原状黄土冻融过程抗剪强度劣化机理试验分析

Experimental analysis on the mechanism of shear strength deterioration of undisturbed loess during the freeze-thaw process

通过对西安Q_3原状黄土在封闭系统冻融作用下的电镜扫描和直剪试验,研究了冻融作用对原状黄土微观结构和强度的影响。试验表明:冻融过程中原状黄土微观结构发生显著变化,大颗粒集粒数量明显减少,小粒径土颗粒所占比重增加,孔隙面积比增加。进一步基于损伤力学理论,得到微观结构冻融损伤度随冻融次数增加呈指数增加趋势,反映出冻融作用一定程度上破坏黄土体的结构强度,但多次冻融后黄土体结构强度趋于稳定的残余强度。冻融过程土样表面结构发生破坏,且含水率越高,土体表面特征破坏越严重。粘聚力随冻融次数增加呈指数衰减趋势,且含水率越高,粘聚力衰减幅值和速率越小;粘聚力随含水率增加表现出线性衰减特征,且冻融后粘聚力与含水率的变化规律近似重合;内摩擦角无明显规律性变化。粘聚强度冻融损伤系数随冻融次数增加呈指数增加趋势,随含水率升高有增大趋势。基于试验数据规律性,进一步提出了原状黄土粘聚强度劣化模型,该模型经试验验证可较好描述原状黄土粘聚强度劣化规律。

SEM and direct shear tests were conducted to study the influence of freeze-thaw action on the microstructure and strength of xi＇an Q_3 undisturbed loess under the closed system.The experimentalresults showed that the microstructure of loess changes significantly during the freeze-thaw process,with the number of large particles aggregate decreasing,the proportion of small particle size increasing,and the ratio of pore increasing.Based on the damage mechanics theory,the freezing-thawing damage degree versus freezing-thawing times was established.Analysis indicated that the freezing-thawing damage degree exponentially increased with the increase of freezing-thawing times,representing that the structural strength of loess can be destroyed by freezing-thawing.However,the structural strength eventually reached a residual strength value after repeated freezing and thawing.Freezing-thawing made the surface structure of soil destroyed,and had a bigger influence with the increace of water content.The cohesion exponentially decreased with freezing and thawing times increasing,and had a smaller attenuation amplitude and rate with the increase of water content.With the increase of water content,the cohesion linearly decreased,and had an approximately same variation after freezing and thawing.The internal friction angle had no obvious variation during the whole freezing-thawing process.The freezing-thawing damage coefficient of cohesion exponentially increased with the increase of freezing-thawing times,and also increased with water content increasing.A cohesion strength deterioration model was developed based on the laboratory test results,and the model can be used to well describe the strength deterioration regularity by the test verification.