Stability Analysis of Upstream and Downstream Dam Slopes with Water Level Drawdown Using Response Surface Function

This paper presents a framework combining uncoupled slope stability analysis with second order polynomial response surface function to fully investigate the stability and reliability of upstream and downstream dam slopes (denoted by UDS and DDS, respectively) during water level drawdown period. The transient seepage analysis with Seep/w is followed by a traditional limit equilibrium method with Slope/w to obtain the minimum factor of safety (denoted by FS_min) at a given time step t and the failure probabilities of UDS and DDS at the time step T_m when a minimum FS_min is achieved are calculated and compared to provide much insights. The proposed methodology is illustrated against a typical soil dam. The comparative studies have indicated that for a given drawdown rate v, the FS_min of UDS first drops dramatically as t increases from 0 to T_m and then it increases gradually to a constant value at T when the steady seepage field is achieved. The FS_min of UDS at T_m decreases significantly as v increases. Both T and T_m values decrease as v increases for UDS. An identical value of FS_min is observed for all v values at their respective T values for a specific drawdown range for both UDS and DDS. The FS_min of DDS first increases significantly and then increases slightly and finally remains at a constant value. Compared with the FS_min at T_m, the failure probability of UDS indicates more significant influence of v on stability of UDS. As v and/or drawdown range increases, the failure probability of UDS has a potential to exceed that of DDS demonstrating the necessity and importance of considering both UDS and DDS in the case of water drawdown period. The cross influence of slope angle of UDS and DDS on the stability of UDS and DDS is found and this influence should be taken into account for stability analysis of dam slopes during water level drawdown conditions.