Rock Cohesion Research Articles

Degradation of rocks on the Moon: Insights on abrasion from topographic diffusion, LRO/NAC and Apollo images

The efficiency of regolith production is key in understanding the properties of airless surfaces. Debris aprons, of fillets, around rocks are a ubiquitous morphology on many surfaces without atmosphere, which origin and evolution are largely unknown. Here we develop a model for the morphological evolution of the rock-fillet system on the Moon that considers fillet material to be produced by the juxtaposed rock under abrasion. We show that rocks of different cohesion have fillets with distinct morphological evolution. Thus, a fillet around a rock allows to disentangle rock cohesion from its surface exposure age. By combing topographic diffusion modeling with images of blocks of known age on the Moon, we find abrasion rates for m–sized rocks to be higher than for cm–sized rocks. Artificial images constructed with model topography indicate that rocks with fillet can be identified in orbital images by a bright halo around a rock and by the fillet shadows. Fillets around lunar rocks are consistent with abrasion by isotropic micrometeoroid bombardment.

Study on distortion energy and stability of gob floor based on stress deviator field

AbstractTo analyze the plastic failure process and mechanism of gob floor in the process of close‐distance coal seams mining, a simplified mechanical model of the lateral abutment pressure of the gob was proposed to derive the stress of surrounding rock. The distribution characteristics of gob floor stress deviator were obtained, and the effect of geological parameters (coal depth H and horizontal lateral pressure coefficient λ) were studied. By combining Mohr–Coulomb theory, the influence of surrounding rock cohesion c, internal friction angle φ, different coal seam depths H and different horizontal lateral pressure coefficients λ on the distribution of plastic zone of gob floor was obtained. The results showed that the distribution of the plastic zone under different geological factors is roughly the same as that of the stress deviator. This also proves that the excavation of the rock layer causes the redistribution of stress, which leads to the plastic failure of the surrounding rock. To verify the rationality of the proposed model, industrial experiments and simulation experiments were carried out. The results of on‐site investigation showed that the stability and integrity of surrounding rock in the concentration area of stress deviator is far less than that in the low concentration area. In addition, the acoustic wave test and the displacement field monitoring result of surrounding rock in the simulation experiments of a similar model are basically consistent with the distribution of the stress deviator field. These results agree well with the distribution of stress deviator obtained by theoretical calculation.

Lateral Loading of a Rock–Socketed Pile Using the Strain Wedge Model Based on Hoek–Brown Criterion

Rock–socketed pile under lateral loading is important in engineering practice. It is very significant to calculate the lateral bearing capacity of rock–socketed piles since few studies focus on this problem. The rock cohesion and instantaneous angle of friction, which have a high correlation with confining pressure, are obtained. Moreover, the strain wedge model is modified from three aspects: the assumption of nonlinear displacement; the stress level related to cohesion and friction angle; and the pile side resistance. Then, the modified strain wedge model is employed to deduce p–y criterion for rock–socketed pile considering Hoek–Brown failure criterion. The fourth-order partial differential equation constructed according to the p–y curve is solved by using the finite difference method. A numerical method with 2 m diameter rock-socketed pile is given to validate the rationality of the proposed method. It is shown that the proposed could predict the pile deformation well, and the responses are considered acceptable.

Experimental study of rock ruptures shear strength anisotropy

Joints significantly reduce rock cohesion compared to unfractured rock, but the question is what effect the possible anisotropy of the shear strength of different types of rupture has on possible subsequent failure. Both natural samples of granodiorite with natural joints and fault surfaces and gypsum models have been tested on the Matest A129 Rock shear box apparatus. The shear strength of preexisting ruptures was measured under a fixed normal stress component. The anisotropy of the shear strength of the joints and fault surfaces reached more than 60 % of maximum strength, which is a very important value for solving structural loads. The shear strength was analyzed with polar plots. The pattern of the real joints typically showed a teardrop shape with one peak of strength in a certain direction and a minimum in the opposite direction. On the contrary, striated fault surfaces are characterized by two axial directions of minimal shear strength, i.e., longitudinal and transverse, and by two axial oblique directions with maximal shear strength, so the strength distribution in the polar graph has a four-cornered shape. The study showed that the anisotropy of the shear strength of various types of ruptures is their important feature.

