Numerical Investigation into the effects of cavities on stability of slopes and seepage through earth dams under rapid drawdown condition

The stability of dams and natural slopes is a significant and interesting issue and one of the key challenges in the civil engineering field. Cavities forming under earth structures are problematic in the field of geotechnical engineering and may lead to structural damage and loss of life and property. This thesis presents a numerical investigation to evaluate the stability of earth dams and seepage through them, considering the combined impact of the presence of cavities in the subsoil and rapid-drawdown conditions. The current work aims to examine the influence of several parameters related to the cavity – such as the location (varying in X and Y directions), shape and diameter of the cavity, and the number of cavities – on the flow rate and the slope stability of the earth dam. The joint effects on stability of cavities and the strength parameters of slopes (soil cohesion and angle of internal friction) were also investigated and analysed parametrically. A numerical simulation was conducted to analyse seepage and slope stability (by calculating the safety factor [SF] against stability) considering the impact of cavities, using the finite element-based PLAXIS 2D software. This study includes two main stages of analysis. The first was performed employing the Mohr-Coulomb (MC) model to model the slope material and the subsoil. However, in the second stage, the Hardening Soil (HS) and MC models were employed to model the slope material and subsoil, respectively.It is concluded that the presence of cavities under the upstream slope impacts both slope stability and the flow rate through the earth dam dramatically, which causes a considerable decrease in the SF and a notable increase in the flow rate: the SF decreased significantly to 0.715, and this value does not satisfy the minimum limit (1.2–1.3) set by the codes of practice for the stability of dam slopes under rapid-drawdown conditions. The existence of a cavity greatly influences the seepage and stability of the slopes on the upstream side; the SF decreased noticeably by 64.9%, compared to 11.9% on the downstream side, while the flow rate increased to 459.8×10-3m/day, as opposed to 2.572×10-3m/day for a cavity-free model. In addition, the location of a cavity in the X direction has more influence on slope stability and seepage than its location in the Y direction, as the SF and flow rate increase or decrease only slightly when changing the cavity depth. Increasing the cavity diameter resulted in a significant reduction in SF values no matter where the cavity was located, either horizontally or vertically. The existence of two or three cavities at the same depth has more of an influence on the slope’s stability compared to them existing at different depths within the same model. Slope stability in the earth dam during rapid-drawdown conditions increases with an increase in the soil cohesion and friction angle; however, these increases are insignificant compared to the major effects of the cavities on the stability. This influence is even bigger if the cavities are positioned in locations with the maximum effect on stability (at critical horizontal positions). Based on what has been concluded from the simulation related to the effect of cavity shape, this proves that the cavity’s horizontal position has more influence on the dam’s stability and the flow rate than its shape. It is found that using the MC model or HS model has a similar impact on the dam’s stability and, no matter where these cavities are found under the upstream or downstream slope, the type of model used does not reduce the cavity’s influence when it exists in crucial positions. It is concluded that the horizontal position of the cavity is the factor that has the most influence on slope stability and seepage.To summarize, the results of this thesis – obtained through executing numerical analyses – could help to give a better understanding and provide a clearer view of the influence of the presence of cavities in the foundations of slopes and earth dams