低固结压力下土–膨润土防渗墙填料渗透和扩散系数测试

渗透和扩散是污染物在防渗屏障中迁移的重要方式,当前对低固结压力下土–膨润土防渗墙渗透系数和扩散系数测试是否必须采用柔性壁渗透仪、是否必须对试样进行固结尚无统一看法。按土–膨润土防渗墙施工工艺制备填料,使用柔性壁渗透仪测试了30,50和100 k Pa有效固结压力下填料渗透系数,进行刚性壁土柱渗透–扩散试验测试了10 k Pa固结压力下填料渗透系数和扩散系数,基于加速沥出试验原理提出快速测定高塌落度填料有效扩散系数的透析试验方法。柔性壁渗透试验结果表明,填料流入和流出渗透系数均随水力梯度增大而增大,存在起始水力梯度,柔性壁渗透试验的起始水力梯度在6.82~8,随固结压力由30 k Pa增至100 k Pa,渗透系数由5.21×10~(-8)降至3.78×10~(-8) cm/s。10 k Pa固结压力下,刚性壁渗透–扩散试验测得起始水力梯度为5.67,渗透系数为7.14×10~(-8) cm/s,试验不存在侧壁渗漏,填料中Cl~-有效扩散系数为3.12×10~(-6) cm~2/s。透析试验填料未经固结,测得有效扩散系数为4.45×10~(-6) cm~2/s。掺入6.02%膨润土后,粉土渗透系数降低约4个数量级,有效扩散系数仅降低约一半,扩散将是膨润土系防渗墙中污染物迁移的主要方式。

Measurement of hydraulic conductivity and diffusion coefficient of backfill for soil-bentonite cutoff wall under low consolidation pressure

Advection and diffusion are important mechanisms of contaminant transport through barriers. Whether flexible-wall permeameter and consolidated specimen must be used in the permeation or diffusion tests on soil-bentonite backfill under low consolidation pressure is still controversial. The soil-bentonite backfill is prepared according to the common construction procedure of cutoff walls. The hydraulic conductivity of the backfill is measured by a flexible-wall permeameter under effective consolidation pressures of 30, 50 and 100 kPa, respectively. The hydraulic conductivity and diffusion coefficient are also measured by rigid-wall column tests. Based on the theory of dynamic leaching tests, a dialysis method is proposed for quick measurement of the effective diffusion coefficient of the backfill. The results of flexible-wall tests show that the hydraulic conductivity of the backfill increases with the hydraulic gradient. There are initial hydraulic gradients ranging from 6.82 to 8 in the flexible-wall tests. The hydraulic conductivity decreases from 5.21×10^-8 to 3.78×10^-8 cm/s as the consolidation pressure increases from 30 to 100 kPa. Under the consolidation pressure of 10 kPa, the rigid-wall column tests give an initial hydraulic gradient of 5.67, a hydraulic conductivity of 7.14×10^-8 cm/s, and an effective diffusion coefficient of 3.12×10^-6 cm²/s. The backfill in the dialysis tests is not consolidated and the effective diffusion coefficient is 4.45×10^-6 cm²/s. With a bentonite content of 6.02%, the hydraulic conductivity of the backfill decreases by 4 orders of magnitude, while the effective diffusion coefficient only decreases by about 50%, so diffusion will be the dominant contaminant transport process in soil-bentonite cutoff walls.