膨胀土的二维膨胀各向异性试验研究

为给膨胀土地区结构物设计计算提供关键参数，研制了二维膨胀仪，选取广西百色中膨胀土和湖北枝江弱膨胀土，制备不同干密度状态的试样，分别开展了恒体积膨胀力、单向膨胀和二维膨胀试验，分析和表征了竖向和侧向膨胀的规律，并比较了两向膨胀的差异性；通过扫描电镜观察，分析了产生膨胀各向异性的原因。结果表明：恒体积条件下最终侧向与竖向膨胀力之比，以及单向膨胀条件下最终侧向与竖向膨胀率的比值，均会随压实度发生变化，干密度越大两向膨胀力(或膨胀率)的差异性越大；两向膨胀力随膨胀应变的变化均表现出前期快速衰减、后期缓慢衰减的趋势，可用幂函数表征，但拟合参数不同；在二维膨胀条件下，竖向膨胀会影响侧向膨胀，不同程度竖向膨胀后的侧向膨胀规律同样可用幂函数表征，拟合结果表明竖向膨胀会改变拟合参数导致曲线形态的改变，继而影响侧向膨胀力随侧向应变的衰减幅度。干密度增大致使黏土矿物片状颗粒整体更趋向于水平定向排列，这是造成膨胀各向异性的主要原因。膨胀过程中土颗粒的重分布会导致各向异性不断变化，故难以通过竖向膨胀力预测侧向膨胀力，在进行相关计算时建议实测侧向膨胀规律。

Experimental Study on Two-dimensional Swelling Anisotropy of Expansive Soils

A two-dimensional dilatometer was developed to determine the key parameters for the design of structures in expansive soil areas. Medium expansive soil from Baise, Guangxi, and weak expansive soil from Zhijiang, Hubei were selected to prepare specimens with different dry densities. Swelling tests of the specimens were conducted under constant volume, one-dimensional swelling, and two-dimensional swelling conditions. The vertical and lateral swelling behaviors were analyzed and characterized, and the differences in swelling behaviors between the two directions were compared. Subsequently, the swelling anisotropy was analyzed using scanning electron microscopy. The test results demonstrate that the ratio these of the final lateral swelling pressure to the final vertical swelling pressure measured under constant volume and that of the final lateral swelling strain to the final vertical swelling strain measured under one-dimensional swelling conditions vary with dry density. The larger the dry density, the greater the difference in the swelling pressures (or strains) in the two directions. With increasing swelling strain, the swelling pressures in both directions decay rapidly in the early stage but slowly in the later stage; this decaying behavior can be characterized by a power function with different fitting parameters. Under two-dimensional swelling conditions, vertical swelling has an impact on lateral swelling. The variation in lateral swelling after vertical swelling to different extents can also be expressed by a power function. The fitting results show that the vertical swelling causes a change in the fitting parameters, which adjusts the features of the curves. This change influences the magnitude of reduction in the lateral swelling pressure with lateral strain. With an increase in dry density, the flaky particles of clay minerals exhibit a stronger orientation toward the horizontal direction, which is the root cause of swelling anisotropy. In addition, the redistribution of soil particles during swelling leads to a constant variation in swelling anisotropy. Therefore, the prediction of lateral swelling pressures based on vertical swelling pressures becomes difficult. Accordingly, the trend of lateral swelling should be measured when calculating the force acting on structures in expansive soil areas.