Study On Compaction Effects During Soil Water Characteristic Curve Measurement

Soils present swelling and shrinkage properties during wetting and drying processes. Bulk density will increase a lot because of the hydraulic and mechanism pressures during the measurement of soil water characteristic curve by centrifuge method. Accordingly, the soil clod undergoes a compaction. The changing bulk density and compaction effect caused by experimental process itself will influence the soil water retention characters. This research investigated the compaction effect during soil water characteristic curve measurement by centrifuge method and special devices for four undisturbed soils. By measuring the soil water characteristic curves under certain bulk densities, we analyzed the effect of changing bulk density during measurement on soil water characteristic curve parameters as well as the effect of soil disturbance on the relationship among mass water, pressure head and bulk density. The results showed that:(1) The bulk density and compaction effect increased with the increase of pressure head during the measurement of soil water characteristic curve. The proposed soil compaction model in this paper can satisfy the boundary conditionρ=ρ0 when pressure was equal to zero. The fittedρmax was smaller than the soil particle density, which made the proposed model present some physical meanings. The improved model showed a better performance than the two-variable L model. The performance and physical meanings of improved model were no better than the three-variable Assouline model. However, the improved model seemed better because of having fewer parameters and simple form.(2) The clear change of bulk density caused by the compaction during the measurement of soil water characteristic curve changed the soil water retention characters. The bulk density was greater, the effective saturation degree under the same pressure head was greater, and the slop of soil water characteristic curve was flatter. The fitted parameters, A andα, for four soils were decreased with the increase of bulk density, and the relationship between parameter and bulk density can be depicted by power function. The fittedλand n increased with the increase of bulk density, and the relationship can be described by linear function. Compared with van Genuchten model, the correlation ship between model parameter of Brooks-Corey and bulk density was greater.(3) The soil texture was heavier, the curve slop of mass water content～bulk density for undisturbed soil was flatter, which was reverse to the packing study. The effect of soil disturbance on three-variable surface was greater for heavier soils, especially at the lower pressure stage. Therefore, undisturbed samples can be substituted by the packing samples only if the packing bulk densities were closely approximate to the sampling bulk densities. For heavier soils, however, the undisturbed samples were strongly recommended because the approximation propose seems unlikely approached.(4) The soil disturbance influence the model performance on the relationship among mass water content, pressure head and bulk density. For the Huangmian soil, the effect of soil disturbance was smaller. When it comes to Lou soil and Red soil, the effect was greater. These results was consitstent with the analysis of soil disturbance effect on measured data.The compaction effect during soil water characteristic curve measurement will change the soil water retention characteristics. Based on the analysis of existed soil compaction models, an improved two-parameter model was proposed. According the measured data, soil disturbance presented a considerable influence on the releationship between mass water content, pressure head and bulk density, especially for the clay soil. At the low pressure and small bulk density of the surface where soil rention characteristics are crucial to the plants survivance, the intensity of soil disturbace was tremendous. Therefor it is quite necessary to take the soil disturbance factor into account when dealing with the soil three-variable surface issue. This research would provide valuable references to the revise the effect of bulk density changes on soil water characteristic curve parameters, and thus be beneficial to improve the precision of model predicting.