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The collapse of gully walls is an important mode of gully erosion, and it depends strongly on two soil shear-strength parameters: cohesion and the internal friction angle. However, little research has examined these parameters in China's black soil region. In the present study, we sampled six groups of surface soils from (our different land uses, and two groups of soils at depths of 30cm and 60cm. The cohesion (C) and internal friction angle (p) of each group were determined at 10 levels of water content. The results show that c increased with increasing water content until a certain level (12% for the surface soils, and 14% for the deeper soils), after which c decreased. Internal friction angle generally decreased with increasing water content. We developed a model predicting collapse of gully walls based on (our parameters: dry bulk density, water content, crack depth, and the width of the soil collapse. The model predicts that soil collapse would occur at two levels of water content: dry and wet conditions. Field data showed that collapse only occurred under the wet condition; the critical mass water content given by the model is between 31.0% and 33.8% moisture. [ABSTRACT FROM AUTHOR]