弱膨胀土在浸水膨胀过程中的微观结构变化特征

为探究膨胀约束条件对膨胀土水化稳定后微观结构的影响,对河南一处膨胀土开展了不同荷载下室内一维膨胀变形试验,并采用真空冷冻干燥法对初始和膨胀稳定后的土样脱湿,然后进行扫描电镜试验和压汞试验,测定土样微观结构类型和孔隙分布的情况。扫描电镜试验结果表明:重塑膨胀土呈紊流结构,颗粒排布没有明显的方向性;在允许体积膨胀的条件下,颗粒中的伊蒙混层羽翼状边缘被充分水化,边缘更为光滑,粒间连接也变弱,土体从紊流结构逐渐过渡为粒状堆叠结构。压汞试验结果表明:不同荷载下土样的孔径分布曲线均为三峰曲线,土中的孔隙按照等效孔径大小可以分为大孔隙、小孔隙、微孔隙和超微孔隙。与初始土样相比,恒体积膨胀条件下,土样中的小孔隙被压缩成微孔隙。在上覆荷载为0 kPa和50 kPa的侧限膨胀条件下,由于上覆荷载小于土的膨胀力,土颗粒能够推开其旁边的颗粒,所以土中各类型孔隙的体积含量均增多。

Microstructural Variation Characteristics of Weak Expansive Soils During Hydration

One-dimensional swelling tests under different load conditions were conducted to obtain the evolution of Nanyang expansive soil’s microstructure. Samples with initial state and samples swelling under different conditions were freezing-dried to get prepared for Scanning Electron Microscope (SEM) tests and Mercury Intrusion Porosimetry (MIP) test. Then the modification in microstructures and pore size distribution were analyzed. SEM tests show that soil-particle arrangement has no obvious directionality. Samples with initial compacted state and swelling under constant volume are in turbulent flow structures, while samples swelling under 0 or 50 kPa surcharge are of granular structures after reaching steady state. MIP test shows that the pore size distribution curves of samples under different swelling conditions are of three peaks. Pores in soils can be classified into macro-pore, fine-pore, micro-pore and ultra-micro-pore. Compared with initial states, fine-pores in samples swelling under constant volume are compressed into micro-pore. The volumes of all kinds of pores in samples swelling under 0 or 50 kPa surcharge loads all increase as the surcharge loads are larger than the expansive force. In this situation, soil particles can push each other away.