Spatial variability of soil undrained shear strength is usually not rigorously considered in the design of basal heave for deep excavations. In this study, the slip circle method is employed to investigate how the required safety factor against basal heave was affected by spatial variability in the context of reliability-based design. The non-stationary random field model is adopted to model spatial variability of undrained shear strength. Results show that the required safety factor obtained with the consideration of spatial variability is much smaller than that without the consideration. Parametric studies show that the vertical scale of fluctuation has a significant influence on the required safety factor: the longer the scale of fluctuation, the larger the required safety factor. For target failure probabilities of 0.01 and 0.001, the corresponding required safety factors are in the ranges of 1.4-1.9 and 1.6-2.5, respectively, for the average value of vertical scale of fluctuation of 2.5 m.