Mechanical Properties And Constitutive Simulation Of Peat Soils

Due to the special soil forming mechanism,peat soil contains a lot of organic matter.Its loose spongy structure,large specific surface area and high adsorption lead to poor geotechnical characteristics of peat soil,such as high organic matter content,large void ratio,low density,high water content,high compressibility and low shear strength.The special microstructure characteristics make the mechanical properties of peat soil show a strong rate correlation.If only a specific rate is used in the test process,then the results are often difficult to be used as a reliable engineering basis.Therefore,it is of both practical value and scientific significance to understand and master the significant rate dependent mechanical properties of peat soil and to establish a constitutive model considering time-dependent characteristics.Supported by the National Natural Science Foundation of China project,the influence mechanism of solid deformation of organic matter on mechanical properties of peat soil and the anisotropic constitutive model(No 41972293),a systematic experimental study on the mechanical properties of Kunming peat soil,such as rate-dependent consolidation and strength,was carried out.Mainly divides into the conventional one-dimensional consolidation of its compression feature springback test(15 days),one-dimensional creep consolidation tests were loaded respectively(15 days),one-dimensional consolidation creep test with graded loading(lasting 60 days),constant strain rate one-dimensional consolidation test(6strain rates),constant stress rate one-dimensional consolidation test(5 stress rate),consolidation permeability joint test(8 strain levels)and so on were used to study the strength characteristics of the specimens Drained triaxial shear test,K₀consolidated drained triaxial shear test and variable rate K₀consolidated undrained triaxial shear test.According to the above test results,the change law of mechanical properties of peat soil is analyzed,and the rate dependent characteristics and formation mechanism are mainly discussed.Furthermore,on the basis of the traditional elastic-plastic model modified Cambridge model,an elastic viscoplastic model of peat soil based on over stress theory is established,and the secondary development is carried out by using ABAQUS finite element software and UMAT subroutine Based on the stress integral algorithm,the numerical calculation program for the elasto viscoplastic constitutive model of peat soil is compiled by using FORTRAN language.Finally,the model is verified by numerical simulation and unit test.The main research contents and achievements are as follows:(1)Through the indoor basic physical properties test and field investigation,it is found that Kunming peat soil has small natural gravity,high natural water content,large initial pore ratio and high organic matter content,which is similar to that of high organic soil or other peat soil,but it is different from ordinary soft clay.(2)Based on a series of conventional one-dimensional consolidation tests,the parameters of Kunming peat soil,such as pre consolidation pressure,compression coefficient,compression index and rebound index,are obtained.The secondary consolidation coefficient and the ratio of secondary consolidation compression to total consolidation compression are studied by loading and grading one-dimensional consolidation creep tests,and the relationship between secondary consolidation coefficient C_?and consolidation pressure p is analyzed Furthermore,the relationship between coefficient of secondary consolidation C_?and compression index C_cof peat soil is studied,and peat soil is divided into high secondary compressibility soil according to coefficient of secondary consolidation compression.(3)The influence of strain rate and stress rate on the compressibility of Kunming peat soil was studied by continuous loading test,and the change rule of permeability with pore ratio in the compression process was studied.Both CRS and CRL tests increase with the speed of the consolidation pressure.In CRL test,the early consolidation pressure is more sensitive to the rate at low speed level;For compression index C_c,in CRS test,the compression index C_cof peat decreases with the increase of strain rate,while in CRL test,the influence of the rate on the compression index C_cis not obvious;for the consolidation coefficient C_v,C_vincreases with the increase of the rate in both tests;During the compression process,the change rule of permeability coefficient with pore ratio conforms to the empirical formula C_k=0.25e₀.(4)The shear strength characteristics of peat soil under different consolidation modes are systematically studied,and the shear rate effect of strength characteristics of peat soil under K₀consolidation is mainly analyzed.Through a series of triaxial tests,the K₀coefficient of peat soil is 0.48,and the critical state stress ratio M is 1.4 under the condition of isotropic consolidation.Comparing the cohesion c and internal friction angle?under the condition of isotropic consolidation and K₀consolidation,it is found that the former c=10.14kpa,?=34.13°and the latter c=3.02kpa,?=29.8°respectively.It can be seen that the c under the condition of K₀consolidation is larger,while the?is slightly smaller.Comparing the undrained shear strength q_uunder the confining pressure of 30k Pa,60k Pa and 90k Pa,it is found that the q_uunder K₀consolidation is 134.2%,165.6%and 199.5%of that under isotropic consolidation,respectively;the results also show that the stress-strain curve of peat soil under undrained condition is softening,while that under drained condition is hardening;the effect of strain rate on the undrained stress-strain curve of peat soil is not obvious The undrained shear strength increases by 22.1%?26.9%when the shear rate increases by 10times(5)Referring to the classical elastic-plastic modified Cambridge model,an elastic viscoplastic model of peat soil is established by combining the overstress theory.Six material constants(compression index?,rebound index k,critical state stress ratio M,Poisson's ratio?,flow parameter?and rate dependent parameter n)in the model can be obtained by conventional consolidation and triaxial shear tests.(6)Through the UMAT sub program interface provided by ABAQUS finite element software,the numerical calculation program of the elasto viscoplastic constitutive model of peat is compiled by using FORTRAN language by using the step explicit stress integral algorithm of sub increment.Based on the established numerical model,the experimental results are simulated.The results of the experiment and the experimental results show that the experimental rules and the results are basically consistent with the results.The model can reflect the mechanical response of peat rate,and verify the reliability of the model.