Direction Of Maximum Principal Stress Research Articles

Propagation prediction of asymmetrically originated fractures by use of displacement discontinuity method

In existing fracture propagation simulation studies, the fracture propagation simulation is based on the assumption that the fracture originates symmetrically with respect to the wellbore. However, the drilling treatments can induce stress reorientation near the wellbore, resulting in asymmetrically originated fractures, which is different from the assumption of existing studies. In such cases, it can be difficult to predict the fracture geometries. In this work, the authors propose a fracture propagation model to simulate the propagation of asymmetrically originated fractures with the displacement discontinuity method. With the aid of the proposed model, the authors investigate the impact of the redistribution of stress field on the propagation of asymmetrically originated fractures and conduct a comprehensive study to investigate the effects of various geological and fluid parameters on the propagation of asymmetrically originated fractures. The results show that the redistribution of the stress field renders the newly generated fracture segments to deviate from the initial originations so that although the initial fracture origination is asymmetric, the asymmetrically originated fractures rapidly propagate along an approximately symmetrical path and propagate parallel to the direction of maximum principal stress as it goes away from the wellbore. The closer the two wings of the fracture are to the symmetric origin, the faster it is for the change of propagation direction to the direction of the maximum principal stress. A large principal stress difference tends to cause asymmetrically originated fractures to quickly deviate from the initial originations and propagate parallel to the direction of maximum principal stress. As the fluid power law index decreases, the fluid viscosity is significantly reduced due to the effect of shear thinning, causing the asymmetrically originated fractures to propagate parallel to the direction of maximum principal stress. The fluid leak-off coefficient only influences the fracture length and width, whereas its influence on the propagation trajectory can be slight.

Analysis of mechanical properties of rice particle beds in confined compression tests and stress transfer predictive model

High-stress breakage during storage and processing is one of the primary factors in the decline of rice quality. To reveal the mechanical response law under this condition, confined compression and numerical simulations were used to analyze the mechanical evolution laws of rice particle beds under high-pressure in this work. The results show that the compaction process progresses from top to bottom. The total number of contact forces in the particle beds increases, the strong contact forces exhibit a diffusion in direction. The maximum principal stress is consistent with the direction of the strong force chain, particle breakage direction corresponds to the maximum principal stress direction. Finally, based on the analysis of the particle beds, a uniaxial confined compression axial stress transfer model for rice particle beds is proposed. The results are helpful for understanding the stress transfer characteristics of rice particle beds, provide basis for the improvement of related equipment.

Modeling the Hydraulic Fracturing Processes in Shale Formations Using a Meshless Method

Complex bedding properties and in situ stress conditions of shale formation lead to complex hydraulic fracturing morphologies. However, due to the limitations of traditional numerical methods, the simulation of hydraulic fracturing in shale formation still needs further development. Based on this, the liquid–solid interaction modes and the SPH governing equations considering liquid–solid interaction force have been introduced. The smoothing kernel function in the traditional SPH method is improved by introducing the fracture mark ξ, which can realize the simulation of rock hydraulic fracturing processes. The stress boundary of the SPH method is applied by stress mapping of “stress particles”, and the feasibility and correctness of the method are verified by two numerical examples. Then, the simulation of hydraulic fracturing processes of bedding shale formations are carried out. With the increase of horizontal stress ratio, the total number of damaged particles decreases, but the initiation and extension pressure increase gradually. The initiation stress of small bedding dip angles (θ < 45°) is larger than that of big bedding dip angles (θ > 45°). The hydraulic fracture propagation range at low horizontal stress ratio is wider and the fracture is along the direction of maximum principal stress, while the hydraulic fracture propagation range at high horizontal stress ratio is limited to the perforation. The hydraulic fracture will propagate through the bedding with small dip angles. However, when the bedding dip angle is larger, the hydraulic fracture will propagate along the bedding direction.

