The failure mechanism and sliding surface are significant for calculating the safety factor of deep excavations. In this study, the discontinuous layout optimization (DLO) method is used to investigate the influence of the increase in soil strength with depth and the width of the excavation on the basal heave failure mechanism. First, the DLO results show that with the soil strength increasing linearly with depth, the sliding surface extension downward is limited. The sliding surface is located below the toe of the retaining structure when the embedded ratio D/H is small, which is different from the traditional assumption. The logarithmic relationship between the normalized sliding radius R/(D + h) and the embedded ratio D/H is obtained. This relationship provides a reference for determining the size of the sliding surface and solving the problem that the safety factor decreases as the embedded ratio increases in Shanghai’s local code. Furthermore, the safety factor Fs first decreases and then gradually becomes stable as the width-to-depth ratio B/H increases. When Fs reaches a stable value, B/H is taken as the critical width-to-depth ratio (B/H)s to classify the wide and narrow excavations, and the linear relationship between (B/H)s and D/H is given.