Shear Crack Research Articles

Plastic Hinge Length for Flanged Reinforced Concrete Shear Walls with Asymmetric Sections

ABSTRACT This research investigates the determination method, spreading mechanism and influencing factors of the plastic hinge length for asymmetrical flanged walls using the finite element method. The plastic hinge length of flanged walls under flange-in-compression loading is generally greater than that under flange-in-tension loading. Based on the results, simple equations for predicting the plastic hinge length of flanged walls and which consider the contributions of the moment gradient, diagonal shear cracks, and strain penetration are proposed. Through comparison with the existing equations and design codes, the predicted plastic hinge lengths obtained from the proposed equations accord better with the test results.

Influence of shear reinforcement on the shear cracking and shear resistance of simply supported beams subjected to concentrated loads and cantilevers subjected to uniformly distributed loads

This paper presents an experimental program on reinforced, rectangular concrete beams with varying amounts of shear reinforcement. A total of ten simply supported beams subjected to two concentrated loads and twenty-four cantilevers subjected to uniformly distributed loads were tested. The specimens had the same cross-sectional properties and stirrup spacing. The single variable was the bar diameter of the stirrups; this was done so that five shear reinforcement ratios ranging between zero and two times the required minimum shear reinforcement ratio, as stipulated by Eurocode 2, could be obtained. The test results indicate significant differences in the shear crack pattern and shear resistance of the simply supported beams and cantilevers – each with the same longitudinal and shear reinforcement. For each amount of shear reinforcement investigated, the shear resistance of the cantilevers subjected to a uniformly distributed load was higher than that of the simply supported beams. Additionally, the amounts of shear reinforcement, specifically amounts below the threshold for the minimum shear reinforcement, could considerably increase the shear resistance of the cantilevers, while having almost no influence on the shear resistance of the simply supported beams. However, the difference in the shear resistance between these two structural types tends to decrease with an increase in the amount of shear reinforcement. A comparison of the shear resistance predicted by formulas included in some of the codes of practice and by a model recently developed by the authors is also provided.

Assessing the Seismic Performance of Exterior Precast Concrete Joints with Ultra-High-Performance Fiber-Reinforced Concrete

This study was conducted to evaluate the seismic performance of an exterior precast concrete (PC) beam–column joint with ultra-high-performance fiber-reinforced concrete (UHPFRC). Currently, 45 MPa non-shrinkage mortar is used as grouting for the connection between PC beams and columns. In this study, PC joint specimens were designed using 45 MPa non-shrinkage mortar and 120 MPa UHPFRC as a grouting agent for connecting PC members. The shear reinforcement effect of UHPFRC was confirmed to reduce shear cracks in the joint core; this trend was similar in the specimens with reduced shear rebars. The maximum moment of the test specimen with the corbel was slightly increased, but there was no significant difference, and the failure pattern also showed similar results to the specimen without the corbel. In the test specimen to which the U-shaped beam was applied, the attachment surface of ultra-high-performance concrete and normal concrete were separated, and a large decrease in strength was observed. Considering workability, U-shaped beam do not seem to have any major merits in general, such as increased strength and difficulty in manufacturing, and it was judged that it was effective to separate the PC beams from the column face through corbels. Shear reinforcement through UHPFRC is effective in relieving congestion by reducing shear reinforcement bars at the joint, and it is judged that it can be used as PC joint grouting due to its excellent fluidity.

Analysis of the Dynamic Mechanical Properties and Energy Dissipation of Water-Saturated Fissured Sandstone Specimens

To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments’ fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing.

Acoustic Emission Waveform Characterization of Crack Origin In Self-Healing of Mortar Due to Internal Carbonation

In this study, self-healing of mortar was achieved by “built-in” carbonation of soluble Na2CO3 and Ca(OH)2. The effect of carbonate and calcium ions, available either internally or externally by conditioning the specimens with Ca(OH)2 and Na2CO3 solution, on the formation of calcite in cracks was investigated. The acoustic events were monitored and compared in the loading process before and after healing. Furthermore, a calibration test was carried out to distinguish the characteristic acoustic emission events of the fracture of the matrix and of healing products. It was found that the distribution of acoustic energy with FMA (frequency at maximum amplitude) and the hits with duration show a consistent trend with that of calibration. The change of tensile to shear cracking ratio in reloading illustrates a self-healing effect of cracks. Meanwhile, X-ray diffraction analysis indicates more calcite formed in the crack of self-healing specimens. The pretreated ceramsite-containing specimens exhibit the predominate self-healing effect due to the internally available CO32– and sufficient Ca2+ as compared to the others.