Propagation and complex morphology of hydraulic fractures in lamellar shales based on finite-discrete element modeling

We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses λ=σ3/σ1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda ={\\sigma }_{3}/{\\sigma }_{1}$$\\end{document} and relative inclination of the bedding. When the lateral pressure coefficient (λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda$$\\end{document}) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for λ≥0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\ge 0.67$$\\end{document}, shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress (σ3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\sigma }_{3}$$\\end{document}) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations (β=60∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta =60^\\circ$$\\end{document}), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

Numerical investigation of mode I fracture toughness anisotropy of deeply textured shale

The fracturing behavior of shale is controlled by the complex interaction between texture plane and rock matrix. This study develops a numerical model of semicircular bending of textured shale, integrating considerations of texture inclination and strength using the finite discrete element method, to examine the impact of shale anisotropy on damage patterns and fracture toughness. The results disclose the following insights: (1) Mode I fracture toughness anisotropy is significant, with fracture toughness showing a negative correlation with texture inclination and a positive correlation with texture strength. (2) Typical failure modes of SCB specimens encompass tensile failure across textured planes, tensile-shear mixed failure, tensile failure along texture planes, and tensile failure along the matrix. (3) For inclination angles ranging from 0° to 90°, cracks tend to deflect along the texture, leading to a fracture surface with high roughness. The fracture surface roughness was higher along the direction of maximum principal stress for the 0° and 90° specimens compared to the Divider specimens, while it was lower along the thickness direction compared to the Divider specimens. (4) The toughening mechanism of textured shales is categorized into three classes: separation along the texture, fracture path deflection, and texture cracking. Texture strength mainly influences texture cracking, while texture inclination primarily affects the deflection of crack paths and separation along the texture. (5) The criterion for predicting modified fracture toughness accurately predicts the anisotropic fracture toughness of shale compared to the conventional criterion for predicting fracture toughness.

Numerical modeling of fracture propagation of supercritical CO2 compound fracturing

The exploitation of shale gas is promising due to depletion of the conventional energy and intensification of the greenhouse effect. In this paper, we proposed a heat-fluid-solid coupling damage model of supercritical CO2 (SC-CO2) compound fracturing which is expected to be an efficient and environmentally friendly way to develop shale gas. The coupling model is solved by the finite element method, and the results are in good agreement with the analytical solutions and fracturing experiments. Based on this model, the fracture propagation characteristics at the two stages of compound fracturing are studied and the influence of pressurization rate, in situ stress, bedding angle, and other factors are considered. The results show that at the SC-CO2 fracturing stage, a lower pressurization rate is conducive to formation of the branches around main fractures, while a higher pressurization rate inhibits formation of the branches around main fractures and promotes formation of the main fractures. Both bedding and in situ stress play a dominant role in the fracture propagation. When the in situ stress ratio (σx/σy) is 1, the presence of bedding can reduce the initiation pressure and failure pressure. Nevertheless, it will cause the fracture to propagate along the bedding direction, reducing the fracture complexity. In rocks without bedding, hydraulic fracturing has the lengthening and widening effects for SC-CO2 induced fracture. In shale, fractures induced at the hydraulic fracturing stage are more likely to be dominated by in situ stresses and have a shorter reorientation radius. Therefore, fracture branches propagating along the maximum principal stress direction may be generated around the main fractures induced by SC-CO2 at the hydraulic fracturing stage. When the branches converge with the main fractures, fracture zones are easily formed, and thus the fracture complexity and damage area can be significantly increased. The results are instructive for the design and application of SC-CO2 compound fracturing.

The Distribution Law of Ground Stress Field in Yingcheng Coal Mine Based on Rhino Surface Modeling