Failure of Rock Slopes with Intermittent Joints: Failure Process and Stability Calculation Models

Abstract Rock slopes with intermittent joints in open-pit mines are complex geological bodies composed of intact rock and discontinuous structural planes, and their stability analysis are necessary for mine disaster prevention. In this study, a series of base friction tests were performed to determine the failure process and displacement field evolution of rock slopes with intermittent joints using the speckle technique of a noncontact measurement system. Next, stability calculation models of the slopes were established from the energy perspective using the plastic limit analysis theory, and the effects of the joint inclination angle and coalescence coefficient of rock bridges on the slope stability were evaluated. The four main conclusions are as follows: (1) The failure of rock slopes with intermittent joints shows the feature of collapse-lower traction-upper push. (2) Based on the failure modes of rock bridges in slopes, the failure of rock slopes with intermittent joints could be divided into three types: tensile coalescence (type A), shear coalescence (type B), and tensile–shear coalescence (type C). (3) Among the three slope types, the stability of the type A slope is significantly influenced by rock cohesion, whereas that of the type B slope is significantly influenced by joint cohesion. The stability of type C slope is significantly influenced by the joint inclination angle and joint friction angle. (4) The local-stable slope is unstable while the first through-tensile crack in the zone of the potential sliding body higher than the critical instability height appeared. This study guides the stability evaluation and instability prediction of jointed rock slopes in open-pit mines.

Optimal Support Solution for a Soft Rock Roadway Based on the Drucker–Prager Yield Criteria

Through theoretical calculation, the stress and deformation of surrounding rock can be analyzed, providing guidance for the support design and optimization of soft rock roadways. In this paper, theoretical solutions for both the optimal support pressure and the allowable maximum displacement of surrounding rock are derived from the Drucker–Prager (DP) yield criteria and the steady creep criterion expressed by the third invariant of deviator stress. The DP criterion with different parameters is compared and analyzed with an engineering example. Then, based on the calculation results the effects of long-term strength, cohesion, and internal friction angle of soft rock on the maximum plastic zone radius and allowable maximum displacement of roadway are discussed. The results show that the optimal support solution of soft rock roadways based on the DP criteria can not only reasonably reflect the intermediate principal stress but can also be used to compare and discuss the influence of different DP criteria on the calculation results. The higher the long-term strength of the rock surrounding a roadway is, the smaller the optimal support force is and the larger the allowable maximum displacement is. When the calculated long-term strength of soft rock can ensure that the deformation of the roadway does not exceed the allowable maximum displacement, the roadway can maintain long-term stability without support. With an increase in the cohesion or internal friction angle of soft rock, the radius of the plastic zone decreases gradually and the allowable maximum displacement is reduced by degrees. The use of grouting and other means to improve the strength of surrounding rock can effectively reduce the roadway deformation and save support costs. This new theoretical solution can consider different intermediate principal stress effects and different DP strength criteria, enabling the parameters to become easier to determine. It has a wider range of applications, and the calculation results better demonstrate the strength potential of the surrounding rock.

Study on Effect of Rock Interface on Hydraulic Fracture in W9 Offshore Gas Field

In-situ principal stresses in ZH formation of W9 offshore gas field are measured by Kaiser effect method, and the horizontal minimum stress profile is interpreted by logging information. Static Young’s modulus and Poisson’s ratio of rock are measured by tri-axial loading experimental system, and cohesion and internal friction angle of rock are calculated by empirical formula based on logging data. Failure mode discrimination equation of hydraulic fracture at the rock interface is established applying stress analysis of hydraulic fracture tips at rock interface combined with tensile failure criterion and shear failure criterion. In-situ stresses and strength parameters in ZH formation are used to calculate and analyze the control effect of interface on hydraulic fracture propagation. The calculation results show it is necessary to control fracture height growth in ZH formation of W9 offshore gas field, which is of great significance on guiding the optimization of fracturing parameters in this field.