Experimental study on failure mechanical properties and acoustic emission characteristics of soft rock–coal combination under dynamic disturbance

This study aims to investigate the failure mechanical properties of deep soft rock–coal combinations when subjected to dynamic disturbance. Hence, uniaxial compression and dynamic disturbance experiments were conducted on samples of muddy sandstone, coal, and soft rock–coal combinations using a creep disturbance dynamic impact loading experimental system. This study analyzed the damage properties of soft rock–coal combinations under dynamic disturbance by combining sample failure characteristics, energy evolution, and waveform signals of samples using the acoustic emission system. Results indicate that the peak strength and modulus of elasticity of the soft rock–coal combinations under uniaxial compression and dynamic disturbance are intermediate between those of coal and rock. The strength deterioration effect of the rock–coal combinations under dynamic disturbance is obvious, and the peak strength of the roof–coal and coal–floor combinations are reduced by 6.17% and 10.30%, respectively. Dynamic disturbances increase the degree of damage to the rock–coal combinations, which is accompanied by large amounts of coal fragments and large fragments of the rock–coal combinations. The failure to the sample is primarily caused by the extension of tensile damage, with the development of shear cracks playing a secondary role. The soft rock-coal combinations undergo small-scale damage at the initial disturbance (N1), with large main frequency eigenvalues. Large-scale damage occurs at the time of failure (N2), with smaller main frequency eigenvalues, sharp fluctuations in the acoustic emission signal, and peaks in the absolute AE energy and counts.

A numerical investigation of slip evolution of shear crack faces oriented arbitrarily under non-steady contact

Shear loading is induced on the surfaces of a crack that is aligned at an arbitrary angle in relation to the direction of compressive loading. Contact faces may experience relative slip if the tangential stress along the crack surpasses the friction threshold determined by the product of normal stress and coefficient of friction. In contrast, other regions remain in stick contact with a lower level of shear stress than the friction limit. The resulting slip motion generates additional stress, potentially driving already slipped points further. The overall motion of crack faces is constrained by the bonding tip. Therefore, the stick-slip conditions can not possibly be determined in advance, and the slip evolution should thus be investigated for the analysis of friction fracture, especially in cases of non-steady contact, such as rolling or loading-unloading contact. This study explores the loading history effect on stick-slip contact of arbitrarily oriented shear cracks. The crack is assumed as distributed glide dislocations based on the discrete dislocation technique. The loading is incrementally applied until the targeted amplitude with the slip displacement gradually accumulating during the contact cycle. The initial contact position is successively changed during a rolling contact process, while the loading amplitude is decreased when followed by an unloading process. The factors that influence the stick-slip patterns of crack faces, including loading history, crack orientation and coefficient of friction, are investigated. The developed methods are expected to provide insight into the study of contact mechanics, and the conclusions can be used to predict the frictional fracture under rolling or loading-unloading contact.

Experimental investigation of the compressive and shear behaviours of grouted and hollow masonry constructed with PVA fibre-reinforced mortar and grout

Polyvinyl alcohol (PVA) fibres are ultra-high-performance fibres that can be used to reinforce cementitious composites. This study experimentally investigated the compression and shear behaviours of grouted and hollow masonry constructed with fibre-reinforced mortar (FRM) used for the joints and/or PVA fibre-reinforced grout (FRG). In this regard, the effect of increasing the fibre content of 6 mm PVA fibres in the FRM on the behaviour of masonry was investigated. Moreover, the effect of changing the fibre length (8 mm and 13 mm PVA fibres) and the fibre content of 13 mm PVA fibres in the FRG on the behaviour of masonry was investigated. This was accomplished by constructing and testing 31 masonry prisms and 29 masonry assemblages. The behaviour of PVA-reinforced masonry was evaluated based on compressive strength, shear strength, modulus of elasticity, modulus of rigidity, compressive strain at peak compressive stress, shear strain at peak shear stress, compression ductility index and compression toughness. In addition, the compression and shear stress-strain curves, failure modes and crack propagation of PVA-reinforced masonry were compared with unreinforced masonry. It was concluded that using PVA fibres in the grout minimized the masonry face shell spalling, and most of the post-peak deterioration appeared in the form of masonry cracks and less face shell spalling compared with unreinforced masonry. In addition, using PVA-FRM only or PVA-FRM with PVA-FRG could increase the compressive strength of grouted masonry. In contrast, using PVA-FRM did not change the compressive strength of hollow masonry. Furthermore, all grouted assemblages with PVA-FRM and PVA-FRG (except for assemblages with 13 mm PVA-FRG with a fibre content of 0.10% by weight of grout) have similar average shear strength compared with unreinforced assemblages. Finally, As the fibre content of 6 mm PVA fibres in the FRM of hollow assemblages increased, the shear strength and the modulus of rigidity decreased. This study highlights the improvements and drawbacks of using PVA Fibres in mortar and grout on the compressive and shear performances of hollow and grouted masonry.

