| 摘要: |
| 【目的】研究冻融作用对重塑黄土渗透特性的影响，为季节冻土区黄土边坡、道路及堤坝等工程的设计和施工提供参考。【方法】以Q3黄土为研究对象，在封闭系统下进行快速冻融循环试验，每个循环为-20 ℃冻结12 h，20 ℃融化12 h，冻融循环次数分别为0，2，4，5，10，14，17和20。将干密度为1.7 g/cm3、含水率为18.0%的重塑黄土样制成长×宽×高为1 cm×1 cm×2 cm的长条形样品，然后置于高低温试验箱进行冻融循环处理；将冻融后的样品风干后进行扫描电镜观察，并定量分析重塑黄土的骨架颗粒形态、连接方式、孔隙形态及孔隙面积比等微观结构特征。同时制作含水率分别为15.0%，18.0%，21.0%，28.0%及饱和含水率的土样，分5层压制成干密度分别为1.4，1.5，1.6和1.7 g/cm3的黄土三轴样，然后置于高低温试验箱进行冻融循环试验；最后在围压分别为100，200，300和400 kPa条件下进行三轴黄土渗透试验，分析围压、含水率、干密度和冻融循环次数对黄土渗透特性的影响。【结果】冻融作用对土体表面结构破坏严重，有横向和纵向裂缝。冻融过程中，重塑黄土土体微观结构发生了明显变化，集团粒数量明显减少，土粒胶结性变差；随着冻融循环次数的增加，土样的孔隙面积比呈指数增大趋势，冻融循环20次后孔隙面积比由0.76增加到0.79。渗透系数随围压的增大表现出指数衰减特征，且围压为400 kPa时渗透系数差异较小；随着干密度和初始含水率的增加，渗透系数表现出抛物线变化特征，且干密度和含水率分别为1.6 g/cm3和21.0 %左右时，渗透系数较大；渗透系数随冻融循环次数增加表现出指数增大特征，冻融循环20次后渗透系数增加1~2个量级。【结论】经历冻融循环后，重塑黄土土体的结构强度遭受一定程度上的破坏，渗透性增强，冻融循环20次后黄土土体渗透特性趋于稳定；冻融过程中产生的裂缝是诱发黄土土体渗透性增强的主要原因；冻融过程中重塑黄土渗透特性与土的初始状态、冻融循环次数及围压密切相关。 |
| 关键词: 重塑黄土 冻融循环 扫描电镜试验 微观结构 渗透系数 |
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| 基金项目:国家自然科学基金项目(51478385, 51208409）；冻土工程国家重点实验室开放基金项目(SKLFSE201312) |
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| Experimental analysis on permeability law of remolded loess under freezing thawing condition |
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XU Jian,WANG Zhangquan,REN Jianwei,et al
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| Abstract: |
| 【Objective】The permeability of remolded loess during freezing thawing process was investigated to provide reference for design and construction of loess slope,road and dam in seasonal frozen ground. 【Method】The impacts of freeze thaw cycles on remolded loess (Q3) were studied by rapid freezing thawing test under closed system.The soil samples placed in the F-T chamber were subjected to 0,2,4,5,10,14,17 and 20 freezing thawing times with constant temperature of -20 °C for 12 h and 20 °C for another 12 h.In order to quantitatively observe and analyze the microstructure of remolded loess under freezing thawing cycles including particle morphology,connection mode,pore morphology and porous area ratio,the scanning electron microscope (SEM) test was conducted on dry samples with dry density,water content and size of 1.7 g/cm3, 18.0 % and 1 cm×1 cm×2 cm,respectively.Meanwhile,triaxial specimen with water contents of 15.0%，18.0%，21.0%，28.0% and saturated water content were compacted to targeted dry densities of 1.4, 1.5,1.6 and 1.7 g/cm3 in five layers. The triaxial permeability test was also conducted with confining pressures of 100,200 300 and 400 kPa to analyze the influence of confining pressure,water content,dry density and freezing-thawing times.【Result】Freezing thawing action destroyed the surface structure strength of soil greatly and transverse and longitudinal crack appeared on the surface.The microstructure of remolded loess changed significantly under freezing-thawing condition.The number of aggregate decreased obviously and the soil cementing property was weakened.Image processing software showed that the void area ratio exponentially increased from 0.76 to 0.79 with the increase of freezing-thawing time.The permeability coefficient of remolded loess exponentially decreased with the increase of confining pressure,and the difference under high confining pressure of 400 kPa was small.The permeability coefficient followed parabola law with the increase of dry density and water content and the permeability coefficient was greater with the dry density and water content of 1.6 g/cm3 and 21.0%, respectively.With the increase of freezing thawing time,the permeability coefficient exponentially increased by 1-2 order of magnitude after 20 freeze thaw cycles.【Conclusion】Freezing thawing action can destroy structural strength of loess and enhance permeability of remolded loess.However,the permeability of remolded loess reached a stable state after 20 freezing-thawing times.Cracks caused by freezing thawing action enhanced the permeability. The effect of freezing thawing on loess permeability was closely related to initial condition of soil,freezing-thawing times,and confining pressure. |
| Key words: remolded loess freezing and thawing cycle SEM test microstructure permeability coefficient |
