape. Meanwhile, the proportion of shear cracks increases gradually.

Research on the effect of rock content and sample size on the strength behavior of soil-rock mixture

The mechanical behaviors of soil-rock mixture (S-RM) are very complicated and significantly affected by its rock block content and gradation composition. A systematic study was performed on how the strength and failure characteristics of S-RM can be affected by the rock block content, the “oversize rock block” processing methods adopted, and the sample size in triaxial tests. Both the deviator stress ratio and friction strength of S-RM increase together with the rock block content, but the former tends to decrease as the confining pressure increases. In the case of S-RM samples with the same gradation, these two parameters tend to decrease as the sample scale becomes larger. The development of deviator stress over axial strain of samples prepared using the equal quantity substitution method is similar to that of the samples of the natural gradation. During the shearing process, some large rock blocks in S-RM will be broken. The breakage mode of the rock blocks can be primarily divided into four categories according to its mechanism, namely, disintegration, corner breakage, fracturing, and breakage along bedding faces. As a result of the breakage, the strength envelope curve of S-RM follows a power function.

Experimental study on law of limit storage energy of rock under different confining pressures

To investigate the effects of different unloading rates on rockbursts, the conventional triaxial compression tests and confining pressure unloading tests have been conducted on rock samples. The variation law of stress and stress thresholds for structural failure of rock sample under different confining pressures was analyzed, and the influence of different unloading confining pressure rates on limit storage energy during rock sample failure process was revealed. Experimental results showed that under uniaxial compression, the stress for structural failure of the rock sample is the same as stress thresholds for structural failure. In the conventional triaxial test, the stresses for structural failure and stress thresholds for structural failure of the rock sample will increase with the increase of confining pressure. Compared with the stresses for structural failure, the stress threshold for structural failure of rock samples increases by 32.88%, 53.63%, and 78.51%, respectively when the confining pressures are 10 MPa, 20 MPa, and 30 MPa. Under different confining pressure unloading rates, the limit storage energy of samples is also different. The higher the confining pressure unloading rate is, the smaller the limit storage energy is. When confining pressure increases, the limit storage energy of rock samples increases. Under the same initial confining pressure and confining pressure unloading rate, the change of axial pressure has little effect on the limited storage energy of rock, i.e., the energy required for rock failure is a definite value under certain conditions and is not affected by the stress loading path. This research can be helpful to understand and assess the rockburst during the deep rock mass excavation by different tunneling methods.

Effect of freeze-thaw cycles on the mechanical properties and constitutive model of saline soil

The freeze-thaw cycle is one of the most common natural physical processes in cold regions and will significantly affect the deformation characteristics of saline soil. To study the mechanical properties and constitutive relationship of saline soil under freeze-thaw cycles, in this paper, a series of freeze-thaw cycle tests and consolidated-drained (CD) triaxial tests were conducted on remodeled saline soil in the Qian'an area of western Jilin, China. Based on the elliptic-parabolic double yield surface constitutive model, a modified model considering the effects of freeze-thaw cycles is established. The results show that the specimen exhibits a strain-hardening stress-strain relationship, and the volumetric strain during shearing exhibits a shear-shrinkage characteristic overall. As the number of freeze-thaw cycles increases, the volumetric strain gradually increases, and the shear strength gradually decreases. As the confining pressure increases, the volumetric strain gradually increases. Then, the elliptic function and parabolic function are selected to describe the volume yield surface and the shear yield surface on the p-q plane respectively. By introducing the correlation flow rule, the functional relationship between the deviating stress increment and the axial strain increment and volumetric strain increment is derived. Based on the results of the triaxial test, the variation in the model parameters with the number of freeze-thaw cycles was determined. The results show that as the number of freeze-thaw cycles increases, c, φ, h, K, n, M1, M2, and a show a decreasing rule, while t shows a gradually increasing rule, and all factors can use logistic function to fit the regression relationship between the model parameters and the number of freeze-thaw cycles. The expression of the model parameters with the number of freeze-thaw cycles as a factor is substituted into the stress-strain increment constitutive equation, and a modified double yield surface model considering the effects of freeze-thaw cycles is established. The calculated values of the model are basically consistent with the measured values. This shows that the double yield surface constitutive model can be applied to saline soil.

