Rock Strength Properties Research Articles

Влияние статической нагрузки от горнотранспортного оборудования на устойчивость уступов, расположенных в скальных породах

Most of the enterprises applying open-pit mining method at deposits face challenges of pit wall steepening when approaching the pit wall to the final position or reducing productivity. The main task when increasing the angle of pit slope is to determine its stability. One of the ways to steepen the open-pit wall is to increase the slope angle of benches composing the open-pit wall. The bench stability is primarily determined by its physical properties and structural uncertainty. The higher the strength properties of rocks, the steeper the slope angle formed in these rocks can be. The "Rules of stability..." provide for the possible accounting the loads from mining equipment when assessing the stability, but this possibility is provided for the calculation of bench stability of open-pits and cuts, formed by semi-hard and dispersed rocks, dumps formed from clay or semi-hard rocks and (or) from a mixture of clay and hard rocks. At the same time, it is of interest to assess the impact of static loading from the located mining transport equipment in a stationary state on the stability of the benches located in hard rocks. It is shown that the load from the track dozer scraping the berms has an impact of no more than 6-7% for a distance of about 15 m from the top crest of a 30-meter long bench with a slope angle of 80° to the edge of the track, which is generally insignificant for maintaining bench stability. The minimum value of the safety factor of the bench with the load from the BELAZ-75306 mining dump truck is at a distance of 8 to 10 m from the top crest to the edge of the wheel. That is, at this distance the mining dump truck has the maximum impact. At 30 m distance from the bench’s top crest, mining dump truck has no impact on the bench stability. The maximum impact of the dump truck on the stability is 22 to 25%.

Evaluation of formation susceptibility and sand production potential in an offshore field, Niger Delta Basin, Nigeria

Wellbore instability and sand production are all common challenges in the Niger Delta oil province, resulting in high drilling and production cost as well as damage to oil facilities. The vulnerability of lithologic formations to wellbore instability and resultant sand production is investigated in the four delineated reservoirs of the “Areo” field, western part of Niger Delta Basin. The foundation for establishing the geomechanical properties in this study was a 1-dimensional mechanical earth model, using gamma ray (GR), density (RHOB), compressional slowness (DTC), and shear slowness (DTS) logs. Within the Areo oil field, two wells (well 001 and well 002) were correlated. The evaluated formations are still primarily composed of compacted shale and unconsolidated sandstone, with reservoir sand units exhibiting lower elastic and rock strength properties than shale units. High compressibility and porosity make sand more brittle, while low compressibility and porosity make shale stiffer due to high moduli. The maximum force that can be applied to a shale unit without causing it to fail is 17.23 MPa, which is the maximum average rock strength of the shale. It means that shale requires more vertical stress or pressure than sand does in order to deform it (15.06 MPa). The three sand prediction approaches used in the analysis of sand production predictions have cut-off values that are higher than the average values of the formations. The Schlumberger sand production index method (S/I) indicates that the reservoir has potential for sand influx in the two wells, with the average of the four reservoirs studied in wells 001 and 002 being 1.55 × 1012 psi and 1.14 × 1012 psi respectively. However, when a formation's sand production index is less than 1.24 × 1012 psi, as it is in this study, the formation is likely to produce sand. These findings support the notion that the defined sandstone units are highly unconsolidated and have a high potential for producing sands; therefore, sand control techniques must be factored into process optimization and cost reduction plans.

A micromechanical-based failure criterion for rocks after high-temperature treatment

The strength properties of rocks after high-temperature treatment are substantially impacted by thermally-induced cracks. In this work, a new parameter of temperature-dependent initial damage or crack density is proposed and a micromechanical-based failure criterion incorporating it is developed to characterize the strength properties of rocks after high-temperature treatment under compressive loading. The mathematical expression of the suggested nonlinear failure criterion coincides with the Hoek-Brown failure criterion for intact rocks. The micromechanical-based failure criterion is then verified using the experimental results of five sets of rocks (Macheng granite, Tournemire shale, red sandstone, coarse marble and veined marble) after high-temperature treatment. The results shown that the strength properties of rocks after high-temperature treatment can be well captured using the failure criterion incorporating temperature-dependent initial damage. Three evaluation indicators (root mean square error, correlation coefficient and mean absolute error) are indicated that the failure criterion incorporating temperature-dependent initial damage has a strong predictive capability. The parameter of temperature-dependent initial damage provides a great description of the effect of thermally-induced damage on the strength evolution of rocks after high-temperature treatment. The proposed failure criterion in this work will become an effective alternative for predicting the nonlinear strength evolution of pre-heated rocks under compressive loading due to its high reliability and easy-to-use.

