FISSURE DEVELOPMENT AND STRENGTH DETERIORATION OF AN EXPANSIVE SOIL EXPOSED TO WETTING-DRYING AND FREEZING-THAWING CYCLES

Expansive soil is highly sensitive to weathering loading such as water infiltration and evaporation, or temperature changes(wetting-drying and freezing-thawing cycles), making it prone to cracking and thus degradation of its mechanical properties. To demonstrate the development of cracking and shearing performance as well as the association between the two, laboratory testing programs on expansive soil exposed to various wetting-drying(WD), freezing-thawing(FT), and wetting-drying after freezing-thawing(WD-FT)cycles were carried out, incorporating a 3D laser scanning and a direct shear device. According to the laboratory results, WD, FT, or WD-FT weathering has a significant impact on cracking development and shear properties of the expansive soil, and the degree of influence showed a non-linear superposition, with obvious coupling effects. Among those, WD weathering has the most significant effect, followed by WD-FT weathering, and FT weathering has the least effect. Both cohesion(c) and internal friction angle(φ)nonlinearly decreased with increases in the number of WD, FT, and WD-FT cycles, and the data points were in a wide distribution band. The fissure rate, D, and the percentage of fissure rate close to the shear surface, D′, were introduced, and the data points of c-D and φ-D′ relations of the expansive soil under different weathering loading show a gentle-steep-smooth change, collapsing into a single curve. Accordingly, a direct link between fissure and shear strength was established, and a formula for shear strength of the expansive soil was proposed by considering the effect of fissure. There was a good comparison between the calculated and measured data that establishes the rationality and validity of the proposed formula.