In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the ...development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased.
•Influence of the intermediate principal stress on deformation, strength, and failure mode of sandstone and coal.•Different Mogi-type strength criteria were discussed and compared.•The interval effect of intermediate principal stress on coal rock strength was discussed.•The envelope of true triaxial strength of rocks on the π plane was discussed.•The influence of weak structures on the failure mode of coal was discussed.
Under compression, gathering data related to the post-failure part of the stress–strain curve requires stiff servo-controlled testing systems. In unconfined conditions, data related to the post-peak ...region of the intact rock parameters are not common as pre-peak and peak state parameters of stress–strain behavior. For problems involving rock in the failed state around structures, proper choice of plastic constitutive laws and post-failure parameters is important for the modeling of the failed state. The aim is to relate commonly used intact rock parameters of pre-failure (tangent modulus
E
i
and secant modulus
E
s
) and peak strength (
σ
ci
) states to parameters of the post-failure state under unconfined compression. Post-failure parameters are the drop modulus (
D
pf
), representing the slope of the falling portion in brittle state, residual strength (
σ
cr
), and dilatancy angle (
ψ
°). Complete stress–strain curves were generated for various intact rock of different origin. Seventy-three post-failure tests were conducted. Samples included in the testing program were chosen to represent rocks of different origin. Specimens of granite, rhyodacite, dunite, quartzite series, glauberite, argillite, marl, and lignite were used in the tests. The results from the pre-failure and peak state testing parts were processed and compared to the post-failure stress–strain parameters. For the estimation of post-failure parameters in terms of the pre-peak and peak states, the functional relations were assessed. It was found that the drop modulus
D
pf
increases with rock strength
σ
ci
, following a power function with an approximate power of two. With an exponential trend, the
D
pf
/
E
s
ratio increases with decreasing
E
i
/
σ
ci
ratio. Relations estimating the residual strength and dilatancy from the pre-peak and peak state parameters are in logarithmic and exponential functional forms, respectively.
The International Society for Rock Mechanics has so far developed two standard methods for the determination of static fracture toughness of rock. They used three different core-based specimens and ...tests were to be performed on a typical laboratory compression or tension load frame. Another method to determine the mode I fracture toughness of rock using semi-circular bend specimen is herein presented. The specimen is semi-circular in shape and made from typical cores taken from the rock with any relative material directions noted. The specimens are tested in three-point bending using a laboratory compression test instrument. The failure load along with its dimensions is used to determine the fracture toughness. Most sedimentary rocks which are layered in structure may exhibit fracture properties that depend on the orientation and therefore measurements in more than one material direction may be necessary. The fracture toughness measurements are expected to yield a size-independent material property if certain minimum specimen size requirements are satisfied.
In natural rock masses, joints or flaws usually occur in sets that are more or less parallel and regularly spaced. Many problems occurred in tunnel engineering mainly result from the failure of ...joints or faults because they are weaker compared with the rock matrix. Therefore, it is important to study the interaction of joints with the tunnel, as well as the interactions between themselves. In this research, rock-like specimens of non-persistent jointed rock masses containing a circular hole are prepared and tested under uniaxial compression. The test results show that the peak strength and elastic modulus of the non-persistent jointed rock specimens display a “U” type variation with respect to the joint inclination. The multiple joints can be simplified and regarded as a combination of three types of two-joints relative location, and the crack coalescence types and failure modes are in accordance with the classic crack coalescence types of the two joints. The crack coalescence between the joints and a circular hole can be categorized into three modes with respect to the joint inclination. The Discrete Element Method (DEM) model of the jointed rock specimen that contains a circular hole is established in particle flow code 2D (PFC2D). The simulation results show a good agreement with the experiment results. Furthermore, the displacement fields at the crack zones reveal that the coalescence of joints is mainly caused by tensile cracks and that shear slipping cracks easily occur at the sidewalls of the circular hole. By comparing the parallel-bond force fields in jointed models containing a circular hole with no a circular hole, it is shown that the circular hole can change the stress state at the top and bottom. The joints at a joint inclination of 90° have the smallest effect on the mechanical behavior of the rock specimen.