Influence of Stress Sensitivity on Water-Gas Flow in Carbonate Rocks

Carbonate reservoirs significantly contribute to exploitation. Due to their strong heterogeneity, it is of great significance to study core seepage capacity and gas-water two-phase flow of reservoirs with various pore structures under different stresses for productivity prediction, gas reservoir development, and reservoir protection. We utilize micrometer-resolution X-ray tomography to obtain the digital rocks of porous, fractured-porous, and fractured-vuggy carbonate rocks during pressurized process and depressurization. The Lattice Boltzmann method and pore network model are used to simulate the permeability and gas-water two-phase flow under different confining pressures. We show that at the early stage of pressure increase, fractures, vugs, or large pores as the main flow channels first undergo compaction deformation, and the permeability decreases obviously. Then, many isolated small pores are extruded and deformed; thus, the permeability reduction is relatively slow. As the confining pressure increases, the equal-permeability point of fractured-porous sample moves to right. At the same confining pressure, the water saturation corresponding to equal-permeability point during depressurization is greater than that of pressurized process. It is also proved that the pore size decreases irreversibly, and the capillary force increases, which is equivalent to the enhancement of water wettability. Therefore, the irreversible closure of pores leads to the decrease of permeability and the increase of gas-phase seepage resistance, especially in carbonate rocks with fractures, vugs, and large pores. The findings of this study are helpful to better understand the gas production law of depletion development of carbonate gas reservoirs and provide support for efficient development.

Dynamic Compression–Shear Response and Failure Criterion of Rocks with Hydrostatic Confining Pressure: An Experimental Investigation

Rocks in the deep underground are likely subjected to both hydrostatic confining pressure and dynamic compression–shear load. Thus, accurately characterizing the dynamic properties and failure mechanism of hydrostatically confined rocks under combined compression–shear impacting is crucial for the stability assessment of deep underground rock structures. In this study, on the basis of an improved split Hopkinson pressure bar (SHPB) apparatus, the combined dynamic compression–shear tests are performed on inclined cylindrical sandstone specimens with hydrostatic confining pressures. During the test, the dynamic force balance of rock specimens can be well satisfied using the pulse shaping technique. Our results show that the hydrostatic confining pressure and dynamic loading rate help strengthen the load-carrying capacity of rocks. In contrast, the shear component in the dynamic load limits the dynamic peak stress of rocks. As hydrostatic confining pressure increases, the failure surface based on the Drucker–Prager criterion gradually expands outward. Under dynamic loading, the compressive deformation modulus of rocks decreases with increasing shear component in the dynamic load, contrary to its response to hydrostatic confining pressure. Fragmentation analysis indicates that the hydrostatic confining pressure and the shear component of dynamic loading restrict the fracture behavior of rocks. Besides, as the specimen inclination angle and the hydrostatic confining pressure increase, the failure pattern of rock specimens changes from the tensile-dominated failure with a truncated conical surface to the shear-dominated failure with a single shear plane along its short diagonal.

Dual attenuation peaks revealing mesoscopic and microscopic fluid flow in partially oil-saturated Fontainebleau sandstones

SUMMARY We conducted stress–strain oscillation experiments on dry and partially oil-saturated Fontainebleau sandstone samples over the 1–2000 Hz band at different confining pressures to investigate the wave-induced fluid flow (WIFF) at mesoscopic and microscopic scales and their interaction. Three tested rock samples have similar porosity between 6 and 7 per cent and were partially saturated to different degrees with different oils. The measurement results exhibit a single or two attenuation peaks that are affected by the saturation degree, oil viscosity and confining pressure. One peak, exhibited by all samples, shifts to lower frequencies with increasing pressure, and is mainly attributed to grain contact- or microcrack-related squirt flow based on modelling of its characteristics and comparison with other experiment results for sandstones. The other peak is present at smaller frequencies and shifts to higher frequencies as the confining pressure increases, showing an opposite pressure dependence. This contrast is interpreted as the result of fluid flow patterns at different scales. We developed a dual-scale fluid flow model by incorporating the squirt flow effect into the patchy saturation model, which accounts for the interaction of WIFFs at microscopic and mesoscopic scales. This model provides a reasonable interpretation of the measurement results. Our broad-frequency-band measurements give physical evidence of WIFFs co-existing at two different scales, and combining with modelling results, it suggests that the WIFF mechanisms, related to pore microstructure and fluid distribution, interplay with each other and jointly control seismic attenuation and dispersion at reservoir conditions. These observations and modelling results are useful for quantitative seismic interpretation and reservoir characterization, specifically they have potential applications in time-lapse seismic analysis, fluid prediction and reservoir monitoring.

