基于VG-PENG收缩特征曲线与收缩各向异性的裂隙率估算模型

为量化农田裂隙发育程度,考虑脱湿过程中土壤孔隙在基质域、沉降域和裂隙域间转化,该研究提出基于土壤收缩特征和收缩各向异性的裂隙体积比率(裂隙率)关于含水率的预测模型。该模型包括3个子模型:改进VG型式的基质域收缩特征VG-PENG模型,描述收缩各向异性的几何因子Logistic模型,基于上述VG-PENG收缩特征模型和几何因子模型的裂隙率预测模型。通过土壤收缩试验和裂隙演化监测试验,采用图像处理技术提取裂隙数据,评价了该模型的优度及适用性。结果表明,VG-PENG收缩模型具有较好的连续性和明确的物理意义,可精确描述土壤收缩特征(R~20.98);该研究引入Logistic曲线描述土壤收缩几何因子,揭示了收缩过程中土壤横向开裂和纵向沉降的各向异性机理,提出了脱湿初期纵向沉降(几何因子趋近1)、中期主沉降-副开裂(几何因子处于1～3之间)、后期趋于稳定3个阶段,Logistic模型可精确描述收缩几何因子随含水率变化;基于VG-PENG收缩模型和Logistic几何因子模型,构建了裂隙率关于含水率的演化模型,该模型呈"S"型曲线,取决于土壤收缩属性及其各向异性特征,裂隙率模拟值和实测值吻合较好,呈显著水平(R~20.90,P0.001)。该研究裂隙率预测模型修正了土壤收缩各向异性在裂隙率估算中造成的误差,并突破性地将VG-PENG收缩特征曲线进一步推演并应用于裂隙率模拟,可方便、快捷地通过土壤收缩数据预测农田裂隙率随含水率演化规律,为膨缩土裂隙流研究提供理论依据和参数基础。

Crack porosity estimation model based on VG-PENG shrinkage characteristic curve and soil shrinkage anisotropy

Soil shrinkage and cracking are essential behavior during dehydration in the environmental geotechnical engineering of farmland. Their quantification can be necessary to determine the soil physical-hydraulic parameters and crack preferential flow in soils. In the present study, a new predicting model was proposed for the crack ratio with respect to soil water content using a VG-based soil shrinkage characteristic curve and soil shrinkage anisotropy factor. The migration and transition of pores were also considered in the solid-liquid-gas phase system in soil matrix-subsidence-crack domains. This model included a VG-based soil shrinkage characteristic model, a shrinkage anisotropy model using a Logistic curve, and the soil cracking ratio model. An indoor experiment was conducted to investigate the soil shrinkage characteristics and cracking behavior. The crack ratio was determined using image processing techniques and morphological features. The experimental data was used to evaluate the fitting of a model for cracking ratio evolution. The results showed that the VG-based shrinkage model (VG-PENG model), three fitting parameters, and two estimated parameters, well predicted the soil shrinkage characteristics in various types of soils (R2>0.97, RMSE<0.04). The Logistic model was first introduced into the expression of soil shrinkage geometry, which was previously used to describe the growth principles under limited resources. Quantification of soil shrinkage anisotropy showed that the soil shrinkage was highly anisotropic. The soil shrinkage exhibited only subsidence with shrinkage geometric factor approximately equivalent to 1 in the early phase of the soil dehydration. The shrinkage showed mainly vertical subsidence with a light horizontal shrinkage (or cracking) in the middle phase, with shrinkage geometric factor varying between 1 and 3. The shrinkage geometric factor tended to be stabilized in the late phase, indicating a residual state of shrinkage. The anisotropic shrinkage with rapid change occurred in the relative water content of 0.3-0.7. The logistic shrinkage anisotropy model well predicted the shrinkage geometric factor with respect to the water content. A new model was also proposed to predict the evolution of crack ratio with respect to water content. The curve of the model showed sigmoid characteristics, depending highly on shrinkage properties and anisotropy. The simulated data showed better agreement with the experimental one, indicating an extremely significant level (R2=0.974, P<0.001). Since the water content within the soil layer was assumed evenly distributed, this model was considered to be appropriate in a relatively limited height of the soil layer. The evolution of crack ratio was predicted from a perspective view of soil physics rather than a mechanical view. Consequently, the soil shrinkage anisotropy was fully integrated into the modelling of the cracking ratio. A significant innovation was also made to apply the VG-type shrinkage characteristic curve to crack ratio prediction. A crack porosity prediction belonged to the field of soil physics to describe the evolution of cracking ratio using the shrinkage curve and geometry factor. The proposed model well predicted the cracking ratio in surface soils with high accuracy and convenience. A further investigation was also needed to explore the efficacy of the crack ratio model on undisturbed soils, and the effects of substrate properties on cracking behaviors. The finding can provide a promising theoretical basis and parameter prediction for soil water-solute movement in soils with variable-solid phase and preferential flow in soil physics and hydrology.