Mechanical properties and meso-structure response of cemented gangue-fly ash backfill with cracks under seepage- stress coupling

Highlight

• Mechanical properties of CGFB with cracks under seepage were studied.

• Seepage and prefabricated cracks aggravate the damage and degradation effect of CGFB.

• Damage mechanism of CGFB with cracks under seepage-stress coupling was analyzed.

Abstract

Seepage and cracks are two important factors affecting the stability of the cemented gangue-fly ash backfill (CGFB, a mixture of coal gangue, fly ash, cement and water). In this study, the CGFB samples with a 45° single crack were prepared and the mechanical properties of CGFB with cracks under different seepage water pressures were studied. Based on the theory of damage mechanics, the damage variable which can evaluate the seepage effect and crack effect was introduced, and the damage evolution law of CGFB with cracks under the seepage-stress coupling was discussed. The research hat: The mechanical properties of CGFB with cracks under seepage-stress coupling are mainly determined by the seepage water pressure, prefabricated crack and their coupling effects. Seepage and crack aggravate the attenuation of the peak strength and elastic modulus of CGFB, but with the increase of seepage water pressure, the deterioration effect of the crack weakens. Seepage and crack have significant effects on the damage of CGFB. Finally, based on the discrete element software, damage mechanism of CGFB with cracks under seepage-stress coupling was analyzed from a microscopic perspective. The research results can lay a foundation for the long-term stability analysis and instability prediction of CGFB with geological defects such as joints and cracks in water-rich mines.

Introduction

In mining projects, resource development must be coordinated with the environment, and green mining of coal resources is the focus of coordinated development [1], [2]. As an effective method to improve the utilization rate of coal resources and control surface subsidence, backfill mining technology has been widely used in China, and has become an important means of green coal mining [3], [4], [5], [6], [7], [8], [9], [10], [11]. The stability of backfill is an important factor to prevent the instability of goaf and surface subsidence [12], [13], [14], [15]. In actual projects, mining disturbance, blasting impact or poor filling effect and other factors can cause a large number of cracks inside the backfill, which is easy to reduce the strength of backfill, and then cause the overall instability of the filling face and surface subsidence [16], [17]. In addition, with the increase of coal mining depth, seepage has become another important factor affecting the stability of the backfill, and seepage and cracks affect the stability of the backfill together [18], [19], [20]. Comprehensively considering the weakening effects of seepage and cracks on the mechanical properties of the backfill is the key to study and evaluate the stability of backfill. Therefore, it is of great theoretical value and practical application significance to study the mechanical properties and meso-structure response of the backfill with cracks under the coupling of seepage and stress.

At present, cemented backfill mining is a common method of backfill mining in mining field [21], [22], [23]. Cemented backfill materials (CBM) usually include ordinary Portland cement (OPC), solid waste materials such as tailings, coal gangue, fly ash (FA) and water. Metal mine CBM usually use tailings as an aggregate, and the product is called cemented tailings backfill (CTB) [24]. Coal mine CBM usually use FA and coal gangue as aggregates, and the product is called cemented coal gangue-FA backfill (CGFB) [25]. In recent years, many scholars at home and abroad have studied the ratio, mechanical properties and water hardening mechanism of CTB and CGFB. Hou et al. [26] assessed the coupled effects of temperature and binder content on the mechanical and physical properties of frozen CTB. Wei et al. [27] obtained the impact factors of hydration heat of cemented tailings backfill by different evaluation indexes. Xu et al. [28] investigated the coupled effect of curing temperature and age on compressive behavior, microstructural and ultrasonic properties of CTB. Wang et al. [29] explored the law of dehydration-consolidation of full tailings filling slurry in the case of Electro-osmosis by the self-made electro-osmotic dehydration and natural dehydration test device. Zhang et al. [30] studied the main factors affecting the mechanical properties of CGFB, determined the optimal ratio, and successfully applied the test results to the site of Daizhuang coal mine. Chen Lei et al. [31] studied the effects of curing cycle, slurry concentration, FA content and cement content on the uniaxial compressive strength of high cemented gangue backfill strength (HCGB) by using single-factor and multi-level test methods. Sun et al. [32] discussed the deformation field evolution characteristics, Poisson’s ratio, and energy release response of GCGFB samples under uniaxial compression based on the digital speckle method (DSCM). Wu and Qi et al. [33], [34] studied the factors that affect the hydration and mechanical properties of CGFB. Du et al. [35] studied the bearing mechanism of unconfined backfill column (UBC) with stirrups by monitoring the internal stress and deformation of CGFB columns. Feng et al. [36] aimed at the stability monitoring of unconfined backfill structures (UBS) in partial filling, and obtained the failure process of CGFBM through uniaxial compression tests. The above research results promote the research and development of CTB and CGFB, but there is no relevant research on the mechanical properties of CBM with cracks under the coupling effect of seepage and stress.

There are few researches on the mechanical properties of CBM under seepage and stress, but there are a lot of researches in rock materials. Haeri et al. [37], [38] found that there are two main types of cracks in fissured volcanic ash Portland cement samples by uniaxial compression experiments: wing cracks and secondary cracks, and the ultimate fracture load of the specimen decreases with the number of cracks increases. Heek et al. [39] found that there are two kinds of mechanical mechanism of crack growth in cracked rock through experimental research: one is the tensile stress acting alone, and the other is the combination of tensile and shear stress. Cao et al. [40] studied the fracture mechanism of sandstone with holes and fissures under osmotic pressure by using a self-developed osmotic pressure device. Wang et al. [41] carried out permeability tests of limestone fractures with different roughness, analyzed the influence of coupling effect of stress and seepage erosion on the surface morphology of rough fractures, and studied the evolution law of seepage characteristics. These research results have promoted the research and development in the fields of seepage effect for fractured rock. However, there are obvious differences between rock materials and CBM. Compared with rock materials, CBM has low strength and many pores and fissures. Therefore, it is necessary and innovative to study the mechanical properties and microstructure response of CBM under the coupling effect of seepage and stress.

In this study, the CGFB samples with a 45° single crack were prepared and the mechanical properties of CGFB with cracks under different seepage water pressures were studied. Based on the theory of damage mechanics, the damage variable which can evaluate the seepage effect and crack effect was introduced, and the damage evolution law of CGFB with cracks under the seepage-stress coupling was discussed. Finally, based on the software of particle discrete element, the effects of seepage and crack on the contact force chain distribution, crack number and distribution of CGFB were analyzed from a microscopic point perspective.