Experimental study on the thermal damage characteristics of cement stone

Aiming at the problem of cement ring sealing failure during deep high temperature shale gas exploitation, comprehensively considering the influence of the characteristics of multi-cluster fracturing of multiple horizontal wells and formation temperature, the cementing cement the southwest region is taken as the research object. After exposure to different temperatures (95?C and 135?C) and for different times (5, 10, and 20 times), axial and triaxial tests with different confining pressures (0, 5 MPa, 15 MPa, and 30 MPa) were carried out. The research shows that: the stress-strain curve of cement stone after heat treatment can be divided into four stages: compaction, elastic, yield, and post-peak stage; as the confining pressure increases, the compaction phase disappears, the yield phase increases, and we see the transition from brittle to ideal plasticity after the peak and as the temperature and number of thermal cycles increase, the cohesive force decreases significantly, and the internal friction angle shows a slight increase. The elastic modulus and the peak strength decreased.

Characterization and mesosimulation of the triaxial compression failure of rock-like materials

The laboratory tests and the numerical simulation of the particle flow code of triaxial compression are conducted to analyze the fractal dimensions of fracture surface, strength properties, energy dissipation, damage patterns, and crack evolution. The following results are obtained. (1) Under low confining pressure, the fracture surface of a brittle specimen is rough, and the angle relative to the maximum principal stress is small. The specimen eventually exhibits a local tensile–shear failure mode. As the confining pressure increases, the strength of the specimen increases, and the yield platform for the stress–strain curve is more obvious. The main fracture surface flattens, and the fracture angle increases as the confining pressure increases. The specimen macroscopically exhibits a shear failure mode. (2) As the confining pressure increases, the trend of transgranular shear failure becomes obvious, and the microfracture surface has a smaller fractal dimension. (3) In the triaxial compression test, energy is stored, dissipated, and released. When the storage limit is reached, the internal elastic strain energy becomes released sharply, and the damage variable increases rapidly. (4) As the confining pressure increases, the number of tensile and shear cracks accumulates in an “S” shape, and the proportion of shear cracks increases gradually.

DEM simulation of the complete triaxial test of sandstone

Complete triaxial tests considering residual strength are of great significance in analyses of the mechanical properties of rock in deep-rock engineering. In this paper, the complete triaxial test of Berea sandstone was quantitatively simulated using the discrete element method. The macro- and micromechanical properties of Berea sandstone were analyzed, and the following conclusions were obtained: (1) As the confining pressure increases, the peak and residual strengths of the sample increases, the stress–strain curves gradually change from softening to hardening, and the volume curves gradually change from dilatancy to shrinkage. (2) When the confining pressure is low, bond failure between particles mainly occurs under the state of tensile stress. As the confining pressure increases, compressive state failure gradually exceeds tensile state failure. (3) Under different confining pressures, the energy dissipated by friction between particles is far greater than other forms of energy dissipation and the energy dissipated by rolling between particles is always greater than the energy dissipated by twisting.

