Weak Planes Research Articles

Development of simplified analytical model to predict the roll separating force in bar rolling

ABSTRACT Prediction of roll separating force (RSF) is necessary to design new rolling mills, optimise the number of passes and for online control of dimensional accuracy. The intricacy of deformation and complex mixed boundary conditions of velocity and friction makes exact analytical prediction of RSF in a bar or shape rolling quite difficult. Hence, the formula for the prediction of RSF while bar rolling is developed in this research using sticking and slipping friction, assuming the process is equivalent to compression of the simple rectangular bar between two flat plates. The concept of the weak plane strain is also incorporated in the proposed formula. Sinel’nikove’s formula derives the contact area between the roll and material. Considering the direct significance of the contact area on the value of RSF, the factors in the said formula are corrected based on a graphical method. Shida’s formula is used to calculate the flow stress. The proposed formula is compared with more than fifty distinct experimental findings found in the literature. It is found that the formula is in good agreement with majority of the experimental data. The derived simplified analytical formula can be successfully applied to most types of passes in such industries.

Maximum Penetration Height and Intrusion Speed of Weak Symmetric Plane Fountains in Linearly Stratified Fluids

The flow behavior of weak symmetric plane fountains in linearly stratified fluids is studied numerically with three-dimensional simulations over a range of the Froude (Fr), Reynolds (Re), and stratification numbers (s). The two main parameters describing the fountain characterization are the dimensionless maximum fountain penetration height (zm) and intrusion velocity (uint), which differ significantly at different flow development stages. It was found that the stratification stabilizes the symmetry of the weak fountains, which makes the fountain become asymmetric at a larger Fr value, and zm at the fully developed stage continues to increase as a result of the intrusion, which continually changes the ambient fluid stratification features, thus the buoyant force. The evolution of intrusion experiences three distinct stages. Both Fr and s have effects on zm and uint, with the effect of Fr usually larger than that of s. The overall impacts of Fr and s can be quantified in terms of Frasb, with a and b varying for different parameters. With numerical results, empirical correlations are produced in terms of Frasb for each relevant parameter, which generally predict the results very well.

The primary influence of shear band evolution on the slope bearing capacity

Slope bearing capacity is one of the most important characteristics in slope engineering and is strongly influenced by weak planes, loading conditions, and slope geometry. By presenting the evolution of slip surfaces, this paper explored how the slope bearing capacity is affected by widely observed influencing factors. The initiation and propagation of slip surfaces are presented in laboratory model tests of slope using the transparent soil technique. Shear band evolution under various weak planes, loading conditions, and slope geometries were experimentally presented, and slope bearing capacities were analyzed with the process of shear band evolution. This paper verified that slip surface morphologies have a strong relation with the slope bearing capacity. The same slip surface morphology can have different evolutionary processes. In this case, it is the shear band evolution that determines the slope bearing capacity, not the morphology of the slip surface. The influencing factors such as pre-existing weak planes, loading conditions, and slope geometry strongly affect the slope bearing capacity as these factors govern the process of shear band evolution inside the slope.

The risk of fluid-injection-induced fault reactivation in carbonate reservoirs: an investigation of a geothermal system in the Ruhr region (Germany)

In this study, we investigate the potential for fluid-injection-induced fault reactivation and induced seismicity risk during simultaneous injection-extraction operation in a theoretical geothermal doublet system in a carbonate reservoir in the Ruhr region. Using a coupled three-dimensional thermo-hydro-mechanical approach, we investigate the probability of injection-induced fault rupture. We perform a sensitivity study assuming variability of the fault and matrix permeability, injection/production flow rates, well placement options, rock thermal properties, and evaluate the influence of thermally induced stresses. The ruptured fault areas were calculated based on a Coulomb friction law and a notion that the shear slip is controlled by the ratio of shear to effective normal stresses acting on a pre-existing plane of weakness in the in situ stress field configuration. Ruptured fault areas in the intrinsically not critically-stressed environment, using location-specific empirical correlations, were used to compute local moment magnitudes of potential earthquakes. Based on this study, we conclude that, in the long-term, thermally-induced stresses play a dominant role during fault reactivation and greatly increase the likelihood for induced seismicity. We, therefore, propose that a minimum safe distance between an injection well and a fault should be based primarily on the radius of a thermal plume generated during the expected lifetime of a geothermal system. Results from this study provide valuable insights for the development of future deep geothermal systems in the Ruhr region and other geothermal reservoirs worldwide.

