Rock Blocks Research Articles

A study on the evolution of microstructure of soil-rock mixtures based on the electrochemical conductive paths model under different rock and moisture content conditions

ABSTRACT To further improve the understanding of the evolution law of the microstructure of SRM in road engineering under moisture and rock content, the electrochemical impedance spectroscopy test under different moisture and rock conditions was carried out. This paper proposed an electrochemical model of different conductive paths based on different cementation states. The impact of rock and moisture content on the characteristic parameters of the conductive path model was analyzed. The results indicated the impedance spectrum curves became more pronounced with the continuous decrease in moisture content and increase in rock content. There was a well-defined exponential relationship between total impedance and both moisture and rock content. The total impedance decreased continuously with increasing moisture content and increased with increasing rock content. The characteristic parameters had practical significance in representing the microscopic structure. The moisture content primarily influenced the variation trends of the representation parameters by altering the channel width, length, and quantity of the continuous pore water solution pathways and the discontinuous rock-soil bonded bodies. The rock content affected the changes by compressing the external and internal pore water solution and soil particles, altering the compression and support between the continuous pore water solution pathways and the block rock framework.

Evidences of deformations of minerals of the Berkut quartz vein (South Urals) affected by dynamometamorphic processes

The article examines petrography and mineralogy of rocks of the Berkut quartz-feldspar vein in the South Urals, which is characterized by intense deformation in comparison with numerous quartz veins of the Kyshtym block. These data allowed the sequence of formation and transformation of the Berkut vein. The vein is a typical vein of replacement, which is composed of blocks of quartz and granite rocks, and contains relics of host rocks (amphibolites and gneisses). Our studies revealed the tectonic transformations of rocks and minerals, as well as the deformation mechanisms with leading plastic deformation and processes of dissolution of minerals under pressure together with brittle fracturing. Dissolution and further deposition led to the crystallization of minerals of new generations (quartz, albite, and oligoclase) and often minerals of different composition (biotite after amphibole). Subsequent metasomatosis is responsible for new generations of minerals (quartz, albite, microcline, muscovite, epidote, zoisite, and clinozoisite) with different morphology and crystal optical properties.

Quality evaluation of surrounding rocks in a mine shaft based on the game gray target model

Accurate evaluation of surrounding rock quality grade can help guarantee the safety of tunnel design and construction; hence, it has great significance in the construction of mine shafts. Accordingly, the uniaxial saturated compressive strength of the rock block Rc, rock quality index RQD, rock softening coefficient kR, integrity coefficient of the rock mass kv, depth H, unit weight of the rock mass γ, coefficient of quantification of the angle between the principal structural plane and shaft axis k2, and weight of the groundwater k1 are first selected as the indexes of assessment to introduce the game gray target model. Then, the gray target model of the surrounding rocks in a mine shaft project is established. The weight coefficient of each index is next calculated using the combination weighting method based on game theory, and the synthetic target center distance of each sample is determined using the gray target model. Finally, the quality level of the asphalt pavement is determined. The suggested model can be used to mine a small data sample to the maximum extent possible, thereby minimizing the information shortage caused by the small sample to a certain extent, to evaluate the final quality grade of the surrounding rocks quantitatively. Thus, the proposed approach provides a new scheme for the future quality assessments of surrounding rocks.

Progressive failure and quantitative stability analysis of rock slope with cross-type discrete fracture network using spring-based smoothed particle hydrodynamics method

The propagation and coalescence of discontinuous cross-type joints are often the primary drivers of rock slope failures. Due to the inherent randomness of these joints, identifying suitable methods to accurately characterize progressive failure and quantitatively assess the stability of slopes with cross-type joints is particularly challenging. To address this, a Discrete Fracture Network-Spring Based-Smoothed Particle Hydrodynamics (DFN-SB-SPH) method is proposed, which considers cross-type DFN generation, rock matrix heterogeneity, cracking, block contact, and deposition within a unified SPH framework. In this approach, damage and contact of blocks are achieved by introducing a fracture sign to enhance the kernel function and applying Newton's law to the damaged particles to realize the point to point contact. A uniaxial compression test on rock samples with double fissures is carried out to verify the accuracy of the proposed method. The influence of joint density, geometric parameters (i.e., trace length, dominate dip angle) of cross joints, and rock mass heterogeneity on the failure characteristics of RSCD are investigated. The results revealed that the DFN-SB-SPH model possesses robust capabilities for crack tracking, mass movement, and contact slipping along fracture surfaces. Moreover, DFN generation, heterogeneous characterization, shear and tensile fractures within a single SPH framework without mesh distortion are realized. Five typical failure modes calculated by DFN-SB-SPH are primarily determined by the combination of joint dip angles and trace lengths. Furthermore, the dip angle, trace length, joint density, and rock mass heterogeneity significantly alter the stability of RSCD with sensitivity coefficients for factor safety of 27.7 %, 44.6 %, 72.9 % and 35.0 %, respectively. Higher dominant dip angles (above 60°) lead to significantly longer run-out distances for RSCD compared to lower angles (below 30°) once instability occurs. The sudden rise in major principal stress at the slope's top and toe, along with the shift of tensile stress concentration towards the middle, predicts the stages of crack propagation and sliding in slope failure.

