Mechanical Properties And Engineering Application Of Cemented Waste Rock Backfill In Deep Mining

The cemented waste rock backfill is a new type of backfill process that mixes the naturally graded waste rock aggregates produced by underground mining with cemented tailings slurry and transports them to the stope for filling.Cemented waste rock filling is a promising backfill method in deep mining because it can realize the utilization of waste rock underground and has the characteristics of high strength,low cost,and safe operation.However,as a heterogeneous composite artificial material,the cemented waste rock backfill’s mechanical properties and failure mechanism have not received enough attention.The research on its influence on deep mining and surrounding rock stability is far less comprehensive than cemented tailings backfill and cemented paste backfill.This paper comprehensively uses laboratory tests,theoretical analysis,and numerical simulation to study the evolution law of cemented waste rock backfill’s mechanical properties under the influence of waste rock aggregate’s content and grading index.A theoretical model of the evolution of cemented waste rock backfill’s mechanical properties was established.The transformation law and mesoscopic mechanism of cemented waste rock backfill’s failure mode were revealed.Finally,the research results of cemented waste rock backfill’s mechanical properties were applied to deep stope backfill to verify the correctness of the theory and simulation test.The research contents are concluded as follow:(1)Study on mechanical properties of cemented waste rock backfill and combination of backfill and surrounding rock.First,the uniaxial compression test and variable angle shear test were designed in detail to study the mechanical properties of cemented waste rock backfill.Laboratory tests show that the mass fraction and gradation index of waste rock aggregate in the cemented waste rock backfill have the optimal values.When the waste rock aggregate’s mass fraction and gradation index are both the optimal values,the cemented waste rock backfill’s strength and deformation modulus are the largest.On the contrary,when the waste rock aggregate’s mass fraction and grading index are small or large,the cemented waste rock backfill’s strength and deformation modulus will decrease.The waste rock aggregate’s optimal gradation index corresponding to different mass fractions is not the same.When the mass fraction of waste rock aggregate is close to the maximum limit,the influence of the gradation index on the waste rock cemented backfill’s mechanical properties is significantly weakened.(2)Propose an evolution model of cemented waste rock backfill’s elastic modulus and compressive strength.Based on solid mechanics and damage mechanics theories,this paper established a theoretical evolution model that can describe the cemented waste rock backfill’s elastic modulus and compressive strength.The parameters of the theoretical evolution model which were fitted according to laboratory test results are within the reasonable range,and the variation laws of elastic modulus and compressive strength calculated by the laboratory test and the theoretical model are similar.These results show that the proposed evolution model can well reflect the influence of waste rock on the cemented backfill’s elastic modulus and compressive strength.(3)Study on the meso-mechanism of the evolution of the cemented waste rock backfill’s mechanical properties.This paper realizes the fine modelling of the waste rock cemented backfill through the discrete element numerical method and proposes a quantitative discrimination method that can accurately identify the failure mode of the numerical model according to the moment tensor theory.The numerical simulation results show that the failure mode of the cemented waste rockfill specimen in the uniaxial compression test is the tensile shear failure.As the mass fraction of waste rock increases,the failure mode of cemented waste rock backfill is closer to pure shear failure.At the mesoscopic level,the evolution of the cemented waste rock backfill’s compressive strength is determined by the three types of contact in the numerical model: the particle with particle(bb),the particle with waste rock(b-p),and the waste rock with waste rock(p-p).The backfill model with the optimal mass fraction and gradation index of waste rock aggregate will have moderate b-p contacts and p-p contacts and greater strength,so it has greater strength.(4)Research on the influence of cemented waste rock backfill on the stress transfer in deep mining.Based on the study of cemented waste rock backfill’s mechanical properties,this paper established the relationship between post-peak strength parameters and minimum principal stress for the traditional strain softening plastic models obeying the Mohr-Coulomb strength criterion and Hoek-Brown failure criterion.A new constitutive model was proposed to simulate the deformation,failure,and brittle-ductile transformation behaviour of the cemented waste rock backfill and rock mass under the three-dimensional stress state.The simulation results of the mining stress transfer process in the deep mine show that the mining stress will transfer to the shallow stope after backfill mining the inclined orebody’s deep area.The stress transfer causes the areas with the maximum horizontal displacement,the highest stress concentration,and the highest risk factor in the formation to appear at the position of 1～2 floors above the deepest stope.Backfill mining the deep area will lead to the continuous transfer of mining-induced stress to the shallow barrier pillars.The shallow backfilled stopes are also in a high-stress state,which may cause engineering disasters such as rock bursts.(5)Study on the improvement effect of the cemented waste rock backfill on stress conditions in the deep stopes.Due to the brittle-ductile transformation behaviour of the rock mass,the barrier pillars’ minimum principal stress increases after being enclosed by the backfill,and its bearing capacity is improved,so it can continuously withstand the transfer stress from the deep.The mining stress transferred to the underlying formation will lead to a stress concentration zone under the deepest stope,the extent of which increases with the mining depth.Replacing lowstrength cemented tailing backfill with cemented waste rock backfill can reduce the stress concentration of the area to be excavated below the deepest stope.The effect of reducing the stress concentration will be more significant when the backfill’s strength and elastic modulus are higher.