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

AbstractGeophysical logs collected from a deep borehole drilled into the North American Craton in Northeastern Alberta shed valuable light on the state of stress in stable cratons. Observed breakout (BO) azimuths rotate between three depth intervals, ranging from 100° at 1,650–2,000 m to 173° at 2,000–2,210 m, and finally to 145° at the bottom. No apparent fractures that might have disturbed the stress field were found. Two potential causes of these rotating BOs can be either a heterogeneous stress field or elastic and strength anisotropy of the formation. The latter interpretation is favored since observed BO azimuths are strongly controlled by rock metamorphic textures, as validated by their close correlations with dip directions of foliation planes and polarization directions from dipole sonic logs. Monte Carlo realizations further demonstrate that anisotropic metamorphic rocks subjected to a uniform horizontal stress direction could result in the observed azimuth‐rotating BOs. Stress magnitudes inferred from this analysis, which incorporates rock anisotropy and weak foliation failure planes, suggest a normal faulting regime and a maximum horizontal compressive direction consistent with that in the overlying Western Canadian Sedimentary Basin (NE‐SW) and the motion of the North American plate. The inferred stress magnitudes are low and Mohr‐Coulomb analyses demonstrate that the formation is not near the critical condition for slip on weak planes. However, more detailed investigations should be conducted since Monte Carlo calculations indicate that analyses from BO widths, particularly when a conventional Kirsch‐based formula is employed, are highly nonunique, allowing for large variations in potential stress states.

Research on disaster mechanism and correlation of natural earthquake and coal and gas outburst

In recent years, with the increasing of mining depth and mining intensity, coal and gas outburst has become one of the most destructive and harmful dynamic disasters in coal mines. Natural earthquakes are the most destructive natural disasters in the earth’s crust. Both of coal and gas outbursts and natural earthquakes are caused by geo-dynamic processes. In order to research the correlation between coal and gas outburst and natural earthquake, in this manuscript, we took Pingdingshan eastern mining area as the research object. The principles of disaster prevention, seismology, statistics and geophysics were taken as the research basis, both the characteristics and laws of natural earthquakes, and the occurrence laws and characteristics of coal and gas outburst were systematically analyzed by geo-dynamic division method. At the same time, the relationship between the two disasters were established. The research results show that natural earthquake is the accumulation and release process of elastic properties in rock mass, which can induce the abnormal emission of gas in coal mines under specific conditions. Especially the “weak plane structure” in geological structure, which is more likely to lead to the occurrence of coal and gas outburst accidents. Tectonic activities and stress field changes have a unified mechanism for the occurrence of natural earthquake and coal and gas outburst, there are correlations between space and intensity. The research results can also provide new ideas for the prediction work of these two kinds of disasters.

Using legacy 3D seismic data and source parameters of mining‐induced earthquakes to mitigate the risk of rockbursting in Kloof Gold Mine, South Africa

AbstractRockbursts pose a significant risk in deep gold mines in the Witwatersrand Basin of South Africa as they may damage the excavation, injure workers, and delay production. We analysed the source mechanisms of 75 large mining‐related seismic events (ML 1.5–2.7) that caused damage to stopes in Kloof Gold Mine and used legacy 3D reflection seismic data to delineate the ore body and geological structures that may be correlated with mining‐related seismic events. The 75 seismic events took place at depths of 1600–4200 m below surface. Most events were located close to stopes mining the Ventersdorp Contact Reef ore body. The S‐to‐P‐wave energy ratio (Es/Ep) from the analysed seismic events ranged from 1.1 to 19.9. The source mechanisms of mining‐induced seismic events aid in understanding the dominant modes of failure. Planes of weakness may be the result of mining‐induced stresses or pre‐existing geological structures such as faults and dykes. The Es/Ep ratio, focal mechanism and moment tensor solutions were correlated to the underground mapped and seismically defined geological structures. Approximately 44% of the events showed strong correlation with the known underground mapped and seismically imaged geological structures (faults and dykes), whereas 56% of the events were related to elements of the mining geometry (dip pillars, abutments and remnants). This information enables mining layouts to be modified to minimize the risk of rockbursting.