Relationship of rockburst and energy dissipation characteristics of brittle rock at great depth

Rockburst is defined as a major geological disaster of hard rock mass. At present, rockburst analysis mostly focuses on its mechanism, prediction, and preventive measures, and great progress has been made. However, a few comprehensive analyses on the rockburst mechanism still need to be investigated from different energy perspectives. From the energy perspective, this study analyzes the relationship between a rockburst and the destructive properties of T2b marble in the deep buried tunnel of the Jinping Secondary Hydropower Station. Our analysis includes strain energy accumulation and dissipation, shear fracture, energy release, and acoustic emission (AE). The obtained results show that in the energy release and dissipation processes, three dilatancy mechanisms trigger a rockburst, namely the tensile fracture extension control mechanism, closed fracture friction control mechanism, and their mutual transformation compound control mechanisms. Different mechanisms lead to different energy mutation patterns that determine the rockburst extent and scale. We also found that the strain energy of brittle rocks linearly increases with the increase of the confining pressure during the failure of the brittle rocks. The energy storage rate of the front peak at the lower confining pressure was significantly greater than that of the back peak. However, the difference gradually decreases as the confining pressure increases. The total shear fracture energy with the increase of the confining pressure first increases and then decreases. The peak before the shear fracture energy nonlinearly increases gradually, and the two fractures can be similar until the brittle–ductile transition. Lastly, the AE experiment shows that during the addition and unloading of brittle rocks at a great depth, three types of energy active periods and three types of energy decay periods are observed. The precursor to an instantaneous rockburst is the second type of energy fading period, while that to a delayed rockburst is the third type of energy fading period. The second type of energetically active period refers to the birth of a rockburst. The third type refers to the occurrence of a standard rockburst or a recurrence, such as a discontinuous rockburst.

Experimental research on the anisotropic properties of sandy slate

In this study, the effect of joint on the anisotropy of a homogeneous, stiff, and intact sandy slate from a typical hydropower station is investigated by selecting and making jointed rock samples with jointed angles of 0°, 30°, 45°, 60°, and 90°. Accordingly, an acoustic measurement and a triaxial compression test are conducted on these jointed rock samples. The research results show that the joint angle significantly influences the rock mass anisotropy. In other words, the average value of the longitudinal wave velocity of the jointed rock samples is smaller than that of the intact rock. Furthermore, the value of the longitudinal wave velocity increases with the increase of the joint angle and is the largest when its spreading direction is in line with the joint inclination. The stress–strain curve, elastic modulus, peak strength, and failure mode of the joint rock samples are found to have anisotropic features. The failure stage occurs in samples with joint angles of 0°, 30°, and 90°. The stress–strain curve of the samples with joint angles of 45° and 60° tends to be horizontal. Meanwhile, the elastic modulus and the compression strength displayed a U-shaped distribution with an increase of the joint angle and the smallest value that belongs to samples with joint angles of 30° and 60°. The peak value ratio of the elastic modulus and the compression strength gradually decrease when the confining pressure increases, indicating that the anisotropy of the samples has weakened (i.e., the increase of the confining pressure decreases the anisotropy of the sandy slate strength). The failure mode of the samples with different joint angles is presented as follows: the samples with a 0° joint angle exhibit a tensile splitting damage; the samples with a 90° joint angle exhibit compression–shear damage; the samples with a 30°joint angle exhibit the combination of slide and compression–shear damage; and the samples with 45° and 60°joint angles exhibit slide damage along the joint surface.

Research on the influences of confining pressure and strain rate on NEPE propellant: Experimental assessment and constitutive model

In order to study the influences of confining pressure and strain rate on the mechanical properties of the Nitrate Ester Plasticized Polyether (NEPE) propellant, uniaxial tensile tests were conducted using the self-made confining pressure system and material testing machine. The stress-strain responses of the NEPE propellant under different confining pressure conditions and strain rates were obtained and analyzed. The results show that confining pressure and strain rate have a remarkably influence on the mechanical responses of the NEPE propellant. As confining pressure increases (from 0 to 5.4 MPa), the maximum tensile stress and ultimate strain increase gradually. With the coupled effects of confining pressure and strain rate, the value of the maximum tensile stress and ultimate strain at 5.4 MPa and 0.0667 s−1 is 2.03 times and 2.19 times of their values under 0 MPa and 0.00333 s−1, respectively. Afterwards, the influence mechanism of confining pressure on the NEPE propellant was analyzed. Finally, based on the viscoelastic theory and continuous damage theory, a nonlinear constitutive model considering confining pressure and strain rate was developed. The damage was considered to be rate-dependent and pressure-dependent. The constitutive model was validated by comparing experimental data with predictions of the constitutive model. The whole maximum stress errors of the model predictions are lower than 4% and the corresponding strain errors are lower than 7%. The results show that confining pressure can suppress the damage initiation and evolution of the NEPE propellant and the nonlinear constitutive model can describe the mechanical responses of the NEPE propellant under various confining pressure conditions and strain rates. This research can lay a theoretical foundation for analyzing the structural integrity of propellant grain accurately under working pressure loading.