Damage evolution and failure characteristics of a cemented gangue backfill considering direct shear and dynamic tests

Backfill mining is an effective technique that can prevent and control the impact of dynamic disasters which can occur in deep underground coal mines. This technique allows to reduce the surface subsidence and achieve a greener mining. The backfill shear mechanical properties under mining disturbances, are of great significance for ensuring the surrounding rock stability in deep mining. Therefore, studying the shear mechanical properties of the cemented gangue backfill body (CGBB) under coupled dynamic and static loads actions in deep coal mining is of great importance. In this paper, shear mechanical tests and the acoustic emission (AE) technology were used to study the shear mechanical response characteristics of CGBB under low-frequency dynamic normal load conditions. The results show that a dynamic normal load with an amplitude of 5 kN improves the shear CGBB strength, while a dynamic load with an amplitude greater than 10 kN will cause a shear strength deterioration, and the degree of deterioration is positively correlated with the load amplitude. Under low-frequency dynamic normal load conditions, the shear CGBB stress-strain curve exhibits classical characteristics. AE indicate that the main CGBB damage changes from a nonlinear deformation to an elastic stage under low-frequency dynamic normal load conditions. During the shear failure process, the cumulative AE counts are positively correlated with the load amplitude. The crack type identification based on RA-AF shows that the low-frequency dynamic normal load affects the CGBB crack type. Under constant normal load (CNL) conditions, tensile or mixed cracks account for 63.95% and are the main fracture mode. Conversely, under low-frequency dynamic normal load conditions, shear cracks account for more than 60% and are the main failure mode. This research provides a reference for the CGBB stability analysis under complex stress conditions for the case of a deep underground coal mining.

Experiment study on shear behavior and properties of granite fractures under real-time high-temperature conditions

Direct shear tests were conducted on granite fractures with similar joint roughness coefficients under real-time high temperatures (up to 400 °C) and constant normal stiffness conditions. In this study, the shearing process was monitored via acoustic emission, and fracture surface morphology was measured with three-dimensional laser scanning. With increasing temperature, peak and residual shear strengths, peak friction coefficient, and maximum normal dilation became higher. In contrast, peak normal displacement and peak shear displacement decreased. The apparent cohesion and internal friction angle exhibited opposite trends as temperature increased, primarily due to competition between thermal hardening and thermal cracking. The elastic strain energy of stick-slip events increased with temperature. From 100 °C to 400 °C, the degradation of fracture roughness and the volume of sheared-off asperities grew. Real-time temperature influences shear properties and is mainly attributed to physical and geometrical changes. Acoustic emission parameters, including count, energy, and hit, show segmented variation during fracture shearing. Based on parameter analyses of the average frequency and rise angle values, during the shearing process, the proportion of shear cracks at each stage varied with temperature. Shear failure was the predominant failure mode. Results from this study provide insight into the shear behavior of granite fractures under real-time high temperatures.