Research on Red-Bed Soft Rock Engineering Properties and Foundation Appraisement of Construction Engineering in Guangzhou

Based on the methods of geological drilling, literature research, induction and statistics, the site investigation of an old industrial park reconstruction project in Haizhu District of Guangzhou city was completed. The landform, formation lithology, structure and hydrogeological characteristics were systematically identified. Combined with indoor test and in-situ test, the variation rules of physical and mechanical parameters of argillaceous siltstone with different weathering degrees in the site were analyzed, and the evaluation results were given. The engineering suitability and the foundation stability are evaluated. The results showed that the weathering of soft rock makes the statistical standard values of compression coefficient, compression modulus, direct fast shear cohesion and internal friction angle of rock and soil decrease. Under the saturated state, the average value of strongly weathered argillaceous siltstone is 1.51 MPa, and that of moderately weathered argillaceous siltstone is 6.2 MPa. The strength of soft rock is low after softening with water, and the differential weathering of rock mass is strong. There are great differences in rock strength, local differences between fully and strongly weathered rock surfaces, and great changes in thickness. The weathering is extremely heterogeneous, in which the influence of structure, lithology, fissures and groundwater played a key role. The foundation is generally in a stable state, and the site is suitable for the construction of the project. The research results can provide reference for similar geotechnical engineering investigation and design.

Evaluation of failure criteria under dynamic loading of the wellbore with strain rates between 10−6 and 100 /s

Dynamic loading is very common during drilling, mining, hydraulic fracturing, and tunnel excavation. This could compromise the long-term stability and integrity of the underground constructions. Under dynamic loading, rocks usually behave differently compared to static loading. Whether the static failure criteria can be used to evaluate structure failure under dynamic loading is a question that has not been answered yet. In this paper, the triaxial compression strength data under different strain rates (between 10−6 and 100/s) for four types of rock (Bedford limestone, Bukit Timah granite, Laurentian granite, and Dry silica sand) have been used to evaluate six different failure criteria. Two methods have been implemented to obtain rock cohesion and internal friction angle, which are the grid search algorithm and the Mohr failure line method. The results show that the grid search algorithm guarantees the best performance of the failure criteria, providing the best-fit rock parameters. The more practical method of the Mohr failure line can also generate reasonable predictions if a safety factor of 1.1 is considered. Both methods show that Modified Lade is the failure criterion that provides the best performance in predicting rock failure under dynamic loading. In the petroleum industry, most of the activities causing dynamic loading of the wellbore are characterized by a magnitude of strain rate lower than 100/s (even lower than 10−3/s). Consequently, this work shows that it is reasonable to assume that the commonly used static failure criteria, especially Modified Lade, could be selected to analyze wellbore stability under dynamic loading conditions effectively.

Analyzing the correlation between stochastic fracture networks geometrical properties and stress variability: a rock and fracture parameters study

In this study, the correlation between geometric properties of the fracture network and stress variability in a fractured rock was studied. Initially, discrete fracture networks were generated using a stochastic approach, then, considering the tensorial nature of stress, the stress field under various tectonic stress conditions was determined using finite-difference method. Ultimately, stress data were analyzed using tensor-based mathematical relations. Subsequently, the effects of four parameters including rock tensile strength, rock cohesion, fracture normal stiffness and fracture dilation angle on the stress perturbation distribution were evaluated. The obtained results indicated that stress perturbation and dispersion are directly related to fracture density, which is expressed as the number of fractures per unit area utilizing the window sampling approach. It was also demonstrated that they are inversely related to power-law length exponent which represents the length of fracture. It was observed that stress distribution, among the evaluated parameters, is more sensitive to the fracture normal stiffness and the effects of rock parameters on stress distribution are negligible. It was concluded that the highest stress distribution is created when the fracture network is dense with fractures having high length and low normal stiffness value.

