间歇循环荷载作用下饱和砂土累积塑性变形及孔压特性研究

Characteristics of the cumulative plastic deformation and pore water pressure of saturated sand under cyclic intermittent loading

长期列车荷载作用会引起砂土地基和填料的强度衰减、累积沉降过大甚至沉陷等病害问题,严重者危及行车安全。揭示病害机制需探明间歇荷载作用下饱和砂土累积塑性变形及孔压特性。对此开展了不同围压、动力幅值下连续加载与间歇加载的动三轴试验,试验结果表明:(1)饱和砂土累积塑性变形-振次曲线呈现锯齿状发展。间歇效应导致卸荷回弹,并显著降低了后加载阶段砂土累积塑性变形,可使连续加载条件下的破坏型“转换”为稳定型。(2)对于塑性安定和塑性蠕变型,孔压-振次关系曲线呈台阶状,第1加载阶段的孔压随振次迅速累积增长,而间歇阶段排水,孔压消散接近或降至0,土体趋密实;在后续加载阶段,孔压累积幅值大幅降低。对于增量破坏型,孔压在第1加载阶段快速增大,试样破坏。(3)建立了表征间歇加载下砂土累积塑性应变两阶段发展特征的预测模型,预测效果良好。(4)间歇效应提高了砂土抵抗塑性变形的能力,连续加载会高估砂土累积塑性应变和低估其动强度。该研究结果对深刻认识间歇循环荷载下饱和砂土的累积变形特性和机制具有重要参考价值。

The long-term train loads can result in problems such as strength attenuation,excessive cumulative settlement and even subsidence in sandy soil foundations and filling materials,which can jeopardize train operation safety.To understand the mechanism of the disease,it is necessary to explore the cumulative plastic deformation and pore water pressure characteristics of saturated sand under intermittent loading.Therefore,dynamic triaxial tests under continuous and intermittent loading with different dynamic amplitudes and confining pressures were performed.The test results showed that:(1)The cumulative plastic deformation-loading cycle curve of saturated sand exhibited a“zigzag”pattern.The intermittent effect led to unloading rebound and significantly reduced the accumulated plastic deformation of sand in the later loading stage,which can transform the failure mode from“destructive”to“stable”under continuous loading.(2)For plastic stability and plastic creep,the pore water pressure-loading cycle curve showed a ladder shape.In the first dynamic loading stage,the pore water pressure increased rapidly with loading cycle while drainage occurred during the intermittent stage,causing pore water pressure was dissipated and approached or equaled zero,resulting in denser sand soil.In the subsequent loading stage,the cumulative amplitude of pore water pressure decreased significantly.For the incremental failure type,the pore water pressure increased rapidly and the specimen was damaged in the first loading stage.(3)A prediction model characterizing the two-stage development of cumulative plastic strain of sand under intermittent loading was established and its prediction effect was good.(4)Intermittent effect increased the resistance of sand to plastic deformation.The cumulative plastic strain of sand was overestimated and the dynamic strength was underestimated under continuous loading.The cumulative deformation characteristics and its mechanism of saturated sand under cyclic intermittent loading can be deeply understood.