干湿、冻融循环对膨胀土土–水及收缩特征的影响
Influences of drying-wetting-freeze-thaw cycles on soil-water and shrinkage characteristics of expansive soil
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摘要: 研究了冻融循环、干湿循环和交替的干湿—冻融循环对压实膨胀土的微观结构、土–水特征曲线和收缩特征曲线的影响。试验结果表明:冻融、干湿以及干湿—冻融循环显著改变了压实膨胀土的大孔隙系统;干湿循环使压实土样的大孔隙消失,但产生可见的宏观裂隙;冻融循环导致土样产生肉眼不可见的微裂隙;干湿—冻融循环既产生可见的宏观裂缝,又产生不可见的微裂隙。宏观及微观裂隙的产生显著降低了膨胀土在低吸力范围内的持水能力以及从饱和状态到干燥状态的收缩量,但对高吸力范围内的持水能力和土样的缩限和塑限无显著影响;收缩特征曲线等比例收缩段和残余收缩段的斜率在冻融循环和交替的干湿—冻融循环后减小,但在干湿循环后基本保持不变;受3种循环作用后的试样的含水率–孔隙比–吸力关系分布在同一个土–水特征曲面上,该曲面可由本文所提的改进土–水特征曲面模型加以描述。Abstract: The effects of freeze-thaw (FT), drying-wetting (DW) and alternative drying-wetting-freeze-thaw (DWFT) cycles on the micro-structure, soil-water characteristic curve (SWCC) and soil shrinkage characteristic curve (SSCC) of a compacted expansive soil are investigated. The experimental results show that the three investigated cyclic treatments significantly change the macro-pore system of soil. The macro-pores formed during compaction disappear during DW cycles while visible cracks are introduced. FT cycles induce invisible micro-cracks, and macro- and micro-cracks are both discovered in DWFT specimens. Macro- and micro-cracks significantly reduce the water retention capacity of soil in the low suction range and the amount of shrinkage from saturated condition to oven dryness. They have no influences on the SWCC in the high suction range and the shrinkage limit and plastic limit of soil. The slope of the SSCC at the proportional and residual shrinkage stages decreases after FT and DWFT cycles but remains constant after DW cycles. The relationships among moisture content, void ratio and suction of soil after different FT, DW and DWFT cycles are distributed on a unique surface which can be reasonably described by the proposed modified model. This study is useful for understanding the hydro-mechanical behavior of expansive soil under the influences of environmental factors.