The distribution law of the ground stress field is of great significance in guiding the design of coal mine roadway alignment, determining the parameters of roadway support, and preventing and controlling the impact of ground pressure in coal mines. A geostress inversion method combining Rhino surface modeling and FLAC3D 6.0 numerical simulation software is proposed. Based on the geological data of the coal mine and the results of on-site measurements, a three-dimensional geological model of Yingcheng Coal Mine is established for the geostress inversion, and the distribution law of the geostress field in Yingcheng Coal Mine is obtained. Research shows the following: (1) The horizontal maximum principal stress values of the Yingcheng Mine are between 33.9 and 35.3 MPa, the horizontal minimum principal stress values are between 23.6 and 25.4 MPa, and the direction of the horizontal maximum principal stress is roughly in the southwest to west direction; (2) the three-way principal stress magnitude relationship is σH > σv > σh, indicating that the horizontal stress dominates in the study area, which belongs to the slip-type stress state; (3) The maximum principal stress of No. 3 coal seam is 33.1–34.8 MPa, the middle principal stress is 27.5–29.2 MPa, and the minimum principal stress is 17.3–22.9 MPa. Due to the influence of topography and burial depth, there is a phenomenon of stress concentration in some areas. By comparing the inversion values with the measured values, the accuracy of the geostress inversion is high, and the initial geostress inversion method based on Rhino surface modeling accurately inverts the geostress distribution pattern of the Yingcheng coal mine.

An anisotropic topology optimization procedure for continuous fiber reinforced polymer structures with biaxial fiber layout for improved intersection point design

Fiber placement technologies allow for manufacturing of complex-shaped composite parts with load-adapted fiber orientation. For design of corresponding structures, topology optimization is well-established. However, anisotropic topology optimization approaches are often limited to unidirectional fiber orientations. In truss-like topology optimized structures, the intersections between the individual trusses are subject to multiaxial stress states. For those, a unidirectional fiber orientation is not appropriate and may not be suitable for manufacturing.In this work we propose a topology optimization procedure that allows for unidirectional but also biaxial fiber orientations. The choice between unidirectional and biaxial fiber layout is based on the stress state within the finite elements. For the biaxial fiber layout, the first fiber angle is aligned with the maximum absolute principal stress direction, while the second fiber orientation is calculated using composite net theory.Results show that considering biaxial fiber orientations leads to fiber orientations that can be better manufactured by fiber placement technologies and are considered suitable for automatic derivation of manufacturing-tolerant placement paths. Furthermore, the stress state at intersections is improved reducing the probability of inter fiber failure. In addition, consideration of biaxial fiber orientations could increase the stiffness at constant weight compared to purely unidirectional topology optimization.

Deformation Behavior and Seismic Characteristics of Sandy Facies Opalinus Clay During Triaxial Deformation Under Dry and Wet Conditions

Unconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave velocity evolution. Dry and wet (≈ 8 and > 95% initial water saturation, respectively) samples with 12.6 ± 0.4 vol% porosity were deformed parallel and perpendicular to the bedding direction at a constant strain rate of 5 × 10–6 s−1. Dry samples revealed semi-brittle behavior and exhibited strain localization at failure, while deformation was more ductile at saturated conditions, promoting stable, slow faulting. Peak strength, Young’s modulus, and number of cumulative acoustic emissions decreased significantly for wet samples compared to dry samples; the opposite was observed for Poisson’s ratio. P-wave velocity anisotropy was significantly altered by differential stress, primarily due to the interplay between pore and fracture closure and stress-induced microcrack formation. For samples that were deformed perpendicular to bedding, we observed a reduction and reversal of P-wave velocity anisotropy with increasing differential stress, whereas anisotropy of parallel samples increased. The results suggest that water saturation reduces the pressure at the brittle-ductile transition and that the elastic properties and anisotropy of sandy facies Opalinus Clay can be significantly altered in an anisotropic stress field, e.g., adjacent to fault zones or tunnel excavations. Changes in elastic anisotropy are primarily controlled by the orientation between the pre-existing microfabric and the maximum principal stress direction, stress magnitude, and the degree of water saturation.

Effect of pH on primary and secondary crack propagation in sandstone under constant stress (creep) loading

