Most of the soil encountered in engineering exists in the form of unsaturated soil.Taking the special red clay in Hainan area as the research object

based on research into the properties of soil and water of the red clay

it is considered that Van Genuchten model reflects better the relationship between saturation and base material of red clay in Hainan.Combined with the expressions of the effective stress of unsaturated soil proposed by Bishop

the development mechanism of swell-shrink ground fissure under rainfall and evaporation was analyzed.Numerical example shows that for unsaturated soil

the coefficient of at rest earth pressure is not a constant

but varies with the depth and steady flow

which is different from that in saturated soil.In practice it has an important guiding significance for the supporting structure design.At the same time

because the coefficient of earth pressure at rest has negative values in unsaturated soil

swell-shrink ground fissure is prone to produce.And in the evaporation condition

theoretical maximum ground fissure depth is greater than that in the absence of ground seepage and rainfall conditions.