| Road diseases in seasonally frozen areas of China are very serious,which directly affect the service life and efficiency of roads.The diseases are the result of comprehensive function of subgrade's property decay and external environmental factors.It's an effective measure to prevent road diseases by modified subgrade filling with excellent performance and stability.Oil shale wastes are solid residues of combustion and distillation of oil shale minerals.With the development and utilization of oil shale resources,a large number of oil shale wastes produce.The mountainous wastes not only occupy valuable land resources,but also pollute farmland and water resources because of the soluble solids,sulfides,organic matter and trace element compounds.The application of oil shale wastes in subgrade has significant advantages such as high utilization rate,low processing cost and strong applicability.Funded by Natural Science Foundation of China “Study on the mechanism of cold resistance layer to cure subgrade frost damage” and Scientific & Technological Planning Project of Jilin Province “Key technology of the application of oil shale wastes and fly ash to subgrade in seasonally frozen areas”,a novel subgrade filling material modifying silty clay by oil shale waste and fly ash is proposed.This novel subgrade material can be called oil shale wastes modified subgrade filling or the modified subgrade filling.The modified subgrade filling is expected to great performance,stability and frost resistance in seasonally frozen areas.The main contents of this paper are as follows:(1)The physical and chemical properties of oil shale wastes,fly ash and silty clay are determined.Based on the principle of maximum recycle of wastes and obtaining an excellent and stable filling material in seasonally frozen areas,the novel subgrade filling,silty clay modified by oil shale wastes and fly ash with optimum composition ratio is proposed.The optimum ratio is determined by compaction,liquid-plastic limits and CBR bearing ratio tests.On this basis,the gradation of grain and chemical composition stability of the novel subgrade filling during mixing and after multiple freeze-thaw cycles are analyzed.Finally,according to the results of anions,cations and trace elements in the infiltration fluid of the subgrade soil,the environmental impact of the modified subgrade filling is evaluated.(2)Based on the specified standard in “Specifications for Design of Highway Subgrades”(JTG D30-2015),the road performance of oil shale wastes modified subgrade filling is studied by two parameters: CBR bearing ratio and dynamic resilient modulus.The influence of compaction degree,test environment factors(whether water immersion)and stress conditions on the two parameters are investigated.The road performance of the modified subgrade filling is evaluated by the contrast of silty clay.In order to provide convenience and accurate design indicators for the practical engineering of oil shale wastes modified subgrade filling,the dynamic resilient modulus prediction models based on stress conditions and CBR bearing ratio are derived.Furthermore,under the guidance of the specification,a structure design of subgrade with oil shale wastes modified subgrade filling is given.(3)In order to assess the strength and deformation characteristics of the novel modified subgrade filling under dynamic traffic loads,the dynamic properties of the modified filling are studied by dynamic triaxial test.The dynamic strengths under different stress conditions and failure cycles are obtained.By contrast,the failure dynamic stress ratio is introduced to reveal the failure characteristics more clearly.Based on the Moore-Coulomb theory,the dynamic cohesion and internal friction angle of the modified filling are calculated,and the variation trends are also analyzed.Moreover,the dynamic modulus is calculated by extracting the strain and stress data from stable hysteresis loops.The Hardin-Drnevich constitutive model is applied to solve the maximum dynamic modulus and shear stress.Furthermore,the normalization model of dynamic modulus and empirical prediction model of maximum dynamic modulus for the modified subgrade filling are derived.(4)The dynamic properties and meso-mechanism of oil shale wastes modified subgrade filling after freeze-thaw cycles are studied.Dynamic resilient modulus and strength are taken as objects to analyze the influence of freeze-thaw cycles and stress conditions on dynamic properties.The damage ratio and its law are also summarized.Based on the SEM(scanning electron microscope)test,the meso-structure parameter of modified subgrade filling after freeze-thaw cycles are investigated.The relationship between mesoscopic parameters and dynamic property parameters are analyzed.Moreover,the BP artificial neural network algorithm is used to construct the relationship model between the dynamic properties and the microstructure parameters.This model is helpful to quantitatively predict the dynamic properties of the modified filling after several freeze-thaw cycles in reality.(5)Considering the excessive deformation of road subgrade after construction for years,the deformation characteristics of oil shale wastes modified subgrade filling is carried out.The large-number cyclic triaxial vibration tests under different stress conditions and freeze-thaw cycles are conducted.Relying on the material shakedown theory,the stress-strain curves of test samples are described and analyzed.The vertical total strain and plastic cumulative strain are extracted.The influence of confining pressure,vertical stress,loading frequency and freeze-thaw cycles on deformation are investigated.For the plastic cumulative strain,the normalized and logarithmic prediction model are proposed.Furthermore,the variation trend of stiffness and dynamic resilient modulus for the modified subgrade filling during large cyclic loading are revealed,which provides valuable guidance for performance evaluation after large-number cyclic loading in practical engineering. |
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