Effect of gradation on macro-meso shear properties of the alluvial-diluvial soil-rock mixture (ADSRM)

Alluvial-diluvial soil-rock mixture (ADSRM) is a unique geological material primarily consisting of pebbles, gravel, and soil. Gradation design significantly enhances the shear strength of ADSRM, offering significant implications for sustainable construction of transportation infrastructure in mountainous regions. Hence, the N-method of gradation theory was employed for the gradation design of ADSRM fillers in this study. Macro-mechanical testing reveals that the graded sample exhibits significantly higher shear strength, equivalent cohesion, and friction angle compared to the ungraded sample, with more pronounced shear dilatancy. Meso-shear properties suggest that the graded sample has a lower percentage of pore area compared to the ungraded sample, but a higher percentage of small pores relative to total pores, and more crushed rock blocks under the same normal stress conditions. Pore abundance indicates that the majority of pores on the shear plane are oblate in shape. The findings from the equivalent diameter and fractal dimension of the rock block contour suggest improved grinding roundness of rock blocks in the graded sample post-shearing. In summary, the quantity of pores and broken rock blocks primarily impacts the strength and equivalent cohesion of the filler, while the abundance, roundness, shape coefficient, and fractal dimension of rock block contour mainly affects the internal friction angle.

Middle Triassic basaltic pyroclastic rocks from the Mt. Medvednica ophiolitic mélange (NW Croatia): petrology, geochemistry and tectono-magmatic setting

Hectometric blocks of Middle Triassic mafic pyroclastic rocks, represented by volcanic agglomerates/breccias and lapilli tuffs, form part of the ophiolitic mélange of Mt. Medvednica, situated in the southwestern segment of the Zagorje-Mid-Transdanubian Zone. These rocks share petrochemical characteristics with pyroclastic derivatives of alkali, within-plate basaltic lavas of Mts. Medvednica, Samoborska Gora, and Kalnik, indicating the occurrence of explosive events preceding the dominant effusive submarine volcanism during the Middle Triassic (Illyrian-Fassanian?) stages. The formation of these pre-ophiolitic pyroclastics is associated with an intracontinental rift setting and reflects melts derived from an OIB-type enriched mantle plume source. These pyroclastics represent uncontaminated melts that erupted through a highly thinned continental crust. In geodynamic terms, the formation of pyroclastites occurred during the Late Anisian-Early Ladinian along the continental margin of Palaeotethys through the proto back-arc rifting of continental lithosphere (Adria Plate), leading to the formation of the Maliak/Balkan Neotethys Rift, in the emerging northwestern segment of Neotethys. The investigated pyroclastic rocks of Mt. Medvednica document the extension in an evolved intracontinental rift basin, which immediately preceded the generation of the initial Neotethyan oceanic lithosphere during the Upper Triassic.

Finite–Discrete Element Method Simulation Study on Development of Water-Conducting Fractures in Fault-Bearing Roof under Repeated Mining of Extra-Thick Coal Seams

