Strain Rate Effect on Mechanical Properties of Cemented Backfill under Dynamic and Static Combined Loading

In order to study the influence of pillar stopping blasting on the stability of cemented backfill, the dynamic impact test under low strain rate (61.1∼86.8 s−1) was conducted on cemented backfill with two kinds of strength using three-dimensional coupled static-dynamic SHPB equipment. At the same time, the strain rate effect of failure mode, dynamic strength factor, and energy transfer of backfill were analyzed. The results show that when the cemented backfill was loaded under different strain rates in the initial three-dimensional static pressure environment, the pore compaction process was no longer obvious but directly entered the elastic deformation stage. Within the range of strain rates, the extreme value of dynamic intensity factor (DIF) of CTB230 was 6.8, while the extreme value of dynamic intensity factor of CTB310 specimen did not appear within the range of strain rates due to the improvement of the internal cementation force between particles. The fracture surfaces of specimens were perpendicular to the direction of load, and the failure mode was mainly the axial tensile failure, and the fracture surfaces were mostly close to the loading end. According to energy calculation, reflected energy accounts for 80.4%∼86.6% of incident energy; dissipated energy, 5.5%∼14.3%; transmitted energy, 5.3%∼7.9%.

Development of Fly Ash Composite Binder and Optimization of Slurry Ratio

In view of the high cost of cement filling, the new cementitious materials are developed by using solid waste resources. Firstly, on the basis of material physicochemical analysis, the fly ash composite cementation ratio test and optimization test are carried out to determine the optimal ratio. Then, the filling body strength and pipeline transportation characteristics test are carried out to analyze they influence law. Finally, the genetic algorithm is used to optimize the slurry ratio. The results show that the strength of cemented backfill increases linearly with the increase of slurry concentration; The slump and bleeding rate of slurry decrease with the increase of slurry mass fraction, and increase with the decrease of binder sand ratio, the optimal proportion of fly ash (FA) based composite binder is w(FA): w(clinker): w (desulfurized gypsum (DG)): w (slag powder (SP)) =40:12:12:36; The optimum slurry ratio is 1:4 of binder/sand and 72% of concentration.

Study on Damage Constitutive Model of High-Concentration Cemented Backfill in Coal Mine

In order to construct the damage constitutive model (DCM) of high-concentration cemented backfill (HCCB) in coal mine, the generalized Hoek-Brown strength criterion was used as the failure criterion. For the difference of theoretical derivation of constitutive relation, a new DCM based on residual strength was proposed. Combined with the conventional triaxial compression test, the correctness and rationality of the DCM were verified. The damage evolution characteristics of HCCB were analyzed, and the physical meaning of model parameters was clarified. The results show that (a) the theoretical curves of stress-strain relation are in good agreement with its experimental curves, which means DCM can simulate the deformation and failure process of HCCB. (b) The damage evolution curve of HCCB is S -shaped. To some extent, the confining pressure can inhibit the development of damage. (c) The parameter F 0 reflects the position of the peak point of the DCM, and parameter n is the slope of the straight line segment in the postpeak strain softening stage, which are, respectively, used to characterize the strength level and brittleness of HCCB. The establishment of DCM of HCCB is helpful to reveal its deformation and failure mechanism and provides theoretical basis for its strength design.

Permeability Tests and Multi-Factor Analysis of Sand-Based Cemented Backfill Five-Component Samples With Different Mix Ratios