Time-dependent, chemically assisted crack propagation behavior in rocks is fundamental to understanding the long-term stability of civil engineering structures. In this study, we investigated the propagation of primary and secondary cracks in macrofractured sandstone in distilled water (pH = 5.6) and in an aqueous solution of hydrochloric acid (HClaq, pH = 1.82) using digital image correlation (DIC), comparing the results to those from experiments performed in air. Results show that for the macrofracture angles γ = 0 and 90°, the nucleation position r of the primary crack is not affected by pH, and always occurs at the fracture tip (r = 0) and fracture center (r = 1), respectively. For γ = 30, 45, and 60°, r increases quasi-linearly with the increase of γ in semi-log plots, and an increase in pH moves the primary crack away from the fracture tip. For a given γ, an increase in pH produces an increase in the kink angle k1 of the secondary crack. The influence of pH on secondary crack propagation is most pronounced at high γ values. As the pH increases, the location of the secondary cracks gradually turns counterclockwise with a decreasing swing range, from 37° at pH 1.82–35° at pH 5.6. The secondary crack is inclined at a maximum of 30° and 32° to the maximum principal stress direction at pH 1.82 and pH 5.6, respectively. Right-lateral shear secondary cracks with well-developed tensile tail fractures are more prevalent in higher pH environments. The comparison with dry experiments indicates that distilled water is more aggressive than HClaq. The significant loss of calcium ions in distilled water and the reduction of large pore volume fraction in HClaq are considered the main reasons for the deterioration of the pH effect.

Research on Wellbore Stability in Deepwater Hydrate-Bearing Formations during Drilling

Marine gas hydrate formations are characterized by considerable water depth, shallow subsea burial, loose strata, and low formation temperatures. Drilling in such formations is highly susceptible to hydrate dissociation, leading to gas invasion, wellbore instability, reservoir subsidence, and sand production, posing significant safety challenges. While previous studies have extensively explored multiphase flow dynamics between the formation and the wellbore during conventional oil and gas drilling, a clear understanding of wellbore stability under the unique conditions of gas hydrate formation drilling remains elusive. Considering the effect of gas hydrate decomposition on formation and reservoir frame deformation, a multi-field coupled mathematical model of seepage, heat transfer, phase transformation, and deformation of near-wellbore gas hydrate formation during drilling is established in this paper. Based on the well logging data of gas hydrate formation at SH2 station in the Shenhu Sea area, the finite element method is used to simulate the drilling conditions of 0.1 MPa differential pressure underbalance drilling with a borehole opening for 36 h. The study results demonstrate a significant tendency for wellbore instability during the drilling process in natural gas hydrate formations, largely due to the decomposition of hydrates. Failure along the minimum principal stress direction in the wellbore wall begins to manifest at around 24.55 h. This is accompanied by an increased displacement velocity of the wellbore wall towards the well axis in the maximum principal stress direction. By 28.07 h, plastic failure is observed around the entire circumference of the well, leading to wellbore collapse at 34.57 h. Throughout this process, the hydrate decomposition extends approximately 0.55 m, predominantly driven by temperature propagation. When hydrate decomposition is taken into account, the maximum equivalent plastic strain in the wellbore wall is found to increase by a factor of 2.1 compared to scenarios where it is not considered. These findings provide crucial insights for enhancing the safety of drilling operations in hydrate-bearing formations.

Numerical analysis on dynamic response and damage threshold characterization of deep rock mass under blasting excavation

The excessive destruction of surrounding rock in deep tunnel will change the original environmental state and destroy the natural ecological balance. Research on the dynamic response characteristics and damage thresholds of rock masses in deep environments plays a crucial role in determining the excavation range of blasted rock and establishing safety construction scheme. This study employs numerical simulation techniques to investigate the dynamic response characteristics of surrounding rock under different ground stress conditions. By introducing the dynamic ultimate tensile strength criterion, critical fracture stress threshold, and maximum damage radius of rock under coupled dynamic-static loading conditions are determined. The research shows that under uniaxial ground stress condition, increasing ground stress inhibits damage to the surrounding rock and the extension of cracks in the excavation area, while imposing restrictions on the attenuation rate of explosive stress. Under bidirectional equal ground stress condition, an increase in lateral pressure coefficient inhibits the development of damage zones along the excavation contour, yet enhances the extension of cracks in the maximum principal stress direction. Moreover, when lateral pressure coefficient becomes excessively large, the attenuation rate of explosive stress significantly increases. Based on the threshold values of peak particle velocity (PPV), the functional relationship is established to predict safety criteria for deep blasting excavation.

