A constitutive model was developed to describe the nonlinear behaviour of intact rocks under static loading. The model includes the pre-peak elastic and the post-peak strain-softening behaviour, as ...well as dilation. The model employs the shrinking of the failure criteria by progress of plastic deformation, to consider the strain-softening behaviour of rocks. A non-associated plastic potential function based on the dilation angle is employed to formulate the plastic deformation and dilation of rocks. Triaxial laboratory test results are used to derive the model equations. The model employs the assumptions that crack propagation in rocks during post-peak deformation is a cohesion-losing process, during which the frictional angle is constant. The assumptions were verified through careful study of the laboratory test results. In the developed model, the dilation angle is associated with the confining stress, friction angle of rocks and uniaxial compressive strength of rocks. Rocks have peak dilation angle at the point of peak strength and after that, it decreases with progress of plastic deformation, and is close to zero when rocks reach residual strength. The developed model is implemented in a finite difference code. The numerical results are compared with the test results, which show that the model captures the post-peak behaviour of rocks well.
•A softening behaviour of sedimentary rocks from peak strength to residual strength is modelled based on Mohr–Coulomb criterion.•Dilation behaviour of sedimentary rocks is defined in terms of mathematical equation.•All of the data are extracted from outcomes of triaxial test on three different rocks such as sandstone, silty-sandstone and mudstone.•Key factors in these proposed equations are cohesion, friction angle and dilation angle.•The observation of test data shows deduction in cohesion, minor change in friction angle and deduction in dilation angle.
•Extension/compression-first wave transmission differs owing to joint opening.•Joint tensile strength determines stress wave propagation under dynamic extension.•Rock bolt and shotcrete lining ...effectively support opening against dynamic extension.
Rock joints are prone to open and fail when subject to severe dynamic extension, which might result in spalling of surrounding rock and collapse of underground openings. However, the understanding of dynamic response of rock joints and underground openings subject to tensile stress wave is still at its infancy. To investigate the effect of tensile wave on the response, stability and supporting of underground openings in jointed rock mass, numerical modelling was carried out herein with the DEM-based universal distinct element code (UDEC) after its validation using explosion testing measurements. Results showed that joint tensile strength has significant effects on stress wave propagation if it is lower than the amplitude of tensile stress wave in the shallow rock mass, where joint opening will occur, no significant portion of wave energy could transmit through the joint, and the transmission coefficient for tension-first wave is lower than that for compression-first wave. The buried depth of underground opening and joint properties including stiffness, spacing and dip angle as well as crossing angle between joint sets could significantly influence stress wave propagation and dynamic responses of underground openings under dynamic extension. In addition, installation of rock bolts with appropriate number and length and shotcreting with sufficient thickness could effectively reinforce the surrounding rock and reduce the area of disturbed zones when against dynamic extension. The findings in this study could be of great significance for the design, supporting and stability evaluation of underground openings.
Mixed mode fracture experiments were conducted on Harsin marble using two disc-shape samples namely the Brazilian disc (BD) and the semi-circular bend (SCB) specimens. For each specimen, a complete ...fracture toughness envelope ranging from pure mode I to pure mode II was obtained. The experimental results indicate that the mixed mode fracture toughness depends on the geometry and loading conditions such that for any similar mode mixture, the BD test data were significantly greater than the SCB fracture toughness results. Therefore, the conventional fracture criteria which present a unique mixed mode fracture curve, fail to predict the test results. It is shown that a generalized criterion, which takes into account the effects of geometry and loading conditions, is able to provide individual fracture curves for theses specimens with very good estimates for the test results obtained from both BD and SCB specimens. The BD and SCB specimens can be suggested as appropriate specimens for obtaining typical upper bound and lower bound envelopes for mixed mode fracture toughness of rocks.