Pre-stressed anchor cable force evolution laws of plane sliding bedding slopes

This paper aims to understand how anchor cable force evolution laws can improve reinforcement designs. The self-developed weak plane sustained deterioration technology was conducted in the indoor test on the reinforced bedding rock slope containing a linear type weak plane. The force evolution laws of anchor cables with the weak plane sustained deterioration was studied. Furthermore, the theoretical equations were derived on the basis of the rigid assumption of the rockmass and the limit equilibrium method, and the equations have been verified to be reliable through comparison with the test results. The numerical simulations in different rockmass mechanical parameters reveal that the theoretical equations are suitable for hard rockmass. The results indicate that, with the weak plane sustained deterioration, the anchor cable axial forces increase gradually, and the axial force increase is inversely proportional to the length of the free sections. The findings could provide the theoretical guideline for the pre-stress safety reserves during the reinforcement designs of anchor cables in different positions of bedding rock slopes.

Landslide Susceptibility mapping using random forest and extreme gradient boosting: A case study of Fengjie, Chongqing

Landslide susceptibility analysis can provide theoretical support for landslide risk management. However, some susceptibility analyses are not sufficiently interpretable. Moreover, the accuracy of many research methods needs to be improved. Therefore, this study can supplement these deficiencies. This study aims to research the evaluation effects of random forest (RF) and extreme gradient boosting (XGBoost) classifier models on landslide susceptibility, and to compare their applicability in Fengjie County, Chongqing, a typical landslide‐prone area in southwest of China. Firstly, 1624 landslides information from 1980 to 2020 were obtained through field investigation, and a geospatial database of 16 conditional factors had been constructed. Secondly, non‐landslide points were selected to form a complete data set and RF and XGBoost models were established. Finally, the area under the ROC curve (AUC) value, accuracy, and F‐score were used to compare the two models. The results show that even though both classifiers have a highly accurate evaluation of landslide susceptibility, the RF model performs better. In comparison, the RF model has a higher AUC value of 0.866, and its accuracy and F‐score are approximately 2% higher than XGBoost. The land use, elevation, and lithology of Fengjie County contribute to the occurrence of landslides. This is due to human engineering activities (such as land reclamation, and housing construction) resulting in low slope stability and landslides in widely distributed sandstone, siltstone, and mudstone layers owing to their low permeability and planes of weakness.

Influences of matrix strength and weak planes on fracture response of recycled aggregate concrete

The fracture behaviours of concrete made with natural aggregate and recycled coarse aggregate (RCA) derived from crushing old concrete are compared. The performances are evaluated using concrete proportioned for different compressive strengths. The RCA from crushed old concrete produces a composite aggregate with mortar and aggregate phases. An examination of the RCA shows pre-existing cracks in the aggregate phase. Crack propagation and crack opening profiles in the fracture response of concrete beams are evaluated using the Digital Image Correlation (DIC) technique. The displacement profiles across the beam obtained using DIC are evaluated to understand the crack growth in the concrete. The cohesive stress response and an energy measure determined from the fracture test are related to physical observations of the fracture surface. The crack path in the concrete and the contribution of the different interfaces depend on the strength of the matrix surrounding the aggregate. In concrete with lower cementitious and higher water contents, the new mortar interface with RCA and the pre-existing cracks in the RCA contribute to the fracture surface in the RCA. While a larger fracture surface area is created in concrete made with RCA, the energy measure and the cohesive stress determined from the fracture test are lower. In concrete proportioned for higher compressive strength, there is a densification of the RCA-mortar interface and the fracture plane is produced through the aggregates. The pre-existing cracks in the RCA create weak planes, which contribute to the fracture surface created. Improving the mortar-RCA interface does not result in an improvement in tensile strength or fracture characteristics since there is also a significant contribution of the weak planes in the aggregate phase of RCA to the failure surface. The measured fracture surface area does not correlate with the energy measure from fracture test response.

