Experimental study on the permeability characteristics and structure damage of Malan loess in Heifangtai area
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摘要:
持续引水灌溉改变了马兰黄土的结构, 降低了土体的抗剪强度, 导致黑方台地区黄土滑坡频繁发生, 严重影响着当地居民生命和财产安全。为了明析马兰黄土的渗透过程, 取黑方台马兰黄土为研究对象, 分别开展核磁共振(NMR)试验及扫描电镜(SEM)试验, 以解释此类黄土在不同初始含水率及不同干密度下的渗透特性及结构损伤微观特征。研究结果表明: 入渗速率与土体初始含水率呈负相关关系, 土体初始含水率越高, 其充水微小孔隙增加速率越慢, 充水中大孔隙增加速率越快; 入渗速率与土体干密度呈负相关关系, 且会率先形成高含水率区域, 土体干密度越大, 其充水微小孔隙增加越慢, 充水中大孔隙增加越快。入渗前后对比发现, 试样初始含水率越高, 微小孔隙增加比例越小, 颗粒间接触方式变化越不明显; 干密度越大的试样不同孔隙体积基本按等量变化, 接触面积明显减少, 形成更多的架空孔隙, 连通性较好, 具有较好的储水能力。入渗后试样原本的致密结构丧失, 颗粒破碎严重, 部分细长状颗粒向似椭圆状颗粒演化, 颗粒间接触方式变为点边接触, 粒间胶结作用遭受损伤破坏, 甚至部分团粒中颗粒分离、脱落, 使得土体强度丧失, 最终导致滑坡发生。研究结果可为黄土滑坡的防治提供依据。
Abstract:Continuous water diversion irrigation has changed the structure of Malan loess, led to the reduction of the shear strength of the soil, and thus the frequent occurrence of landslides in the Heifangtai area, which seriously affected the lives and property safety of local residents. In order to analyze the infiltration process of Malan loess, nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) tests were respectively carried out to investigate the permeability and structural damage microscopic characteristics for loess at different initial water contents and different dry densities. The obtained results show that the infiltration rate is negatively correlated with the initial moisture content of the soil. The density is negatively correlated, and it will take the lead to a higher water content area. The water content is higher, the increase in the proportion of micropores is smaller, and the less obvious the change in the contact mode between particles; the dry density of the sample is larger, the different pore volumes basically change in the same amount, the contact area is significantly reduced, and more overhead pores are formed, which are connected to each other, leading to good water storage capacity. After infiltration, the original dense structure of the sample is lost, and the particles are severely broken. Some slender particles evolve into nearly round particles or elliptical particles, and the contact mode between particles becomes point-edge contact.The intergranular cementation is damaged and destroyed, and even some particles in the aggregates are separated and fall off, resulting in the loss of soil strength and eventually leading to landslides. The results can provide a basis for the prevention and control of loess landslides.
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图 1 1990-2014年灌溉量与滑坡发生的关系
Figure 1. Relationship between irrigation amount and landslide occurrence from 1990 to 2014
图 2 黑方台黄土滑坡取样点位置图
Figure 2. Sample point location map of loess landslide in Heifangtai area
图 4 不同初始含水率重塑黄土在入渗下的充水孔隙分布图
a.初始含水率15%重塑黄土充水孔隙体积分布；b.初始含水率15%重塑黄土不同充水孔隙体积柱状图；c.初始含水率17%重塑黄土充水孔隙体积分布；d.初始含水率17%重塑黄土不同充水孔隙体积柱状图；e.初始含水率19%重塑黄土充水孔隙体积分布；f.初始含水率19%重塑黄土不同充水孔隙体积柱状图
Figure 4. Water-filled pores distribution during infiltration in the cases of different initial water contents
图 5 干密度为1.5 g/cm3的重塑黄土在入渗下的充水孔隙分布图
a.干密度1.5 g/cm3重塑黄土充水孔隙体积分布; b.干密度1.5 g/cm3重塑黄土不同充水孔隙体积柱状图
Figure 5. Water-filled pores distribution during infiltration in the cases of dry density 1.5 g/cm3
图 6 不同试样入渗过程核磁共振成像分析(1~12为成像次数)
a.干密度1.36 g/cm3、初始含水率15%入渗成像图；b.干密度1.36 g/cm3、初始含水率17%入渗成像图；c.干密度1.36 g/cm3、初始含水率19%入渗成像图；d.干密度1.5 g/cm3、初始含水率15%入渗成像图
Figure 6. NMR analysis of different samples during infiltration
图 7 渗透前后试样SEM图像
a.干密度1.36 g/cm3、初始含水率15%，渗透前; b.干密度1.36 g/cm3、初始含水率15%，渗透后; c.干密度1.36 g/cm3、初始含水率19%，渗透前; d.干密度1.36 g/cm3、初始含水率19%，渗透后; e.干密度1.5 g/cm3、初始含水率15%，渗透前; d.干密度1.5 g/cm3、初始含水率15%，渗透后
Figure 7. SEM images of samples before and after infiltration
图 8 试样入渗前后二值化处理效果图
a.干密度1.36 g/cm3、初始含水率15%，入渗前；b.干密度1.36 g/cm3、初始含水率15%，入渗后；c.干密度1.36 g/cm3、初始含水率19%，入渗前；d.干密度1.36 g/cm3、初始含水率19%，入渗后；e.干密度1.5 g/cm3、初始含水率15%，入渗前；f.干密度1.5 g/cm3、初始含水率15%，入渗后
Figure 8. Effect of binarization treatment before and after sample infiltration
图 9 各试样渗流前后不同孔隙所占比例图
Figure 9. Proportion of different pores in each sample before and after seepage
图 10 渗透前后各试样颗粒丰度
Figure 10. Particle abundance of each sample before and after infiltration
图 11 渗透前后土体结构损伤示意图
Figure 11. Schematic diagram of soil structure damage before and after seepage
表 1 黄土土样的基本物理参数
Table 1. Physical parameters of loess samples
物理参数 干密度ρd/(g·cm-3) 天然含水率
ω/%塑限/% 液限/% 塑性指数 颗粒组成wB/% 黏粒(< 0.002 mm) 粉粒([0.002, 0.075]mm) 砂粒(>0.075 mm) 测试值 1.36 7.13 15.7 26.4 10.7 6.18 79.04 14.78 
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表 2 入渗试验工况
Table 2. Infiltration test conditions
序号 试验条件 干密度ρd/(g·cm-3) 初始含水率
ω/%流量/(mL·min-1) N1
N2
N3不同初始含水率 1.36 15
17
190.45 N4
N5不同干密度 1.36
1.5015 0.45
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