The formation of water-conducting fractures in overlying strata caused by underground coal mining not only leads to roof water inrush disasters, but also water-conducting fractures penetrate the aquifer, resulting in the occurrence of a mine-water-inrush disaster and the loss of water resources. It destroys the sustainability of surface water and underground aquifers. This phenomenon is particularly significant in extra-thick coal seams and fault-bearing areas. Numerical simulation is an effective method to predict the failure range of mining overburden rock with low cost and high efficiency. The key to its accuracy lies in a reasonable constitutive model and simulation program. In this study, considering that the three parts of penetrating cracks, non-penetrating cracks, and intact rock blocks are often formed after rock failure, the contact state criterion and shear friction relationship of discrete rock blocks and the mixed fracture displacement–damage–load relationship are established, respectively. Combined with the Mohr–Coulomb criterion, the constitutive model of mining rock mass deformation–discrete block motion and interaction is formed. On this basis, according to the engineering geological conditions of Yushupo Coal Mine, a numerical model for the development of water-conducting cracks in the roof with faults under repeated mining of extra-thick coal seams is established. The results show the following: The constitutive relation of the continuous deformation–discrete block interaction of overlying strata and the corresponding finite element–discrete element FDEM numerical program and VUSDFLD multi-coal seam continuous mining subroutine can numerically realize the formation process of faults and water flowing fractures in overlying strata under continuous mining of extra-thick multi-coal seams. The toughness of sand mudstone is low, and the fracture will be further developed under the repeated disturbance of multi-thick coal seam mining. Finally, it is stabilized at 216–226 m, and the ratio of fracture height to mining thickness is 14.1. When the working face advances to the fault, the stress concentration occurs in the fault and its overlying rock, which leads to the local fracture of the roof rock mass and the formation of cracks. The fault group makes this phenomenon more obvious. The results have been preliminarily applied and tested in Ningwu mining area, which provides theoretical support for further development of roof water disaster control under the condition of an extra-thick coal seam and avoids the loss of water resources in surface water and underground aquifers.

Magnetic restoring forces on rocking blocks

AbstractThis study investigates the idea of adding an extra magnetic restoring force to a rocking block to improve its overall dynamic performance. The proposed concept ensues by introducing a pair of identical magnets to the rocking block. Both magnets are considered lumped on their respective volume centers and are embedded within the rocking block and the supporting base. When properly magnetized, this pair of magnets provides the rocking block with an extra magnetic restoring force which, although it takes on its maximum value when the two magnets are in contact, decreases as the distance between the two magnets increases. The proposed concept, subjected to pulse‐type base excitations, reveals the inherent problem of magnetic restoring forces. From the overturning spectra of the rocking block, it is found that there are cases where the block fails (overturning) in the presence of magnets, while the same free‐of‐magnets block rocks safely (no overturning) when its own weight acts as the only restoring force. This interesting finding appears to be counterintuitive. Is it possible that by providing additional restoring force the block is “driven” to overturn? This study shows that when the rocking block returns toward the vertical position, the angular velocity, in the presence of magnets, is higher than the angular velocity, in the absence of them. This increase in the angular velocity is a direct outcome of the nature of the magnetic restoring forces, and it is mainly the reason that causes the overturning of the rigid block during its free vibration regime. To mitigate the shortcomings of using magnetic restoring forces, the idea of a semi‐active control of the pair of magnets is introduced and explained in detail. This paper concludes with the advantages and potential disadvantages of the overall performance of rigid blocks in the presence of magnetic restoring forces.

The Proterozoic–Cambrian salt detachment zone at Lakhar Kuh, Iran: a seismic-scale analogue for salt stringers, detachment folding and multiphase structural development

Abstract In the Middle East, significant evaporite units formed in the latest Precambrian–Cambrian, Triassic, Jurassic, Cretaceous and Cenozoic. The Precambrian–Cambrian period gave rise to the Ara Salt carbonate stringer plays, southern Oman and the giant Zagros anticline traps (Hormuz Formation). While outcrops of salt diapirs are common, basal detachment exposures are extremely rare. The Lakar Kuh area of Central Iran reveals the Precambrian–Cambrian basal salt detachment, on satellite images, in natural cross-section view through the entire Phanerozoic sedimentary section east of Ravar. This view illustrates how older diapiric structures (pillows and normal faults) were the focus of later contractional folds, particularly in the Jurassic and Cenozoic. The salt detachment zone contains many floating blocks (stringers) of clastic, carbonate and igneous rocks. Some blocks were stoped from the overlying beds, while most were layers originally interbedded with the evaporites. Block size, distribution and orientation is highly variable, and folding is infrequent. Lakar Kuh encompasses several key themes generally pertinent to structural geology and salt system research: the presence of sedimentary stringers within evaporites, multiple detachment levels within a thick (>5 km) stratigraphic section, detachment folding, multiphase salt activity, reactivation of older structures by newer ones and multi-stage development of salt bodies.

