Study On Mechanical Behavior Of Turfy Soils Under Cyclic Loading Based On The Discrete Element Method

Turfy soil is a special soil with numerous undecomposed plant fibers and humus,which has undesirable engineering properties such as low bulk,high porosity ratio,high compressibility,high water content,high permeability,and high organic matter content.Since the soil matrix of turfy soil contains a large amount of plant roots that have not been fully decomposed,it can be regarded as a mixture of the soil matrix and plant fibers.Human engineering has involved the distribution area of turfy soil nowadays,and different forms of loading are produced by different engineering activities.Therefore,the differences in mechanical properties of turfy soil beneath the foundations subjected to multiple loading forms are deserved to be investigated.To further understand the mechanical behavior of soil-plant fiber mixtures under different forms of cyclic loading in safety engineering,this research combines the laboratory test data and numerical simulation results to analyze the mechanical properties of turfy soil from a combination of macroscopic and mesoscopic perspectives.The main works of this research are as follows:(1)Three samples of turfy soil with different plant fiber contents were selected to investigate the dynamic properties of turfy soil under cyclic loading using the laboratory equipment of the global dynamic system(GDS).It was found that the dynamic strength of turfy soil increased with the plant fiber content,and the dynamic deformation decreased with the plant fiber content.The stiffness and the elastic properties of the turfy soil are stronger as the plant fiber content increases,consequently,the recoverable elastic deformation of the turfy specimen is smaller.(2)The DEM model of soil matrix-plant fibers is constructed using the software platform PFC3 D which is based on the particle flow DEM theory.The construction of the turfy soil DEM model is based on the basic physical properties and mesoscopic structural characteristics of realistic turfy soil.The parameter calibration of the turfy soil DEM model is completed based on the study results of the group about the remodeled turfy specimen and the cyclic loading data in this research.After the procedure of calibration,the predicted DEM responses agree well with the laboratory data,thus indicating that the dynamic loading behavior of turfy soil can be captured by the proposed DEM model.(3)A series of consolidation-drained cyclic triaxial test simulations using the DEM was conducted for the DEM model of turfy soil.The multi-level cyclic loading with different waveforms(triangular,sinusoidal,square)was performed in this research.The macroscale behavior of turfy soils under exterior loading(traffic loads and earthquakes)with different forms are analyzed.Macroscopically,the multilevel cyclic stress path with different waveforms has a certain effect on the dynamic deformation of turfy soil.With the same number of cycles,the square waveform produces the largest compression strength and deformation for the sample during the loading process,while the triangular waveform has lower strength than sinusoidal waveforms.With the number of cycles per stage increased,the deformation resistance of the turfy soil specimens gradually increased.This indicates that the deformation resistance of the turfy soil gradually decreases and gradually develops into a destabilization-prone soil with the increase of earthquake magnitude.(4)Microscopically,the presence of plant fiber was found that can enhance the strength of the turfy specimens,including inducing the specimens to store energy,enhancing the frictional forces at the contact interfaces,and providing tensile force to support the surrounding soil.The contribution of plant fibers to the strength of turfy soil is most evident under square waveform loading.The maximum bending degree of plant fibers in the turfy soil under square waveform loading,which indicated that the plant fibers were disturbed to the greatest extent under square waveform.Therefore,the deformation of the turfy soil specimen under square waveform was also the greatest.