Granite residual soil is a special regional soil with special mineral composition and pore structure characteristics, which is easy to induce serious geological disasters or engineering problems, so it is particularly important to study its mechanical properties of unsaturated soil and its control mechanism. However, the effects of dry density and initial water content on soil–water characteristic curve (SWCC) and their mechanisms are still unclear. Therefore, samples with different dry densities (1.30 g/cm3, 1.50 g/cm3, 1.70 g/cm3) and initial water content (14 %, 20 %, 22 %) were set up in this paper. SWCC test was conducted on the two groups of samples under the humidification path and dehumidification path using filter paper method. Combined with scanning electron microscopy (SEM) test and pore-size distribution (PSD) test, the influence mechanism of different micro-pore structure on SWCC and hysteresis characteristics of granite residual soil was analyzed qualitatively and quantitatively. The results show that the samples with different dry densities basically coincide with each other in the high suction segment. The larger the dry density is, the smaller the range of transition zone in the low suction segment is. As the initial water content of the sample increases from 14 % to 22 %, SWCC changes from a single increase curve to a double increase curve, and the corresponding pore-size distribution curve (PSDC) changes from a trimodal curve to a bimodal curve. The SWCC of granite residual soil has obvious hysteresis effect, and the hysteresis area becomes smaller with the increase of dry density. The inflection point exists in the hysteresis area of SWCCS with higher initial water content. The distribution range of macropore is determined by dry density, and the distribution range of small pore is determined by initial water content. The transformation of SWCC from a single increase curve to a double increase curve is mainly controlled by the distribution range of small pores. The bottleneck effect in the migration process of pore water in the soil and the pore redistribution during water intake and water loss are the main reason for the hysteresis of SWCC. The results of this work provide some guidance for the study of unsaturated soil mechanical properties of granite residual soils.

花岗岩残积土是一种特殊的区域土，具有特殊的矿物成分和孔隙结构特征，容易诱发严重的地质灾害或工程问题，因此研究其非饱和土的力学特性及其控制机制显得尤为重要。然而，干密度和初始含水量对土水特征曲线（SWCC）的影响及其机制仍不清楚。因此，本文设置了不同干密度（1.30 g/cm3、1.50 g/cm3、1.70 g/cm3）和初始含水量（14%、20%、22%）的样品。采用滤纸法对两组样品在加湿路径和除湿路径下进行SWCC测试。结合扫描电镜（SEM）测试和孔径分布（PSD）测试，定性和定量分析了不同微孔结构对花岗岩残积土SWCC和滞后特性的影响机制。结果表明，不同干密度的样品在高吸力段基本一致。干密度越大，低吸力段过渡区范围越小。随着样品初始含水量从14%增加到22%，SWCC从单次增加曲线变为双次增加曲线，相应的孔径分布曲线（PSDC）从三峰曲线变为双峰曲线。花岗岩残积土的SWCC具有明显的滞后效应，且滞后面积随着干密度的增加而变小。拐点存在于初始含水量较高的SWCCS滞后区。大孔隙的分布范围由干密度决定，小孔隙的分布范围由初始含水量决定。SWCC由单增量曲线向双增量曲线转变主要受小孔隙分布范围的控制。土壤孔隙水运移过程中的瓶颈效应以及吸水失水过程中孔隙的重新分布是造成SWCC滞后的主要原因。该工作结果对花岗岩残积土非饱和土力学特性的研究具有一定的指导意义。