Research on the Movement of Overlying Strata in Shallow Coal Seams with High Mining Heights and Ultralong Working Faces

To study the roof movement and ground pressure evolution characteristics of an ultralong working face in a shallow coal seam with a high mining height, the Shangwan Coal Mine in the Shendong mining area was used as the research background, and the physical and mechanical parameters of the surrounding rock were determined through rock mechanics experiments. A physical simulation model was built considering the 7 m mining height of the 12301 fully mechanized working face of the Shangwan Coal Mine to simulate and study the evolutions of the movement, fracture and collapse of the coal seam, direct roof, and basic roof and overlying strata during the mining process. The mechanical characteristics of the support, mechanism of roof collapse, and changes in the working resistance of the support were analysed and simulated. The research results indicate that when mining at a height of 7 m, the direct roof and basic roof strata collapse in layers; the basic roof strata collapse backwards, the rock block arrangement is more irregular, and the range of the basic roof that can form structural rock layers extends higher. After the basic roof rock fractures, it cannot form a masonry beam structure and can only form a cantilever beam structure. The periodic fracture of the cantilever beam causes periodic pressure on the working face. These research results are of great significance for planning the further mining of shallow coal seams with high mining heights and ultralong working faces in the Shendong mining area, as well as for improving the control of overlying strata.

Investigation on vibration characteristics of rock based on circular plate and cylinder model: dimension, geometric shape and boundary condition

Abstract The declaration on the “natural frequency of rock” exists in many engineering areas, and it has caused many misunderstandings. Different from the mass-spring model usually used, the circular plate model and cylinder model are respectively established to clarify the relationship between the vibration characteristics (including natural frequency and vibration mode) and their influencing factors of rock by modal analysis. The effect of dimension, geometric shape, and boundary condition on the vibration characteristics of rock with plate structure is investigated, in which the semi-analytical solutions agree well with the simulation results. By using the cylinder model based upon the Lamé-Navier Eq., the effect of such influencing factors on the vibration characteristics of the block rock sample is further studied and verified by numerical simulation and experimental results. The results suggest that the natural frequency of “rock” (including the experimental rock sample) is strongly dependent on the dimension, geometric shape, and boundary condition. The resonance frequency observed in the excitation experiment is not only closely associated with the natural frequency of a specific order, but also dependent on the dominance of the particular vibration mode. These findings contribute to a better understanding of the rock-breaking mechanism under dynamic loads with a certain excitation frequency.

Research on the mechanism and application of wedge cutting blasting with hole-inner delay

To increase the efficiency of deep-hole blasting driving in mine rock tunnels, an innovative pattern of wedge cutting blasting with hole-inner delay was proposed. First, the blasting mechanisms of conventional and innovative wedge cutting patterns were theoretically investigated. The results showed that the resistance from large upper rock blocks and the clamping action from the surrounding rock were the major challenges of conventional cutting methods. For the innovative cutting pattern, under the conversion of the spatial distribution and release sequence of blasting energy, the first blasting of the upper charge can strengthen the breaking of the upper rock mass and create a new free surface, which provides favorable conditions for the delayed blasting of the bottom charge. Second, finite element models of two cutting patterns were established and solved, and the simulation results visually revealed the propagation of a stress wave. Critically, the stress strength in the upper cavity increased by 66–83% under the action of the upper charge, which was conducive to the breaking of the upper rock mass and the generation of a new free surface. Therefore, the rock mass in the bottom cavity can be readily broken and discharged. Ultimately, field applications were executed in a rock tunnel. Compared with a conventional cutting pattern, the proposed innovative cutting pattern can prominently increase the cycle advance and hole utilization and greatly reduce the unit consumption of explosives and detonators. This research confirms the usability of the innovative wedge cutting pattern with hole-inner delay in deep-hole blasting driving of rock tunnels.

Mechanism of radial chain rockbursts in a deep granite TBM tunnel under the influence of weak band

Radial chain rockbursts (RCRs) in deep-buried TBM tunnels have prolonged durations and can pose significant safety risks. Further research is needed to deepen our understanding of the mechanisms of recurrence of rockbursts under the influence of structure plane. In this study, the geological characteristics, occurrence characteristics, microseismic (MS) activity laws, and rock mass fracture mechanism of several RCRs influenced by weak band were summarized to reveal the mechanism of recurrence of RCRs. Research shows that under the influence of weak band, (1) RCRs tends to occur in the area where the dip angle is large, and the strike intersects with the tunnel direction at a large angle and forms a small angle with the maximum principal stress. The width of the weak band typically ranges from 0.01 to 1 m, which is positively correlated with the final depth of rockburst pit. The strength of the weak band is relatively “soft” compared to the surrounding bedrock. (2) The depth of each rockburst pit in RCRs gradually decreases, the time interval between adjacent rockbursts gradually increases, and the maximum thickness of rock block in each rockburst gradually increases. (3) The maximum MS energy of MS events and the maximum apparent stress gradually decreased in the development process of RCRs. The region of stress concentration gradually extended to deeper zone, and the distance difference between stress concentration region and sidewall during each rockburst occurrence stage gradually decreased. (4) The effect of weak band on RCRs attributes to the excavation induced local stress concentration near the weak band, and the interface between the weak band and surrounding rock controls the boundary of the rockburst body. The “chain effect” of RCRs mainly shows that the promotion effect of the previous rockburst on the subsequent rockburst, by promoting the stress and rock fracture transferring to the deep zone, and the proportion of shear fractures increases.

