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膨胀土具有裂隙性,反复胀缩裂隙的发展破坏了土体的连续性及均匀性,使土体表现出明显的各向异性,依托连续介质理论定义的土体表征单元体(REV)难以满足实际要求.针对不同次数现场原位干湿循环作用下的原状土试样进行3种不同尺寸(30 cm2, 900 cm2, 1963 cm2)的直接剪切试验及裂隙图像分析,从土体表面裂隙率(CIF)和黏聚力参数对膨胀土REV尺寸的影响进行研究.结果表明,未干湿循环时,原生裂隙使土体黏聚力参数表现出非连续性,试样尺寸越大,土体测试黏聚力越小,次生裂隙的发展进一步加剧了试样尺寸对测试黏聚力的影响;土体REV尺寸随着胀缩裂隙的发展而增大,且其增长速率逐渐减小,3～5次循环后趋于稳定;在同一循环条件下,依托黏聚力参数确定的REV尺寸(REV-C)约为依托裂隙率确定的REV尺寸(REV-CIF)的1.75～2.97倍.CIF可作为表征土体裂隙状况的基本指标,在胀缩裂隙影响下,膨胀土的CIF均值与不同最大允许误差所确定的REV尺寸呈正相关,且相关系数均大于0.9.本研究提出了一种基于裂隙图像确定裂隙膨胀土REV-C的方法,并给出研究区域内膨胀土在不同暴露时间下直接剪切试验的REV尺寸大小.“,”Cracks resulting from cyclic wetting and drying of expansive soils create discontinuities and anisotropy in the soil. The representative elementary volume (REV) defined by the continuous-media theory cannot be applied to cracked expansive soils that are considered discontinuous media. In this study, direct shear tests of three different scales (30 cm2, 900 cm2, 1963 cm2) and crack image analysis were carried out on undisturbed soil samples subjected to drying-wetting cycles in-situ. The REV size of expansive soil was investigated using the crack intensity factor (CIF) and soil cohesion. The results show that soil cohesion decreased with increasing sample area, and the development of secondary cracks further exacerbated the size effect of sample on cohesion of the soil. As shrinkage cracks developed, the REV size of the soil gradually increased and plateaued after 3-5 cycles. Under the same drying-wetting cycle conditions, the REV size determined using soil cohesion (REV-C) is 1.75 to 2.97 times the REV size determined using CIF (REV-CIF). Under the influence of shrinkage cracks, the average CIF is positively correlated with the REV size determined using different maximum permissible errors, with the coefficient of correlation greater than 0.9. A method for determining the REV-C based on crack image analysis is proposed, and the REV-C of expansive soil in the study area under different exposure times is given.