Porosity, permeability and rock mechanics of Lower Silurian Longmaxi Formation deep shale under temperature-pressure coupling in the Sichuan Basin, SW China

To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and Yongchuan areas of the Sichuan Basin for porosity and permeability experiments and a triaxial compression and sound wave integration experiment at the maximum temperature and pressure of 120 °C and 70 MPa. The results show that the microscopic porosity and permeability change and the macroscopic rock deformation are mutually constrained, both showing the trend of steep and then gentle variation. At the maximum temperature and pressure, the porosity reduces by 34%–71%, and the permeability decreases by 85%–97%. With the rising temperature and pressure, deep shale undergoes plastic deformation in which organic pores and clay mineral pores are compressed and microfractures are closed, and elastic deformation in which brittle mineral pores and rock skeleton particles are compacted. Compared with previous experiments under high confining pressure and normal temperature, the experiment under high temperature and high pressure coupling reveals the effect of high temperature on stress sensitivity of porosity and permeability. High temperature can increase the plasticity of the rock, intensify the compression of pores due to high confining pressure, and induce thermal stress between the rock skeleton particles, allowing the reopening of shale bedding or the creation of new fractures along weak planes such as bedding, which inhibits the decrease of permeability with the increase of temperature and confining pressure. Compared with the triaxial mechanical experiment at normal temperature, the triaxial compression experiment at high temperature and high pressure demonstrates that the compressive strength and peak strain of deep shale increase significantly due to the coupling of temperature and pressure. The compressive strength is up to 435 MPa and the peak strain exceeds 2%, indicating that high temperature is not conducive to fracture initiation and expansion by increasing rock plasticity. Lithofacies and mineral composition have great impacts on the porosity, permeability and rock mechanics of deep shale. Shales with different lithologies are different in the difficulty and extent of brittle failure. The stress-strain characteristics of rocks under actual geological conditions are key support to the optimization of reservoir stimulation program.

Energy Evolution Characteristics of Water-Saturated and Dry Anisotropic Coal under True Triaxial Stresses

During deep underground coal mining, water-injection-related engineering methods are generally carried out to reduce the hazards of coal dynamic disasters. The energy evolution characteristics of coal can better describe the deformation and failure processes, as it is more consistent with the in situ behavior of underground mining-induced coal. In this study, experimental efforts have been paid to the energy evolution characteristics of water-saturated and dry anisotropic coal under true triaxial stresses. The effects of water saturation, intermediate stress, and anisotropic weak planes of coal on the true triaxial energy evolution were systematically evaluated. The results show that the overall energy is weakened due to the water adsorption for water-saturated coal samples. The water-weakening effect on the overall energy of water-saturated coal is more pronounced when perpendicular to the bedding plane direction than in the other two cleat directions. The accumulation elastic energy anisotropy index of dry and water-saturated coal samples is higher than 100.00%. Both accumulation and residual elastic energy of dry and water-saturated coal samples show an increasing-then-decreasing trend with intermediate stress increase. The results obtained in this study help understand the in situ behavior of coal during deep underground mining and control coal dynamic disasters.

Mechanical properties of selected granitoids along the Karora hydropower project, Besham, Northern Pakistan: Insights from petrography and joint analysis

The stability of tunnel site depends upon the deformation behaviors of the surrounding rocks, including joint density, mineral composition, micro cracks, and grain boundaries. The aim of this study was to analyze the mechanical properties of the Besham Complex in Northern Pakistan in terms of petrography and joint analysis. The Besham complex is prone to deformation as it is bounded by the Main Mantle Thrust (MMT) in the north, the Thakot fault (Tf) in the east, and the Swat block in the west. In the study area, the deformation was observed at the mesoscopic scale, while alteration in grains and fractures were determined at a microscopic scale. Detailed petrographic study of the representative samples from each station revealed that the area is dominantly composed of granite, quartz syenite, quartz monzonite, granodiorite, quartz-rich granitoid, monzogranite, alkali feldspar granite, and quartz monzodiorite. Extensive field work was performed to determine the joint volume (JV), in which the number of joints per cubic meter was recorded for each joint set. The rock quality designation (RQD) was calculated through JV analysis. An RQD value <25% suggested that most rocks are very poor quality, while samples with RQD values between 25% and 50% are poor quality. The petrographic analysis revealed that the dominant contents of the rock samples are feldspar (55.6%), quartz (27%), and mica (17.4%). Petrographic control over surface joint volume was established, suggesting that abundant feldspar, coarse grain, anhedral to subhedral grain shape, microcracks, and alteration of minerals provide planes of weakness for joints and thus reduce the RQD value. Therefore, some serious precautions and supporting measurements, such as shotcrete, rock bolts, metal mesh, and steel arch supports, are essential to avoid severe geo-hazards during further excavation.

