Paddy Soil Compaction Risk Assessment Based On Soil Compaction Model

Soil compaction by field traffic has turned out to be a major cause for soil degradation worldwide.Changes of compacted soil physics infringes existing balance among soil physics,soil biology and chemistry,leading to soil degradation.In south China paddy regions rice is cultivated in wet paddy soils which are generally high in clay content and are subject to severe traffic compaction.Thus field trafficking is a major cause for soil degradation.As the paddy system provides 55%grain food supply,interruption of soil compaction as a measure to prevent soil degradation as a threat for food security should be taken seriously and a technical framework is urgently required.In doing so a major problem is related to the unknown mechanism governing soil compaction and a lack of risk assessment of soil compaction.Based on the previous achievements,the following 3 stages were concerned in this work:(1)tyre-induced stress is distributed on the soil surface;(2)soil stress transmission within the soil volume;(3)stress-induced soil compaction.Proper definition on the soil stress is a key element connecting the first and the third stages.Therefore,the key of this research was taken on the second stage of soil compaction(i.e.,soil stress transmission).In considering the concentration factor of the S?hne model as a starting poit,the affecting factors of soil properties on stress transmission was systematically analyzed.The S?hne model was then evaluated with both analytical and experimental measures.The evaluated model was then applied in the field to predict stress propagation within soil profile and thus soil compaction risk was assessed.Detailed tasks includs:1.Proposition and experimental evaluation on stress transmission coefficient.The S?hne model considers the influence of both the loading condition and soil properties,the key of which is a proper selection on suitable concentration factor.However,existing methods of concentration factor selection are confusing,the underlying reason of which is a lack of proper consideration on how the concentration factor varies with soil properties.Theoritical analysis on the concentration factor further improved the S?hne theory by proposing a dimensionless factor,?z/?o which was defined as Stress Transmission Coefficient,i.e.STC.Two soils,i.e.one paddy soil and one loess soil,were used in controlled moisture contents and bulk densities to evaluate the soundness of STC.2.Totally 30 states of soil were used in uni-axial compression test,in which soil stress sensors were applied to monitor soil stress and for the calculation of concentration factor.Coefficient analysis were made among concentration factor,soil moisture content,bulk density and precompression stress.Results showed that STC increased with improved soil moisture content.A negative correlation was found between precompression stress and STC.STC-based calculation on the concentration factor yielded a range of 2.64-12.39,which was similar with the findings from the past studies.However,the STC-related results carried information of the changes of the soil environment.It thus indicates that STC could be a stable dimensionless index providing a relatively more reasonable approach for the calculation on the concentration factor.This approach not only explains the soil properties more accurately,but also contributes to the improvement on the soil compaction model.3.The theory and the application of STC in the field was investigated.Both theoretical derivation and experimental evaluation confirmed the validness of the proposed STC as a tool for soil compaction study.Whereas in applying STC to the field another obstacle appeared as how to measure the field-state STC.The definition of STC implies that two methods can be used for the determination on field-state STC,i.e either with indoor experiment or field measurement.For laboratory study,the ?STC theory was further developed,which was fulfilled by sampling the soil from the field and measured in the lab with consolidation experiment For the field testing,an in-situ plate sinkage testing plateform was developed and applied to monitor both soil surface contacting stress and propagated stress within soil profile.Based on the two methods,comparison was made among ?STC-derived results of the concentration factor,laboratory tested results on field-samples(50 mm diameter)with two thicknesses(100 mm and 150 mm),and the in-situ results with controlled plate dimensions(80,100 and 150 mm)and soil depths(100 and 150 mm).Field-state STCs were thus thoroughly investigated under different plate sizes,different soil depths and different conditions.The acquired results were further evaluated with in-situ plate sinkage test and in-situ wheeling test,in which soil stress was monitored with soil stress sensors.The acquired results were then compared with that derived from the S?hne model.It was found that the ?STC-derived results of the two soil depths were 0.30 and 0.15,respectively.There was not marked difference between the measured result and that collected from the plate sinkage test(for all plate sizes of 30,100 and 150 mm).It was also found that there was no significant difference among different plate sizes.For a given soil state STC changes with soil depth,which means that laboratory approach for soil STC is a reliable technique providing confident reaults.Calculated concentration factors under relevant loading conditions and soil properties explained the influences of plate size and soil depth on it,which decreased with the increase plate size,and increased with soil depth.For both plate sinkage test and in-situ wheeling test,both STC theory and the S?hne model predicted soil stress changes with loading conditions successfully.4.A technical rating system for soil compaction risk was constructed.Proper assessment of soil compaction risk provides a means of visually presentation on soil compaction.In considering both the soil compaction theory and the reported results,a relatively acomplished rating system was constructed for agricultural applications.The rating system was made of three components.In the first stage Hallonborg model was used to calculate the contacting area of the tyre-soil interaction,from which equivalent radius was determined.In the second stage the S?hne model was applied to calculate soil stress within the soil profile.In the third stage soil precompression stress was applied as a comparison with STC,with which soil compaction risk was evaluated in each soil layer.The proposed rating system was then applied to evaluate soil compaction risks by a tractor in a farming field in Babaiqiao,Luhe,Nanjing.The X type sampling scheme was used and 25 soil samples were collected with ?50×50 mm soil cores for a sampling depth of 200 mm.Totally 400 soil cores were collected and laboratorially tested on both the STC and precompression stress.The monitored field was then trafficked with Yanmar YT704,John Deree 5-854 and Dongfeng 754,respectively.Furferl.1 software was applied for the plotting of soil compaction risks under these tractors.The rated results indicated that Yanmar YT704 resulted into the highest soil compaction,followed by John Deree 5-854.Dongfeng 754 led to the lowest soil compaction risk.Axial load,tyre sectio size,radius,inflation pressure and soil precompression stress are the main factors in determining soil compaction risk.Reduction of soil compaction risk requires proper selection of axial load,change of tyre or adjust inflation pressure for reduced soil surface contacting pressure,or waiting for suitable soil workability.In case of unavoidable soil compaction risks,it is necessary to perform soil compaction risk rating,which would provide referencing basis for subsquent remediate approaches with subsoiling or deep tillage.