New Insights into the Internal Structures and Geotechnical Rock Properties of the Giant San Andrés Landslide, El Hierro Island, Spain

The San Andrés landslide on El Hierro (Canary Islands) represents a rare opportunity to study an incipient volcanic island flank collapse with an extensive onshore part. The presented research improves the knowledge of the internal structure and rock characteristics of a mega-landslide before its complete failure. The investigation combines multiple geophysical measurement techniques (active and passive seismic) and remotely sensed, high spatial resolution surveys (unmanned aerial vehicle) with in situ and laboratory geotechnical descriptions to characterize the rock properties inside and outside the San Andrés landslide. The available geophysical and geological data have been integrated into 3D geomodels to enhance their visual interpretation. The onshore geophysical investigations helped detect the possible San Andrés landslide sliding surfaces at depths between 320 m and 420 m, with a rather planar geometry. They also revealed that rocks inside and outside of the landslide had similar properties, which suggests that the previous fast movements of the landslide did not affect the bulk properties of the displaced rocks as the failure chiefly occurred along the weakened sliding plane. Uniaxial strength tests on basalt rocks further indicate a high variability and spatial heterogeneity of the rock strength properties due to the different types of volcanic rocks and their texture. The new information on the rock properties and structural setting of the San Andrés landslide can now be used to develop realistic geotechnical slope models of the onshore part of the flank collapse that are possibly applicable for slope stability or deformation calculations. It will also help assess related hazards marked by a low occurrence probability and a high impact potential.

Boudinage and the rheology of syntectonic migmatites in the high-strain Taili deformation zone, NE China

Abstract This paper presents a detailed field characterization of boudinage in a high-strain zone several kilometers wide in Northern China to establish relationships between boudin types and rheological contrasts between different parts of migmatites during the migmatization process. This zone contains nearly all types of boudins: foliation boudins, blocktorn boudins, pinch- and- swell structures, tapering boudins, object boudins, and modified boudins. These boudinage structures record the different stages of melt-involved and solid-state deformation. The boudinage of migmatites is significantly controlled by the evolving rheological contrasts between the leucosome and melanosome. During the melting stage, the deformation and boudinage of rocks are controlled by the melt fraction. Migmatite strength progressively decreases with increasing melt fraction. The occurrence of melt-filled foliation boudins and melanosome block boudins suggests that the residuum and melanosome are more competent than the leucosome. During solid-state deformation after crystallization, the existence of recrystallized solid-state leucosomes and the intrusion of pegmatites cause the migmatite strength to increase. The relationship is reversed: the leucosome is much more competent than the melanosome. The type and geometry of boudins and pinch- and- swell structures are correlated to the fraction of leucosome in the migmatites. The mechanical strength and strain localization of migmatites after crystallization depend on the presence and volume fraction of the different mineral phases, as well as the mineral grain size. The type and geometry of boudins suggest that the effective viscosity of migmatite can be ranked, from high to low, as: quartz veins; coarse-grained, thick pegmatite; coarse-grained, diatexite migmatite; medium-grained leucosome; and fine-grained melanosome. While experiencing partial melting and migmatization, a rheologically homogeneous protolith is turned into two dominant lithologic domains: a competent diatexite migmatite domain and an incompetent melanosome migmatite domain. Spatially, with the increasing leucosome fraction in migmatites, the migmatite rheology of rock changes from homogeneous to heterogeneous and anisotropic, and then back to homogeneous. The strain distribution likewise changes from uniform to partitioned, and then back to uniform. This evolutionary process of strength and rheological properties of rocks during migmatization may promote strain localization at mid-crustal conditions.