Laboratory investigation on cryogenically induced fractures in shale with beddings

Hydraulic fracturing is essential for effective production of shale oil and gas reservoirs. Liquid nitrogen (LN2) fracturing is promising to enhance oil and gas production. However, the cryogenically fracturing mechanisms of LN2 in shale are still not clear, which are analyzed through combining the laboratory experiments and numerical simulations in this work. Laboratory fracturing shales using the cryogenic LN2 are carried out with the digital image correlation (DIC) recording the fracturing processes. Three typical modes of induced fractures in shale with beddings are observed, including the splitter (mode-S), arrester (mode-A) and divider (mode-D). The mechanisms and detailed fractures of the three modes are analyzed, which strongly depend on the weak plane and thermal conductivity anisotropy. A numerical model is developed to simulate the thermal-mechanical coupling, based on which the applicability of the tensile strength criteria (TSC) and fracture toughness criterion (FTC) are analyzed. It shows that TSC and FTC determine the lower and upper bounds of the induced fracture lengths. TSC is suitable to specify the lengths of mode-A fractures, while FTC is applicable to specify the lengths of mode-S and mode-D fractures. The developed model is used to simulate the branch fracture propagations in a shale reservoir in Triassic Yanchang Formation under the in-situ temperature and stress conditions. The results suggest that mode-D fractures are mostly induced by the cryogenic LN2, which can form a complex fracture network in shale. The findings on fracture lengths and modes can provide references for the shale fracturing treatments with LN2.

Mechanism Analysis of Floor Water Inrush Based on Criteria Importance though Intercrieria Correlation

Floor water inrush has always been an important problem affecting coal mine safety. The water inrush from the deep coal seam floor is the final manifestation of the water inrush channel formed by the expansion and penetration of rock mass cracks under the combined action of a high ground temperature, high ground stress, high karst water pressure and strong mining disturbance. The fault is a structurally weak plane developed in the complete rock layer, which can be regarded as a permeable channel with good permeability. In order to explore the location of confined water from the fault to rock stratum caused by the tensile fracture of a fault during floor water inrush, floor water inrush is regarded as the result of the joint action of water pressure and mining stress. The COMSOL Multiphysics numerical simulation software was used to simulate the advancing process of the working face of the coal seam with a fault structure. Different fault dip models were established for comparative analysis. The objective weight CRITIC based on data volatility was used to analyze the water pressure and principal stress near the fault. The results show that the uplift strength of highly confined water increases with the increase in the fault dip angle, and the weight of water pressure near the fault has a certain relationship with the location of the water body entering the rock stratum from the fault.

INVESTIGATING THE ANISOTROPY STRENGTH INDEX (ASI) FOR SOME EGYPTIAN ORNAMENTAL STONES

The nature of ornamental stones is anisotropic. The heterogeneous characteristics of the anisotropic rocks vary with direction. The highest to lowest strength ratio is known as the anisotropy strength index (ASI). A thorough investigation of the ASI is necessary to determine the best-directed loads for these rocks. On core specimens that have been bored parallel and perpendicular to the weakness planes, this is estimated using both uniaxial compression and point load testing. For this examination, four different rock types had cores that were drilled conventionally and in line with weakness planes. The research shows that drilling cores to weakness planes at a normal or nearly normal angle (90° to 60°) yields the best, most dependable ASI. According to the current study’s findings, the ASI fluctuates depending on how uniformly the mineral content and texture of rocks are. A suggested way to calculate the ASI and the load point strength is also included. This study reveals that the employment of ornamental stone as is (for example, precipitation position is more robust and reliable than that perpendicular form) is critical in determining the resilience of this type of rock and its spatial implementation (e.g. flooring).