Experimental study on the significance of pressure relief effect and crack extension law under uniaxial compression of rock-like materials containing drill holes

The drilling pressure relief technology is an effective way to reduce the accumulation of elastic energy in the tunnel envelope, which can reduce the risk of regional ground pressure occurrence. However, there is a lack of theoretical guidance on which drilling parameter has the greatest degree of influence on the effectiveness of pressure relief. The uniaxial compression tests were conducted to study the relationships between drilling parameters (the diameter, depth, and spacing) and the mechanical properties and deformation modulus of specimens. The results show that: (1) The drilling diameter (DDR) and drilling depth (DDH) of single-hole specimens negatively correlate with the peak-failure strength and deformation modulus, while the drilling spacing (DS) of double-hole specimens positively correlates with the peak-failure strength and deformation modulus. It shows that the borehole diameter has a more significant effect on the decompression effect. (2) With the help of the Grey Relational Analysis, the factors affecting the peak-failure strength and deformation modulus of the drilled specimens were ranked in significance. From the largest to the smallest, they are DDR, followed by DDH and DS. (3) The role of the pressure relief mechanism is to transfer the high stress in the shallow part of the roadway to the deep part, reduce the peak strength of destruction and deformation modulus of the peripheral rock in the drilled section, so that the characteristics of the mechanical behavior of the rock are significantly weakened, and the range of the area of the drilled hole decompression is enlarged. During the loading of the borehole, the borehole stress field dominates in the early stage, and cracking starts near the borehole along the direction perpendicular to the direction of maximum principal stress (horizontal direction). In the later stage, the maximum principal stress field dominates and vertical cracks with large widths appear. During crack expansion, the plastic energy dissipation effect is enhanced and the deep impact conduction path is weakened, thus protecting the roadway. This study determined the significance of the pressure relief effect of different drilling parameters, which can guide reasonable modifications of drilling parameters in the field.

Theoretical Analysis of the Active Earth Pressure on Inclined Retaining Walls

The estimation of earth pressure is crucial in the design of retaining structures. The evaluation of vertical retaining walls has been well studied within the framework of the differential flat element method in prior investigations, in which the vertical stress and maximum principal stress are assumed to be uniformly distributed. Inclined retaining walls have been successfully adopted in excavation engineering. Due to the inclination of retaining walls, the maximum principal stress direction rotates approximately parallel to the inclined wall back, which affects the active earth pressure on the walls. This paper provides an analytical solution to evaluate the active earth pressure on inclined retaining walls. A numerical model is first established to analyze the characteristics of the principal stresses and vertical stress distribution of soil behind walls with various inclination angles. An idealized vertical stress field containing two zones is developed, and a hyperbolic function is proposed to illustrate the distribution of vertical stress at various depths. Subsequently, the relationship between the nonuniform characteristics of the vertical stress and normal stress acting on a differential flat element is established based on a circular stress trajectory. The active earth pressure along the inclined wall is then obtained based on the balance of the forces on the differential elements. The predicted data from the proposed analytical solution are compared with the previous experimental, numerical, and theoretical results with excellent agreement, demonstrating the accuracy of the proposed method.

Evolution of tensile fractures in feldspar porphyroclast and its implication in paleostress estimation

Fractures are ubiquitous brittle features of the upper crust. Syntectonic granites are often replete with fractures of various attitudes, cross-cutting each other, which develop under the prevalent states of stress. In this article, we study the origin and mechanism of formation of tensile fractures restricted within rectangular alkali feldspar porphyroclasts found in Closepet granite (Dharwar Craton, India), and venture on the possibility of using them as potential paleostress marker. The study is conducted using finite-element-method (FEM) to assess the distribution of induced stresses within the porphyroclasts considering the geometry and varying mechanical properties of the porphyroclast and the embedding matrix. We infer that fractures in the porphyroclast develop due to amplification and concentration of far-field stresses within it. The direction of maximum principal compressive stress as indicated by the fractured porphyroclasts lies in close proximity to the paleostress direction derived from fault-slip analysis of adjacent bodies such as Chitradurga Schist Belt and Koppal syenite (Dharwar craton). Therefore, we advocate that fractured feldspar porphyroclasts are a reliable indicator of paleostress orientations.

