The Study On Stress Condition Of Shallow Soil Layer Under Cyclic Loading

With the wide use of agricultural machinery,the continuous improvement of agricultural machinery load,the problem of soil compaction has become increasingly prominent.The effect of tire on soil compaction is not limited to the Plough Layer,but also affects the stress conditions of deeper soil layers.The previous researches on farmland soil compaction have been mainly concentrated in transient,one time compaction effect.With the actual situation there are differences,In this study the loading process that can cause farmland soil compaction will be considered as long-term,cyclic loading.Conventional tillage and no tillage can not solve the problem fundamentally.So it is necessary to carry on systematic research to the soil stress state from the point of view of soil mechanics,and to find the corresponding solutions to the problems of farmland soil compaction.Farmland soil can be divided into three layers: surface layer,core layer,bottom layer.There are two layers in the surface soil,which have different soil properties: Plough layer and plough pan.Because of the long-term effect of plough,there are obvious mechanical differences between the plough layer and plough pan,which will have some influence on the change of the stress state of the soil.The compaction caused by soil tillage layer can be alleviated,but compactions in the plough pan,subsoil deep the underground is not easy to ease,so the focus of this paper is on the change of stress conditions in this two layers.Prior to the study,the author made a reasonable simplification of the cyclic load on the soil,and selected the cross section between the tire and soil to study.Based on the application of FRIDA tire soil interaction model,the distribution of load on the soil surface is calculated when the tire is contacted with the soil.ABAQUS user defined program DLOAD is used to program and call the trapezoidal cyclic load of farmland soil.Through the use of ABAQUS finite element software,based on the critical state model,the author simulated the stress state of the shallow soil layer of the farmland soil under the cyclic loading caused by agricultural machinery.The results show that there are differences in the change of stress state in the single compaction and different loading conditions.The change of stress state,void ratio distribution,settlement under cyclic loading are higher than those under single compaction,and the difference can not be ignored.the stress underconcave load is higher than the stress under uniform or convex load.The stress value of tillage layer and plow bottom is close to each other along with the increase of compaction times.With the increase of the tire width,between the plow and subsoil the maximum stress position decreased gradually.During the first rolling,the ratio of the porosity of the plough layer and the bottom of the plough layer accounted for 59%,49%of the total change of the 25 times.As the number of compaction increases,the rate of void ratio decreases gradually.The amount of subsidence caused by primary compaction accounts for more than 95% of the total amount of subsidence caused by the 25 times compaction.Finally,the optimizatison of the control of agricultural machinery load,the selection of operation mode,the number of tires and the selection of expansion pressure are put forward.