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

Sammanfattning

Temporary excavations in soft clay present significant stability challenges in urbaninfrastructure construction, especially when slopes are steep or kept open for extended periods. Current geotechnical practice assesses short-term stability usingundrained analysis, an assumption that does not account for the time-dependentprocesses that occur as pore pressures dissipate and soil behaviour transitions fromundrained to drained conditions. This transitional phase is well recognized in theory but lacks a consistent framework for practical application, which can lead toeither overly conservative or insufficient stability analysis. This thesis investigatesthe time-dependent slope stability of temporary excavations in soft clay throughcombined numerical modelling. Finite Element analyses were performed in PLAXIS2D to simulate the generation and dissipation of negative pore pressures followingexcavation, which were subsequently exported to SLOPE/W where Limit Equilibrium analyses were conducted to evaluate the factor of safety over time. Three slopegeometries with inclinations of 1:1, 1:1,5 and 1:2 were analysed. A sensitivity analysis was further conducted to evaluate the influence of key material parameters on theresults. The results show that negative excess pore pressures generated due to stressrelief during excavation provide a temporary but significant stabilising contributionin short term. As consolidation progresses, these pore pressures dissipate and thefactor of safety gradually converges towards long-term drained values. Slope geometry was found to govern both the magnitude and the rate of this transition: steeperslopes reach drained conditions more rapidly, while flatter slopes maintain shortterm stability over longer periods. The undrained assumption was found to be validfor 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 wasidentified as the most critical parameter for stability throughout the consolidationperiod, while the Young’s modulus and permeability governs the rate of transition.The results further show that a traditional combined analysis without accountingfor transient negative pore pressure underestimates short-term stability comparedto the method in which distribution of pore pressure was included. Overall, theresults demonstrate that the transition from undrained to drained conditions is agradual process, and that capturing the transient pore pressure response is essentialfor accurately representing the temporary stabilising effect of excavations.