Implications of multi-stage deformation on the differential preservation of Lower Paleozoic shale gas in tectonically complex regions: New structural and kinematic constraints from the Upper Yangtze Platform, South China

The strength of tectonic deformation in tectonically complex regions, particularly multi-stage superimposed tectonic regions, can have a significant impact on shale gas preservation conditions. However, differential tectonic deformation has still not been quantitatively constrained, and its impact on the preservation conditions of shale gas is thus poorly understood in the northern Guizhou Province, southwest China. This study investigates the differential preservation conditions of marine shale gas in four eroded synclines (including Shixi, Daozhen, Zhongguan, and Fuyan Synclines) of the northern Guizhou Province. Interpretation of structures in field outcrops, geometry and kinematic analysis, and typical gas reservoir anatomy are combined to quantitatively explore the effect of superimposed deformation on the preservation conditions of shale gas. According to a set of new structural and kinematic constraints, the Riedel shear model for strike-slip faults is firstly conducted in the eroded synclines in the northern Guizhou Province, which innovatively established a linkage between detailed structural analysis and evaluation of shale-gas preservation conditions. We demonstrate that superimposing of deformations plays a role in differential accumulation in eroded synclines. The results indicate that (1) fault strike in the Shixi Syncline is more concentrated than Zhongguan and Fuyan Synclines, whereas fault strikes in the Zhongguan and Fuyan Synclines are more scattered. There is only one direction of maximum principal stress (σ1) in the Shixi and Daozhen Synclines. There are two maximum principal stress directions (σ1) in the Zhongguan and Fuyan Synclines. The Zhongguan and the Fuyan Synclines have tensional environments, whereas the Shixi and the Daozhen Syncline have compressional settings; (2) the formation dip angle, distance to a class II fracture, average detachment layer thickness, intensity of superimposed modifications, and interlimb angle may be positively correlated with gas content; (3) the progressive transformation pattern is established. These findings lay the foundation for the future research on the differential preservation mechanism of eroded synclines and may be useful for enhancing research on worldwide strike-slip faults controlling hydrocarbon accumulation.

Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel

To explore the influence of lithology on the failure behavior of layered tunnel, true triaxial compression experiments were undertaken on cubical phyllite and yellow sandstone samples containing a “D” shaped hole. The failure progress of the hole sidewalls was monitored and captured using a miniature camera. The results reveal that the initial vertical failure stress of the phyllite samples presents a “U” shaped change as the bedding angle increases. At the bedding angle of 45°, compared with the initial vertical failure stress of the yellow sandstone tunnel, that of the phyllite tunnel is lower, resulting in larger rock fragments and deeper V-shaped grooves. The failure pattern of the phyllite tunnel is primarily manifested as extensive shear sliding failure, and the failure is more severe. The failure of the yellow sandstone tunnel is primarily characterized by the sequential laminar fracturing along the maximum principal stress direction, predominantly manifesting as tensile failure. The primary factors influencing the failure of two types of layered rocks are the significant variations in the clay mineral content within the rocks. For the phyllite, it contains nearly one-third of montmorillonite (a clay mineral). This results in the formation of weak bedding planes within surrounding rocks, which induces shear slip failures along these bedding planes. In contrast, the yellow sandstone has a lower clay mineral content, leading to the absence of distinct weak bedding planes within the surrounding rock. In this case, bedding planes present ignorable effect on the surrounding rock.

Joint Studies In The Western Part Of Ikom-Mamfe Basin, Lower Benue Trough, Nigeria

The Ikom-Mamfe basin, also called the Mamfe embayment, is a bifurcation of the Benue trough in southeastern Nigeria and southwestern Cameroun. The basin is about 130km long, 60km wide basin and is filled with about 4km thick sedimentary rocks. Deformational events led to gentle folding of rocks in the basin and emplacement of magmatic rocks. Systematic and detailed field studies were carried out in eight (8) locations within the study area, in which the orientations of 1437 joints and 102 veins were acquired. Two sets of orthogonal joints observed in the study exhibited symmetry with respect to the directions of the fold axes and were dominated by high-angle dips averaging about 80o. This implies a syntectonic origin of these structures also during their propagation, the maximum principal stress directions (σ1) were orientated in two different directions NW- SE and NNE-SSW for DI and D2 respectively. The relative strength of deformation during DI was greater than D2 and it was also unfolded that the ‘ac’ extension joints trending NW- SE and NNE-SSW hosted relatively thicker veins in both DI and D2. These joints served as better conduits for mineralizing fluids and also most suitable directions for future prospecting.