Simple Slope Stabilization on Quartz Sandstone using Horizontal Drain

The maintaining stability of public facilities, especially public roads, it is very necessary, so the stabilization efforts must be carried out. The water from seepage that comes out of sandstone layer carries material with fine particles, it causes the rock cohesion to decrease and thus leading to scouring. As a result of the scouring, the layer becomes overstep and can disturb road stability. This study was conducted to provide simple but measurable recommendation for maintaining road stability after the previous stabilization effort failed. The method used a fluid mechanics approach in which water from the formation was given space to come out of the formation without carrying fine particles (cement). The analysis was carried out using the finite element method by installing horizontal drain pipe. The result of analysis shows that the horizontal drain installation helps water to come out without creating an overstep layer. The recommended horizontal drain is 8 m long with a slope of at least 3% and a length of 1 m that must enter the sandstone formation as a water source. With this method, the road stability can be maintained and the stabilization can be carried out in an easy, inexpensive, and applicable way.

Mining reasonable distance of horizontal concave slope based on variable scale chaotic algorithms

Abstract Horizontal concave slope structure is a common mining structure. It is the basis of mining safety to grasp the reasonable mining distance of horizontal concave slope. Therefore, based on variable scale chaotic algorithm, the reasonable mining distance of horizontal concave slope is studied. The safety factor is used to describe the reliability of the slope. When the slope is in a reliable state, it is a reasonable mining distance. Based on the Warehouse–Mohr strength theory, the four key parameters of rock cohesion and internal friction angle of horizontal concave slope and major and minor stress of element nodes are considered. The similarity of mining safety factor of horizontal concave slope is measured and the objective function is constructed. The target is searched based on variable-scale chaotic algorithm. The output result is the optimal solution of reasonable mining distance for horizontal concave slope. The experimental study shows that the reasonable mining distance of horizontal concave slope can be obtained effectively based on variable scale chaotic algorithm, and the slope height and slope angle are inversely proportional to the mining reliability of horizontal concave slope.

A coupled thermo-hydrologic-mechanical (THM) model to study the impact of hydrate phase transition on reservoir damage

Thermal fluid injection is proven to be one of the effective methods to dissociate hydrate. The injection of thermal fluid can destroy the hydrate phase equilibrium, resulting in hydrate decomposition and hydrate saturation decline, which further cause reservoir damage. Previous works separated the study of hydrate saturation variation caused by thermal fluid injection and the reservoir damage caused by hydrate saturation variation. In this paper, a novel thermo-hydrologic-mechanical (THM) coupling model is proposed to simulate the thermal fluid injection in hydrate reservoir, and its effects on reservoir damage were analyzed. The results showed that thermal fluid injection causes the decomposition of hydrate, resulting in the decrease of hydrate saturation. The solid phase hydrates decompose into methane and water, which increases the pore pressure of hydrate reservoir and further affects the in-situ stress field. The change of effective stress induces pore deformation, and the hydrate decomposition also releases the space occupied by hydrate in reservoir pores, both affecting the reservoir permeability. Hydrate phase transition weakens the cementation of hydrate on rock particles and reduces the reservoir rock cohesion. In addition, the elastic modulus of hydrate decreases due to hydrate decomposition. The temperature and injection rate of thermal fluid are the main external factors affecting the hydrate phase transition. With the increase of thermal fluid temperature, hydrate saturation decreases due to hydrate decomposition, resulting in variation of reservoir parameters. Enhancing fluid injection rate can increase the reservoir pore pressure, which inhibits the hydrate decomposition.

Direct shear experiment on salt rock incorporating loading rate effect

To investigate the shear behavior of salt rock under different loading rates, direct shear testing was conducted on high-purity salt rock obtained from Pakistan using a YZW-100 apparatus. The experimental results of the shear stress, deformation, and failure characteristics of the salt rock showed that the loading rate is an essential factor influencing the shear strength parameters of the rock. When the shear loading rate was increased, the cohesion of the salt rock significantly increased; however, the internal friction angle slightly decreased, nearly remaining unchanged. The shear failure mode of the salt rock manifested by shear stress-displacement curves tended to be ductile failure with increased normal force. The residual stress of the salt rock was large, indicating its high friction-bearing capacity. The dilatancy deformation was also subjected to shear loading. The fracture section of the salt rock exhibited visible local scratch marks. These experimental results are helpful for further understanding the failure mechanism of salt rock and the stability of underground gas storage under different gas injection and production scenarios.