Quantitative Risk Assessment of Rock Instabilities Threatening Manazan Caves, Karaman, Türkiye

Located in the southern part of Central Anatolia Region, Manazan Caves is situated in the east of Yeşildere Valley. It is a five-storey mass dwelling carved into a human-made high rock mass. Although there are no exact data to determine when the caves were carved, it is assumed that the caves were carved and settled in the Byzantine period during the 6th − 7th centuries. The geological and geotechnical data analyses were conducted in Manazan Caves. As a result of these analyses, it was observed that rock blocks of different sizes fell into the field in various times of the year, and there were instabilities in some individual rock blocks. In the study area, the movement mechanisms of rock slopes due to discontinuity were analysed by using various computer programs (DIPS, RockFALL, GEO5); and in order to ensure the safety of life and property and to protect this heritage cave, the current rockfall potential and risk along with slope instabilities were determined, and the fundamental prevention measures to be taken were discussed.

Influence of weak interlayer thickness on mechanical response and failure behavior of rock under true triaxial stress condition

In this study, we meticulously investigate the influence of the thickness of weak interlayer on mechanical response and failure behavior of rock mass near the free surface of tunnel, with the aid of a true triaxial test system. We simulated the true triaxial stress condition of rock mass containing weak interlayer proximate to the excavation boundary, where one face remains free, while others are subjected to loading stress and increasing tangential stress. The experimental results reveal a distinct change in the fracture morphology near the free surface of rock mass as the thickness of weak interlayer increases, concomitant with a gradual decrease in the fractal dimension. With the increase of the thickness of weak interlayer, the failure characteristics of rock mass change from a “binary” to a “unitary” feature, i.e. mixed tension-shear failure near the free surface and shear failure far away from the free surface changes to mixed tension-shear failure that only occurs near the free surface. Moreover, an inverse relationship is observed between the bearing capacity of the rock mass and the thickness of weak interlayer. Conversely, the peak strain in the tangential stress direction increases with the increase of the thickness of weak interlayer, illustrating that the slip shear strain of rock block positioned above the weak interlayer is gradually formed, and governs the strain behavior of rock mass in the tangential stress direction with the increases of the thickness of weak interlayer. By employing the finite-discrete element method (FDEM), we replicated the failure process of surrounding rocks containing weak interlayers near the free surface of tunnel, demonstrating considerable congruity with our physical test results. In light of the failure characteristics of surrounding rocks containing weak interlayers, we advocated a localized area priority reinforcement support strategy, prioritizing regions with weak interlayers. Numerical simulations verified the effectiveness of this approach in controlling fracture proliferation in surrounding rocks and inhibiting the splitting failure of surrounding rocks containing weak interlayers near the free surface of tunnel.

The application of flexible and porous concrete structures in training works and scour protection

Concrete armour units such as Xbloc have been applied as armour layer for breakwaters and bank protections for years. These smartly shaped blocks were invented to offer efficient protection at more exposed locations by applying blocks that work together with neighbouring units to withstand high wave loads (Interlocking). Also material use was lowered since steeper slopes could be realized compared to conventional rock structures. Xstream and Xstone can be applied in a large range of applications that are traditionally built with loose rock, pitched rock or smooth block revetments such as: - Bed protection (e.g. offshore wind foundations) - River training works - Detached breakwaters - Quay wall protection in harbours - Revetment structures This abstract elaborates on why flexible and porous structures made of these small-scale [0.2 – 0.6m] concrete armour units are an interesting, low CO2 and eco-friendly alternative for bed protections and river training works.

