3-D seepage analyses of earth dam repairs using slurry cutoff wall

The studied earth dam was remediated works for seepage and instability problems in the downstream area. The effectiveness of slurry cutoff walls was investigated on seepage behavior. The three-dimensional (3-D) finite element (FEM) analysis at normal high-water level (NHWL) for steady-state flow was established without and with a cutoff wall. Based on the results, the overall effectiveness of the cutoff wall was 85%, which exceeded the predicted design efficiency criterion of 75%. The embankment, residual soil, and phyllite were the three major materials that mainly intercepted by cutoff wall, with decreasing flows of 72–89%, 85–94%, and 55–94% respectively. The phyllite and quarzitic schist below cutoff wall coverage, the flows increased by 179% and 33–61%. Thus, extension of the cutoff lower tip to cover the full depth through phyllite and into the quarzitic schist is highly recommended. The cutoff wall effectiveness reduced potential wet areas on downstream area by 97%, indicating lower risk of seepage erosion. Major reductions in the phreatic line drop were observed of 12–20 m, indicated the hydraulic gradient across the wall thickness of 2.00–3.33 which are less than 4.00 against wall blowout. The piezometric heads of the existing field monitoring are higher than the model without the cutoff wall indicated the possibility of internal erosion. However, this would be clearly solved after cutoff wall construction. The estimated newly installed piezometer heads across the cutoff wall are planned. The model estimates the heads from dam axis to downstream area would lower from 160 to 140–145 m Mean Sea Level (MSL). That would certainly increase the safety on seepage and stability of the downstream slope.