Microseismic activity characteristics and range evaluation of hydraulic fracturing in coal seam

Accurately evaluating hydraulic fracturing (HF) activity characteristics and impact range is critical to optimizing the HF scheme design. This study proposes an adaptive threshold peak detection method and a wavelet transform-support vector machine (WT-SVM) polarity analysis method based on microseismic (MS) monitoring data from a coal mine in Guizhou, China. The results show that the adaptive threshold peak detection method effectively improves the accuracy and stability of the P-wave initial arrival pickup for low signal-to-noise ratio (SNR) signals. The main frequency band of the MS signal of HF is focused on 150–300 Hz. The improved particle swarm optimization algorithm was used by introducing a random-weight strategy to determine the high-density impact areas of the HF within approximately 20 m on both sides of the fracturing borehole. Density cloud map analysis revealed a small area of the blank zone at the junction of the fracturing section, which provided a basis for fracturing parameter design and scheme optimization. The directional characteristics of the MS waveform were quantitatively described by polarity analysis, where the dip angle distribution ranged from 39° to 49°, the strike angle ranged from 61° to 74°, the slip angle was concentrated near 44°, and the stretch angle ranged from 61° to 74°. The WT-SVM proposed in this study overcame the effects of low SNR signals and the limitations of a small-area station network; 62% of the shear ruptures and 38% of the tensile ruptures of the HF were obtained. Shear cracks were the primary expression of the HF rupture type.

Numerical study on the characteristics of roadway failure and instability in coal seam with rock parting

In order to explore the mechanism of rockburst in coal seam with rock parting, a combination of on-site and numerical experiment is used to study the failure and instability process, crack propagation mechanism, and influencing factors. The following four points were addressed: (1) the instability is a process that roadway in coal seam with rock parting go through from stable locking in the initial stress unloading stage to slipping unlocking, and then to spatter ejection in slipping dynamic load disturbance stage. (2) The fracture development caused by unloading excavation of coal seam with rock parting will change from shear crack to tensile crack. In this process, coal-rock contact surface slip and coal-rock fracture are coupled with each other. (3) The greater the mining depth is, the greater the lateral pressure coefficient is, and the higher the rockburst risk is. On the contrary, the lower the risk of rockburst. (4) When choosing the support form of roadway in coal seam with rock parting, the two supporting forms of bolting (cable) and supplementary masonry support should be preferred. The results enrich the theory of the dynamics of surrounding rock fracture in coal mine, further clarify the potential dangers to mining-area roadways and working faces, and provide technical information to ensure the safe and efficient mining of bifurcated coal seam.

Punching performance of flat slabs with openings accounting for the influence of moment transfer and shear reinforcement

The arrangement of openings adjacent or near to slab-column connections is very common practice in flat slab design due to a number of advantages for technical installations. Such location is however conflicting with the most critical region of the slab, where punching shear failures may develop and typically govern the design at ultimate limit state. This requires in many cases arranging shear reinforcement to enhance the performance of the connections both in terms of resistance and deformation capacity. Such shear reinforcement is in addition particularly useful when unbalanced moments are transferred from the slab to the columns, which is a phenomenon typically detrimental for the punching shear resistance of inner connections. Despite the practical relevance of this case, there is currently no experimental evidence on the response of shear-reinforced slab-column connections with adjacent openings and moment transfer. Design is thus performed on the basis of simplified rules, modifying the resistance of connections without openings. In the present study, a comprehensive test programme addressed at this issue is presented in an effort to improve the current state-of-knowledge. Five slabs representing several practical cases and potential arrangements of shear reinforcement were tested. The experimental results are investigated in detail and compared to codes of practice, highlighting a number of deficiencies of current simplified rules. The phenomena is finally investigated on the basis of the mechanical model of the Critical Shear Crack Theory, showing a general frame for modelling and design of such detail.

Dynamic strength and full-field cracking behaviours of pre-cracked rocks under impact loads

The impact loads have a significant influence on the dynamic strength and cracking behaviours of pre-cracked rocks, which plays a key role in the stability and safety of rock engineering. In this study, impact tests were performed by using a split Hopkinson pressure bar (SHPB) in conjunction with the high-speed photography technique. The dynamic strength of pre-cracked rocks under different strain rates and flaw angles was analysed, and the full-field cracking behaviours were elucidated by the evolution of strain localization zones based on the digital image correlation (DIC) analysis. By means of discrete element method (DEM), the impact tests were numerically reproduced to study the strength enhancement and the cracking processes. The results illustrate that the increase in the strain rate and flaw angle would lead to the increasing dynamic strength, the change of tensile and shear crack number is consistent with the strength variation, implying that the microcracking processes has a significant influence on the rate dependency of dynamic strength. As for the cracking behaviours, the strain localization zones would form around the flaw, and the crack branching phenomenon could be witnessed in dynamic tests. Furthermore, the complexity of cracking patterns becomes larger with the increasing strain rate, while the samples with the large flaw angle are difficult to generate strain localization zones and macroscopic cracks. By using a DIC-based method to analyse the crack initiation mechanisms, three types of stress-induced cracks were identified, i.e., mode I, II, and mixed mode I/II cracks. Among these cracks, the mode II cracks are the dominate type of cracks. Based on the mathematical analysis of Griffith model, the combined action of the tangential stress distribution and sliding deformation might be the reason for the crack initiation, and the rate-dependent strength might be induced by the increase of the maximum tangential stress regarding external stress.

