大振次交通荷载对压实黄土微观结构变化的影响

Effect of Traffic Loading with Large Cyclic Number on Microstructure of Compacted Loess

长期交通荷载作用下路基土体累积应变的持续发展在引发路面沉降的同时也会影响土体的动力特性。当前研究大多停留在宏观角度,并未从微观上合理揭示其应变发展、动力参数变异的机理。在我国西北地区的公路交通建设过程中,黄土是应用最为广泛的路基填料。为了揭示长期交通荷载作用下压实黄土的宏观力学响应与其微观结构变化的内在联系,采用动三轴试验得到了长期交通荷载作用下压实黄土的累积应变和动模量随振次的发展规律,进而结合扫描电镜对振动前后压实黄土试样的微观结构进行了对比分析。研究结果表明,长期交通荷载作用下压实黄土的累积应变及动模量随振次的发展表现为明显的两阶段变化特征。按照塑性应变速率发展规律可以对其两阶段变化进行定义,分别为"对数下降段"和"稳定震荡段"。长期交通荷载作用前后压实黄土的微观结构变化主要表现为"凹坑"和贯通孔隙的闭合、颗粒之间胶结的断裂以及颗粒的破碎和重分布。推测"对数下降段"累积应变的快速发展和动模量的逐渐增加是"凹坑"和贯通架空孔隙闭合的宏观表现,"稳定震荡段"累积应变的持续发展和动模量的逐渐衰减则是颗粒之间胶结的断裂以及颗粒的破碎和重分布的宏观表现。建议在工程中通过预振等措施消除对数下降段的累积塑性应变,以期有效控制运营期沉降。

The accumulated strain of subgrade soil under long term traffic loading might induce the pavement settlement and affect the dynamic characteristic of soil. However, most of the researches have forced on the macro-behavior of soil, and microcosmic mechanism of accumulation of strain and variation of dynamic parameters is rarely observed. Loess is the most widely used roadbed filler in the traffic construction of northwest region of China. In order to uncover the inner connection between the macro-behavior and the variation of micro-structure of compacted loess, the variations of accumulated strain and dynamic modulus of compacted loess versus the cyclic number under long term traffic loading are obtained using dynamic triaxial test. Furthermore, the scanning electron microscopy is used to comparatively analyse the microstructures of test sample of compacted loess before and after cyclic loading. The test result indicates that （ 1 ） the variations of accumulated strain and dynamic modulus of compacted loess under long term traffic loading behave two- stage characteristics, which can be named as ＂logarithmic drop stage＂ and ＂stable-oscillation stage＂ in terms of the change of plastic strain rate; （2） the changes of traffic loading mainly behave as the closure of ＂pit＂ and particles, the broken and redistribution of particles ; （3） be drawn that the rapid increase of accumulation strain logarithmic drop stage are macro-behaviors of the closure microstructure of compacted loess under long term connected pore, the breakage of the cement among based on the above statement, a conclusion might and gradually increase of dynamic modulus in the of ＂pit＂ and connected pore, while the continuous improvement of accumulation strain and gradually reduction of dynamic modulus in the stable-oscillation stage are macro-behaviors of the breakage of the cement among particles and the broken and redistribution of particles. It is suggested that the accumulated plastic strain in the logarithmic drop stage should be eliminated through pre-vibration to control the pavement settlement during operation.