Time-dependent crack development processes around underground excavations

The time-dependence of rock strength is an issue that cannot be ignored in the rock engineering field. This paper summarizes previous study results on the degradation of the uniaxial compressive strength of rock (including component cohesion and internal friction angle), tensile strength and Young's modulus over time. Based on previous studies, the degeneracy formulas of intact rock cohesion and internal friction angle are derived. Taking circular tunnel excavation under hydrostatic pressure as an example, the surrounding rock response is studied under different degradation degrees of rock strength and stiffness, including the radius of the excavation damage zone, damage degree of surrounding rock, lining support pressure and tunnel boundary displacement. The results show that with the increasing degree of degradation of the surrounding rock compressive strength, the above four parameters increase in the form of an exponential function and that weakening of the tensile strength had little effect on the surrounding rock response. In addition, as the degradation degree of rock stiffness increases, the radius of the excavation damage zone remains unchanged, whereas the other three parameters values are reduced. An actual engineering case simulation using the combined finite-discrete element method shows that results consistent with field observations can be obtained if the rock strength degradation law proposed in this paper is adopted.

The Roles of Elastic Properties, Magmatic Pressure, and Tectonic Stress in Saucer‐Shaped Sill Growth

AbstractNear‐surface igneous sills commonly exhibit saucer‐like shapes, formed due to interaction with the Earth's surface once some critical sill length is reached relative to its depth. Sill geometry has been strongly linked to the host material conditions, particularly in terms of the elastic properties and shear cohesion of the host rock, operating as primary controls on sill geometry. Here we conduct dynamic numerical simulations for sill growth in the near surface, in which we vary the host rock properties, magma pressure profile internal to the sill ∆P, and the externally applied tectonic stress σr, to consider their contributions to sill geometry. We find that elastic properties alone have little impact on sill geometry. Saucer shapes reflect the additive stress components of the magma overpressure within the sill, and the tectonic stress, controlled by the locus of the maximum energy release rate Gmax. Initially, Gmax is in‐plane with the sill but deflects to ~25° at a critical base length rc relative to depth, due to interaction between the sill and the free surface. Increasing σr decreases this angle and increases rc of the sill. ∆P controls sill growth rate but has little effect on overall geometry. Host rock cohesion and elastic properties control the absolute magnitudes of σr required to effect a change in sill geometry.

Comprehensive weighted matter-element extension method for the safety evaluation of underground gas storage.

This study focuses on a safety evaluation method for underground gas storage. Gas storage is usually constructed underground in complex environments, and the service life of such facilities is limited. To ensure the secure and long-term operation of gas storage facilities, safety evaluation has become the focus of management. The present paper provides an effective method for safety evaluation. An index system was established as the foundation of the analysis for this evaluation, and the matter-element extension method was applied to obtain a quantitative evaluation result. For the weight values of each index in the matter-element extension method, this paper presents a comprehensive weight computation method based on vague sets and entropy. By application of this method, the safety level of a gas storage facility in the Jintan salt mines (in Jiangsu, China) was calculated, and the evaluation result was 4.6433, which meant the safety level was V and the underground gas storage was slightly at risk. It indicated that the influence on the overall safety and tightness of this gas storage could be ignored in the operation process, but the frequency of regular monitoring should be increased. The defective indexes were also obtained, such as salt rock cohesion, the roof thickness, the volume contraction ratio, the interlayer content, the height of the casing shoe and the adjacent cavity pressure difference, which need to be monitored and modified. This paper evaluated the safety of the underground gas storage from a unique perspective. It is expected that the results of this research will contribute to the maintenance and operational decisions, and provide a reference for management in the energy industry.

Insights into the Compositional Evolution of Crustal Magmatic Systems from Coupled Petrological-Geodynamical Models

