Experimental And Modelling Studies On Creep Behavior Of Unsaturated Silty Clay In Fuyang

In this paper,based on a series of triaxial creep tests of unsaturated soil,the creep characteristics of remolded silty clay samples in the south of Fuyang under different matric suctions,net confining pressures and deviatoric stresses were studied.Combined with the creep test results,an improved fractional-order Nishihara model was proposed by applying the fractional calculus and rheological theories.Aiming at the defects of traditional fruit fly optimization algorithm,an improved fruit fly optimization algorithm based on Ginicoefficient-guided variation(GiniFOA)was proposed and applied to intelligent parameter identification of fractional-order creep model.Through the simulated and measured curves were compared and analyzed,which verified the advancement of the improved fractional-order Nishihara model and the reliability of GiniFOA for parameter identification of the fractional-order system.The main conclusions are as follows.(1)The triaxial creep test of unsaturated soil shows that the creep increases with deviatoric stress.Under the same deviatoric stress,soil creep decreases with the increase of matric suction or net confining pressure.The matric suction has a specific hardening effect on the soil,but this hardening effect will gradually stabilize with the increase of matric suction.It is also found that the stress-strain isochronous curves of test soil have good similarities and show prominent nonlinear characteristics under different stress conditions.Therefore,it is necessary to establish a creep element model to describe this nonlinear characteristic.(2)Considering the nonlinear characteristics of the measured curve of creep test,an improved fractional-order Nishihara model was discussed with the Harris function nonlinear Hooke body,the fractional Abel Newton body,and the fractional Abel friction element to describe the hardening or softening effect of soil.The improved model can simulate the accelerated stage of soil creep.And its strain-stress-time expression was derived through the fractional differential equation theory and the fractional Laplace transform.At the same time,the three-dimensional form was further extended to consider the influence of matric suction.(3)Given the characteristics of a large number of parameters and a large amount of identification calculation of the fractional-order model,a new method of parameter identification based on GiniFOA was discussed.By introducing the Ginicoefficient,the algorithm can give early warning when it falls into local extremum,and then guide the individual to start the mutation strategy,and jump out of the local extremum in time,which overcomes the defects of the common mutation strategy based on variance or standard deviation.The convergence accuracy and stability of GiniFOA were analyzed by 26 benchmark functions,and the test results are compared with 11 common swarm intelligence algorithms.The comparison results show that GiniFOA has better convergence accuracy,robustness and anti-interference.(4)Based on the MATLAB platform,the improved fractional-order Nishihara model discussed is compiled into the fitness function code that adapts to the GiniFOA interface.Then the GiniFOA is used to identify and fit the intelligent parameters of 12 creep measured curves obtained from the creep test.The results are compared and analyzed with the parameters identification results of nine widely used swarm intelligence algorithms.The fitting results show that the improved fractional-order Nishihara model can describe the whole creep process for the silt clay in the Fuyang area.By comparing the fitness function convergence curves of parameter identification with eight widely used swarm intelligence algorithms,it is found that the proposed intelligent identification method has better fitting accuracy,applicability,and reliability.