Relationship of petrographic and mineralogical characteristics with mechanical strength properties of granitic rocks: a case study from the Biga Peninsula, NW Turkey

This study was carried out on granitic rocks from the Biga Peninsula in order to investigate petrographic, and mineralogical properties in comparison with mechanical strength properties. This study was conducted in four different locations where granitic rocks outcrop from north to south in the Biga Peninsula: Lapseki (Şevketiye granite), Ezine (Kestanbol granite), Bayramiç (Yassıbağ granite), and Edremit (Eybek granite). Field observations and detailed petrographic studies of representative samples indicate that they are quartz monzodiorite (Şevketiye and Yassıbağ), quartz monzonite (Kestanbol), and granodiorite (Eybek) in composition. All the rocks have similar mineral composition, comprising mainly feldspar (plagioclase-alkali feldspar), quartz, mafic minerals (biotite and amphibole), accessory minerals (sphene, zircon, opaque minerals), and secondary minerals (sericite, epidote, chlorite) with differences in the percentage of modal mineralogy, textural details, and weathering. Physical and mechanical properties were also defined in this study, these include water absorption, porosity, uniaxial compressive strength (UCS), tensile strength (TS), Schmidt hardness tests (SHT), Los Angeles test (LA), and frost resistance test (FRT) with sodium sulfate (Na2SO4). The results of strength tests show that the Şevketiye granite had the lowest strength values with the highest values for Los Angeles abrasion, frost resistance, water absorption, and porosity. The Eybek granite exhibited the highest value for strength, with the lowest values for Los Angeles abrasion, frost resistance, water absorption, and porosity. A comparison of the petrographic properties of all studied rocks with their respective strengths show that modal mineralogy, degree of alteration, and texture influence the strength of the rocks. Additionally, an increasing propensity for UCS and TS values was found with increasing SHT values. The LA abrasion values and the FRT values were found to increase with decreasing Schmidt hardness test values for the tested samples. The found results of this study could help in the application of these rocks as row material.

Results of Research Related to Reducing Energy Consumption in the Rock Wining Process

Energy consumption is the main parameter describing the efficiency of the rock winning process in mechanical excavation. Because the rock winning process is a very complex one, the strength properties of rocks prescribed by classical rock mechanics is not sufficient: Knowledge of cutting forces values, and of the shape and volume of removed chips – based on tool geometry and cutting head kinematics – has significant importance in the optimal design of winning machines, and in improving their operating parameters. The paper deals with some results in research performed at the University of Petroșani on these issues, both in theoretical findings and experimental assays in laboratory and on site.

Evaluation of gold ore properties and their impact on grinding operations

Purpose. The purpose of this paper is to evaluate the efficiency of grinding operations in terms of how mechanical properties (e.g., strength properties and ore texture) affect the Bond Work Index. Methods. The specimens have been collected in the Eastern Desert of Egypt, namely Abu Marwat, Hamash and Al Sadd. As a result, strength parameters such as compressive strength, cohesiveness and hardness have been assessed. Ore texture, mineral content and bonds between tiny fabric units have been examined using X-Ray Diffraction (XRD) and thin section. Findings. This research shows that as the strength properties of the rock increase, the Bond Working Index also increases. Moreover, the results indicate that the level of cohesion of ore minerals with the surrounding tailings, on the one hand, and the variance in the tailing content, on the other hand, play a significant role in the processing operation, given the discrepancy in the Bond Work Index for the six Abu Marawat gold ores of 18.8%. Originality. This research attempts to develop a methodology for assessing the efficiency of grinding operations as a function of rock strength properties and ore texture in relation to the Bond Work Index. Practical implications. Ore texture is one of the most important factors influencing the grinding process. Since grinding consumes a considerable amount of energy, the economic evaluation is based on increasing the grinding efficiency. According to previous research, the petrographic, which varies from sample to another, has an impact on the mechanical properties as well as the grinding operations.

Influence of Rock Properties on Structural Failure Probability—Caprock Shale Examples from the Horda Platform, Offshore Norway

In any geological subsurface fluid injection, a viable top seal is required to contain the vertical movement of the injected fluid plume. However, seal integrity assessment is challenging because of the uncertainties possessed by various parameters. A probabilistic solution might be more appropriate when significant uncertainties are present. In this study, we evaluate Drake shale caprock structural reliability using a stochastic method. Drake shale is the primary top seal in the Aurora CO2 storage site, located in the Horda Platform area in the northern North Sea. Based on the lithological variations, Drake caprock shale is divided into two parts designated by upper and lower units. Six model scenarios from the upper and lower Drake units have been tested. The probabilistic structural failures of varying model scenarios are estimated using the First-Order Reliability Method (FORM). Drake Formation shale shows a considerably low probability of failure (~0) with a high reliability index in the initial stress-state condition and after-injection scenarios. Moreover, the parameter sensitivity study indicates that horizontal stress and cohesion are the most influential input parameters during reliability estimation. Comparative analysis between the caprock properties and failure probability reveals that rock strength properties such as cohesion and friction angle strongly dictate the probability of failure estimation. Moreover, comparing two caprock shale formations indicates that the structural failure values are not correlatable; hence, a formation-specific failure assessment is recommended.

