The shear strength at a point in soil mass A is denoted by τf(x,z), where x and z are the horizontal and vertical coordinates, respectively. The friction angle φ is taken to be 0° in this study for simplicity, i.e., τf(x,z) = c(x,z). The shear strength τf(x,z) is simulated as a stationary Gaussian random field with inherent mean E(τf) = μ and inherent standard deviation [Var(τf)]0.5 = σ. The Coefficient Of Variation (COV) of this random field is equal to σ/μ. To define the correlation structure of τf(x,z) between two locations with horizontal distance = Δx and vertical distance = Δz, an auto-correlation model is considered in this study: the single exponential model (SExp) (Vanmaqrcke 1977, 1984). And the two-dimensional (2D) stationary Gaussian random field τf(x,z) can be readily simulated by the Fourier series method (Jha & Ching 2013).