饱和细粒土固结过程的三维孔隙演化特征

THREE-DIMENSIONAL PORES EVOLUTION CHARACTERISTICS DURING CONSOLIDATION PROCESS OF SATURATED FINE-GRAINED SOIL

孔隙作为软土的重要组成单元，在软土性质发生改变时其变化最直接、最明显。研究软土固结过程中孔隙演化特征，对于认识软土排水固结机制具有重要理论意义。本文对大亚湾饱和细粒土在梯度压力下的孔隙结构进行微纳米尺度定量分析：将圆柱形土样进行真空冷冻升华干燥，利用同步辐射显微CT获取分辨率1.625μm的二维切片，应用Avizo软件的灰度阈值截断法将二维切片重建三维结构，采用形态学算法对三维孔隙结构进行量化和表征。研究表明：自沉状态下超过90%孔隙的等效直径为4～10μm，只有少数孔隙的等效直径大于40μm；土样在100kPa压力作用后，大孔隙数量迅速减小，小孔隙数量迅速增加，表明初始状态下孔隙对压力最为敏感；大中孔隙容易被压缩消灭或被分裂为小微孔隙；随压力增加孔径变化趋于平缓，小孔隙和微孔隙占优势，孔隙的抗压能力与大小成反比。经梯度压力作用后土体从絮凝结构逐渐变成片叠结构，颗粒之间平行排斥，孔隙丰度逐渐减小，形状变得细长，方向趋于水平。本研究利用同步辐射显微CT技术结合三维可视化软件Avizo，建立具有真实孔隙结构特征的软土三维模型，从微纳米尺度分析了饱和细粒土的固结蠕变机理。

Soil mechanical properties change during the consolidation progress, which is related to the changes for pores of saturated clays. Therefore, a thorough investigation of pore evolution characteristics during the process of saturated clay consolidation has important significance for understanding and applying saturated clay drainage in construction. The pore evolution characteristics of the Daya Bay soil are investigated. In order to decrease the void ratio efficiently, saturated fine-grained soil under the pressure gradient is tested. Firstly, cylindrical soil samples are dried by a cryogenic chamber. Secondly, Synchrotron Radiation CT is used to scan the dried samples to obtain high-precision slices. Then three-dimensional models are restructured with Avizo software using grayscale threshold method. Furthermore, porous structures are quantitatively estimated with several of morphological algorithms. Studies show that pores, under the weight settlement, more than 90% are located in 4-10μm and only a few are larger than 40μm. Soil samples, under the pressure of 100kPa and with large pore content will decrease rapidly while its small pores will increase rapidly, demonstrating that pores are more sensitive to pressure under the initial state. Medium and large pores can be split into small micropores or easily eliminated after being compressed. The results show that aperture with increasing pressure changes gently, small pores and micro pores have more advantages. It can be conclude that small and micro pores have stronger anti-pressure ability. After the pressure gradient, the original flocculation structure gradually changes to the flat stacked structure, particles repel parallels, porosity abundance decreases gradually and pores become elongated in a level direction. In this study, a three-dimensional model with real pore structure characteristics is established using the combination of CT scanning and Avizo（an advanced 3D visualization software）.It analyzes the consolidation mechanism of the saturated fine-grained soil from the microscopic perspective.