Fault slip events of rock masses around underground tunnels, such as irreversible displacements and slip-type rock bursts, can easily occur after external disturbances. To investigate the mechanism ...and threshold conditions of fault slip events triggered by external disturbances, a series of slip experiments using red sandstone blocks were performed with various horizontal pulling forces and vertical dynamic disturbances. The experimental results show that the relative vertical displacements of the adjacent blocks were the main reasons for anomalously low friction phenomenon; the initial stress state strongly affected the final residual horizontal displacements and the threshold conditions of fault slip events; as the shear force ratio β increased, the stabilities of the rock blocks became increasingly sensitive to disturbance energies, and the threshold disturbance energies for causing fault slip events decreased; and when β was close to 1, even a considerably small disturbance triggered a sustained slip failure. Based on energy analyses, a theoretical model was developed, and a dimensionless energy parameter I was introduced to quantifiably characterize the threshold conditions of different types of fault slip events. The threshold condition for rock blocks to start slipping was that the I-value should reach a threshold value, while the threshold conditions of a sustained slip failure were that the I-value should reach a larger threshold value and β should simultaneously exceed a certain value. The expression for the I-value was given and, can reflect the dependency relationships between the disturbance energies, shear force ratios and rock parameters, i.e., the yield strain, the length to height ratio and the ratio of the elastic coefficients. The experimental results and the threshold conditions derived from the theoretical model are essential when carrying out stability analyses and geohazard predictions in rock engineering.
•Novel experimental system for fault slip events induced by external disturbances.•A series of slip experiments using red sandstone blocks are performed.•Influences of the initial stress states on final residual displacements and threshold energy conditions are discussed.•The threshold energy conditions of irreversible displacements and sustained slip failures are quantifiably characterized.
•The methods of geological investigation, 3D scanning and CT scanning were adopted.•Natural structures of columnar jointed rock mass were investigated.•P-wave velocities of large scale columnar ...jointed basalt cores were tested.•Relationship between P-wave anisotropy and columnar joint was determined.•Distribution of internal implicit joint and its affect to anisotropy were analyzed.
Researches about the structural characteristics of rock mass are the basis for understanding their failure modes, mechanism properties as well as evaluation of structural stability during the process of excavation. Natural columnar jointed basalt has complex structures and exhibits remarkable anisotropy and size effect. In this study, the observation window method was firstly applied to describe the natural columnar joints and pillar shapes at engineering scale and then the structural characteristics of columnar joints were analyzed. The roughness of columnar joints scanned from large-size rock core obtained at drainage tunnel #1-1 was secondly investigated based on the 3D scanning method. Some techniques, such as field statistical analysis and CT scanning were used to analyse the characteristics of internal implicit joints in columnar jointed basalt. Thirdly, the large scale columnar jointed basalt cores were tested by P-wave, which assisted to investigating the anisotropy of columnar jointed basalt. The experimental and analytical results indicate that, at the engineering scale, natural columnar jointed basalt is heterogeneous, exhibiting the features of mosaic structure and mainly quadrilateral in shape. At the large-size rock core scale, the roughness of columnar joints is low with the value of 5.558. At the basalt column scale, the internal implicit joints are not randomly distributed, instead, showing obvious tendency. The internal implicit joints and columnar joints are important factors contributing to the special failure mode of columnar jointed basalt. At the rock core scale, with increasing angle between the P-wave propagation direction and column, the anisotropy of columnar jointed basalt core gradually increases and the degree of anisotropy is proportional to the number of columnar joints. The internal implicit joints could also affect the P-wave anisotropy of basalt column and the results in this study can provide some references for projects related to columnar jointed rock mass.
The W-Sn Panasqueira ore deposit is a magmatic-hydrothermal system, which includes a high grade quartz-vein type mineralization and underneath disseminated greisen-type mineralization located in the ...upper part of a two-mica granite. We investigate genetic and chronological relationships between greisenization of the Panasqueira granite and the formation of ore-bearing quartz veins by monitoring major and trace elements variations in quartz-muscovite assemblages composing the two-mica granite, greisen and ore-bearing quartz veins. Greisen is marked by an overall depletion in Mg, Ti, Ca, Na, Ba, Sr, REE and enrichment in Fe, Li, Rb, Cs, Sn, W that reflect the breakdown of feldspars and biotite and implication of W-Sn-bearing fluids during greisenization. Muscovite from greisen and mineralized quartz veins are enriched in granophile elements (F, Rb, Cs, Li, Sn, W and Zn) compared to magmatic muscovite from the two-mica granite. Trace elements contents in quartz depict evolutionary trends with progressive enrichment in Ge and B and depletion in Al, Ti and Li between magmatic and hydrothermal quartz that emphasize the progressive evolution and cooling of the magmatic-hydrothermal system of Panasqueira. Multivariate statistical approach applied on quartz and muscovite data demonstrates similarities in composition between quartz and muscovite from greisen with those composing ore-bearing quartz veins. These similarities suggest that greisenization and the formation of mineralized veins result from the same hydrothermal event and derived from the same source of hydrothermal fluids. Apatite from greisen and quartz vein yielded respectively U-Pb ages of 292±10 Ma and 295±5 Ma confirming that greisenization and the formation of mineralized veins occurred roughly at the same time. These ages also overlap with the cooling age of the Panasqueira granite (296±4 Ma), indicating a temporal and genetic link between greisenization, W-Sn mineralization and the granite crystallization. Temperatures of the magmatic-hydrothermal system constrained by Ti-in quartz thermometry depicts a cooling trend from magmatic quartz of granite (700-600°C) to hydrothermal quartz of greisen (500-400°C) and veins (450-350°C). These results provide evidences that greisenization and the formation of W-Sn bearing quartz veins occurred at the magmatic-hydrothermal transition, during which orthomagmatic fluids rich in volatiles, incompatible elements and metals (W and Sn) were exsolved at the final stage of solidification of the Panasqueira two-mica granite.
