Study On Constitutive Models Of Bioenzyme-treated Expansive Soil

In engineering practice,lime,cement and other inorganic materials are often used to improve expansive soil.The results showed that bio-enzymes can improve the physical and mechanical properties of expansive soils,but there are few studies on constitutive models of bioenzyme-treated expansive soils at home and abroad.It is necessary to study the constituti've models of bioenzyme-treated expansive soils in order to reveal its engineering characteristics and provide basis for engineering design of expansive soils.Thus a series of triaxial tests were carried out on bioenzyme-treated expansive soils.Based on the Cambridge model and Yin's Double Yield Surfaces Model,the correlation relationships between the content of bio-enzymes and the constitutive models were established.The evolution law of the stress-strain relationships of the improved expansive soil under different bio-enzyme content was obtained.Finally,the related models were modified to improve the description accuracy of the improved soil.The following research progress and innovative conclusions are obtained:1.The physical and mechanical properties of enzymatic modified expansive soils were studied through a series of physical properties tests,expansive properties tests and triaxial shear tests.The experimental results are as follows:(1)The experimental results of physical properties of expansive soil improved by biological enzymes show that the liquid-plastic limit decreases with the increase of the amount of biological enzymes;the amount of biological enzymes has little effect on the optimum water content,the optimum water content is about 18%;the amount of biological enzymes has a great influence on the standard moisture content of expansive soil,It can be described by quadratic function.(2)Expansion properties of expansive soils improved by enzymes were tested.The results show that the content of enzymes had great influence on free expansion rate,unloaded expansion rate and loaded expansion rate.Bioenzymes could effectively improve the expansibility of expansive soils.(3)The triaxial test results of enzymatic modified expansive soils show that the stress-strain correlation is strain-hardening,and the hardening law conforms to the theory of isotropic hardening and kinematic hardening.Bioenzymes can effectively improve the compressive and shear properties of the improved soils.2.Based on the triaxial shear test results,the theoretical study of the constitutive model of single yield surface was carried out.The influence of bio-enzyme dosage on the parameters of modified Cambridge model was analyzed.Based on the Wei Rulong-Khosla-Wu model,the effect of bio-enzymes on plastic shear deformation in elastic walls and yield surface shapes was considered.Finally,a modified MCC model and a modified Wei Rulong-Khosla-Wu model were established,in which the disturbance factor was the amount of bio-enzymes and the content of biological enzymes was the unified disturbance function.The following theoretical research results are obtained.(1)The modified Wei Rulong-Khosla-Wu model can describe the stress-strain relationships of the test results most accurately,and the modified MCC model can greatly improve the accuracy of the stress-strain relationship of bioenzyme-treated expansive soils compared with the MCC model.(2)The content of bio-enzymes significantly affects the slope of isotropic consolidation line.With the increase of the amount of bio-enzymes,the slope of rebound curve increases.The test results show that bio-enzyme have a great influence on the parameters of the stress-strain relationships of expansive soils,which indicates that the compression resistance of the expansive soils improved by bio-enzymes increases under the same loading conditions.(3)The stress-strain correlation is strain hardening.According to the test results,the hardening law conforms to the theory of isotropic hardening and kinematic hardening.The modified Wei Rulong-Khosla-Wu model can be considered by considering the shape of yield surface.(5)In this paper,the plastic deformation in the elastic wall is considered by modifying the elastic modulus.3.The elastic-plastic "hat" constitutive model of single yield surface can not well describe the large plastic shear deformation and even failure of soil under the stress path of constant deviating stress and reduced confining pressure.Therefore,based on a series of triaxial drainage shear tests,the elastic deformation evolution law of the improved expansive soils with different enzymes content was analyzed,and the evolution law of yield surface shape related to shear compression and shear expansion as well as the evolution law of failure line equations were studied.On this basis,the influence of bio-enzyme dosage on the parameters of Yin's Double Yield Surfaces Model was analyzed.The amount of bio-enzyme was introduced as the correction factor to modify Yin's Double Yield Surfaces Model,and finally the Yin's Double Yield Surfaces Model was modified.The following theoretical research results are obtained.(1)The modified Yin's Double Yield Surfaces Model can well describe the constitutive relationship of expansive soils improved by bio-enzymes.Comparing the theoretical and experimental values of the modified Yin's Double Yield Surfaces model,they are in good agreement.(2)Biological enzymes can increase the elastic deformation of the improved expansive soil,and the parameters related to elastic deformation increase with the increase of the amount of biological,enzymes.With the increase of enzymatic incorporation,the shape of the shear yield surface shows an evolutionary law far from the origin of the number axis,while the shape of the dilatancy yield surface shows an evolutionary law that is more and more far away from the number axis and gradually convex.(3)Biological enzymes can effectively improve the shear resistance of expansive soils.By studying the evolution law of failure line equation,it is concluded that biological enzymes can significantly enhance the cohesion and internal friction angle of soil,but have little effect on the failure ratio of soil.Based on single yield surface models and double yield surfaces models,the constitutive relationships of bioenzyme-treated modified expansive soils is studied,which enriches the relevant theory of critical state soil mechanics.The proposed modified model can explain the stress-strain relationship of enzymatic modified expansive soils well,and it can also provide some reference for the stress-strain study of other improved soils.The further engineering application of enzymatic modification of expansive soil can provide some reference value.