Rock mass substitute geometry theory (SGT) and 3D centroid sliding pyramid (CSP) method

Discontinuous analysis methods are typically used for the stability analysis of fractured rock masses with multiple sets of joints. Among various discontinuous methods, the key block theory (KBT) requires less data input, has a higher computational efficiency, and is suitable for wide-ranging engineering applications. Classical discontinuous numerical methods typically require complete boundary information for rock block units. However, current information-gathering methods can only capture information on rock surfaces. The proposed rock mass substitute geometry theory (SGT) overcomes this limitation by using local geometric information to equivalently represent the original geometric body simplifying subsequent analyses. A 3D centroid sliding pyramid (CSP) method is introduced for motion state analysis of a rock block that uses only the collinear faces of a block’s free surfaces. A comparison of CSP with KBT proves that CSP not only addresses the issue of collecting only surface information of rock masses but it significantly minimizes the computation scale. Additionally, a 3D discontinuous analysis program was developed, encompassing the entire process of rock mass information gathering, 3D discontinuous modeling, and 3D CSP analysis. The effectiveness and efficiency of the method were validated using stochastically generated models, while its practical engineering applicability was demonstrated at an engineering site.

Crack and failure behaviors of sandstone subjected to dynamic loads visualized by micro-computed tomography

Microscopic dynamic failure behaviors of rocks are significant to rock engineering, which is still insufficiently understood. This study combines split Hopkinson pressure bar (SHPB) and micro-CT (computed tomography) to explore the microscopic failure characteristics of sandstone under impact loading. SHPB is responsible for the dynamic test, and micro-CT is responsible for pre- and post-test inspections. The results show that the pores and defect influence the dynamic strength but do not alter the overall trend of increased strength with a higher impact level. The dynamical crack development is then analyzed. Three types of cracks (i.e. I-, Y-, and H-type) are identified to describe the crack development. When rock is simply fractured, only I-type crack exists due to tensile failure, and it grows irregularly. As the strain rate increases, I-type crack is transformed into Y- and H-type crack due to shear failure. Crack coalesces at that moment, and the complexity increases along the impact direction. The coalescence occurs preferentially in the area with more pores, and around a third of pores are involved, where the maximum contribution area is in the middle of sample. Microcracks are formed inside the rock blocks, and rock grains and fragments fill in the cracks. The dynamic crack development is accompanied by microcracks, while rock grains and fragments result from the development of these microcracks. In addition, the influence of a semi-penetrating defect perpendicular to the impact direction is investigated. The defect can impede stress transfer and concentrate energy consumption. The findings are expected to enhance understanding of rock dynamics and support rock engineering development.

Granulites of the Larba Block of the Dzhugdzhur–Stanovoi Superterrane: Reconstruction of the Formation Conditions

Abstract —We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur–Stanovoi superterrane. The Larba block is dominated by basic schists and garnet–biotite–orthopyroxene and garnet–biotite–cordierite–sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P–T parameters. The P–T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7–8 kbar, 800–850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4–5 kbar and T = 630–700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P–T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, TNd(DM) = 2.57–2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8–3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.

Estimation of shear strength parameters of intact limestones using miniature triaxial test

Determination of the strength parameters of intact rock samples is an important requirement in rock engineering. On the other hand, proposing a simple method as an index to estimating the strength parameters is one of the tasks of engineering geology. The conventional triaxial test (CTT) is one approach used to determine shear strength parameters (φ and c) that requires standard core cylindrical samples. As it is not possible to prepare standard core samples or collect rock block samples to prepare standard cores under all conditions, the shear strength parameters should be determined using samples having non-standard geometry and dimensions. For this purpose, a miniature triaxial test (MTT) device is introduced and used as an index test to apply triaxial loading on small cubic rock samples in order to estimate their shear strength parameters. The repeatability of the MTT results was determined by performing 27 MTT on 6 × 6 × 6 mm3 polymethyl methacrylate (PMMA) cubic samples. The results showed that the values measured by the device were repeatable with over 97% similarity. The internal friction angles (φ) of the mortar samples obtained from the CTT on cylindrical samples and MTT on the 6 × 6 × 6 mm3 samples are similar. However, the cohesion (c) value measured from the MTT test is nearly double the value obtained from the CTT test. The shear strength parameters of 12 different intact limestone samples from the Asmari Formation located in the Siyakouh section were obtained by performing CTT on standard NX core samples and MTT on the 6 × 6 × 6 mm3 samples. The correlation between the φ and c from the CTT and MTT had coefficients of determination (R2) of 0.92 and 0.88, respectively. The precision of the empirical relation for the CTT-MTT correlation was evaluated and showed that shear strength values estimated by the MTT had about 90% similarity with the measured shear strength values from the CTT.