Time-dependent slope stability of temporary excavations in soft clay: A numerical study of the undrained to drained transition using combined analysis

Abstract

Temporary excavations in soft clay present significant stability challenges in urban infrastructure construction, especially when slopes are steep or kept open for ex tended periods. Current geotechnical practice assesses short-term stability using undrained analysis, an assumption that does not account for the time-dependent processes that occur as pore pressures dissipate and soil behaviour transitions from undrained to drained conditions. This transitional phase is well recognized in the ory but lacks a consistent framework for practical application, which can lead to either overly conservative or insufficient stability analysis. This thesis investigates the time-dependent slope stability of temporary excavations in soft clay through combined numerical modelling. Finite Element analyses were performed in PLAXIS 2D to simulate the generation and dissipation of negative pore pressures following excavation, which were subsequently exported to SLOPE/W where Limit Equilib rium analyses were conducted to evaluate the factor of safety over time. Three slope geometries with inclinations of 1:1, 1:1,5 and 1:2 were analysed. A sensitivity analy sis was further conducted to evaluate the influence of key material parameters on the results. The results show that negative excess pore pressures generated due to stress relief during excavation provide a temporary but significant stabilising contribution in short term. As consolidation progresses, these pore pressures dissipate and the factor of safety gradually converges towards long-term drained values. Slope geome try was found to govern both the magnitude and the rate of this transition: steeper slopes reach drained conditions more rapidly, while flatter slopes maintain short term stability over longer periods. The undrained assumption was found to be valid for approximately 5 days for the 1:1 slope and 15 days for the 1:1,5 and 1:2 slopes, after which a combined analysis should be applied. The effective friction angle was identified as the most critical parameter for stability throughout the consolidation period, while the Young’s modulus and permeability governs the rate of transition. The results further show that a traditional combined analysis without accounting for transient negative pore pressure underestimates short-term stability compared to the method in which distribution of pore pressure was included. Overall, the results demonstrate that the transition from undrained to drained conditions is a gradual process, and that capturing the transient pore pressure response is essential for accurately representing the temporary stabilising effect of excavations.