Characteristics and control factors of tectonic fractures of ultra-deep tight sandstone: Case study of the Lower Cretaceous reservoir in Bozi-Dabei area, Kuqa Depression, Tarim Basin, China

The Bozi-Dabei area in the Kuqa Depression host high-quality reservoirs in the Bashijiqike Formation and Baxigai Formation sandstones of the Lower Cretaceous, in which reservoirs yield significant industrial gas flow despite being situated at a considerable burial depth of 8200 m. The geological history of the target formation involves multiple phases of tectonic movements, resulting in the development of multi-genetic fractures that enhance the reservoir's storage and seepage capacity. Based on the results of the drilling core, field profile survey, imaging logging, and experimental analysis, this study presents an analysis of fractures in the Lower Cretaceous dense sandstone reservoir of the Bozi-Dabei area, andclarifies the characteristics and controlling factors of the multi-genesis and multi-period fractures. Additionally, it proposes an effective fracture development model that accounts for geo-stress control. In the Bozi-Dabei area, the prevailing high extrusion stress environment has led to the development of predominantly regional tectonic fractures and fault-related fractures, with relatively gentle deformation-related fractures. The results of a combination of multi-attribute data determination techniques, including fracture filling, inter-cutting relationship, fracture filling isotope, inclusions, and cathode luminescence tests, this study reveals that the reservoir fractures have experienced three major periods of tectonic movement. The regional tectonic fracture development is mainly controlled by stratigraphic lithology and thickness, while the proximity influences fault co-derived fractures to the fault and the relative positions of the upper and lower plates of the fault. The shift in the direction of the late horizontal maximum principal stress leads to the opening or closing of early fractures under different conditions in the Bozi-Dabei area, consequently affecting the degree of fracture opening and effectiveness. Notably, when the horizontal maximum principal stress is deflected to intersect with early fractures at a smaller angle or even superimpose, the fracture effectiveness of the related group system in the deflection direction improves, resulting in an overall coordination. The distribution characteristics of the fracture system in this highly productive reservoir are the result of dominant configurations from multi-phases of geological activities.

Stress distribution and failure characteristics around U-shaped caverns with different height-to-width ratios under biaxial compression

Understanding the stress distribution and failure characteristics around a U-shaped cavern is vital to the stability analysis of rock structures such as tunnels and roadways. In this study, the stress distribution around a U-shaped cavern with different height-to-width (h/w) ratios under biaxial compression was first analyzed based on the complex variable theory. Then, numerical simulations were conducted using PFC2D to study the failure process and energy evolution around a U-shaped cavern with different h/w ratios. Theoretical solution shows that the maximum stress concentration factor occurs at the bottom corner of the U-shaped cavern. With the h/w ratio increasing from 0.75 to 3.0, the stress concentration factor around the sidewall shows a declining trend, while the opposite trend is observed in the vault and floor. Numerical results show that when the maximum principal stress is parallel to the vertical direction, the thin slab buckling occurs in the sidewall of the cavern with low h/w ratios (h/w = 0.75 and 1.0), accompanied by the release of relatively high kinetic energy. Instead, the wedged-shaped slab spalling occurs in the sidewall of the cavern with high h/w ratios (h/w = 1.5, 2.0 and 3.0), which can be considered as a static process. Although the U-shaped cavern with low h/w ratios has higher integrity after failure, more stain energy is accumulated in the remaining surrounding rock, which is likely to cause rockbursts under a higher stress state or dynamic disturbance. When the maximum principal stress is horizontal, the rock slab separates from the cavern floor and presents a reverse trend with the increasing h/w ratio. The above results indicate that the h/w ratio and the direction of maximum principal stress should be considered comprehensively in the design of the support system of U-shaped caverns.