The effects of water content on transversely isotropic properties of organic rich gas shale

Organic-rich gas shales are often composed of thin laminated structures and, thus, may be considered as transversely isotropic material. The shale's anisotropic properties are affected by many factors, including confining pressure, water content, total organic content (TOC), etc. In this work, ultrasonic velocity measurements were performed at various confining pressures and water content conditions to study the effects of confining pressure and water content on anisotropic properties at Eagle Ford shale. The test results show that Eagle Ford shale has weak elastic anisotropy. In addition, the results confirm that transversely isotropic media is an appropriate model to characterize Eagle Ford shale.Both the compressional (P) and shear (S) wave velocities increase as the confining pressure increases, especially at low confining pressure range. The horizontal elastic modulus, vertical elastic modulus and shear modulus also increase with the increase in confining pressure. Varying the confining pressure show minimal to no effect on the horizontal and vertical Poisson's ratio. On the other hand, the increase in the confining pressure decreases both P- and S-wave anisotropy. The study shows that P-wave anisotropy is more sensitive to confining pressure than S-wave anisotropy. The imbibed water significantly decreases P- and S- wave velocities. For a given sample at the same confining pressure, the decrease of P-wave/S-wave velocity is proportional to the unit increase of water content, which is defined as water content impact factor. Increasing water content also significantly reduces horizontal Young's modulus, vertical Young's modulus and shear modulus. Water makes the shale softer. The study shows that increase in the water content decreases the horizontal Young's modulus, the vertical modulus as well as the shear modulus by 8%, 11% and 15%, respectively. Both horizontal and vertical Poisson's ratio increase with increasing water content. Rock samples tested confirm that the increase in water content increases the P- and S-wave anisotropies by approx. 2.1% and 2.6%, respectively. The presence of water narrows the difference of VP and VP(900) at small angle of the near vertical range and, therefore, decreases the near vertical P-wave anisotropy.

Mechanical Properties and Acoustic Emission Characteristics of the Bedrock of a Hydropower Station under Cyclic Triaxial Loading

The bedrock of hydropower stations is susceptible to irreversible failure due to dynamic unloading and loading related to earthquakes, rock bursts or blasting. To ensure operational safety, the damage evolution process of bedrock under cyclic loading must be investigated. Hence, this study investigates the mechanical properties and acoustic emission (AE) characteristics of the bedrock from different positions of the Badantoru hydropower station under different confining pressures. First, the relationship between the dynamic elastic modulus and residual strain is analyzed, and then the brittleness and damage of the rocks are quantitatively characterized. Subsequently, the failure process and crack development of the rocks are revealed from the obtained AE characteristics. The results show that as the applied confining pressure increases, the peak differential stress and fatigue life of the rock specimens increase; whereas, the shear failure angle decreases. Under the same confining pressure, the rocks with a greater number of internal joints and pores (Dam Site I) have a lower bearing capacity under cyclic loading, a greater brittleness after strain softening, and a smaller shear failure angle. With increasing plastic strain, all of the specimens exhibit cyclic softening, and an obvious four-stage fatigue damage evolution can be observed.

Seepage characteristics of a fractured silty mudstone under different confining pressures and temperatures

To investigate the influence of confining pressures and temperatures on the seepage characteristics of fractured rocks, seepage tests were conducted on a fractured silty mudstone using a self-developed experimental system, and the effects of different factors on coefficient of permeability were discussed. The results showed that the increasing confining pressure will gradually decrease the coefficient of permeability, and this process is divided into two stages: 1) the fast decrease stage, which corresponds to a confining pressure less than 30 kPa, and 2) the slow decrease stage, which corresponds to a confining pressure larger than 30 kPa. Unlike confining pressure, an increase in temperature will increase the coefficient of permeability. It is noted that fracture surface roughness will also affect the variation of coefficient of permeability to a certain extent. Among the three examined factors, the effect of confining pressure increases is dominant on fracture permeability coefficient. The relationship between the confining pressure and coefficient of permeability can be quantified by an exponential function.

