Abstract: In the present investigation, laboratorial test is conducted to study the evolution law of the fissures of Nanyang expansive clay under wetting and drying cycles. Oven drying method and vacuum saturating method are employed to simulate the drying and wetting process respectively. In order to record the development of fissures, the specimen is weighed and photographed regularly during the drying process. By employing the vector diagram technology to vectorize the fissures photos; the geometric characters of fissures are extracted before calculating the fissures ratios. Study of the regularity of fissures ratios changes proves that: the essential factor that influences the stretching degree of fissures is not water content, but the gradient of water content, which is mainly governed by the spatial distribution of desorption rate, the permeability characteristic of clod and the size of clod. Under wetting and drying cycles, the fissures of expansive clay will develop gradually, and it is mainly represented by the increase of total area and total length of fissures, but the role of wetting and drying cycles is limited, the development of fissures will cease when it comes to a certain degree due to the undersize of the clods. Expansive soil mass will encounter plastic deformation under wetting and drying cycles; this plastic deformation is not relevant to the swell-shrink characteristics of expansive clay, the development of fissures and the damage of soil mass integrity are the direct causes of this plastic deformation; a higher degree of fissures development gives rise to a more significant plastic deformation

Key words: expansive clay, wetting and drying cycles, evolution law of fissures, fissures ratio, vector diagram