The purpose of this study is to understand the mechanism of the overall shear strength for a soil mass in the presence of spatial variability. Random field finite element analyses are used to simulate spatial average shear strengths and overall shear strengths that govern the failure of a soil mass. Based on the simulation results, it is found that the statistics of spatial average strengths can be estimated by Vanmarcke's theory. However, the overall shear strength is found to be close to the average shear strength along the actual slip curve, rather than the spatial average shear strength over the entire soil mass. A probabilistic model is proposed for the average shear strength along the actual slip curve. This model considers the effects of different stress states and orientation of the spatial variability. The model is further validated by the overall shear strengths simulated by the random field finite element analyses.
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