干湿-冻融循环条件下湿陷性黄土剪切及压缩特性的劣化规律

Deterioration law of shear and compression characteristics of collapsible loess under dry-wet and freeze-thaw cycles

新疆北疆地区某输水明渠工程跨越大面积湿陷性黄土区域,湿陷性黄土经多年降雨、蒸发及季节气温交替变化后力学特性衰减严重,极易造成渠基塌陷及渠坡滑动破坏等工程现象。为深入探究其劣化机制,通过开展干湿-冻融循环条件下湿陷性黄土的直剪、压缩及其微观扫描试验,从宏-微观两个尺度分析其剪切强度、压缩特性的劣化规律及其损伤物理机制。研究结果表明(1)直剪试验:随着干湿-冻融循环次数的增加,峰值抗剪强度呈急速减小-速率减缓-趋于稳定3阶段发展趋势;黏聚力呈指数形式减少,第1次循环减小幅度最大,5次循环后趋于稳定状态,劣化度达到44.55%;内摩擦角变化幅度在2.1°以内,最大劣化度为7.04%,受干湿-冻融循环的影响较小。(2)压缩试验:压缩曲线可依据固结压缩屈服应力σ_(k)分为弹性变形和弹塑性变形两阶段,σ_(k)随循环次数增加前移;压缩系数、压缩指数与循环次数呈指数、幂函数形式减小,土体的整体压缩性减小;回弹指数与循环次数呈线性正相关。(3)微观结构:通过微观电镜扫描(scanning electron microscope,简称SEM)试验分析,在循环作用下,大孔隙减小,中、小孔隙增加,排列方式趋于无序状;大粒径颗粒逐渐转化为中、小颗粒,形态趋向于拟圆形;利用关联度分析发现孔隙大小及其角度是影响剪切强度的主要因素;引入Pearson相关系数发现颗粒形态及孔隙大小对压缩指标的影响程度最大。

A water conveyance open channel project in the northern Xinjiang region crosses a large area of collapsible loess.The mechanical properties of the collapsible loess have undergone severe degradation after years of exposure to rainfall,evaporation,and seasonal temperature fluctuations,making it highly susceptible to engineering phenomena such as channel foundation collapse and slope failure.To delve into the deterioration mechanism,direct shear,compression,and microscopic scanning tests were conducted on the collapsible loess under dry-wet&freeze-thaw cycles.The deterioration patterns of shear strength and compression properties,as well as their damage mechanisms,were analyzed at both macro and microscopic scales.The results of the study indicate(1)Straight shear test:with increasing the number of dry-wet-freeze-thaw cycles,the peak shear strength exhibits a three-stage trend:rapid decrease,decelerated rate of decrease,and eventual stabilization.The cohesion decreased exponentially,with the largest reduction occurring during the first cycle,and stabilizing after 5 cycles,reaching a degradation degree of 44.55%.The change in internal friction angle,which varied within 2.1°,was less affected by the wet-dry-freeze-thaw cycles,with a maximum degradation of 7.04%.(2)Compression test:the compression curve can be divided into two stages of elastic deformation and elastic-plastic deformation according to the consolidation yield stress σ_(k),and σ_(k) shifts forward as the cycle times increase.The compression coefficient and compression index decreased exponentially or in a power function form with increasing cycle times,indicating reduced overall compressibility of the soil body.(3)Microstructure:through scanning electron microscope(SEM)analysis,under cycling,the number of large pores decreased while the number of medium and small pores increased,with the arrangement tending towards disorder.Large particles gradually transformed into medium and small particles,and their morphology tended to become rounded.Correlation analysis indicates that pore size and its angle are the main factors influencing shear strength.Pearson’s correlation coefficient reveals that particle morphology and pore size have the greatest influence on compression indices.