A Case Study: Shear Interface Testing of a Constructed Geosynthetic Barrier

Since geosynthetic interfaces often serve as a weak plane on which sliding may occur, shear strengths of these interfaces need to be carefully assessed. In this case study, geosynthetic and soil samples were exhumed from the base and side slope of a constructed facility to generate site specific data. The importance of using representative materials and performing the shear interface testing under conditions similar to those expected in the field are discussed. Once the test data was available, they were used in a stability analysis software package to determine the Factor of Safety (FoS) of the barrier system. Lastly, to illustrate the risk of using literature shear strengths parameters without confirming them during construction, the FoS using actual shear strengths will be compared to the design FoS which used literature shear strengths.

CHIBOK RIFTO-MAGNETICS AND GEOLOGY

On sheet 134 of the aeromagnetic map of Nigeria is a prominent northeast (NE) trending negative anomaly that extends for >20 km. This work reinterprets the anomaly using oasis montage™ 7.0 software, by inverse geological modelling of magnetic profiles. The aim of this work is to identify the Chibok magnetic anomaly as a plate tectonic feature, and as part of the West African Rift System. Three profiles were modelled across the residual magnetic anomaly, which yielded a depth range of 1 to 2 km. The result furnished a subsurface picture of ‘horst and graben’ which are characteristic of rifts. The sedimentary infill of the interpreted rift is considered to be sands/or sandstone due to the low magnetic anomaly. Sandstone has very low magnetic susceptibility [30 x106 emu (S.I.)] compared to granite and basalt. Geological data showed NW, NE, N-S, and E-W structural trends on the granitic basement. These manifest as faults, shear zones, joints and foliations. Emplacement of the Tertiary basalt in the NW of the area of study area showed planes of crustal weakness in the Precambrian basement. A palinspatic map of the area identified the pre-Tertiary geology. The sediment infill of the interpreted rift may serve as very rich aquifer for groundwater abstraction for irrigation in this semi-arid zone. The evidence of the plate tectonic nature of the interpreted rift indicated its alignment with offshore transform faults like Chain and Charcot as well as the Cameroun Volcanic Line.

Instability Mechanism of Main Control Weak Structural Plane of Bedding Slope Under Explosive Stress Wave

Instability Mechanism of Main Control Weak Structural Plane of Bedding Slope Under Explosive Stress Wave

Fracture prediction of tight sandstone reservoirs using outcrops and log curve-based extremum method: A case study of the Chang 7 Member of the Yanchang Formation in Block X, Ordos Basin

The lack of seismic data and special log data on the Chang 7 Member of the Yanchang Formation in Block X of the Ordos Basin poses a major challenge to the study of the development characteristics and identification of fractures that greatly influence on-site production. To overcome this obstacle, this study determined the macroscopic characteristics of fractures in the study area based on field outcrop profiles and indoor core observations. The results are as follows: (1) Fractures in the study area are dominated by regional fractures in weakly deformed structural areas, with three sets of fractures mainly occurring. Core observations show that fractures in the study area have high dip angles, small extensions, and small openings; (2) The vertical extended lengths of these fractures are significantly controlled by rock mechanical layers; (3) Fractures show three types of extended morphologies when meeting weak planes. Vertically, there is an exponential correlation between the sand body thickness and the fracture linear density. The argillaceous shale thickness has a logarithmic relationship to the fracture linear density when it is < 1 m and has an exponential relationship to the latter otherwise. The planar distribution of fractures is significantly affected by layer thickness and rock mechanical properties. Given the stable occurrence, high dip angles, and small extensions of regional fractures, this study highlighted the abnormal increase or decrease in log curve values of fracture segments by calculating the extrema of log curves. The purpose is to identify the locations and quantity of fractures using conventional log curves in the case of limited drilling data. This log curve-based extremum method enjoys a high resolution. For both coring and non-coring wells in the study area, there is a positive correlation between their fracture linear density determined based on the production segment thickness of vertical wells and their daily liquid yield in the first week of well test and pilot production. This result indicates that the method for identifying natural fractures proposed in this study is highly reliable. Moreover, this study provides key parameter controls for predicting the three-dimensional distribution of fractures. The understanding of the fracture spatial distribution is also a key external factor that affects the design and construction of horizontal well fracturing.