Seepage-Induced Piping Risk At The Downstream Toe: A Stability And Vulnerability Assessment Of The Bohol Qawlo Cfrd, Somaliland

Abstract

Water scarcity is a persistent challenge in arid regions like Somaliland, necessitating the development of vital infrastructure such as the Bohol Qawlo Embankment Dam in the Gabiley Region. However, constructing hydraulic structures on permeable sandy colluvium foundations introduces significant geotechnical risks, particularly regarding seepage control and internal erosion. This study evaluates the performance of the Bohol Qawlo Concrete-Faced Rock fill Dam (CFRD) by investigating its seepage behavior, slope stability, and load-deformation characteristics under critical loading conditions. The research utilized the GeoStudio 2018 numerical modeling suite, employing the Finite Element Method (SEEP/W and SIGMA/W) and the Limit Equilibrium Method (SLOPE/W). The seepage analysis confirmed that while the upstream concrete face effectively serves as an impermeable barrier, a critical hydraulic vulnerability exists at the downstream toe. A peak seepage flux of 2.4× 10-4 m3/sec/m2 was recorded at the interface between the dam body and the colluvium foundation. In the absence of a transition filter, this high-velocity flux presents a high risk of particle migration, internal erosion (piping), and progressive foundation washout. Conversely, the structural slope stability analysis demonstrated robust performance, with the Factor of Safety (FOS) for the downstream slope calculated at 1.490 for End-of-Construction, 1.731 for Long-Term Steady State, and 1.477 under seismic loading. These values significantly exceed the minimum safety criteria established by the U.S. Army Corps of Engineers (USACE). Deformation analysis further revealed high structural stiffness, with a maximum vertical settlement of only 0.024 m (0.15% of the dam height). The study concludes that while the dam is mechanically stable, immediate remedial measures, specifically an engineered toe drain with an inverted filter, are required to mitigate hydraulic risks and ensure long-term sustainability.