Anti-strike Capability of Steel-fiber Reactive Powder Concrete

Penetration and contact explosion tests on reactive powder concrete (RPC) containing 5 per cent steel-fiber were carried out to investigate the anti-strike capability of steel-fiber reactive powder concrete (SFRPC). The penetration tests consisted of two sample groups corresponding to hit speeds 308 m/s - 582 m/s and 808 m/s - 887 m/s, respectively. The contact explosion tests were carried out in an explosion test pit using TNT with charges in the range 0.5 kg - 3.0 kg. The tests results show that the anti-strike capability of SFRPC targets is much better than ordinary C30 concrete. The penetration depths of the SFRPC targets were less than half those evaluated values of the C30 concrete targets. The areas of the blasting funnels and the explosion cavity radii in the SFRPC plates are also much less than the calculated results in ordinary C30 concrete, being about one quarter of those of the latter.Defence Science Journal, 2013, 63(4), pp.363-368, DOI:http://dx.doi.org/10.14429/dsj.63.240

Uncertainty Analysis on Risk Assessment of Water Inrush in Karst Tunnels

An improved attribute recognition method is reviewed and discussed to evaluate the risk of water inrush in karst tunnels. Due to the complex geology and hydrogeology, the methodology discusses the uncertainties related to the evaluation index and attribute measure. The uncertainties can be described by probability distributions. The values of evaluation index and attribute measure were employed through random numbers generated by Monte Carlo simulations and an attribute measure belt was chosen instead of the linearity attribute measure function. Considering the uncertainties of evaluation index and attribute measure, the probability distributions of four risk grades are calculated using random numbers generated by Monte Carlo simulation. According to the probability distribution, the risk level can be analyzed under different confidence coefficients. The method improvement is more accurate and feasible compared with the results derived from the attribute recognition model. Finally, the improved attribute recognition method was applied and verified in Longmenshan tunnel in China

Bulk Density Adjustment of Resin-Based Equivalent Material for Geomechanical Model Test

An equivalent material is of significance to the simulation of prototype rock in geomechanical model test. Researchers attempt to ensure that the bulk density of equivalent material is equal to that of prototype rock. In this work, barite sand was used to increase the bulk density of a resin-based equivalent material. The variation law of the bulk density was revealed in the simulation of a prototype rock of a different bulk density. Over 300 specimens were made for uniaxial compression test. Test results indicated that the substitution of quartz sand by barite sand had no apparent influence on the uniaxial compressive strength and elastic modulus of the specimens but can increase the bulk density, according to the proportional coarse aggregate content. An ideal linearity was found in the relationship between the barite sand substitution ratio and the bulk density. The relationship between the bulk density and the usage of coarse aggregate and barite sand was also presented. The test results provided an insight into the bulk density adjustment of resin-based equivalent materials

Strain Rockbursts Simulated by Low-Strength Brittle Equivalent Materials

This paper presents experimental study on rockbursts that occur in deep underground excavations. To begin with, the boundary conditions for excavation in deep underground engineering were analysed and elastic adaptive boundary is an effective way to minimize the boundary effect of geomechanical model test. Then, in order to simulate an elastic adaptive loading boundary, Belleville springs were used to establish this loading boundary. With the aforementioned experimental set-ups and fabrication of similarity models for test, the phenomena of strain mode rockbursts were satisfactorily reproduced in laboratory. The internal stress, strain, and convergences of the openings of the model were instrumented by subtly preembedded sensors and transducers. Test results showed that, with an initial state of high stress from both upper layers’ gravitational effects and in situ stress due to tectonic movements, the excavation brings a dramatic rise in the hoop stress and sharp drop in radial stress, which leads to the splitting failure of rock mass. Finally a rockburst occurred associated with the release of strain energy stored in highly stressed rock mass. In addition, the failure of the surrounding rock demonstrated an obvious hysteresis effect which supplies valuable guide and reference for tunnel support. Not only do these results provide a basis for further comprehensive experiments, but also the data can offer assisting aids for further theoretical study of rockbursts

Experimental Study on the Influence of Moisture and Clay Content on Stress Wave Attenuation Characteristics of Filled Joints