Mechanical properties and failure behaviour of anchored granite containing two filled edge-notched fissures in triaxial compression

High in-situ stress commonly exists in deep rock engineering and causes great difficulties in underground mining support systems. To elucidate the relationship between the anchoring effect of bolts and confining pressures, triaxial compression tests were performed to study the deformation characteristics, mechanical properties, and failure behaviour of anchored granite with two pre-existing fissures. The results demonstrated that the deformation characteristics or mechanical properties of anchored granite, including triaxial compressive strength, elastic modulus, and stored or dissipated energy, increased with increasing confining pressure under low confining pressures. Rockbolt boreholes significantly weaken the mechanical properties and failure behaviour under high confining pressures and have a negative impact on the peak axial strength and stored or dissipated energy of the anchored granite. The fissure angle and confining pressure have a direct impact on the failure mode of anchored granite. With increasing confining pressures or fissure angles, shear cracks and anti-wing cracks instead of tensile wing cracks are more likely to cause ultimate failure. In addition, four modes of crack coalescence were identified: no coalescence, two types of direct crack coalescence, and indirect crack coalescence.

Mechanical properties and energy damage evolution mechanism of fiber-reinforced cemented sulfur tailings backfill under uniaxial compression.

This paper studies mechanical properties and energy damage evolution of fiber-reinforced cemented sulfur tailings (CSTB) backfill. The effects of fiber length and fiber content on the stress, toughness and failure properties of the CSTB were systematically revealed. In addition, the energy index evolution law was studied, and the energy damage evolution mechanism of CSTB was revealed. The results show that the deformation failure of fiber-reinforced CSTB mainly goes through four stages: initial crack compaction, linear elastic deformation, yield failure and post-peak failure. The peak stress and residual stress of the CSTB firstly increase and then decrease with the increase of fiber content and the addition of fiber can promote the change from brittle failure to ductile failure of the CSTB. Adding appropriate amount of fiber can improve the toughness of CSTB, and the influence degree of fiber length on the toughness index of CSTB is 6mm>12mm>3mm. The total strain energy increases linearly along the variation of fiber content, while the elastic strain energy and dissipated energy increase exponentially at the peak stress point. In the process of CSTB deformation and failure, "gentle-linear growth-slow growth-rapid decline" is for elastic strain energy, while "gentle-slow growth-rapid growth-linear growth" is for dissipation energy. The damage and failure of CSTB mainly experienced four stages: initial damage, slow growth of damage, accelerated damage and damage failure, and the damage evolution curve also showed the changing characteristics of "gentle-slow growth-rapid growth-linear growth". The CSTB without added fiber showed obvious "Y-type" and "linear-type" shear failure characteristics and the phenomenon of shear cracks penetrating the backfill appeared. No big shear crack occur when it is damaged, showing that the fiber addition restrain the crack growth and improve the overall crack resistance of the CSTB. Hydration products are obviously distributed on the surface of the fiber, which indicates that the fiber will be evenly dispersed in the CSTB and form a certain bonding force with the cement-tailings matrix, thus improving the overall mechanical properties of the CSTB.

Understanding the evolution mechanism and classification criteria of tensile -shear cracks in rock failure process from acoustic emission (AE) characteristics

This study aimed to investigate the crack evolution and failure mechanism of rocks by conducting uniaxial compression tests (UCT) and Brazilian indirect tension tests (BITT) using four categories of rock specimens. Real-time acoustic emission (AE) signals were detected and recorded during rock failure processes by an AE monitoring system. The study focused on the accumulative AE hits curves, stress–strain curves, and other AE energy characteristics to divide the failure process into five stages, and determine the stress thresholds. Furthermore, the transition trend of rock crack types was analyzed based on RA-AF data. A novel classification criterion of crack types based on clustering and statistical theory was proposed, and the crack boundaries of the four rocks were determined. Additionally, the tension to shear ratios of the rocks in descending order were fine-grained granite, granite, yellow sandstone, and white sandstone. Finally, the crack development trend and failure characteristics of the rock were analyzed using the crack scale and crack dividing line. The study found that the failure mode of the rock could be predicted accurately by AE parameter analysis at Unstable Crack Growth (Stage-IV).

