The main aim of this investigation is to develop backfill concrete including coal gangue and metakaolin to reduce solid waste. For this purpose, a total of 30 concrete mixtures were designed by the inclusion of 0%, 25%, 50%, 75% and 100% coal gangue as coarse aggregates and 0%, 10% and 20% metakaolin as binder at 0.55 and 0.45 water to cement ratios. The compressive strength was tested after 3, 7 and 28 days for a total of 90 samples. Meanwhile, the influences of coal gangue and metakaolin on the elastic modulus, ultrasonic pulse velocity, rebound number and open porosity were explored. Then, the relationship between physical and mechanical properties was revealed by design code expressions and empirical models. Furthermore, an extreme learning machine was developed to predict compressive strength by concrete mixtures. The results show that the inclusion of coal gangue results in a poor performance in physical and mechanical properties of concrete. However, the drawbacks of concrete containing coal gangue can be compensated by metakaolin. The predicted results of design code expressions and empirical models are closed to the experiment results, with a 10% error. In addition, the findings reveal that the extreme learning machine offers significant potential to predict the compressive strength of concrete with high precision.

This paper studies the early mechanical properties of fiber-reinforced cemented tailings backfill (CTB) and discuss its modification mechanism. The effects of fiber types and addition (polypropylene fiber, basalt fiber and glass fiber) on unconfined compressive strength of CTB were studied by unconfined compressive strength test (UCS). Scanning electron microscopy (SEM) was used to investigate the microstructure of fiber-reinforced CTB. Based on the theory of interface mechanics and the contact mechanism of fiber interface, the evolution mechanism of fiber-reinforced CTB interface characteristic stiffness was further explored. The results show that the fiber type and content have a significant effect on the strength of CTB, and the optimum addition of fibers is 0.4%. The strength of fiber-reinforced CTB samples increased first and then decreased with the increase of fiber content. The stress of CTB sample without fibers reaches the maximum value when the strain is 1.01%, while introduction of basalt fiber increases that value to 3.74%. In addition, the microstructure characteristics show that the hydration products around the fiber make the CTB sample have better compactness, and fibers can effectively inhibit the crack development of the CTB samples. Finally, using the theory of interface mechanics, it is found that the interface stiffness of CTB sample with basalt fibers is the largest, but the interface contact stiffness increases first and then decreases with the increase of fiber content, which is consistent with the law of macroscopic strength change.

In cold regions, rock’s load-bearing capacity will be greatly diminished in severe settings. This research investigates the influence of freezing and thawing on the physical and mechanical characteristics of sandstone, as well as the strength under complicated stress conditions. Uniaxial compression of sandstone samples was performed following freeze-thaw cycles, and the changes in elastic modulus and peak stress of dry sandstone and saturated sandstone were investigated under various freeze-thaw cycles. ANOVA was used to analyze whether there were significant differences between the number of freeze-thaw cycles and the peak strength and elastic modulus. A three-parameter strength prediction model based on σC, k0, and m is created based on the critical failure energy function. Experimental data is utilized to assess the model’s correctness, and the model is used to forecast the strength of dry and saturated sandstone during freeze-thaw cycles. The result indicates that peak stress and elastic modulus of dry and saturated sandstone show a steady attenuation pattern as the number of freeze-thaw cycles increases. Brittle failure is the failure mode of dry sandstone, whereas brittle failure to plastic failure is the failure mode of saturated sandstone. After 60 freeze-thaw cycles, peak stress in dry and saturated sandstone was reduced by 55.3% and 56.8%, respectively. The three-parameter model can predict the triaxial compressive strength of specimens under different confining pressures through the uniaxial compressive strength of specimens with an error range of 1% to 13%.

The liquefaction of tailings sand caused by seismic loads is a major problem in ensuring the safety of tailings ponds. Liquefaction may cause uncontrolled fluidized failure of the dam body, causing considerable damage to the lives, property and environment of people downstream. In this paper, a prototype tailings sand is used as the material to consider the main factors affecting liquefaction (i.e., dynamic load, soil quality, burial and static conditions). By embedding acceleration, pore pressure and earth pressure sensors in the rigid design of the self-designed rigid model box, different types of seismic waves of different ground motion amplitudes (PGA) were induced in a shaking table test of tailings sand liquefaction. The seismic intensity, waveform (class II, III and IV seismic waves) and active earth pressure of the PGA characterizing dynamic factors were obtained, and the static factors were characterized. The dynamic shear stress ratio, the peak acceleration of the earthquake, the pore pressure of the drainage factor and the buried depth (overlying effective pressure) characterize the soil conditions. SPSS software was used to analyze the factor dimension reduction, and the most suitable factors for factor analysis were obtained. Particle Swarm Optimization (PSO) was used to optimize the parameters, and the improved PSO-SVM algorithm was compared with the existing genetic algorithm (GA) and grid node search (GS). The algorithm used in this paper is fast and has a relatively high accuracy rate of 92.7%. The established threshold model method is of great significance to predict the liquefaction of tailings sand soil under the action of ground motions and to carry out safety managemenin advance, which can provide a certain reference for the project.