Artificial filled joints made of sand–clay mixtures with different clay weight fractions and saturations have different wave attenuation capacities. In this paper, the high amplitude impact test of sand–clay mixtures was carried out by using split Hopkinson pressure bar (SHPB) equipment. The results showed that with the increase of clay weight fraction, the particle crushing decreased continuously, while the wave attenuation coefficient decreased first and then increased. When the weight fraction of clay was 50%, the wave attenuation coefficient reached the minimum among the tested working conditions, and the ratio of transmitted energy to incident energy reached the maximum. With the increase of saturation, the particle crushing decreased first and then increased, while the wave attenuation coefficient increased first and then decreased. When the saturation was 25%, the wave attenuation coefficient reached the maximum, and the proportion of transmitted energy to incident energy reached the minimum. Because of the lubrication of water reduced the friction between particle, the specimen more prone to deformation and particle crushing reduced. As the saturation increased, this effect gradually decreased. In the case of the wave absorbing layer of protective works, special attention should be paid to the adverse effects caused by groundwater

Experimental Study on the Influence of Moisture and Clay Content on Stress Wave Attenuation Characteristics of Filled Joints

Artificial filled joints made of sand–clay mixtures with different clay weight fractions and saturations have different wave attenuation capacities. In this paper, the high amplitude impact test of sand–clay mixtures was carried out by using split Hopkinson pressure bar (SHPB) equipment. The results showed that with the increase of clay weight fraction, the particle crushing decreased continuously, while the wave attenuation coefficient decreased first and then increased. When the weight fraction of clay was 50%, the wave attenuation coefficient reached the minimum among the tested working conditions, and the ratio of transmitted energy to incident energy reached the maximum. With the increase of saturation, the particle crushing decreased first and then increased, while the wave attenuation coefficient increased first and then decreased. When the saturation was 25%, the wave attenuation coefficient reached the maximum, and the proportion of transmitted energy to incident energy reached the minimum. Because of the lubrication of water reduced the friction between particle, the specimen more prone to deformation and particle crushing reduced. As the saturation increased, this effect gradually decreased. In the case of the wave absorbing layer of protective works, special attention should be paid to the adverse effects caused by groundwater

On the Experimental Determination of Poisson’s Ratio for Intact Rocks and Its Variation as Deformation Develops

Poisson’s ratio is of crucial importance for the theoretical and numerical analysis of rock engineering. It is an elastic parameter of the material and the ratio of the absolute value of lateral strain and axial strain when the material is under uniaxial tension or compression. However, it was rarely investigated compared with deformation modulus and strength. Rock materials are different from metal materials. The pure elastic deformation stage is usually very short or nonexistent in the process of uniaxial tension or compression. In this paper, in order to explore the behavior of Poisson’s ratio, uniaxial compression tests according to The International Society for Rock Mechanics and Rock Engineering are performed on standard specimens of granite, marble, red sandstone, carbonate rock, coral concrete, etc. According to the results, Poisson’s ratio, both the secant Poisson’s ratio and tangent Poisson’s ratio, increase with the externally applied stress. Therefore, regarding it as an elastic constant is worthy of a second thought. If the midpoint of the stress interval is fixed in the 50% of uniaxial compressive strength, the average Poisson’s ratio is almost impervious to the varying span of the stress interval. In addition, the average Poisson’s ratio is immune to the nonlinear deformation in the early loading stage. Thus, the average Poisson’s ratio is a better index than the secant Poisson’s ratio in describing the relationship between axial and lateral strains of hard rocks. The determination of Poisson’s ratio of soft rocks needs further investigation because Poisson’s ratio tends to exceed the theoretical limit in relatively low stress levels. The proposed viewpoint provides a deeper insight into the testing, determining, and using of Poisson’s ratio

Sodium Metasilicate Cemented Analogue Material and Its Mechanical Properties

Analogue material with appropriate properties is of great importance to the reliability of geomechanical model test, which is one of the mostly used approaches in field of geotechnical research. In this paper, a new type of analogue material is developed, which is composed of coarse aggregate (quartz sand and/or barite sand), fine aggregate (barite powder), and cementitious material (anhydrous sodium silicate). The components of each raw material are the key influencing factors, which significantly affect the physical and mechanical parameters of analogue materials. In order to establish the relationship between parameters and factors, the material properties including density, Young’s modulus, uniaxial compressive strength, and tensile strength were investigated by a series of orthogonal experiments with hundreds of samples. By orthogonal regression analysis, the regression equations of each parameter were obtained based on experimental data, which can predict the properties of the developed analogue materials according to proportions. The experiments and applications indicate that sodium metasilicate cemented analogue material is a type of low-strength and low-modulus material with designable density, which is insensitive to humidity and temperature and satisfies mechanical scaling criteria for weak rock or soft geological materials. Moreover, the developed material can be easily cast into structures with complex geometry shapes and simulate the deformation and failure processes of prototype rocks