Geomechanical Feasibility Analysis of Salt Cavern Gas Storage Construction in Sanshui Basin, Guangdong Province

Salt cavern gas storage has become the key project of current and future underground gas storage (UGS) facilities construction due to their efficient peak-shaving and supply assurance capacities. The Sanshui Basin in Guangdong Province, China, is rich in salt resources and has high-purity salt rock, which is a potential area for the construction of salt cavern underground gas storage. To speed up the large-scale construction of underground gas storage in China and promote the sustainable development of the natural gas market, it is very necessary to analyze the geomechanics of the target salt layer and study the feasibility of gas storage construction. Based on comprehensive experiments of rock mechanics and thermodynamics, the strength, creep and temperature-sensitive mechanical properties of the target rock in Sanshui Basin were studied. Then, according to the geological conditions of Sanshui salt formation, a three-dimensional geological model was established to analyze the stability of salt cavern gas storage under the injection-production operation. The results show that the average tensile strength and uniaxial compressive strength of salt rock are 1.51 MPa and 26.04 MPa, respectively, showing lower strength. However, under triaxial compression, the compressive strength of salt rock increases significantly, and there is no obvious shear failure phenomenon observed. Moreover, after the peak strength, the salt rock still has a large bearing capacity. In addition, under the confining pressure of 30 MPa, the strength of salt rock decreases by 8.3% at a temperature of 60 °C compared with that at room temperature, indicating that the temperature has a low, modest effect on the mechanical properties of salt rock. The stability analysis shows that, under an injection-production operating pressure of 10–23 MPa, the displacement, plastic zone range and volume convergence rate of single cavity and cavity group are small, and the cavity shows good stability. Overall, the target salt formation in Sanshui Basin, Guangdong Province, presents a good geomechanical condition suitable for the construction of underground salt cavern gas storage. This study can provide a reference for the development and design of salt cavern UGS.

RISKS IDENTIFICATION OF THE GEOTECHNICAL SYSTEM STABILITY LOSS DUE TO GEOMECHANICAL AND HYDROGEOLOGICAL FACTORS

Problem statement. The most dangerous of mining technologies is the construction of workings underground and their maintenance in good condition. Uncontrolled deformations of the rock mass, sudden rock falls, destruction of the lining and blockages of workings are emerged. Personnel injuries from these risk factors, directly related to geomechanical processes, reach half of the total number of victims. Therefore, the issue of risks identifying for geotechnical system stability loss due to geomechanical factor, taking into account the rocks water saturation (a factor that significantly affects the strength properties of rocks) is relevant and serves as the foundation for the development of efficient and safe mining technologies. The purpose of the article. To substantiate methods for risks identifying of the underground workings stability loss due to geomechanical and hydrogeological factors for the mining enterprises conditions. Research results. The potential risk of an emergency situation for each of the geotechnical system elements is proposed to be determined: by the parameters values at the current time, by the parameters deviation amplitude and by the parameters deviation speed from the equilibrium point during the previous period of time. For forecasting the stress-strain state of rocks, the methods of implementing geomechanical models, assessing risks and scenarios of situation development are improved. On the basis of rock deformations monitoring, the risks of longwall support stability loss due to roof collapse were established. To identify the risks arising from the influence of the rocks flooding factor on their strength, and, accordingly, on the mine workings stability, rock samples were studied. It was found that when rocks are watered, the risks increase linearly with an intensity of 0.62...0.71 for mudstones, 0.49...0.58 for siltstones, 0.25...0.37 for sandstones in the depth range of 400 ... 800 m. For automated safety systems, it is proposed to process these parameters using fuzzy logic methods. Scientific novelty. The method for risks identifying of the geotechnical system stability loss has been further developed, which is distinguished by: determining regularities of changes in risk indicators of the controlled object dangerous state by a statistically significant number of geomechanical and hydrogeological parameters of monitoring previous events or by trends and forecasts of further events; using fuzzy logic models that take into account the parameters deviations of each geotechnical system elements and the rate of change for these deviations over a period of time. Conclusions. The obtained dependencies determine the input conditions for identifying of the flooded rock massif stability and creating technologies for mine workings support. The investigations made it possible to answer a number of previously unresolved problems associated with the risks of the mine workings stability loss, to ensure higher efficiency and mining operations safety.