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•A FDEM coupling model is fist applied to deformation and failure of soft rock induced by humidity.•The damage zone and failure pattern of soft rock roadway are significantly affected ...by humidity.•Effects of element size and humidity diffusion coefficient on humidity damage zone are studied.
In underground space engineering, the humidity in the environment has a great influence on the structural performance of soft rock roadway. It not only changes the stress state of rock mass, but also weakens the physical and mechanical properties of surrounding rock, resulting in many accidents such as swelling deformation and even collapse of the roadway. In this paper, the deformation and failure process of soft rock roadway in high humidity environment are simulated by using the humidity diffusion-deformation-fracture coupling model based on the finite-discrete element method (FDEM). Firstly, the coupling model is verified through two simple examples with analytical solutions. Then, the humidity diffusion and floor heave of soft rock roadway after excavation are studied by using the coupling model. The results show that only the humidity of the surrounding rock surface changes greatly at the beginning of excavation and the roadway floor heave caused by humidity is small but with a large change rate. However, with further diffusion of humidity, the displacement of the floor increases slowly at a constant rate. Besides, several main factors affecting floor heave are studied, including humidity swelling coefficient, initial elastic modulus, and initial humidity of floor. Finally, the coupling model is used to simulate the roadway failure process induced by humidity diffusion. The numerical results provide a better understanding of the deformation and failure behavior of soft rock roadway in high humidity environment.
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•Post-Cryogenian (∼600 Ma) iron formations reported for the first time from North Qilian.•Kawa IF shows intimate association with submarine volcanism and hydrothermal activities, in ...response to the breakup of Rodinia.•The Ediacaran seawater was characterized by deep ferruginous conditions.
Iron formations (IFs) are relatively rare in the Neoproterozoic, and a few examples are closely related to the Cryogenian (∼717–635 Ma) Snowball Earth glaciations. In this study, we investigated the rare IFs formed during the Ediacaran period (635–541 Ma) which are hosted in volcano-sedimentary successions belonging to the Zhulongguan Group in the North Qilian Orogenic Belt in northwestern China. Detrital zircon U–Pb data from the tuffaceous breccia associated with the Kawa IF in this region show a large variation in ages between 749 Ma and 2706 Ma. However, two ferruginous tuff samples yielded zircon 206Pb/238U ages of 601.9 ± 6.1 Ma and 598.2 ± 5.5 Ma, respectively. These ages indicate that the Kawa volcano-sedimentary successions and associated IF were deposited at ∼ 600 Ma. Banding/lamina are not well developed in the IF and the rock is dominantly composed of hematite and jasper, with minor clay minerals and breccia. The IF shows high TFe2O3 and SiO2 contents (TFe2O3 + SiO2 = 83–90 wt%) with relatively elevated Al2O3, TiO2, HFSEs (e.g., Nb, Hf and Th) and rare earth element (REE) concentrations. The geochemical features are consistent with chemical sediments with significant clastic material input (∼20%). The rocks also show light REE depletion (La/Yb)PAAS = 0.23–0.34, slightly positive Eu anomalies (Eu/Eu*=1.03–1.12), low Y/Ho ratios of 27.17–29.53, and slightly negative εNd(t) values (−2.03 – −0.70), indicating a submarine hydrothermal source for the iron with dilution by detrital components and seawater. The lack of negative Ce anomalies and redox-sensitive trace element enrichments (e.g., Mo, U, and V), the presence of chamosite, and the relatively high TFe2O3 contents in the ferruginous tuff and slate suggests anoxic and ferruginous conditions. Our results link the post-Cryogenian Kawa IF with submarine volcanism and hydrothermal activity, in response to the breakup of Rodinia.
