Study On Mechanical Characteristics Of Limestone Under Loading And Unloading

At present,with the development of China’s water conservancy,transportation,mining,underground safety engineering,etc.,the stability of surrounding rock excavation is becoming increasingly important during the construction of underground projects.During the excavation of surrounding rock,the stress redistributes,and the loading and unloading stress states are more complicated.The results of rock mechanics loading theory are more perfect,but they are different from the actual situation.The mechanical properties and constitutive relations of rocks under loading and unloading conditions are quite different.Based on this,this paper takes typical limestone in underground caverns of Shuibuya Water Control Project as the research object,and conducts uniaxial and conventional triaxial compression and unloading tests of limestone by combining laboratory test,theoretical analysis and numerical simulation.The strength,deformation and failure characteristics of limestone under loading and unloading conditions are studied.Based on this,the constitutive model is derived,and the numerical analysis of the supporting engineering is carried out.The main research contents and results are as follows:1)The RMT-150 C rock mechanics system was used to conduct uniaxial and conventional triaxial compression tests of limestone,and the mechanical characteristics of limestone under load were analyzed.The results show that limestone has obvious brittleness and shows brittle ductility as the confining pressure increases;in the phase of online elasticity,the elastic modulus of limestone under different confining pressures is not much different;peak strength,peak deformation,elastic modulus Residual strength and confining pressure are all non-linear growth relationships;columnar splitting failure under uniaxial action and uni-slope shear failure under triaxial compression.2)Through conventional triaxial tests under different unloading rates,the mechanical properties of limestone under unloading conditions are analyzed.The results show that under low confining pressure conditions,it is easier to cause brittle drop of limestone stress;unloading will reduce the strength parameters of limestone,and unloading is more likely to cause limestone failure than loading;The friction angle will increase and the cohesive force will decrease accordingly.Tensile shear failure was shown during the unloading test.There were many tensile cracks along the perimeter of the main shear failure surface,and the phenomenon of fragmentation from the main shear to the periphery appeared.3)In order to reflect the strain softening characteristics and residual stage of limestone under loading conditions,the expression of strain softening model was derived based on the elastoplastic theory and Mohr-Coulomb strength criterion.Compared with the test results,it is found that the theoretical model curve is basically consistent with the test results.Based on the mechanical properties of limestone under unloading conditions,from the point of view of damage mechanics,a constitutive model of unloading damage is derived.Through the test results,it is found that although there is a certain deviation between the theoretical model curve and the test results in the residual stage,the trend is generally consistent and has certain rationality.4)According to the unloading damage constitutive model of limestone,using the source code of FLAC3 D software’s own model,a program for custom constitutive model was obtained.On the basis of that,the numerical simulation calculation of the excavation of the underground powerhouse cave of Shuibuya is carried out.Compared with the measured data,it is found that the numerical simulation results are consistent with the measured data,indicating that the model has certain value and significance in actual engineering.