Inversions of landslide strength as a proxy for subsurface weathering

Distributions of landslide size are hypothesized to reflect hillslope strength, and consequently weathering patterns. However, the association of weathering and critical zone architecture with mechanical strength properties of parent rock and soil are poorly-constrained. Here we use three-dimensional stability to analyze 7330 landslides in western Oregon to infer combinations of strength - friction angles and cohesion - through analysis of both failed and reconstructed landslide terrain. Under a range of conditions, our results demonstrate that the failure envelope that relates shear strength and normal stress in landslide terrain is nonlinear owing to an exchange in strength with landslide thickness. Despite the variability in material strength at large scales, the observed gradient in proportional cohesive strength with landslide thickness may serve as a proxy for subsurface weathering. We posit that the observed relationships between strength and landslide thickness are associated with the coalescence of zones of low shear strength driven by fractures and weathering, which constitutes a first-order control on the mechanical behavior of underlying soil and rock mass.

New Models for Predicting Pore Pressure and Fracture Pressure while Drilling in Mixed Lithologies Using Artificial Neural Networks.

Precise prediction of pore pressure and fracture pressure is a crucial aspect of petroleum engineering. The awareness of both fracture pressure and pore pressure is essential to control the well. It helps in the elimination of the problems related to drilling, waterflooding project, and hydraulic fracturing job such as fluid loss, kick, differential sticking, and blowout. Avoiding these problems enhances the performance and reduces the cost of operation. Several researchers proposed many models for predicting pore and fracture pressures using well log information, rock strength properties, or drilling data. However, some of these models are limited to one type of lithology such as clean and compacted shale formation, applicable only for the pressure generated by under compaction, and some of them cannot be used in unloading formations. Recently, artificial intelligence techniques showed a great performance in petroleum engineering applications. Hence, in this paper, two artificial neural network models are developed to estimate both pore pressure and fracture pressure through the use of 2820 data sets obtained from drilling data in mixed lithologies of sandstone, carbonate, and shale. The proposed artificial neural network (ANN) models achieved accurate estimation of pore and fracture pressures, where the coefficients of determination (R2) for pore and fracture pressures are 0.974 and 0.998, respectively. Another data set from the Middle East was used to validate the developed models. The models estimated the pore and fracture pressures with high R2 values of 0.90 and 0.99, respectively. This work demonstrates the validity and reliability of the developed models to calculate pore and fracture pressures from real-time surface drilling parameters by considering the formation type to overcome the limitation of previous models.

Thermal damage and acoustic emission characteristics of granite based on Brazilian splitting test

Rapid and non-destructive evaluation of the thermal stability of surrounding rocks and rock strength is important in field construction. Physical properties and acoustic emission (AE) characteristics of granites can be changed at high temperatures. Thus, investigations on the relationships between rock strength and physical properties could help to evaluate the thermal stability via a physical measuring method. In this study, granite selected from a geothermal drilling project was heated at 25–600°C and different physical properties and tensile strength were measured. Moreover, the thermal damage degree was evaluated and the tensile failure pattern was quantitatively characterized. Notably, the tensile strength of granites could be indirectly acquired through P-wave velocity, hardness, colour, mass loss and volume expansion. The experimental results showed that the variations could be roughly divided into two stages. (1) At 25–400°C, the mass, volume, P-wave velocity, hardness, tensile strength, colour and AE parameters changed slightly. The failure mode and damaged section did not differ significantly within this temperature range. (2) At 400–600°C, these physical and mechanical properties and AE parameters changed significantly. The formation of micro-cracks was aggravated by the loss of structural water and phase transition of quartz, leading to complex failure modes and jagged morphology of damaged sections. The thermal damage factor and the compressive damage factor defined by the P-wave velocity at different conditions increased greatly. In summary, there was no major damage and the internal structure was still relatively intact at 25–400°C. However, the granite showed serious damage in the high-temperature environment (400–600°C), resulting in great changes in physical and mechanical properties and AE characteristics.

