Experimental Study And Numerical Simulation On Resilient Modulus Of Subgrade Soil-Rock Mixture

With the continuous acceleration of China’s highway construction,the current highway construction has been shifted from the central and eastern regions of China to the western and southern regions.In the process of road construction in the western and southern mountainous areas,the lack of suitable roadbed fillers are became a thorny issue in the engineering.In mountain project,if only traditional subgrade soil is used as subgrade filler,the material is difficult to obtained,and the earthwork needs to be transported from other areas,and with the increasing in the cost of earthwork transportation a burden will be increased for the total cost of the project.The natural gravel formed during the road construction process in mountainous areas,which is mixed with the subgrade soil to form an soil-rock mixture is used as subgrade fillers,which can not only effectively improves the bearing capacity of the subgrade and but also can solves the problem of earth consumption caused by the subgrade filling.At the same time,it also reduces the cost of engineering projects and achieves green economic development.Based on the above considerations,this thesis is supported by a grant from the National Natural Science Foundation project,"Calculation Method and Evolution Mechanism of Dynamic Resilient Modulus of Subgrade Structures under the Action of Traffic Load and Humidity Change".(Project No.51908562).And it takes the Expressway Construction Project of Guangzhou to Foshan as the research background.The soil-rock mixture which is formed by bad subgrade soils(granite residual soil)of the project and the commercial crushed rock is selected as the research objects,the experimental study and numerical simulation on resilient modulus of subgrade soil-rock mixture have been carried out.Through the research,the influence of composition and physical state on the dynamic resilient modulus which is an important mechanical index of materials is investigated.On this basis,according to the experimental study results,a model used for estimating the resilient modulus of soil-rock mixture is proposed,which provides a theoretical basis for the estimation of the dynamic elastic modulus of the roadbed filler composed of soil-rock mixture.Moreover,the discrete element method is used to model and analyze the soil-rock mixture,and the change laws of the mesostructure inside the soil-rock mixture under the action of dynamic load are ascertained.The research results provide experimental and theoretical basis for the analysis of the evolution law of the dynamic resilient modulus of soil-rock mixture,the estimation of the value of the dynamic resilient modulus,and the evolution law of the mesoscopic behavior of the soil-rock mixture under the action of dynamic load.It not only has an important theoretical significance,but also has the broad engineering application prospect.The main research works carried out in this thesis and the relevant conclusions obtained are as follows.1.The main research works carried out1)A series of basic physical tests are carried out on soil-rock mixture under different rock contents,including compaction test and limit moisture content test.Through the experimental study,the relations between the maximum dry density and the optimal moisture content as well as rock content are explored.2)By comparing the existing dynamic triaxial loading sequence,the loading sequence that meets the working conditions of the roadbed of the main application area for the soil-rock mixture has been selected,and test samples for different working conditions(rock content and degree of compaction)for the dynamic triaxial tests of soil-rock mixture are prepared,to study the variation laws between dynamic resilient modulus and physical state and stress state.3)By reviewing the existing prediction models of dynamic resilient modulus,based on the dynamic triaxial test results of soil-rock mixture and the three-parameter model of NCHRP1-28A,a prediction model of dynamic resilient modulus of soil-rock mixture is proposed in which the physical composition and the degree of compaction as well as the stress state are all taken into account synthetically.4)The discrete element analysis software is used to reconstruct the triaxial test of the soil-rock mixture,and the dynamic triaxial tests under repeated loads have been simulated numerically.Furthermore,the variation laws of micromechanical behavior of soil-rock mixture under dynamic load have been studied,and then the evolution laws for the coordination numbers and the anisotropy have been all investigated.2.Main research progress and related conclusions1)Basic physical property test results show that the higher the rock content of the samples,the higher the corresponding compaction curves,indicating that the greater the maximum dry density and the lower the optimal moisture content,and vice versa.Meanwhile,it is found that the compaction curve of soil-rock mixture is similar to that of the fine-grained soil,which shows that the compaction process of it does not have the compaction feature of the coarse-grained soil.2)Under various conditions of rock content,the resilient moduli of soil-rock mixtures are all decreased with the increasing of