Seismic Performance Modeling in Earth-Fill Dams with Geotechnical Anomalies: A Comparative Study of MCC and Mohr-Coulomb Approaches

Abstract

This study presents a seismic analysis that includes comprehensive geotechnical and numerical assessment of aging earth-fill dams (Al-Moshanaf

 earth-fill dam) in southern Syria where expansive clay soils is widespread, that dam has experienced upstream slope failure in recent years. Field sampling and laboratory testing were conducted to evaluate soil variability across dam zones, revealing significant heterogeneity in consolidation and swelling behavior. Using field investigations, lab tests, and simulation techniques, the study characterizes compressibility behavior, variations in moisture content, unit weight and specific gravity in geotechnical anomalies zones across the dam. Twelve representative samples underwent consolidation testing (ASTM D4546) and (ASTM D2435), showing variability in compression indices (Cc = 0.23–0.37) and swelling ratios (Cs/Cc = 5–56%), between samples located directly under upstream face and samples from inside dam body at the same level, exhibiting significant fine particles loss and lower moisture content due to long-term exposure and seepage-induced suffusion. To simulate the dam's response under different loading conditions, advanced numerical modeling using FLAC3D v6 was performed, employing both Mohr-Coulomb and Modified Cam-Clay (MCC) constitutive models. Interface elements were introduced to simulate inter-zonal discontinuities. Static analysis under gravity and reservoir loading revealed that zones with high compressibility and swelling index ratios underwent substantial displacements. Results revealed settlements up to 14.4 cm under gravity and reservoir loads, with effective stress reductions up to 62% when pore pressure effects were included. Dynamic simulations under seismic loading showed, that in the case of applying elastic constitutive model, dam failure occurs after 5 sec and acceleration is amplified 42 times at dam crest, while in the case MCC model, failure occurs after 1.88 sec and acceleration is amplified 3.7 times at the crest which. by comparing both cases, elastic constitutive model analysis showed maximum displacements toward downstream, and at dam crest 3.5m, toe 7.8m, while at dam heel displacements were toward upstream by 0.8m.  in the case of MCC model, displacements occur in toward downstream, and at dam crest 0.65m, toe 1.2m, while at dam heel the maximum displacements occur toward upstream by 32m, which is the more realistic results taking into consideration static analysis and actual field conditions. as a result, the MCC model better replicated the observed deformation behavior, including plastic displacements and collapse, supported by laboratory-derived parameters. This study demonstrates that spatially resolved material zoning improves predictive accuracy, validates the use of MCC for seismic analysis in heterogeneous dam structures, and establishes a reliable methodological foundation for future dynamic safety evaluations.