Stress Behavior In Agricultural Soil Compacting Process And The Development And Application Of Its Monitoring System

The development and the wide use of agricultural machinery has, especially in recent decades, led to considerable soil compaction, a problem which was regarded unavoidable in agricultural production. The direct effects of soil compaction include the increase of soil compactivness, reduced porosity and increased soil bulk density. This would further result in decreased air permeability, reduced water infiltration and saturated hydraulic conductivity, and increased soil strength and mechanical impedence for crop rootings. Soil compaction is a main index for soil physical degradation, and thus the descriptive indices for soil compaction and the quantification of this process is a primary concern, which provides a referencing basis for the reduction and amelioration of soil compaction, avoids soil resources destruction and allows a sustainable development of agricultural production.The severe consequences associated with soil compaction were progressively realized by relevant scientists. Many researchers have made efforts to monitor soil compactions. Soil physical indices of bulk density, porosity, soil strength were used as indictors in their research. These indicators, and crop performance and the yield were combined to quantify the post-compaction soil state. Reports on the dynamical monitoring of soil compaction occurred in the compacting process were less found. Hence a research based on stress-strain method for the soil compaction study has its significance.A soil stress monitoring system consisted of pressure sensors and a signal applifier was constructed to test the stress distribution process in soil. This laboratory test work helped to illustrate the mechanism of strength distribution in soil and served as a theoretical reference for further researches.The controlling factors considered in this work include soil moisture content and soil bulk density. The reason for the selection of these two factors include:1) the easiness and the feasibility consideration for the test work, and2) the primary influence of soil moisture content and soil bulk density on stress distribution. In addition, the change of these two factors was able to indicate the environmental variations happened in the’structured soil’when an equilibrium physical and chemical state was reached. The laboratory test work provided the relationship between the stress experience by soil and the resultant soil density, serving as a basis for field experiment. In field test an in situ compacting stress monitoring system for soil compaction was used to illustrate the stress distributing process under the passage of agricultural machineries. Soil pre-compaction stress was also monitored in the process to clearify the relationship between soil pre-compaction stress and soil compaction.Main contributions include:1. By referencing to the soil pressure sensors supplied in the market, an improved sensor with a much reduced size of φ18x6mm, which was specifically designed for agricultural soil stress test, was developed and calibrated. The calibration revealed a satisfactory linear characteristics and a reduced hysterisis error, indicating a possible negative influence from a big sized pressure sensor. Comparing the K values of the sand calibration and the oil calibration proved the inadequency of oil calibration reconmendated by the sensor suppliers.2. Laboratory test apparatus was designed, including sample cores, static compression rig and a loading mechanism. The indoor experiment was then performed by adjusting basic soil parameters (moisture content and density). Results of the experiment not only confirmed the mechanisms of additional stress but also revealed the influencing effects on soil stress distribution from different factors.3. An in situ soil stress monitoring system was constructed for the measurement of stress distribution under a tractor wheeling action, modeled SNH550. By changing the number of wheeling passage and the sensor distribution in soil layers, soil stress distribution mechanism in agricultural soil was illustrated.4. In solving the problem of the long duration for traditional pre-compression stress test, a quick uni-axial compression test method was used, paving a new way for the field soil pre-compression stress test. One model of the NewHalland SNH550tractor was used for soil compaction, and compcting result was measured with soil stress monitoring system. Based on this the relationship between the number of tractor passage and soil pre-compression stress was established.