octahedral shear stress.This rule shows that the soil-rock mixture has significant stress dependence under cyclic loading(that is,the resilient modulus is related to the level of deviatoric stress).Similarly,under the condition of the same octahedral shear stress,the resilient modulus values of plain soil samples and soil-rock mixture samples are all increased with the increasing of the bulk stress.Under the same conditions,the larger the bulk stress,the higher the resilient modulus value.This indicates that the resilient modulus of the soil-rock mixture also has a significant dependence on the bulk stress.Similarly,under the conditions of various compaction degrees,the resilient modulus value of soil-rock mixtures is increased approximately linearly with the increasing of rock content.Meanwhile,its growth rate also shows a certain non-linear growth trend.This shows that with the increasing of rock content,the gravel gradually forms a skeleton structure in soil and it becomes the main body to bear the external load.The stiffness of the crushed rock is much greater than that of the soil particles,and the gradual increasing in the force between soil particles will cause changes in the mesostructure of soil sample.These changes will help to increase the overall stiffness of the soil sample.Therefore,for the subgrade filled with soil-rock mixture as filler,it must be fully compacted to improve its compaction degree,which helps to improve its resilient modulus and reduce the deformation of the subgrade during its service period.3)By comparing and analyzing the different characteristics of the existing common prediction models,the rock content and compaction degree are embedded in the classic NCHRP1-28A three-parameter model in a reasonable form.And then a prediction model for dynamic resilient modulus which can takes into account the physical composition and physical state as well as stress state of material has been established.At the same time,the new model is compared with the traditional NCHRP1-28A three-parameter model.The results show that the proposed new model has higher prediction accuracy(R2=0.97)relative to the NCHRP1-28A model(R2=0.85).This shows that,the model is more accurate in predicting the dynamic resilient modulus of soil-rock mixture which is a particular geotechnical material.Moreover,through the comparative analysis of the estimated results of this model and the test results of two different crushed rock materials,it is verified that the predicted results of this model and the test results are in good agreement.This shows that this new model,which takes into account both the stress state and the physical state parameters,is more suitable for the prediction of the dynamic resilient modulus of the mixtures of geotechnical materials.4)After using discrete element analysis software to simulate the dynamic triaxial test of the soil-rock mixture,it is found that under different conditions of rock content,the mesostructure difference of the soil-rock mixture is very obvious.With the increasing of rock content,its contact force distribution will be changed from "soil-soil" contact to "soil-rock" contact,and with the further increasing of rock content,it will be eventually reached to "rock-rock" contact.At this time,the gravel skeleton plays a leading role in bearing and resisting external loads in the mechanical response of the specimen.Meanwhile,with the increasing of rock content,the rate of soil-rock mixture coordination number is decreased faster and faster.This shows that the contact state of the soil particles in the sample is changed significantly with the changing of rock content,and the formation of the crushed rock skeleton leads to an increase in the suspended particles of soil.Through the further research,it is found that the principal direction angle(θc)of sample is monotonically decreasing from the initial state to the intermediate state during a cycle period of load action.But the principal direction angle of the sample is monotonically increasing from the peak state to the end state.Moreover,the anisotropy coefficient of the sample is always at a low value,and its change is relatively small during the loading process.It is not difficult to find that under low stress state,the anisotropy inside the sample mainly depends on the anisotropy of the crushed rock,which is mainly related to the shape and the distribution of the particles.However,in the discrete-element triaxial test simulated in this study,due to the low stress level,the stress-induced anisotropy changes are small.To sum up,the effect laws of stress state and physical state on the dynamic resilient modulus of subgrade soil-rock mixture are explored through the basic physical properties test and indoor dynamic triaxial test.And the evolution mechanism of mesostructure of soil-rock mixture under the action of dynamic load is revealed.Furthermore,a prediction model of dynamic resilient modulus for soil-rock mixture is established,which can synthetically consider the composition and physical state as well as stress state of material.The research results provide a reliable theoretical basis for the prediction of the dynamic resilient modulus of the soil-rock mixture,and these also provide a reference basis for the promotion and application in similar projects.