农田干缩裂隙形态特征及其裂隙率预测模型研究

Morphological Characteristics of Dry Shrinkage Cracks and Prediction Model of Crack Porosity in Farmland Soils

农田干燥收缩产生的裂隙影响土壤水力-物理结构特征,并为农田灌溉水或污染物运移产生优先通道。为揭示农田裂隙形态和基质收缩等演化特征,并对土壤裂隙率进行预测,基于室内土壤收缩及裂隙发育模拟试验,采用图像处理及形态学算法分析了裂隙几何特征,选取非侵入性局部收缩分析方法量化基质域收缩特性;根据脱湿过程中土壤孔隙由基质域向沉降域和裂隙域中的转换机理,结合收缩特征曲线VG-PENG模型、收缩几何因子Logistic函数2个子模型,提出土壤裂隙率预测模型,并对不同厚度土壤试样的裂隙率进行验证。结果表明,厚度在土壤开裂过程中对裂隙形态影响显著,表现为随着土壤厚度加大,土壤表面大裂隙宽度总体加大,且裂隙骨架密度减小,土壤收缩开裂进程放缓;土壤在干燥过程中,基质收缩呈现非均匀特征,并向块区型芯聚集,裂隙边壁区域呈现出局部集中变形区,且收缩量随土壤厚度增加而增大。裂隙率预测模型结合土壤收缩非均匀性特征,从土壤物理角度预测了裂隙率关于含水率的演化过程,有效模拟了裂隙率随含水率演化规律(模型决定系数R2>0.91)。模型弥补了以往裂隙模型未考虑收缩各向异性的不足,可为裂隙流模型构建及其土壤基质水力特征提供参数基础,有助于制定膨缩土壤修复及水分管理方案。

The cracks caused by drying and shrinkage in farmland affected the soil hydraulic and physical structure characteristics and provided preferential channels for the transport of irrigation water or pollutants.In order to reveal the evolution characteristics of crack morphology and shrinkage of matrix in farmland and predict the crack porosity,experiments were carried out in a greenhouse.Digital image processing and morphological algorithm were used to analyze the geometric characteristics of cracks during the dry-wet cycle.Non-invasive local shrinkage analysis method was used to quantify the shrinkage characteristics of matrix domain.According to the transformation mechanism of soil pores from matrix domain to subsidence domain and crack domain during dewetting process,a prediction model of soil crack porosity was proposed,which included two sub-models:VG-PENG model and Logistic function of shrinkage geometric factor.Based on the measured crack porosity experiment,the universal applicability of the model was verified for the crack porosity of soil samples of different thicknesses.From the perspective of soil physics,the evolution process of crack porosity with respect to water content was effectively predicted.The results showed that layer thickness had significant influence on crack morphology during soil cracking.With the increase of soil layer thickness,the width of large cracks on the surface of the reconstituted soil was increased,the possibility of secondary cracking was decreased,and the density of cracks was decreased,and the process of soil shrinkage and cracking was slowed down.In the process of drying and shrinking,the soil showed the phenomenon of agglomeration to block core.The minimum value deformation near the core in the shrinkage block area tended to be 0,and the edge wall area of cracks showed a concentrated deformation area,and the shrinkage was increased with the increase of soil layer thickness.The vertical uniformity of drying rate and drying degree also changed with soil layer thickness.Thicker soils usually had wider cracks in the soil surface and larger clumps of soil formed by cracking.In the cracking process,the shrinkage anisotropy showed vertical subsidence in the first stage,vertical subsidence in the middle stage with slight horizontal shrinkage,and horizontal cracking in the late stage.Considering the anisotropy of soil shrinkage,the prediction model of crack porosity proposed made up for the shortcoming that the previous crack models did not consider the shrinkage anisotropy,and the model had good effect and was generally applicable in different soil thicknesses(R2>0.91).The study quantified the development characteristics of crack and matrix at soil samples of different thicknesses,which provided the parameter basis for the construction of crack flow model and the hydraulic characteristics of soil matrix,and contributed to the formulation of expanded soil remediation and water management schemes.