دوماهنامه
نوع مقاله : مقالات پژوهشی
نویسندگان
1 دانشجوی دکترای، مهندسی عمران -آب و سازههای هیدرولیکی-دانشگاه مراغه، مراغه، ایران
2 گروه مهندسی عمران- دانشکده فنی و مهندسی- دانشگاه شهید مدنی آذربایجان
3 دانشجوی کارشناسی ارشد، مهندسی عمران - مهندسی آب و سازه های هیدرولیکی دانشگاه شهید مدنی آذربایجان
4 کارشناسی ارشد -گروه مهندسی عمران- خاک و پی -دانشگاه آزاد اسلامی تبریز - تبریز- ایران
کلیدواژهها
عنوان مقاله English
نویسندگان English
: Earth-fill dams with clay cores are pivotal hydraulic structures for potable water supply, agriculture, and flood management; however, over 35% of their historical failures are attributed to seepage and internal erosion. This study focuses on the Nahand Earth-fill Dam (East Azerbaijan Province) and numerically investigates the impact of a 10% increase in the geometric dimensions of the vertical-horizontal chimney drain specifically increasing length from 15 m to 16.5 m and thickness from 1.5 m to 1.65 m using the Finite Element Method (FEM) in GeoStudio 2024. The 10% increment step was selected based on initial sensitivity analysis and reference benchmarks. The numerical model was developed using a mesh of 12,800 quadratic elements, incorporating realistic boundary conditions (upstream hydraulic head of 35 m and downstream head of 5 m). Comprehensive validation was performed using 18 years of instrumentation data from 22 piezometers and 6 observation wells, achieving a coefficient of determination (R2) of 0.982 and a Root Mean Square Error (RMSE) of 0.68 m. The results indicated that this geometric optimization led to: a 22.3% increase in controlled seepage discharge (from 0.008 to 0.0098 m3/sm^3/sm3/s per unit width), a 31.7% reduction in average pore water pressure within the clay core (from 2.180 to 1.123 kPa), and a 9.3% decrease in total hydraulic head loss. Furthermore, the exit gradient remained below the safety threshold of 0.8 in 94.2% of cases, while the Factor of Safety (FOS) for downstream slope stability improved by 25.5% (from 1.45 to 1.82). Additionally, 3D arching effects resulted in an 18.4% reduction in total discharge compared to 2D analysis. Monte Carlo simulation with 10,000 iterations demonstrated a reduction in failure probability to less than 0.3%. Model calibration via the Levenberg-Marquardt algorithm confirmed high precision in replicating the dam’s long-term hydraulic behavior. This cost-effective optimization significantly enhances drainage efficiency and overall structural safety, providing a practical framework with 94.5% generalizability for the design of similar embankment dams.
Keywords: Numerical Modeling, Hydraulic Conductivity, Alluvial Layer, Seepage Flow, Dam Stability
Introduction: Clay-core earth dams are indispensable hydraulic structures for water supply, irrigation, and flood control, yet more than 35% of their historical failures stem from excessive seepage and internal erosion. The hydraulic behavior of alluvial foundation layers, characterized by high permeability and variable thickness, exacerbates these risks by promoting uncontrolled seepage, pore-water pressure buildup, and slope instability. At Nahand Dam (East Azerbaijan Province, Iran), situated on a narrow U-shaped valley with a 16 m thick alluvial layer over Eocene marls, the existing horizontal chimney drain (15 m length, 1.5 m thickness) inadequately mitigates these issues under a 35 m upstream head and 5 m downstream head. Despite advances in numerical modeling, the synergistic effects of drain geometry on seepage control, hydraulic gradients, and stability particularly in heterogeneous 3D geometries remain underexplored. This study addresses this gap by numerically evaluating a targeted 10% dimensional increase in the chimney drain (length to 16.5 m, thickness to 1.65 m) using the finite element method, with the aim of optimizing drainage efficiency, reducing seepage-related hazards, and enhancing overall dam safety for similar structures.
Materials and Methods: The numerical analysis was conducted using GeoStudio 2024 (SEEP/W and SLOPE/W modules) based on site-specific geotechnical and hydrogeological data from Nahand Dam. The 2D and 3D models incorporated realistic geometry (35.5 m dam height, 730 m crest length, central vertical clay core), material properties (Table 3: saturated hydraulic conductivities from 1×10⁻⁹ m/s for the clay core to 5×10⁻² m/s for filters/drains; Table 4: mechanical parameters calibrated via 124 triaxial tests and inverse analysis), and boundary conditions (fixed upstream head of 35 m at normal reservoir level 1607 m asl; downstream head of 5 m). A mesh of 12,800 quadratic triangular elements (2D) and 48,600 tetrahedral elements (3D) was employed, with adaptive refinement ensuring convergence (grid sensitivity: <0.3% change in seepage and pressure). Five scenarios were simulated: baseline (S0: 15 m × 1.5 m) and incremental 10–50% increases in drain dimensions, under steady-state, transient (e.g., 45-day rapid filling, 7-day sudden drawdown), and coupled hydro-mechanical conditions. Validation against 18 years of instrumentation data (22 piezometers, 6 observation wells; R² = 0.982, RMSE = 0.68 m) confirmed model fidelity (98.2% agreement). Uncertainty was quantified via global Sobol sensitivity analysis (10,000 samples) and Monte Carlo simulation (10,000 runs) on key parameters (core permeability, drain dimensions).
Results and Discussion: The 10% increase in chimney drain dimensions yielded substantial hydraulic and stability enhancements. Controlled seepage discharge rose 22.3% (0.008 to 0.0098 m³/s per unit width), facilitating efficient pore-water pressure dissipation, with average core pressures declining 31.7% (2.180 to 1.123 kPa) and total hydraulic head loss reduced by 9.3%. Exit gradients remained below the safe threshold of 0.8 in 94.2% of cases, minimizing internal erosion risk. Downstream slope factor of safety improved 25.5% (1.45 to 1.82) under steady-state conditions, with greater gains (up to 31.5%) in transient scenarios (e.g., rapid filling post-earthquake, PGA = 0.45 g). Three-dimensional arching effects reduced overall discharge by 18.4% compared to 2D models, redistributing flow toward abutments in the narrow valley. Permeability dominated over thickness (745.92% vs. 16.85% seepage impact), as confirmed by Sobol indices (S₁ core k = 0.88–0.91). Monte Carlo results indicated failure probability <0.3%, underscoring the cost-effective nature of this optimization. These outcomes align with and extend prior studies (e.g., Salmasi & Abraham, 2022; Djehiche et al., 2023), providing a validated framework for drain design in alluvial foundations.
Conclusion: This study demonstrates that a modest 10% enlargement of the horizontal chimney drain at Nahand Dam markedly improves seepage management, reduces pore pressures and hydraulic gradients, and bolsters slope stability across steady and transient regimes. The resulting enhancements 22.3% higher controlled discharge, 31.7% lower core pressures, and 25.5% greater factor of safety mitigate internal erosion and uplift risks while accounting for 3D effects. Monte Carlo analysis affirms high reliability (<0.3% failure probability), offering a practical, generalizable (94.5% applicability) blueprint for optimizing drainage in similar clay-core earth dams. These findings advocate for precise, data-driven drain modifications to ensure long-term structural integrity amid evolving hydrological demands.
کلیدواژهها English
