Research On A Whole-process Brittle Creep Failure Model For Hard Rock

With the construction of underground rock engineering,the creep behavior of hard rocks has attracted more and more attention,which is one of the most important factors that affect the safety and stability of underground rock engineering.Hard rocks are mostly brittle,so their creep always cracks suddenly at little deformation,causing great lost.This paper mainly focuses on the creep failure of hard brittle rocks.By analyzing curves of creep,it is found that the processes of creep failure test contain typical creep and brittle failure.The two stages are connected by a failure rule that judges when the creep will fail.In this way,the time when the creep will fail and the stress and strain after fail will be describable quantitatively.By the means of summarizing and analyzing conventional compression tests and creep tests of three kinds of hard rock,relations of the two kinds of test on the Young modulus and fail strain are found.The fail rule is concluded by test data that when creep fails the creep strain is equal to the ultimate strain of the conventional compression tests.Then constitutive relation of creep failure for hard rock is established.The Finite element program of the constitutive relation based on Abaqus is developed and verified.Then the program is applied to calculation of Dagangshan tunnel excavation.This paper mainly contains content and conclusions as follow.(1)Based on creep phenomenon in rock engineering and test data,the creep failure process can be summarized from instantaneous elastic(plastic)stage to visco-elasto-plastic stage to brittle-plastic failure stage to residual strength stage..So it is necessary to joint instantaneous elasto-plastic model to creep model to describe creep failure process.(2)The creep test data and conventional test data of diabase,fine sandstone and red sandstone are compared and analyzed.The strength of rock after creep failure will soon decrease to residual strength,the same in conventional tests.The Young modulus in creep tests tends to keep close to that of same rock and same confining pressure in conventional tests,but varies with confining pressure and axial pressure.The axial strain of creep failure is close to that of same rock and same confining pressure in conventional tests,but the former tends to be smaller than the latter(except the creep test of diabase with 15MPa confining pressure).(3)Creep strain of rock will decrease rock strength.Creep fails when rock strength decreases below the load.The failure rule has been established based on test data.When creep strain increases with time to ultimate strain in conventional test of same rock and same confining pressure,the brittle creep failure will happen.The one-dimensional and three-dimensional constitutive equations are deduced.The full stage model can describe brittle creep failure and instantaneous elasto-brittle-plasticity as well.(4)The constitutive equations are transformed into the finite element incremental format.The FEM program of constitutive relation of creep failure model is developed in Abaqus and its function is tested on a cylindrical specimen.The simulation results calculated by the program in elasto-brittle-plasticity and brittle creep failure match well with corresponding test data.(5)The brittle creep failure model this paper raises is applied to calculation of Dagangshan tunnel excavation,and the simulation results are compared to those by elasto-brittle-plastic model and elastic visco-elasto-plastic model.Compared with the brittle creep failure model,the elasto-brittle-plastic model cannot consider the influence of rheology on stress and strain of surrounding rock and area of failure.The failure area of elasto-brittle-plastic model is smaller than that of the brittle creep failure model.Compared with the brittle creep failure model,the elastic visco-elasto-plastic model cannot show that the strength of surrounding rock will decrease to residual strength after rheology failure.The elastic visco-elasto-plastic model cannot calculate failure area or rheology failure time.The rationality and superiority of the brittle creep failure model is demonstrated by the engineering example.