循环荷载下农田浅部土层应力状态研究

发布时间：2018-04-01 21:38

本文选题：农田土压实　切入点：循环荷载　出处：《河南工业大学》2017年硕士论文

【摘要】：近年来,随着机械化耕作大规模普及,农业机械在提高工作效率的同时,也对土壤造成了破坏,特别是重型农业机械的使用。农机作业时轮胎对土壤压实产生的影响并不局限于土壤耕作层,也会影响更深层次土壤的性质。以往对农田土压实的研究主要集中在短暂,单次的压实效果上,这与实际情况存在差异,本次研究将农田土压实过程看作是农机长期、多次碾压的结果。普通的翻耕和免耕作并不能从根本上解决压实造成的危害,因此需要从土力学角度对土壤应力状态进行系统的研究,并找出应对农田土压实问题的解决办法。农田浅部土层一般可以分为三个层次:表层土,心层土,底层土。其中表层土又存在土壤性质差异较大的两个分层:耕作层和犁底层。耕作层由于长期受到犁锄作用的影响,与耕作层性质存在着明显的差异,这将对农田土压实后土体应力状态的改变造成一定的影响。由于耕作层造成的压实作用可以通过松土缓解,而犁底层、心土层处于地下较深位置,造成的压实不易缓解,所以本文在研究耕作层土壤性质改变时,也将研究耕作层以下这两个层面性状的改变。在进行模拟之前,对农机作业时对土壤产生的循环荷载进行了合理简化,并选取轮胎与土体接触的横断面进行研究。通过对FRIDA载荷分布模型的应用,对农机土壤碾压时对土体表面产生荷载分布进行了计算。并使用ABAQUS用户自定义程序DLOAD对农田土梯形循环荷载进行了编程与调用。通过ABAQUS有限元软件的使用,采用临界状态模型,对农田土在大型农业机械作业时产生的循环荷载下浅部土层的应力状态进行模拟。结果显示,单次压实中,加载条件不同,压实产生的结果在应力状态改变上存在差异,土壤中应力的改变同时会使孔隙比发生变化。凹形荷载分布下应力较均布荷载与凸型荷载应力高。耕作层与犁底层应力值大小随着压实次数的增加而相互接近;随着轮胎宽度的增加,犁底层与心土层之间应力最大值位置逐渐降低。初次碾压时,耕作层与犁底层孔隙比变化占25次总变化量的59%与47%。随着压实次数的增多,孔隙比的下降的速率逐渐减小。初次压实造成的沉陷量占到了25次压实引起的沉陷量的95%以上。多次压实后土体性状的改变较单次压实在应力状态、孔隙比分布、沉陷量、均有提高,其中压实效果随压实次数的增多而不明显。最后对农业机械荷载的控制,作业模式的选择、轮胎组数及膨胀压的选择提出了优化性建议。
[Abstract]:In recent years, along with the large-scale popularization of mechanized farming, agricultural machinery not only improves the working efficiency, but also causes damage to the soil, especially the use of heavy agricultural machinery.The effect of tire on soil compaction is not limited to the soil tillage layer, but also affects the properties of deeper soil.Previous researches on farmland soil compaction were mainly focused on the transient and single compaction effect, which was different from the actual situation. In this study, the process of farmland soil compaction was regarded as the result of long-term and multiple compaction of agricultural machinery.Ordinary tillage and no-tillage can not fundamentally solve the damage caused by compaction, so it is necessary to systematically study the stress state of soil from the perspective of soil mechanics, and to find out the solution to the problem of soil compaction in farmland.The shallow soil layer of farmland can be divided into three levels: surface soil, core soil and bottom soil.There are two layers in topsoil: ploughing layer and plow bottom layer.Due to the effect of ploughhoe on the cultivated layer for a long time, there are obvious differences between the cultivated layer and the ploughing layer, which will have a certain effect on the change of soil stress state after compaction of farmland soil.Because the compaction caused by tilling layer can be alleviated by loosening soil, and the core soil layer is in the deep underground position at the bottom of the plow, the compaction caused by the compaction is not easy to ease, so this paper studies the change of soil properties in tilling layer.Changes in the traits of the two levels below the tilling layer will also be studied.Before the simulation, the cyclic load generated by agricultural machinery was reasonably simplified, and the cross section of the contact between tire and soil was selected for study.Through the application of FRIDA load distribution model, the load distribution on soil surface during soil compaction of agricultural machinery was calculated.The trapezoidal cyclic load of farmland soil is programmed and called by ABAQUS user defined program DLOAD.Through the use of ABAQUS finite element software, the critical state model is used to simulate the stress state of shallow soil layer under cyclic load caused by large-scale agricultural machinery.The results show that under different loading conditions, the results of compaction are different in stress state change, and the change of stress in soil will make void ratio change at the same time.The stress under concave load distribution is higher than that under uniform load and convex load.The stress values of ploughing layer and plough bottom are close to each other with the increase of compaction times, and the position of maximum stress between plough bottom and core soil gradually decreases with the increase of tire width.During the first compaction, the change of the porosity ratio between the plow bottom and the tilling layer accounts for 59% and 47% of the total change of 25 times.With the increase of compaction times, the decreasing rate of void ratio decreases gradually.The settlement caused by initial compaction accounts for more than 95% of the subsidence caused by 25 compaction.The change of soil properties after multiple compaction is higher than that of single compaction in stress state, void ratio distribution and settlement, and the compaction effect is not obvious with the increase of compaction times.Finally, the optimization suggestions are put forward for the control of agricultural machinery load, the choice of working mode, the selection of tire number and expansion pressure.
【学位授予单位】：河南工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：S152.9

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