Research on Acoustic Emission Characteristics and Crack Evolution during Rock Failure under Tensile and Tensile- and Compressive-Shear Stress States

Tensile, compressive-shear, and tensile-shear failure are three typical failure modes of rock- and lining-support structural materials in underground engineering. Comparative studies of the acoustic emission (AE) evolution characteristics of specimens of the same shape and size under different stress states are of great significance in determining universal disaster warning guidelines. Based on a self-developed multi-functional test system, direct tensile, compressive-shear, and tensile-shear tests were conducted on intact and jointed rock-like specimens, comparing AE amplitude and peak frequency parameters under different failure modes from the perspectives of crack scale and crack type evolution. All failure tests were monitored with microcrack propagation at an early stage until ultimate rupture occurred at the peak-load moment. As a result, the “quiet-period” could only be observed from the compressive-shear test. The AE signals distributed in three bands can be used as an indicator of failure identifications. Tensile and shear cracks can be identified by strong and weak amplitudes of low- and high-frequency signals. These results enhance the knowledge of the failure modes of rock mechanics for more applications in monitoring disasters in rock engineering.

Experimental Investigation on the Rehabilitation of RC Flat Slabs Using CFRP Sheets to Enhance Punching Shear Capacity

In this paper, the feasibility of strengthening a flat column–slab connection within the carbon fiber reinforced polymer (CFRP) has been investigated through experimental study. The experimental program includes a set of nine reinforced concrete flat slab specimens. Three unaltered specimens served as control slabs, while an additional six samples were strengthened with various CFRP configurations to enhance their shear capacity. The strain distribution, ductility, punching shear resistance, stiffness, and crack formation were studied. The result of experimental studies showed that in the direct method of strengthening in which two layers of unidirectional CFRP sheets were employed in two opposite directions, the ultimate punching shear resistance improved by 64%, 44.7%, and 15.3%, with respect to the location of the column connection, as compared with the control specimens. In the case of using one layer of unidirectional CFRP strips, the punching shear resistance was enhanced by approximately 16% and 39%, considering the configuration of CFRP sheets and the amount of strengthened and adhesive layers used. Following the outcomes of this research, the application of CFRPs in improving the resistance capacity of flat slabs against the punching shear is considerable. The reported outcomes were compared with the latest provisions of ACI to show the efficiency of the presented strengthening. Finally, a parametric study was performed assuming different loading locations to assess the effect of the loading region on the response of RC slabs. Results indicate that approaching the loading location toward the RC slab supports results of an increase in the load-bearing capacity and a reduction in the ductility of the RC slab.

Experimental study on the effect of unloading rate on gneiss rockburst

Rockburst are often encountered in tunnel construction due to the complex geological conditions. To study the influence of unloading rate on rockburst, gneiss rockburst experiments were conducted under three groups of unloading rates. A high-speed photography system and acoustic emission (AE) system were used to monitor the entire process of rockburst process in real-time. The results show that the intensity of gneiss rockburst decreases with decrease of unloading rate, which is manifested as the reduction of AE energy and fragments ejection velocity. The mechanisms are proposed to explain this effect: (i) The reduction of unloading rate changes the crack propagation mechanism in the process of rockburst. This makes the rockbursts change from the tensile failure mechanism at high unloading rate to the tension-shear mixed failure mechanism at low unloading rate, and more energy released in the form of shear crack propagation. Then, less strain energy is converted into kinetic energy of fragments ejection. (ii) Less plate cracking degree of gneiss has taken shape due to decrease of unloading rate, resulting in the destruction of rockburst incubation process. The enlightenments of reducing the unloading rate for the project are also described quantitatively. The rockburst magnitude is reduced from the medium magnitude at the unloading rate of 0.1 MPa/s to the slight magnitude at the unloading rate of 0.025 MPa/s, which was judged by the ejection velocity.