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Floor heave is a disaster that is frequently encountered in tunnel engineering. Existing explanations of floor heave mechanisms have many limitations, and the most important of which is that the stress concentration, release and transfer phenomena and the influences of the in-situ stress lateral coefficient and tensile strength are ignored. Therefore, the combined finite-discrete element method (FDEM) is used to study the failure process of floor rock masses and propose a new mechanical mechanism of floor heave that can consider the influences of the in-situ stress lateral coefficient and tensile strength. The type I fracture energy corresponding to different tensile strengths is calibrated using a direct tensile simulation test. Then, the floor heave mechanism is investigated under different lateral coefficients and tensile strengths, and five different floor heave modes are proposed under different in-situ stress lateral coefficients and tensile strengths. The floor heave modes controlled by the various in-situ stress lateral coefficients and tensile strengths are different, but they can all be explained by the maximum concentrated tangential stress. [ABSTRACT FROM AUTHOR]

A large amount of slates is distributed along Kangding No. 2 tunnel of Sichuan–Tibet Railway (China). The bedding of the Kangding slate is rich, showing apparent anisotropy of strength. For underground tunnels excavated in the anisotropic rock mass, the rock material is vulnerable to damage in tension because of its low tensile strength. Therefore, the tensile strength and fracture modes are important for the support design of layered surrounding-rock tunnel. In this study, first, a series of direct tensile tests were carried out on slate samples with five kinds of bedding dip angles. Second, the effect of bedding dip angle on the slate's tensile strength, failure modes, and acoustic emission characteristics was analyzed. Third, based on the Nova–Zaninetti criterion and the experiment results, a new phenomenological anisotropic criterion was proposed by introducing anisotropic coefficient, which could describe the nonlinear variation trend of tensile strength with foliation angles. Compared with four typical tensile strength criteria, the accuracy and universality of the new criterion were verified. Finally, the new criterion was implemented into the combined finite-discrete element method (FDEM) to conduct direct tensile simulation of slate with different foliation angles. The results indicated that the direct tensile strength followed an S-shaped increasing trend with the variation of foliation angles. The direct tensile failure modes of slate samples with different foliation angles could be divided into layer activation failure, mixed failure, and rock matrix failure. The acoustic emission process of slate with different bedding inclination under direct tension could be roughly divided into the following three stages: quiet period—stepped saltatory increase—drastic increase. The fitting effect of the new criterion was the highest among Kangding slate and other six typical layered rocks, which showed that the new criterion was suitable for predicting its Brazilian strength and direct tensile strength. At the same time, the numerical simulation results were in good agreement with the experimental results, which further verified the accuracy of the new criterion. Highlights: The acoustic emission characteristic parameters helped to reveal the direct tensile failure characteristics of layered rocks. A new phenomenological anisotropic tensile failure criterion was proposed. The direct tensile test data and the Brazilian splitting test data were used to study the accuracy and universality of the new anisotropic tensile failure criterion. FDEM was used to study the accuracy of the new anisotropic tensile failure criterion. [ABSTRACT FROM AUTHOR]

• A new phenomenological tensile failure criterion with an anisotropic coefficient was proposed. • Direct tensile test of the Kangding slate was carried out to verify the accuracy of the new criterion. • The tensile strength data of eight typical layered rocks were collected and compared with the other six typical tensile strength criteria to verify the universality and superiority of the new criterion. • Based on the sensitivity analysis of tensile rate, the loading rate of 0.01 m/s was recommended for the samples with various foliation angles. • Layer thickness has little effect on tensile strength, but a significant effect on tensile failure modes. The tensile strength is a key parameter for the design of many geotechnical engineerings such as tunnel support, slope support, and petroleum drilling. The tensile strength in layered rocks, however, is anisotropic due to the existence of bedding planes. In this paper, to study the anisotropic tensile behaviors, a series of direct tensile tests were conducted using Kangding slate with five different foliation angles (β = 0°, 30°, 45°, 60°, and 90°). Based on the N-Z criterion, a new tensile failure criterion with the anisotropic coefficient was proposed. The tensile strength data of nine typical layered rocks were collected to evaluate the prediction ability of seven anisotropic tensile failure criteria including the new criterion. Embedding the new criterion into FDEM, the loading rate-dependent and layer thickness-dependent tensile mechanism of layered rock under direct tensile test were simulated. The experimental and numerical simulation results showed that the foliation angle significantly affected the tensile strength and failure modes of the slate. With the increase of foliation angle (β), the tensile strength of nine typical layered rocks, including Kangding slate, presented a non-linear increase trend. Compared with the other six typical anisotropic tensile failure criteria, the new criterion has the best prediction capability on the tensile strength of rocks with four different anisotropic degrees, and therefore the new criterion was recommended. The new criterion was embedded in FDEM to carry out direct tensile simulation of slate with different β. The numerical simulation results were in good agreement with the experimental results, further verifying the accuracy of the new criterion. The loading rate sensitivity analysis indicated that a tensile rate of 0.01 m/s was recommended to assure quasi-static loading of the specimen. Through the sensitivity analysis of layer thickness, it is concluded that layer thickness has very little influence on tensile strength, but has a very significant influence on the final failure modes of the specimens. [ABSTRACT FROM AUTHOR]