Extensive coal mining involves the challenge of liberating coal resources under buildings, railways, and water bodies. Sand-based cemented backfill (SCB) mining is considered an effective method to solve this problem while utilizing solid wastes in large quantities. Since the groundwater seeping into SCB pores in a complex mining environment deteriorates the SCB strength and stability, the permeability optimization of SCB samples under multi-factor conditions by adjusting their mix ratios is very topical. Therefore, in this study, a large number of SCB samples were prepared using aeolian sand as aggregates, cement and fly ash as cementing materials, and quicklime and water reducing agent as additives. The mass of the aeolian sand was a fixed value, while other ingredients' content ratios were expressed as a percentage of aeolian sand mass. With all other factors being constant, the level of one factor was changed at a time for univariate analysis. Four levels were set up for each of the four factors, and 16 tests were performed for a total of 13 mix ratios with an axial pressure of 1 MPa and confining pressure of 3 MPa. The effects of mix ratios, pore size, porosity, and surface structure on SCB's permeability were analyzed in detail. Experimental results show variations of quicklime and fly ash contents significantly changed the SCB's permeability, and variations of cement and water reducing agent contents had a minor impact on SCB's permeability. SCB's permeability positively correlated with porosity, primary pore size, and compactness of surface structure. These findings are considered instrumental in improving the SCB waterproof performance.

Environmental Safety Analysis of Red Mud-Based Cemented Backfill on Groundwater

As one of the main industrial solid wastes, there are a large number of free alkaloids, chemically bound alkaloids, fluoride, and heavy metal ions in Bayer process red mud (BRM), which are difficult to remove and easily pollute groundwater as a result of open storage. In order to realize the large-scale industrial application of BRM as a backfilling aggregate for underground mining and simultaneously avoid polluting groundwater, the material characteristics of BRM were analyzed through physical, mechanical, and chemical composition tests. The optimum cement–sand ratio and solid mass concentration of the backfilling were obtained based on several mixture proportion tests. According to the results of bleeding, soaking, and toxic leaching experiments, the fuzzy comprehensive evaluation method was used to evaluate the environmental impact of BRM on groundwater. The results show that chemically bound alkaloids that remained in BRM reacted with Ca2+ in PO 42.5 cement, slowed down the solidification speed, and reduced the early strength of red mud-based cemented backfill (RMCB). The hydration products in RMCB, such as AFT and C-S-H gel, had significant encapsulation, solidification, and precipitation inhibition effects on contaminants, which could reduce the contents of inorganic contaminants in soaking water by 26.8% to 93.8% and the leaching of toxic heavy metal ions by 57.1% to 73.3%. As shown by the results of the fuzzy comprehensive evaluation, the degree of pollution of the RMCB in bleeding water belonged to a medium grade Ⅲ, while that in the soaking water belonged to a low grade II. The bleeding water was diluted by 50–100 times to reach grade I after flowing into the water sump and could be totally recycled for drilling and backfilling, thus causing negligible effects on the groundwater environment.

Study on the strength property and ratio parameter selection of waste rock cemented backfill

When tailings are used for cemented backfill preparation, the extremely fine unclassified tailings may lead to slow consolidation and low strength of backfill material. Wasted rock as an additional filling aggregated was suggested to optimize the gradation composition of aggregate by scholars over the world. In this paper, the effect of waste rock addition, cement-tailing ratio and slurry concentration on strength and flow properties of waste rock cemented backfill were studied. The results indicate the strength of waste rock cemented backfill was significantly higher than that of unclassified tailings cemented backfill under same cement consumption, which the average strength improvements were 2.02MPa, 0.98MPa and 0.46MPa under cement-tailing ratio of 1:4, 1:8 and 1:10. With the increase of waste rock addition, the strength change of waste rock cemented backfill was less obvious, but the flow property (yield stress) of filling slurry was improved. Further analysis of the slurry stability illustrates that, with the increase of waste rock addition, the bleeding rate demonstrated a trend similar to that observed for the flow property, however, in an adverse manner. Overall, the optimal slurry concentration of 80% and waste rock addition of 40%～50% were determined. Based on the strength requirement, cement dosage was selected, which the cement-tailing ratio of top 10m and the bottom 10m was 1:8, the cement-tailing ratio of the centre stope was 1:10. The research findings can provide a reference for the ratio parameter determination of extremely fine unclassified tailings backfill of similar mines.