Rock mass management to ensure safe deposit development based on comprehensive research within the framework of the geomechanical model development

Purpose. To create and study a three-dimensional geomechanical model in order to determine the parameters of the open-pit walls and benches, ensuring safe and economically feasible mining, as well as predicting unstable zones within the open pit. Methods. A comprehensive methodological approach is used, including a systematic analysis of scientific, normative and methodological literature; analyzing the results of previously performed studies on the object; engineering-geological surveys in the near-edge rock mass of the Kurzhunkul’ deposit; laboratory testing of rock strength properties; determining the rock mass rating according to the MRMR classification; kinematic analysis of bench faces; calculating the stability of the Kurzhunkul’ deposit final boundary using the limit equilibrium method; numerical modeling of the rock mass stress-strain state at the Kurzhunkul’ deposit using the finite element method. Findings. The paper represents the results of data collection and analysis for the development of a geomechanical model of an operating iron-ore open pit in the Republic of Kazakhstan. Comprehensive geomechanical studies to substantiate the optimal parameters of the Kurzhunkul’ deposit walls and benches on the limiting contour, as well as calculations to determine the degree of the open-pit walls and benches stability have been performed. Based on the results of studying the geological-structural configuration of the deposit, as well the mathematical modeling data of stability and acting stresses, subsequently entered into a unified digital database, weakened zones have been identified. Originality. For the first time, the geomechanical model has been created for the conditions of the Kurzhunkul’ deposit, which makes it possible to combine in one database all the parameters that affect the safety of mining operations. The model takes into account structural disturbances of the rock mass that have an adverse impact on stability. Practical implications. The developed model gives a visual representation of the rock mass state at various sites of the deposit, simplifies the selection of design sections for stability calculations, facilitates the choice of optimal technical solutions and analysis, especially for complex geological structures with multiple geotechnical or geological units with different texturing and inclination.

Experimental investigation of the mechanical properties of mud shale under water-bearing conditions and its applications.

Mud shale, used in drilling engineering, is prone to hydration and expansion, resulting in creep deformation that leads to wellbore shrinkage and pipe sticking incidents. Studying the creep characteristics of mud shale is significant for designing a reasonable well structure and determining the lower limit of drilling fluid density. The influence of moisture content on rock strength and creep mechanical properties were studied using water absorption, uniaxial compression, and creep tests. Test results show that with an increase in the moisture content, the mud shale was damaged and softened; moreover, the elastic modulus decreased with increase in moisture content. Under the same load level, the instantaneous strain increased with increasing moisture content. Under different loading stresses, the creep of the rock had nonlinear characteristics, which could be divided into three different creep stages: attenuation, second, and accelerated creep. A new improved creep model based on the Nishihara model was established to describe the accelerated creep characteristics of mud shale under different moisture contents. The ageing degradation and water-bearing weakening effects were introduced. The Levenberg–Marquardt nonlinear least-squares method was applied to invert the creep parameters. The results show that the simulated creep curves, generated using the new creep model, conform to the experimental ones. The relationship between the drilling fluid density and wellbore shrinkage ratio can be defined using this model; it provides a reference for reasonably determining the drilling fluid density.

A study of rock response to failure in the context of the bending properties and comparison with uniaxial tensile and compression behaviour

The phenomenon of rock bending occurs during underground exploitation, construction of underground excavations and tunnels, and even rising heading – shafts. It is also common in building engineering, e.g., in the case of floors. Rocks and concretes as granular materials on the aggregate scale are fractured as a result of exceeding shear and tensile strength. In a complex state of stress – bending, crack propagation occurs from tensioned to compressed fibres. Three-point bending tests of medium-grained quasi homogeneous and isotropic sandstone were tested for strength and deformation properties of rocks. The E deformability modules for compressed and tensioned fibres as well as strains at failure were determined. The results of three-point bending were compared with the results of uniaxial compression and direct tension. Clear differences were found in the values of strengths, moduli of deformation and strains at failure. The bending strength B of about 9.5MPa is almost 3 times greater than the direct tensile strength σT of about 3.2MPa and is 1/10 of the ultimate uniaxial compression strength σC . With three-point bending, the values of the moduli E are equal to: for tensioned fibres about 6.7GPa, for compressed fibres 14.6GPa; in uniaxial compression tests about 13.0GPa and in direct tensile tests 4.8GPa. Rock material was also failure at various strains values at the ultimate strength. In the case of three-point bending tests, the strains at failure were equal to: for tensioned fibres about 0.125%, and for compressed fibres 0.065%; in uniaxial compression tests εz were equal to about 0.63% and in direct tension tests 0.07%.

