فصلنامه
نوع مقاله : مقالات پژوهشی
نویسندگان
1 گروه زمینشناسی، دانشکده علوم زمین، دانشگاه شهید چمران اهواز، اهواز، ایران
2 شرکت آب منطقهای تهران، تهران، ایران
کلیدواژهها
عنوان مقاله English
نویسندگان English
Introduction
Water conveyance tunnels are critical infrastructures for sustainable water supply, particularly in arid and semi-arid regions. However, tunnel excavation can significantly alter groundwater systems, resulting in potential hazards such as sudden groundwater inflow, reduction in aquifer storage, and deterioration of groundwater quantity and quality. In fractured and karstic environments, these impacts may extend over considerable distances and potentially modify regional hydrogeological regimes.
To address these challenges, various predictive approaches have been developed, including numerical modeling, parametric methods, hybrid techniques, and empirical models. Although numerical models can provide detailed insights into groundwater responses to tunnel excavation, their application generally requires extensive datasets and complex calibration procedures, which may limit their applicability in heterogeneous geological settings. Consequently, parametric and empirical approaches, such as the Drawdown Hazard Index (DHI) and Tunnel Impact Score (TIS), have received increasing attention because of their flexibility, simplicity, and suitability for preliminary assessments and real-time decision-making.
The Hezarmasjed water conveyance tunnel, located in northeastern Iran within the Kopet-Dagh tectonic zone, traverses a highly fractured carbonate system hosting numerous springs. Given the dependence of local communities on these springs, assessing the potential impacts of tunnel excavation is essential. This study aims to identify the main controlling factors governing variations in spring discharge and to evaluate spring vulnerability using both the DHI and TIS approaches.
Materials and Methods
The study area is situated in a structurally complex mountainous region characterized by active tectonics, heterogeneous lithology, and significant hydrogeological sensitivity. The tunnel, approximately 8.8 km in length, intersects several geological formations, including carbonate units (Mozduran and Tirgan) acting as primary aquifers, and shale-dominated formations with low permeability. Groundwater flow is predominantly controlled by fracture networks and karstification processes, resulting in rapid hydrological responses to precipitation events.
A total of 45 springs were investigated at varying distances from the tunnel axis. Field data included hydrogeological observations, Lugeon permeability tests, and structural analyses. Permeability values generally ranged from 1E-7 to 2.5E-6 m/s, indicating low to moderate hydraulic conductivity, with higher values in fractured and karstified zones.
Two complementary methods were employed for impact assessment. The DHI method (Dematteis et al., 2001) is a semi-quantitative parametric approach based on seven key parameters, including fracture frequency, rock mass permeability, overburden thickness, plastic zone radius, fault–spring interaction, spring type, and distance from the tunnel. These parameters were normalized and combined to calculate a continuous index representing the risk of spring discharge reduction.
The TIS method (Hassanpour et al., 2021) is an empirical approach that evaluates spring vulnerability based on four main factors: tunnel water inflow (40% weight), hydraulic connectivity (40%), distance from the tunnel (15%), and aquifer recharge capacity (5%). The method provides a classification of impact severity, ranging from negligible to significant discharge reduction.
Results and Discussion
The DHI results indicate that spring vulnerability is primarily controlled by structural and hydrogeological parameters, particularly fracture density and permeability. Calculated DHI values range from 0.1 to 1.5, demonstrating a wide spectrum of potential impacts. Approximately 11% of springs are classified as highly to critically affected (DHI ≥ 0.6), while the majority (about 89%) are expected to experience minor to moderate discharge reductions. Springs with high DHI values are typically located in highly fractured carbonate formations with strong hydraulic connectivity, where tunnel excavation acts as a drainage boundary, intercepting groundwater flow paths (Vincenzi et al., 2022; Saberinasr & Dashti Barmaki, 2023).
In contrast, springs located in low-permeability formations or at greater distances from the tunnel exhibit minimal sensitivity, with DHI values below 0.2. More than 70% of springs fall within an intermediate range, indicating moderate but persistent reductions due to partial hydraulic connectivity.
The TIS results highlight the importance of dynamic hydrogeological parameters, particularly tunnel water inflow and hydraulic connectivity. TIS values range from 1.6 to 6.9, reflecting significant variability in system response. Approximately 44% of springs are classified as unaffected, 44% experience moderate discharge reduction, and around 12% are subject to significant decline.
High-risk springs, such as Sirzar and Bamchenar, exhibit strong hydraulic connectivity and are located within sensitive recharge zones. Notably, the results indicate that hydraulic connectivity can dominate system behavior even at distances exceeding 1000 m, confirming previous findings that connectivity is more influential than geometric proximity (Hassanpour et al., 2021). Conversely, springs located in areas with higher recharge capacity and weaker hydraulic connectivity show minimal impact, even when tunnel inflow is relatively high. This behavior reflects the buffering capacity of aquifer systems, which can partially compensate for tunnel-induced drainage.
A comparative analysis of the two methods demonstrates that the DHI approach emphasizes intrinsic geological and structural conditions, providing a continuous representation of vulnerability, whereas the TIS method focuses on functional aquifer responses and offers a more practical classification of impact severity. The consistency between both methods in identifying high-risk springs supports the reliability of the integrated approach.
Conclusion
This study demonstrates that tunnel excavation can significantly affect groundwater systems, particularly in fractured and karstic aquifers. The integrated application of DHI and TIS methods provides a comprehensive framework for assessing these impacts.
The DHI method is effective for identifying intrinsic vulnerability and worst-case scenarios, highlighting the role of geological structures, permeability, and fracture density. Approximately 11% of springs are at high to critical risk, while most are expected to experience moderate impacts.
In contrast, the TIS method offers a more operational perspective by incorporating dynamic hydrogeological parameters, classifying springs into significant, moderate, and negligible impact categories. The results emphasize the dominant role of hydraulic connectivity and aquifer recharge capacity in controlling system response.
Overall, the combined use of parametric and empirical approaches enhances the reliability of impact assessments and provides a robust basis for groundwater management in tunnel projects. This integrated methodology is particularly suitable for complex hydrogeological environments where data limitations restrict the application of numerical models.
Acknowledgement
The authors gratefully acknowledge the Fanavaran Tarh Jame Consulting Engineers Company for providing essential datasets, technical documentation, and field measurements that enabled a comprehensive and reliable analysis of the hydrogeological system. Their professional support played a crucial role in the successful completion of this study.
کلیدواژهها English
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