Rockburst has become a main factor threatening the safe construction of deep-buried tunnels. To monitor microseismic (MS) event and comprehend evolution process of rockbursts, a MS monitoring system was established in a deep-buried tunnel excavated by drilling and blasting. The mechanisms of three typical rockburst cases were investigated through the analysis of the MS sequence and development process. The results showed that the MS sequence of first rockburst case was a main-aftershock type, corresponding to a strain rockburst that occurred in intact surrounding rocks. Most of the MS events were failure in tensile, with few MS events presenting mixed failure mode. For the second rockburst case, the MS sequence was a fore-main-aftershock type, corresponding to a strain-structural slip rockburst that occurred in surrounding rocks with a considerable number of small-scale structural planes. The MS events in tensile failure were predominated, and a few MS events were failure in mixed and shear. The MS sequence of the third rockburst case was a new type observed in the present study, i.e. main shock type, corresponding to a strain-structural slip rockburst. It occurred in high strength surrounding rocks with rigid structural planes, which easily induced collapse in the rockburst process. The MS events in tensile failure were predominated. The precursor phenomena of former two cases of the rockburst can be identified through the analysis of MS sequence and source parameters. While before the third rockburst case occurred, no or a few MS events were obtained. It is difficult to predict this type of rockburst based on source parameters. For this case, destress blasting method was adopted. The number of high-energy MS events decreased abruptly, and no intense rockbursts occurred. This method can effectively release the concentrated stress of the surrounding rocks and reduce the risk of rockbursts. The results of this study could provide guidance and reference for the early warning and prevention of rockbursts in deep-buried tunnels. • The temporal and spatial evolution process of microseismicity of three typical rockbursts were studied. • The evolution mechanism of three typical rockbursts was revealed. • Mitigation measures were adopted to reduce the risk of rockbursts. [ABSTRACT FROM AUTHOR]

• An optimized random mesostructure generation algorithm for concrete is proposed. • A meso-scale concrete model for failure analysis is achieved in FEM/DEM frame. • Mesomechanism of concrete dynamic tensile fracture behaviour is studied. As a typical multi-phase material, due to its heterogeneous mesostructure, concrete exhibits complicated mechanical responses when subjected to high-rate loads. Considering concrete is much weaker in tension than in compression, it is of significance to investigate the mesomechanism of the dynamic tensile fracture behaviour of concrete under high-rate loading conditions. In this study, the meso-heterogeneity of concrete is considered by random mesostructure generation with efficient algorithm of Entrance block between A and B (E (A , B)). A 2D meso-scale model is developed by implementing a combined FDEM scheme with zero-thickness cohesive elements in ABAQUS software. The proposed model is validated by quasi-static uniaxial tension and uniaxial compression tests as well as dynamic Brazilian tensile split Hopkinson pressure bar (SHPB) test and then used in a series of Brazilian tensile SHPB tests to investigate the influences of the loading rate, aggregate area fraction and the mechanical properties of the interfacial transition zone (ITZ) on dynamic tensile behaviour of concrete. The numerical results indicate that the dynamic tensile strength increases with increasing loading rate and the fragmentation experiences a transition from parse fracturing to dramatic pulverization. A higher aggregate content has an adverse effect on the dynamic tensile strength of concrete. Moreover, the results also suggest that the mechanical properties of the ITZ play a significant role on the dynamic tensile behaviour of concrete. [ABSTRACT FROM AUTHOR]

The moisture environment where the rock is located during the freeze-thaw (F-T) weathering plays a significant role in the deterioration of physical and mechanical properties. This study explores the effect of the moisture conditions on the degradation of the granite dynamic mechanical properties during F-T cycles. Saturated granite samples were positioned in varying moisture conditions (i.e., sealed with cling wrap, half-submersion and completely submersion in water) when they were under different F-T cycles ranging from 20 to 100. Dynamic tensile and compression tests were performed on the fresh and the weathered samples. By appropriately adjusting the gas pressure, the loading rate was controlled in the range of 471–563 GPa/s for the dynamic tensile test and 1944–2144 GPa/s for the dynamic compression test. The deterioration characteristics were examined by the changes in the open porosity, P-wave velocity, and dynamic tensile and compressive strengths. The experimental results for the sealed samples indicate that with the increase of the F-T cycles the moisture content is unchanged while the degree of saturation slightly decreases. Whereas in the half- and completely submerged samples the moisture content slightly increases and the degree of saturation remains. Furthermore, the water migration activities in half-submerged samples are more pronounced than completely submerged samples. The half-submerged samples having the highest degree of saturation are more susceptible to the F-T weathering and therefore, exhibit the lowest dynamic mechanical properties followed by the completely submerged samples and the sealed samples. • The effects of moisture conditions on the F-T induced degradation of the BTS d and UCS d were studied. • The relations of BTS d and UCS d with V p for the samples after F-T cycles were established using power functions. • The half-submerged samples were found to be the most susceptible to the F-T weathering. [ABSTRACT FROM AUTHOR]