Study on the Influencing Factors of Specific Yield of Unconfined Aquifer Using Laboratory Column Drainage Experiment
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摘要: 给水度是重要的水文地质参数之一，为探讨潜水含水层给水度的影响因素，设计均质粗砂、均质细砂、上细下粗、上粗下细4种土柱的排水实验，按照一定间距分段降低地下水位，每段降深设计不同排水时间，求取不同水位埋深的给水度.当地下水位埋深小于支持毛细水高度，给水度受地下水位埋深影响显著，其关系可以用非线性函数来表示.排水时间越长，给水度越大，当土柱分段排水时间超过1 h，给水度值稳定.0.6~0.9 mm粒径的粗砂给水度大于0.2~0.4 mm的细砂；当地下水位在土层分界面下20 cm时，上细下粗土柱给水度显著增大，上粗下细土柱给水度显著减小.给水度的大小与地下水位埋深、排水时间、岩土颗粒和潜水面附近及之上非均质结构等因素有关.Abstract: Specific yield is one of the most important hydrogeological parameters of unconfined aquifer. In order to study the influencing factors of specific yield, the laboratory column drainage experiment was developed to estimate specific yield of homogeneous and stratified heterogeneous sand columns. The groundwater level was decreased according to segmentation of 2 cm or 5 cm and each section kept different drainage times to obtain the specific yield value of different depth to water table. When the depth to water table was within the capillary rise height, the relationship between specific yield and water table depth can be expressed by a nonlinear function. The specific yield value increased with the drainage time and it became constant when the drainage time of each drawdown section was more than one hour. Specific yield value of coarse sand with particle size 0.6-0.9 mm was always larger than that of fine sand with particle size 0.2-0.4 mm. When groundwater level was 20 cm under the layer interface, two different columnar structures of upper-coarse-lower-fine and vice versa exerted distinct impacts on the magnitude of specific yield. For shallow unconfined aquifer, specific yield varied with the initial depth to water table, duration of drainage, soil texture and heterogeneity near the water table.
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Key words:
- specific yield /
- soil column drainage /
- depth to water table /
- drainage time /
- heterogeneity /
- hydrogeology
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图 2 均质土柱给水度随地下水位埋深和排水时间变化曲线
a.均质细砂；b.均质粗砂
Fig. 2. Specific yield versus depth to water table in homogenous column for different drainage times
图 3 均质土柱不同地下水位埋深支持毛细水高度的分布
a.第一阶段；b.第二阶段；c.第三阶段
Fig. 3. Variations of capillary rise height with depth to water table in homogenous column
图 4 层状土柱给水度随地下水位埋深和排水时间变化曲线
a.上细下粗土柱; b.上粗下细土柱
Fig. 4. Specific yield versus depth to water table in stratified heterogenous column for different drainage times
图 5 上细下粗土柱中不同地下水位埋深的支持毛细水高度
a.水位在土层分界面之上；b.水位距离分界面小于6 cm；c.水位距离分界面小于20 cm；d.水位距离分界面大于20 cm
Fig. 5. Variations of capillary rise height with depth to water table in upper-fine-lower-coarse column
图 6 上粗下细土柱中不同地下水位埋深的支持毛细水高度
a.水位在土层分界面之上；b.水位距离分界面小于6 cm；c.水位距离分界面约20 cm；d.水位距离分界面大于45 cm
Fig. 6. Variations of capillary rise height with depth to water table in upper-coarse-lower-fine column
表 1 实验土柱结构和排水时间
Table 1. Experimental soil column structure and drainage time
土柱编号 土柱结构 土柱装样长度 分段排水时间设计 1 均质粗砂 120 cm 5 min、10 min、20 min、30 min、1 h、2 h、3 h 2 均质细砂 100 cm 5 min、10 min、20 min、30 min、1 h、2 h、3 h 3 上细下粗 细65 cm，粗65 cm 30 min、1 h、2 h、3 h 4 上粗下细 粗50 cm，细70 cm 30 min、1 h、2 h、3 h 
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表 2 实验用砂孔隙大小和相应的毛细上升高度
Table 2. Pore size of experimental sands and capillary rise height
砂粒类型 颗粒大小(mm) 孔隙大小（mm) 毛细上升高度(cm) 立方体孔腹 立方体孔喉 四面体孔喉 立方体孔腹 立方体孔喉 四面体孔喉 细砂 最小 0.2 0.146 0.083 0.031 20.49 36.23 96.77 最大 0.4 0.293 0.166 0.062 10.25 18.12 48.39 平均 0.3 0.220 0.124 0.047 13.66 24.15 64.52 粗砂 最小 0.6 0.439 0.248 0.093 6.83 12.08 32.26 最大 0.9 0.659 0.373 0.140 4.55 8.05 21.51 平均 0.75 0.549 0.311 0.116 5.46 9.66 25.81
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表 3 不同排水时间土柱给水度与地下水位埋深的关系
Table 3. Relationship between specific yield and depth to water table for different drainage times
排水时间 细砂拟合式 粗砂拟合式 5 min Sy= 0.215/(1+e ((WTD-33.7)/(-9.332)))-0.008 Sy= 0.297/(1+e((WTD -14.63)/(-3.102)))-0.004 10 min Sy= 0.233/(1+e ((WTD-30.23)/(-8.358)))-0.010 Sy= 0.301/(1+e((WTD -14.26)/(-8.358)))-0.010 20 min Sy= 0.256/(1+e ((WTD-29.33)/(-9.22)))-0.012 Sy= 0.306/(1+e((WTD -14.08)/ (-2.786)))+0.001 30 min Sy= 0.286/(1+e ((WTD-26.44)/(-5.251)))+0.003 Sy= 0.309/(1+e ((WTD -13.63)/(-2.57)))+0.003 1 h Sy= 0.288/(1+e ((WTD -23.98)/(-2.645)))+0.005 Sy= 0.312/(1+e ((WTD -13.78)/(-2.011)))+0.034 2 h Sy= 0.290/(1+e ((WTD -26.31)/(-4.446)))+0.011 Sy= 0.334/(1+e ((WTD -14.47)/(-1.973)))+0.019 3 h Sy= 0.301/(1+e((WTD -24.89)/(-3.856)))-0.000 Sy= 0.344/(1+e ((WTD -12.65)/(-2.459)))+0.002
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