Traditional hydraulic fracturing techniques generally form main hydraulic cracks and airfoil branch fissures, but main hydraulic cracks are relatively few in number. Hydraulic fracturing after water ...pressure control blasting can transform the structure of coal and rock mass. Experiments prove that it is an effective method for increasing the number and range of hydraulic cracks, as well as for improving the permeability of coal seams. The technical principle is as follows. First, a hole is drilled in the coal seam and is injected with a gel explosive (a mining water-proof explosive). Then, water is injected into the hole to seal it, at low enough pressure to prevent cracks from forming. Third, water pressure blasting is done by detonating the explosive. The water shock waves and bubble pulsations produced by the explosion cause a high strain rate in the rock wall surrounding the hole. When the stress imposed on the rock wall surrounding the hole exceeds its dynamic critical fracture strength, the surrounding rock breaks and numerous circumferential and radial fractures propagate outward. Lastly, water injection processes, such as general injection, pulse injection, and/or cyclic injection, are carried out to promote hydraulic fracturing. Depending on the fissure water pressure, detonation fissures continue to expand and additional hydraulic fractures with a wider range are formed. Under the effect of detonation pressure, joints and fissures in the coal mass open and propagate, leading to reduced adhesive forces on structural surfaces and thereby enhancing coal cutting. Therefore, this method improves the permeability of the coal seam, effectively weakens the strength of the coal and rock mass, and reduces the surrounding rock stress of the weakened area, effectively solving the problem of having a small number of big cracks. It is a useful technical approach for improving top coal caving, preventing rock burst, preventing coal and gas outbursts, and raising the gas extraction efficiency in colliery.
► We conducted a test of hydraulic fracturing after water pressure control blasting. ► This is an effective method for increasing the number and range of hydraulic cracks. ► It decreases adhesion force of the structural planes and cuts the coal and rock mass more severely. ► We proposed the technology of that for increasing permeability and weakening strength. ► The technical principle and process were given.
The Solonker Suture Zone is commonly recognised as the location of the Late Permian to Early Triassic closure of the Palaeo-Asian Ocean in the southeastern segment of the Central Asian Orogenic Belt. ...However, the absence of typical suture-related features, as a consequence of uncommon collisional geometries, gave it a cryptic nature. Thus, the tectonic setting, which led to suturing, still remains enigmatic. A geochemical characterisation of Permian sedimentary and volcanic rocks across the suture was carried out. Supplemented with Hf and Nd isotopic analyses, this approach enables not only a better definition of such regional suture, but also estimates on the long-controversial issue of net crustal growth in accretionary tectonic environments.
The results indicate short sedimentary transport distances between the arc basins and their provenances, of which the studied volcanic rocks were a major contributor. Similar enrichment and depletion patterns with respect to N-MORB and average continental crust further corroborate a close source–sediment relationship. Immobile element provenance analyses indicate that the active continental northern margin of the North China Craton was a major source for arc basins to the south of the Solonker Suture Zone. To its north, arc basins are interpreted to be sourced by a more complex mixture of provenances, e.g., the Baolidao volcanic arc suite and the heterogenous Precambrian basement of southern Mongolia. An overall collisional tectonic setting across the suture is recognised. The geochemical signature of sedimentary rocks to the south of the suture points at an active continental arc setting, whereas the bimodal geochemical distribution of the samples to the north shows a contemporaneous active oceanic island arc as well as a passive margin environment. These features favour a double-sided subduction of the Palaeo-Asian Ocean beneath the North China Craton and the Mongolian Arcs throughout the Palaeozoic, including back-arc basin opening north of the suture in the Permian.
Analysis of Hf and Nd isotopic compositions revealed that magmas to the south were produced involving significant crustal contamination, thus having less radiogenic compositions. North of the suture, however, isotopic compositions tend to be more radiogenic, implying a more juvenile contribution. However, the bulk isotopic compositions are close to present-day CHUR, suggesting that crustal reworking appears to be equally balanced by juvenile addition during the Palaeozoic closure of the Palaeo-Asian Ocean across the Solonker Suture Zone.
•Permian arc basins and their provenances are geochemically characterised.•Distinct sedimentary provenances are recognised across the Solonker Suture Zone.•Net crustal growth across the Solonker Suture Zone varies from marginal to significant.
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