Creep Mechanical Properties and Constitutive Model of Hard Brittle Mud Shale

AbstractMud shale is characterized by low strength and strong swelling, with rheological effects and deformation caused by drilling fluid and formation water. Establishing a rheological model to characterize the deformation characteristics is key to solving the problem of wellbore stability. The influence of moisture content on rock strength and creep mechanical properties were studied by means of water absorption, uniaxial compression, and creep tests. The tests showed that with the increase in moisture content, the elastic modulus and strength of hard brittle mud shale decreased. Further, under the same load, the instantaneous strain increased with increasing moisture content. Meanwhile, under various loading stresses, rock creep exhibited non-linear characteristics, which can be divided into three different creep stages: attenuated creep, stable creep, and accelerated creep. Starting with a non-linear viscous dashpot, and then introducing aging degradation and water-bearing weakening effects, based on the water-bearing creep characteristics of hard brittle shale as well as the modeling ideas of the classic component combination model, a new improved creep model based on the Nishihara model was established to describe the characteristics of the accelerated creep stage of hard brittle mud shale with various moisture contents. Subsequently, the Levenberg–Marquardt non-linear, least-squares method was adopted to invert the creep parameters. The results showed that the simulated creep curves achieved by employing the new creep model were consistent with the experimental results, thereby confirming the ability of the new non-linear creep model to provide a theoretical reference for the study of wellbore stability of hard brittle mud shale.

Introducing Tree-Based-Regression Models for Prediction of Hard Rock TBM Performance with Consideration of Rock Type

Prediction of machine performance is a fundamental step for planning, cost estimation/control and selection of the machine type when using a tunnel boring machine (TBM). Penetration rate (PR) and machine utilization (U) are the two principal measures of TBM performance for evaluating the feasibility of using a machine in a given ground condition. However, despite the widespread use of TBMs and established track records, accurate estimation of machine performance could still be a challenge, particularly in complex geological conditions. Since different types of rocks have varied texture (cementation and grain size), and respond differently to cutting forces in the TBM tunnelling, incorporating the effects of rock type in performance prediction models can improve the accuracy of the estimates. The aim of this study was to develop models for predicting penetration rate of hard rock TBMs in different types of rock based on field penetration index (FPI), using multivariable regression analysis and machine learning algorithm, including classification and regression tree (CART). The proposed models offer estimated FPIs in different rock types, rock strength, and rock mass properties in the form of graphs (diagrams), which can be used to estimate TBM penetration rate. The proposed models have been developed based on the analysis of a comprehensive database of TBM performance in various rock types and offers more accurate estimates of machine performance by incorporating many of the key parameters available in typical geotechnical reports and contract documents. The models also exhibit sensitivity to rock mass parameters for predicting the penetration rate.

Semi-analytical Method for Modeling Wellbore Breakout Development

Borehole breakout initiation, progression, and stabilization are modeled using a semi-analytical method based on Melentiev’s graphical conformal mapping procedure, and Kolosov–Muskhelishvili complex stress potentials. The only input data required are the elastic moduli of the rock, the Coulomb strength parameters (cohesion and angle of internal friction), and the far-field stresses. The stresses around the borehole wall are computed, the region in which the rock has failed is then “removed”, creating a new borehole shape. This process is iterated until a shape is obtained for which the breakout will progress no further, and a stable state has been reached. This modeling shows that stresses around the flank of the breakout evolve so as to reduce the propensity for shear failure, which helps to explain why the breakout width remains relatively constant throughout the process, even as the breakout region deepens radially. The failed area around the borehole becomes smaller and more localized, as the breakout tip sharpens and deepens. Using the Mogi–Coulomb failure criterion, a good match is obtained between the modeled breakout geometry and the geometry observed by Herrick and Haimson in laboratory experiments on an Alabama limestone. The new method leads to a correlation between breakout geometry, rock strength properties, and in situ stress. The paper ends with a critical discussion of the possibility of inferring the in situ stress state from observed breakout geometries.