Full-Scale Experimental Evaluation of Internal HDPE–PP Cut-Off Reinforcement for Mitigating Seepage–Overtopping Levee Failure

Seepage prior to overtopping can weaken levees and accelerate breach development under prolonged high-water conditions. This study examined a full-scale sand levee reinforced with an internal high-density polyethylene–polypropylene (HDPE–PP) cut-off wall under sequential seepage–overtopping conditions. A single reinforced levee was tested using the same geometry and a comparable experimental procedure to those of previous unreinforced experiments. An inflow of 1–2 m3/s was supplied for approximately 40 h to induce seepage, followed by increased inflow to induce lateral overtopping. Pore-water pressure, distributed temperature sensing (DTS), and visible and thermal imaging were used to characterize hydraulic responses and failure progression. The reinforced levee showed lower landside pore-water pressure and no visible seepage emergence during the seepage stage. During overtopping, visible surface erosion progressed more slowly than in the previous unreinforced seepage–overtopping case, although it remained greater than in the overtopping-only case. After exposure, the HDPE–PP cut-off wall temporarily obstructed overtopping flow, while local loss of wall continuity was followed by concentrated flow and rapid erosion. These findings indicate that HDPE–PP cut-off reinforcement can mitigate seepage-related deterioration and delay subsequent overtopping failure under the tested conditions, although the adverse influence of prior seepage was not completely eliminated.