AbstractThe evolution of crustal magmatic systems is incompletely understood, as most studies are limited either by their temporal or spatial resolution. Exposed plutonic rocks represent the final stage of a long-term evolution punctuated by several magmatic events with different chemistry and generated under different mechanical conditions. Although the final state can be easily described, the nature of each magmatic pulse is more difficult to retrieve. This study presents a new method to investigate the compositional evolution of plutonic systems while considering thermal and mechanical processes. A thermomechanical code (MVEP2) extended by a semi-analytical dike/sill formation algorithm, is combined with a thermodynamic modelling approach (Perple_X) to investigate the feedback between petrology and mechanics. Melt is extracted to form dikes while depleting the source region. The evolving rock compositions are tracked on markers using a different phase diagram for each discrete bulk-rock composition. The rock compositional evolution is thus tracked with a high precision by means of a database with more than 58 000 phase diagrams. This database describes how density, melt fraction, chemical composition of melt and solid fractions and mineralogical assemblages change over crustal to uppermost mantle P–T conditions for a large range of rock compositions. Each bulk rock composition is composed of the 10 major oxides (SiO2–TiO2–Al2O3–Cr2O3–MgO–FeO–CaO–Na2O–K2O–H2O) including an oxygen buffer. The combined modelling approach is applied to study the chemical evolution of the crust during arc magmatism and related melt extraction and magma mixing processes. Basaltic sills are periodically injected into the crust to model heat/magma influx from the mantle. We find that accumulated sills turn into long-lived mush chambers when using a lower rock cohesion or assuming a higher intrusion depth. Associated partial melting of crustal host rocks occurs around densely distributed dikes and sills. High silica rocks (e.g. granites) are generated by partial melting of the host rocks, melt segregation within dikes, and from fractional crystallization of basalts. Although the volume of these rocks is relatively small in our models compared to rocks with a mafic to intermediate composition, they provide important information about the processes of magma differentiation within arc continental crust.

Seismicity induced during the development of the Rittershoffen geothermal field, France

The development of the Rittershoffen deep geothermal field (Alsace, Upper Rhine Graben) between 2012 and 2014 induced unfelt seismicity with a local magnitude of less than 1.6. This seismicity occurred during two types of operations: (1) mud losses in the Muschelkalk formation during the drilling of both wells of the doublet and (2) thermal and hydraulic stimulations of the GRT-1 well. Seismicity was also observed 4 days after the main hydraulic stimulation, although no specific operation was performed. During chemical stimulation, however, no induced seismicity was detected. In the context of all field development operations and their injection parameters (flow rates, overpressures, volumes), we detail the occurrence or lack of seismicity, its magnitude distribution and its spatial distribution. The observations suggest the presence of the rock stress memory effect (Kaiser effect) of the geothermal reservoir as well as uncritically stressed zones connected to the GRT-1 well and/or rock cohesion. A reduction of the seismic rate concurrent with an increase of injectivity was noticed as well as the reactivation of a couple of faults, including the Rittershoffen fault, which was targeted by the wells. These results are derived from the homogeneous and consistent catalogue of more than 1300 local earthquakes that is provided. This reference catalogue is based on a standard detection method, whose output was manually verified and improved. The given absolute locations have been computed in a calibrated, geologically realistic 3D velocity model. Our work builds on previous analyses addressing the seismicity induced by the GRT-1 hydraulic stimulation and places the results into a historical context, thus considering the full dynamics of the observed phenomena. This paper also complements existing descriptions of the hydrothermal characteristics of the deep reservoir by providing insights separate from the wells.

Sensitivity Analysis Of Influencing Factors On Tunnel Stability In Bad Geological Slope Sections

Based on a tunnel through slope of Yongji highway project, relying on ABAQUS geotechnical numerical analysis software, using the appearance of plastic penetration area for instablilty criterion, choosing 11 typical influential factors as independent variables to tunnel stability, the orthogonal test array L50(511) is used to analyze sensitivity of influence factors under evaluating indexes of safety factor to the tunnel stability and the maximum principal tensile on the inner surface of surrounding rock. Research results show that: there are some certain differences between calculation results of safety factor and maximum principal tensile, but overall presentation represents favorable consistency, and the sensitivity of influence factors to the stability of tunnel through unfavorable geological slope can be preferably annlyzed by orthogonal test. The influence of the cohesion of rock c, the horizontal distance d between tunnel centerline and slope toe, the inclination angle of slope α are significant. Comprehensive consideration of safety factor and maximum principal tensile of tunnel circumference is more perfect than one-sided safety factor when performing the analysis of tunnel stability. The research results of this paper can provide reference to design, construction of similar tunnels.