轮式和履带式车辆行走对农田土壤的压实作用分析

由履带式行走机构代替轮胎被认为是减缓大型农业车辆对土壤压实的有效手段之一。与轮胎相比,履带具有更大的接地面积,能够有效减小车辆对土壤的平均压力。然而履带与土壤接触面间的应力分布极不均匀,应力主要集中在各承重轮下方,履带减缓土壤压实的能力是目前有待研究的问题。该研究通过在土壤内埋设压力传感器,测试比较了相近载质量的轮胎和履带式车辆作用下,0.15和0.35 m深度土壤内的最大垂直及水平应力,同时研究了车辆行驶速度对土壤内垂直及水平应力大小的影响。基于土壤压实分析模型计算了轮胎和履带压实的0.1~0.7m深度土壤内的最大垂直及水平应力分布。通过对0.15和0.35 m深度的土样进行室内测试,比较了轮胎和履带式车辆压实对土壤透气率、先期固结压力及干容重大小的影响。结果表明,履带相比较于轮胎,能够减小土壤内的垂直及水平应力,但垂直应力的减小量比水平应力大;轮胎对0.15和0.35m深度土壤作用的平均最大垂直应力分别约为履带的2.2及2.0倍,而平均最大水平应力仅分别约为履带的1.2及1.1倍。轮胎作用下的最大垂直及水平应力在表层土壤内明显大于履带,但两者的应力差值随着土壤深度的增加逐渐减小,分别在0.7和0.4 m深度时无明显差别。轮胎和履带压实作用下,0.15和0.35 m深度土壤内的垂直及水平应力均随车辆行驶速度的增加而减小,履带作用下的应力减小速度大于轮胎。履带作用下0.15和0.35 m深度内土壤的透气率均明显小于轮胎,但土壤的先期固结压力及干容重无显著区别。研究结果为可为农业车辆行走机构的选择及使用提供参考。

Compaction effects of wheeled vehicles and tracked on farmland soil

Soil compaction induced by vehicle traffic has aroused more concerns due to its negative impacts on soil functions and ecosystems. 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 the track and the soil are complex, the stress distribution at the interface between the track and the soil is uneven, which may reduce the effectiveness of the track in decreasing soil stress. Thus, to determine the ability of track to reduce the soil compaction compared with tire is importance. The objective of this study is to investigate 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. The test was conducted on the sandy loam soil. Three repeated measurements were carried out on tracked harvester and tractor with similar axle load. To measure soil stress, the load transducers were embedded in the centerline of the tire and track at a depth of 0.15 and 0.35 m respectively. After vehicles pass, soil samples are collected at the depth of 0.15 and 0.35 m in the test area. Soil pre-compression stress, air permeability and dry bulk density were measured in the lab. The soil stress in the depth range of 0.1-0.7 m was calculated by using the soil compaction model. 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. Under the effect of tyre compaction, the average vertical stress at the depth of 0.15 and 0.35 m is 2.2 and 2.0 times of track compaction respectively, whereas the average horizontal stress of tyre compaction was only approximate 1.2 and 1.1 times larger than that of the track respectively. Interestingly, no difference in vertical and horizontal stress was found between the track and the tire compaction at the depth of 0.7 and 0.4 m respectively. It indicates that the track is more effective in reducing the soil compaction for the topsoil than for the subsoil. 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 compation. In other words, the ability of the track to reduce soil compaction is weakened due to the uneven stress distribution and the longer compaction time. Though the calculated soil stress is generally in line with the measurement results, the measured vertical stress is lower than the calculated value for the tire compaction but higher for the track compaction, the measured horizontal stress are lower than the calculated values for both the tire and the track compaction. The accurate prediction of the distribution of stress on tire/track surface and the reasonable selection of concentration factor are the key to model calculations. Future research may focus on the impact of support roller configuration and diameter on the distribution of soil stress of track compaction. The research results are helpful to improve the uniformity of soil stress distribution under the track compaction, so as to reduce soil compaction.