动荷载作用后饱和黏土强度衰变规律及微观结构机理研究

Soft clay foundation and slope engineering widely exist in China's south-east coastal areas. The clay may suffer structural damage and strength degradation under dynamic loading disturbance, which could lead to disastrous consequences for the worst. However, strength degradation resulting from dynamic loading has given no consideration to related engineering design and still been at the stage of exploration at home and abroad nowadays. So, stress distribution rule of clay foundation under traffic loading is studied in this project, and principal factors affecting soil dynamic response are refined through monitoring dynamic stress. With consideration of traffic loading and complicated loading, the project could analyze change law of undrained shear strength,and establish development model through interior dynamic testing. Then, quantificational analysis for thixotropy and consolidation is studied when time effect is involved. At last,principal microstructure parameters associated with macroscopical mechanics index and reveal mechanism of strength development could be refined by microstructure observation means. Through the studies in this project, the result could provide a basis of how to design soft clay foundation and slope engineering when refering to strength degradation under dynamic loading reasonably and safely.

软黏土地基工程、边坡工程广泛存在于我国东南沿海地区，在动荷载的扰动下，黏土结构性易遭到破坏，强度将不可避免发生衰减，严重的更会引发灾难性后果。而现阶段的相关工程设计并未考虑动荷载作用导致的土体强度衰减，国内外相关研究也尚处于探索阶段。 为此，本项目通过对车辆荷载作用下土体动应力进行监测，总结交通荷载在黏土地基中应力分布规律，提取影响土体动力响应的主要因素。通过室内动力试验，逐一分析饱和黏土受交通荷载、复杂荷载作用后，土体不排水强度的衰变规律，建立动后黏土强度发展模型。考虑时间效应，定量分析触变性及固结对土体强度恢复的影响。从微结构角度出发，提炼关键微观参数，并与宏观力学指标关联，揭示强度变化机理。通过开展本项目的研究，为动荷载作用下考虑强度衰减时软黏土地基、边坡工程设计的合理性、安全性提供理论依据。

本项目以滨海相黏土为研究对象，以黏土宏细观力学指标演化特佂为主线，进行了现场测试、室内三轴及电镜等宏细观试验，主要研究内容及结果如下：（1）扭剪和单剪作用后黏土强度普遍衰减20%~50%。随着扰动次数的增加，粘聚力和内摩擦角均表现出前期下降迅速，之后放缓并稳定。（2）对土体细观参数与宏观强度做线性相关分析。发现颗粒数量、圆度均为负相关，其中颗粒数量的相关程度最大，而等效直径、面积比和形态比相关度较小。扭剪作用下，纵断面与横断面孔隙增长率相近，这主要是由于土柱轴向与径向分别存在扭剪应变和切应变；而单剪过程中，土柱只在轴向作用剪应变，使得纵断面的孔隙增长速率大于横断面。（3）针对天津滨海软黏土开展长期触变性试验研究。静置100min黏土强度增长3kPa；之后静置一年强度增长1.8kPa。若强度增长速度不变，恢复到原状土强度需要4年时间。影响强度恢复的因素中，静电力作用遵循能量最低原理；氧化铁胶体是构成天津软黏土胶体的重要部分；颗粒剥落引起的强度损失不可逆；良好的排水条件对强度增长有积极意义。（4）利用动三轴仪研究了静偏应力与动应力组合作用下黏土动变形、动弹性模量及动孔压的变化规律。静偏应力为0时，土体产生拉应变，其随动应力增加不断增大，量级10-3。静偏应力大于0时，土体产生压应变，此时若动静应力比小于1，压应变随动应力增加不断增大；若动静应力比大于1，压应变不断减小。相同静偏应力下，动弹性模量随动应力增加不断减小；相同动应力下，动弹性模量随静偏应力增加不断减小，且土体更易应变软化。（5）针对砂-粉黏混合料冻后强度变化进行研究，发现细粒土的存在一方面抑制了冰的生成，另一方面相互吸附产生粘聚力，二者此消彼长决定了强度变化。当孔隙间冰晶不能联结成整体，抗压强度将迅速下降。抗压强度随粉黏含量和含水率的增加呈先增后减趋势，最优粉黏含量为60%，最优含水率为16%。