An integrated study on two subduction-related units (the Heilongjiang Complex and magmatic rocks in the Lesser Xing'an-Zhangguangcai Range) was carried out to explore the Paleozoic to early Mesozoic ...tectonic framework of the eastern Northeast China, with a focus on the Jiamusi block. The Heilongjiang Complex, which is considered as the suture between the Jiamusi and Songliao blocks, is composed of blueschist, amphibolite and ultramafic rocks. The blueschist from the Yilan area shows geochemical features similar to OIB and has a 206Pb/238U age of 288±2Ma, which is interpreted as the crystallization age of protolith formed in an ocean island setting. In contrast, amphibolite from the Xiachengzi area shows geochemical features similar to N-MORB. Zircons in this amphibolite show magmatic character and yield a 206Pb/238U age of 248±4Ma, which is interpreted as the protolith age. Calcic amphiboles in the amphibolite give a 40Ar/39Ar plateau age of 195±3Ma, which is interpreted as the metamorphic age of the Heilongjiang Complex. Magmatic rocks in the Lesser Xing'an-Zhangguangcai Range mostly belong to I-type granites that were formed in an active continental margin setting. Zircon U-Pb ages of these granites range from ~200Ma to ~174Ma with a westward younging trend shown as two groups of emplacing ages of ~200Ma in the east and ~180Ma in the west. Our new data support an early occurrence (likely since the early Permian) of a large ocean (namely the Mudanjiang Ocean) in the area, whose subduction resulted in the metamorphic unit (the Heilongjiang Complex) and the magmatic unit (the Lesser Xing'an-Zhangguangcai Range) in the Jurassic.
•The metamorphism of the Heilongjiang Complex likely occurred during the Early Jurassic.•The protoliths of metabasalts have affinities to OIB and N-MORB.•The Mudanjiang Ocean began to exist from early Permian and started to subduct in the Jurassic.
Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and ...temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressure-controlled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixed-mode deformation processes triggered by high-pressure injection.
This open access volume explores the impact of globalization on the contemporary study of deep-time art. The volume explores how early rock art research’s Eurocentric biases have shifted with ...broadened global horizons to facilitate new conversations and discourses in new post-colonial realities. The book uses seven main themes to explore theoretical, methodological, ethical, and practical developments that are orienting the study of Pleistocene and Holocene arts in the age of globalization. Compiling studies as diverse as genetics, visualization, with the proliferation of increasingly sophisticated archaeological techniques, means that vast quantities of materials and techniques are now incorporated into the analysis of the world’s visual cultures. Deep-Time Images in the Age of Globalization aims to promote critical reflection on the multitude of positive – and negative – impacts that globalization has wrought in rock art research. The volume brings new theoretical frameworks as well as engagement with indigenous knowledge and perspectives from art history. It highlights technical, methodological and interpretive developments, and showcases rock art characteristics from previously unknown (in the global north) geographic areas. This book provides comparative approaches on rock art globally and scrutinises the impacts of globalization on research, preservation, and management of deep-time art. This book will appeal to archaeologists, social scientists and art historians working in the field as well as lovers of rock art.
The deformation modulus of heavily jointed andesitic rock mass was investigated by using 41 Menard pressuremeter test results and geotechnical information obtained from seven geotechnical boreholes. ...The geotechnical borehole logs and laboratory test results provided by a mining company were used for the assessment of rock mass characterization. The log and the test results were taken into account in the rock mass classification work. The well-known empirical equations were employed to predict the deformation modulus of the rock mass. The predictions and the pressuremeter test results were compared and their performance indicators were presented. Non-linear multiple regression methods were used for predictive modelling of the deformation modulus of the rock mass by considering the available data. Rock Quality Designation (RQD), discontinuity condition rating (Dc) of the Rock Mass Rating (RMR) system and uniaxial compressive strength of the intact rock (σci) were taken into consideration as input parameters in four new prediction equations to be used for determination of deformation modulus. One more equation was proposed by the addition of the depth as a predictive parameter. The influence of the depth and vertical to horizontal stress ratio (k) on the deformation modulus was preliminarily examined by using finite element modelling. Initially, the rock mass was assumed to be an isotropic elastic-brittle-plastic medium. Later on, the rock mass was modelled as a discontinuum by imposing a discrete fracture network (DFN). Keeping all of the mechanical properties constant, different depth and k parameters were applied to the pressuremeter models. No influence of the depth and k on the deformation modulus was observed for the isotropic medium while the depth contributed to an increase in the modulus in the anisotropic discontinuum analysis. The numerical modelling findings constituted a basis for the inclusion of the depth parameter into the new predictions.
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