Traffic Induced Soil Plastic Deformation And Compaction By Tired/Tracked Combine Harvester On The Paddy Field

With the promotion and application of tracked combine harvester with large feeding capacity(? 8.0kg/s),the weight of combine harvester is increasing continuously.Serious soil damage and compaction problems in the paddy soil has been found during the harvest time,resulting in degradation of soil functions,crop lodging,ecosystem damage and decline of crop yield,which seriously affects the sustainable development of paddy soil.One of the first visible signs that the paddy soil has been damaged is excessive plastic deformation of trafficked area during the unilateral brake steering of tracked harvester,which have been correlated with the reduction of vehicle's steering ability.The brake force exerted on the inner track will cause the track shearing and pushing the soil beneath it result in serious soil damage.Thus,it is necessary to seek a new steering mode for tracked harvester in order to mitigate the soil damage and improve the vehicle's steering ability.Replacing tires by tracks is considered as a technical method to mitigate the soil compaction,which increases the vehicle's contact area and decreases the mean ground pressure.However,the interactions between track and soil are complex,leading to unevenly distributed stress on track-soil interface.It may reduce the effectiveness of the track in decreasing soil stress.Thus,it is necessary to estimate the distribution of soil stress beneath the tracked and the ability of tracks to reduce the soil compaction as compared to the tire,and to seek a way to improve the uniformity of stress under the track.The work of this paper mainly includes the following aspects:1.Based on the Wong's general theory,the slip and skid of track during the steering of tracked vehicles was analyzed.A dynamic model for tracked vehicles steering on the soft ground,by taking track slip an skid effects into consideration,has been established.The field test was conducted by using tracked harvester steering on the paddy field.The validity of the model was verified by comparing the calculated torques with the measured values.Based on the theoretical model,the moment ofsteering resistance,soil disturbed area and trajectory of track were calculated to analysis the soil damage under two steering modes of tracked harvester.2.The field test was conducted by using tracked harvester steering on the paddy field of three different soil water content.The track trajectory left on the ground after the vehicle turns were measured using self-made soil profile measuring instrument.The soil damaged area(vehicle trafficked area)and soil damaged severity(maximum height of the soil pile)were measured as two indicators to evaluate the soil damage.The test results were compared with the model calculation results.Based on the results of theoretical analysis and field test,the optimum steering mode of tracked combine harvester under different soil conditions was put forward to mitigate the damage of paddy soil.The pivot steering mode is effective in the reduction of soil damaged area,but produces less damaged severity when the track sliding coefficient is exceeding the critical sliding coefficient.3.In order to estimate the ability of tracks to reduce the soil compaction as compared to the tire,both field test and model simulation were conducted.The impact of undercarriages(tire vs.track)on the magnitude of soil stress and soil functions(i.e.pre-compression stress,air permeability,dry bulk density),as well as the impact of vehicle velocity on the magnitude of soil stress was investigated.To measure soil stress,the load transducers were embedded in the center-line of the tire and track at a depth of 0.15 and 0.35 m respectively.Soil cores were sampled at 0.15 and 0.35 m depth in the experimental plots after the vehicle traffic.Soil pre-compression stress,air permeability,and dry bulk density were measured in the lab.The results show that compared with tires,the vertical and horizontal soil stresses are reduced under the track compaction.However,the reduction of the vertical stress was greater than that of the horizontal stress.The vertical and horizontal compaction stresses of tire and tracked vehicle decrease with the increase of vehicle speed,but the speed of stress reduction under the track compaction is faster than that of tire.The measured air permeability of track compaction is obviously larger than that of tire,whereas the measured dry density and pre-compression stress displayed no significant difference between the tire and track.This means that the ability of the track to reduce soilcompaction was weakened due to the undistributed stress and the longer compaction time.Improving the uniformity of stress distribution under the track is the key to improve the ability of track to mitigate the soil compaction.4.The field test was conducted to estimate the distribution of soil stress beneath the track and the impact of track tensions on the distribution of soil stress.The soil stress was measured by embedding the load transducers beneath the centerline of the track at 0.35 m depth.A laser sensor was set on the same line with load transducers at soil surface to track the position of track wheels.The influence of the track tension,the size and distribution of the supporting wheel,the soft and hard degree of the soil,the load of the tracked vehicle,the structure parameters of the track teeth(the height of the track teeth and the arrangement of the track grousers)on the stress at same traffic speed.the maximum stress(the maximum value of each stress peak)and the average maximum stress(the average value of each stress peak)under the track were used to evaluate the uniformity of stress.The test results can make us better understanding of the soil compaction process during the traffic of tracked vehicles,and can also provide us a guidance on optimizing the structure of tracked undercarriages to improve the uniformity of soil stress beneath them.