Mechanical Characteristics And Damage Mechanism Of Cemented Tailings Backfill Under The Coupled Hydro-mechanical Effect

In the upward horizontal cut and fill mining method,the backfill not only maintains the stability of the surrounding rock of the upper and lower wall,but also acts as a working platform for further upward mining.As a result,the backfill is subjected to multiple effects such as rolling,blasting and water soaking,various internal and external defects are formed,which weakens the strength of the backfill,reduces its bearing performance,and even directly leads to collapse,threatening the production safety of the mine.In this paper,the mechanical characteristics and damage mechanism of cemented tailings backfill under the coupled hydro-mechanical effect are studied.The waterbearing characteristics of backfill under the action of water immersion are clarified.Starting from the static load and dynamic load,the mechanical properties of submerged backfill under different loading methods were explored,the corresponding damage constitutive models were constructed,and the damage characteristics were explored.The microstructure of the backfill was analyzed by scanning electron microscopy(SEM).Finally,the damage mechanism of cemented tailings backfill the coupled hydromechanical effect was revealed.The main research work and conclusions are as follows:1)A low-field NMR-based water immersion test of cemented tailings backfill was carried out with the initiating immersion age and water immersion time as variables.Water has both promoting and weakening effects in the formation process of filling body strength.The promoting effect is reflected at the early stage,and the weakening effect is stronger than the promoting effect.Initiation age plays a key role in the water intake of the backfill under the action of water immersion.The smaller the initiation age,the larger the pore aperture of the backfill,and the faster it reaches saturation at the initial stage of immersion.With the increase of soaking time,the water gradually decreases,and the larger the initiation age,the smaller the pore aperture of the backfill.With the increase of time,the filling body gradually reached saturation,and the water volume showed a slow growth trend.2)Experiments on the mechanics of waterlogged cemented tailings backfill under uniaxial compression were carried out with initiating immersion age and water immersion time as variables.By designing comparison experiments between control and immersion groups,the deformation characteristics,strength characteristics,failure modes and energy characteristics of the backfill were analyzed.Water immersion enhances the plastic deformation capacity of backfill,and weakens its compressive strength and energy storage capacity.The strength reduction rate of backfill is consistent with the change law of water content.The smaller the initiation age,the greater the strength reduction rate.Shear failure is the main components of backfill after water immersion under uniaxial compression,the tensile failure is the auxiliary components.The dissipated energy is mostly friction dissipated energy.3)Based on Hopkinson pressure bar test system,an experimental system with 7075 aluminum alloy as the bar was proposed to study the dynamics of backfill.The dynamic experiment of backfill under water immersion was carried out.Under the effect of low impact velocity,water immersion enhanced the shear slip action of backfill and weakened its impact resistance.The main tensile shear failure occurred in backfill,and the smaller the immersion age,the more significant the shear failure.Under the action of high impact velocity,the filling body mainly causes shear failure to form fragments,and the impact is dominant,while the effect of water immersion is not significant.The variation trend of backfill strength difference of soaking age at 7d with the soaking time is opposite to that of water content with soaking time,the variation trend is consistent with that of soaking age at 7d,14 d and 28 d.The increase of impact velocity increases the decrease rate of dynamic compressive strength.4)The sectional damage model under uniaxial compression and the dynamic damage model considering water content and strain rate under impact were constructed respectively,and the damage characteristics of waterlogged backfill under each loading mode were analyzed.Water immersion under uniaxial compression increases the damage before the peak strain,increases the damage rate before the peak strain,and decreases the damage rate after the peak strain.The impact load water immersion mainly increased the peak damage degree of backfill and the proportion of the damage before peak strain to the whole damage.The significance of the effect of flooding on the damage characteristics of backfill body is negatively correlated with the initiation age.The increase of impact velocity mainly increased the significant effect of water flooding on damage characteristics of filling body.5)The damage mechanism of cemented tailings backfill under the action of waterforce coupling is revealed.The physical properties of backfill under water immersion are mainly affected by the physical bound water and free water inside.On the one hand,the physical bound water reduces the bearing capacity of the gel matrix;on the other hand,its lubrication reduces the friction coefficient between the tailings particles and the original cracks of backfill,promoting the dislocation slip of the tailings particles and crack propagation.Free water in the pores both promotes and hinders the crack propagation in the filling body.Under uniaxial compression,the loading rate is slow,and the free water in the pores produces pore water pressure similar to that in the wedge to promote the crack propagation.Under the impact,the loading rate is faster,and the free water in the pore forms meniscus before reaching the crack tip,which impedes the crack propagation.Figure 104;Table 12;Reference 118...