Shale Brittleness Index Based on the Energy Evolution Theory and Evaluation with Logging Data: A Case Study of the Guandong Block.

Shale brittleness is a key index that indicates the shale fracability, provides a basis for selecting wells and intervals to be fractured, and guarantees the good fracturing effect. The available models are not accurate in evaluating the shale brittleness when considering the confining pressure, and it is necessary to establish a new shale brittleness model under the geo-stress. In this study, the variation of elastic energy, fracture energy, and residual elastic energy in the whole process of rock compression and failure is analyzed based on the stress–strain curve in the experiments, and a shale brittleness index reflecting the energy evolution characteristics during rock failure under different confining pressures is established; a method of directly evaluating the shale brittleness with logging data by combining the rock mechanic experiment results with logging interpretation results is proposed. The calculation results show that the brittleness decreases as the confining pressure increases. When the confining pressure of the Kong-2 member shale of the Guandong block is less than 25 MPa, the brittleness index decreases significantly as the confining pressure increases, and when the confining pressure is greater than 25 MPa, the brittleness index decreases slightly. It is shown that the shale brittleness index is more sensitive to the confining pressure within a certain range and less sensitive to the confining pressure above a certain value.

Numerical simulation of hydraulic fracture deflection influenced by slotted directional boreholes using XFEM with a modified rock fracture energy model

Directional hydraulic fracturing (DHF) has been used in conductivity improvement of oil, gas and geothermal energy reservoirs and hard rock roof breaking in coal mines. Although the new DHF method that integrates fracturing borehole and slotted directional borehole (SDB) has been recently introduced, the mechanism and effectiveness of SDB on hydraulic fracture guiding remain open issues. In this study, numerical simulation based on extended finite element method (XFEM) is performed to investigate the effect of SDB on hydraulic fracture deflection. A modified rock fracture energy model is proposed and performed by a user subroutine USDFLD with XFEM, which considers the hardening property of rock shear fracture energy as confining pressure increases. The effects of stress field and borehole line azimuth on hydraulic fracture deflection and bottomhole pressure are investigated. Hydraulic fracture morphologies of three kinds of multi-borehole fracturing technology are compared, the control mechanism of SDBs on hydraulic fracture is explained, and the bottomhole pressure relief effect, borehole spacing and potential application prospects are discussed. Result indicates that the modified model shows good accuracy and applicability in predicting hydraulic fracture propagation, and the hardening property of shear fracture energy should be considered when using XFEM and cohesive element method. The feature “first drop and then rise” of bottomhole pressure curve is an important sign that imply the hydraulic fracture propagates into the influence radius of SDB and can be used as an indicator to evaluate the effectiveness of DHF. Slot increases the influence radius of directional borehole and changes the local stress direction, thus allowing SDB-based DHF technique to be applied to the conditions where the stress difference ratio does not exceed one. When the stress difference ratio is greater than one, the pressurized SDB is more suitable.

Improvement of a Hypoplastic Model for Granular Materials Under High-Confining Pressures

The behavior of granular materials during loading depends on the level of stresses. When confining pressure increases, the peak shear strength, the residual shear strength and the stiffness gradually decrease; besides, the volumetric behavior is shown to be influenced by the stress level. In this paper, such effects, due to changes in stress levels, have been incorporated into a modified von Wolffersdorff hypoplastic model. For this purpose, reference void ratios and exponent α and β, the parameters of the original hypoplastic model are modified using experimental data. The performance of the proposed model is demonstrated by using simulated triaxial tests on Hostun sand with cell pressures up to 15 MPa. The study shows the ability of the improved model to highlight the behavior characteristics of granular materials in dilatancy and (peak) resistance under high stress better than the original model.