The Impact of Groundwater on the Landslide Occurrence in the Southern Slope of Shah Neshin Mountain

ABSTRACT
Landslides are a critical geohazard, often triggered by seismic activity, intense rainfall, and water table fluctuations. Understanding the complex interplay of these factors is a key task in hazard assessment and mitigation strategies. This comprehensive study investigates the regional potential for water flow dynamics, displacement mechanisms, and landslide genesis. Based on a dual approach of rigorous fieldwork and meticulous laboratory analysis, approximately 5 kg of fine-grained clay and 100 kg of coarse-grained material were collected for in-depth mechanical property testing. These investigations focused primarily on assessing Atterberg limits and shear strength characteristics. The results showed a striking correlation between the water table and the site's structural integrity. In particular, the marl and alluvial layers exhibited a significant decrease in resistance, ranging from 40% to 55% and 60% to 80%, respectively, in different regions of the study area. In addition, the cohesion of the layers decreased with increasing slope steepness, resulting in a reduction in internal friction angles. This empirical evidence highlights the region's susceptibility to increased landslide risk, particularly in precipitation-induced surface water infiltration and potential seismic upheaval, such as the powerful 7.3 magnitude earthquake in Sarpol Zahab in 2017. These combined factors highlight the imminent threat of landslides and call for proactive risk management and disaster preparedness measures
 
Extended Abstract
Introduction
The landslide on the southern slope of Shahneshin Mountain (Male Kabud) lies within the western region of Kermanshah province, positioned between 45°53' to 45°54' east longitude and 34°31' to 34°33' north latitude, covering an area of 6 square kilometers, this landslide occurred approximately 8 kilometers north of Sarpol-Zahab city. This area's geological structure, influenced by tectonic activities and various climatic periods, has shaped distinct landforms as rocky outcrops, fractures, sharp ridges, valleys, wall-like abysses (known as Giloi), dolines, karens, screes, and springs. Abundant springs and water seepage along valley sides and slopes suggest a proximity of groundwater to the surface, notably visible as springs emerging from joints and cracks during heavy rainfall. This area is situated in the tectonically active Zagros earthquake zone and experiences significant seismic activity. The mountainous terrain, unique geological compositions, and alternating layers of marl and limestone create a predisposition for landslides in this region. These conditions form a landscape where the convergence of factors sets the stage for landslide occurrences.
 
Methodology
This article aimed to assess the impact of groundwater on potential landslide occurrences and their influence on underlying formations. It delved into various parameters, including surface water dynamics, cracks, faults, groundwater flow, and geotechnical soil properties. The investigation relied on a comprehensive analysis combining field operations and laboratory studies. Identifying the area's cracks and faults was crucial to understanding surface water infiltration and groundwater flow patterns. Satellite imagery facilitated fault direction measurement, while Branton's compass aided in assessing seams, cracks, and primary ruptures resulting from recent landslides. These observations were then translated into directional and slope representations via rose diagrams using Stereonet software. This multifaceted approach allowed for a comprehensive evaluation of the terrain's vulnerabilities and potential triggers for landslide occurrences.
In the subsequent phase, the influence of groundwater on the geotechnical properties of soils underwent a thorough examination. This involved assessing Atterberg limits, shear resistance, adhesion, and internal friction characteristics. To evaluate the alluvial materials and marl beds, samples were obtained both in their dry state and following a 40 mm rainfall event on the slip surface. Laboratory analysis focused on geotechnical features such as Atterberg's limits encompassing shrinkage, plastic, and liquid limits; tests for plastic and liquid limits adhered to Iranian Standard Number 10731. Additionally, direct cutting tests were conducted to measure parameters like adhesion (c) and friction angle (φ). Results from assessing Atterberg's limits and shear strength of alluvial materials and marl beds revealed a noteworthy trend as moisture content increased; the beds exhibited decreased resistance, particularly in marl layers. Given the prevalence of marl formations across numerous sections of the study area, the potential impact of groundwater on slope stability warrants comprehensive investigation.
 
Results and discussion
"One of the pivotal factors influencing the occurrence of significant landslides on the southern slope of Shah Neshin Mountain is the presence and influence of groundwater. The stratigraphic column of the studied area comprises limestone-dolomite and marl beds affected by faults, joints, and cracks. This unique combination, including the almost pure Asmari carbonate formation sequences, intense tectonic activity, and numerous joints and fissures, has fostered the development of karst formations within the mountain. The alternation of permeable limestone and impermeable marl beds results in surface water infiltrating through the limestone layers and accumulating on the impermeable marl beds. This water accumulates atop impermeable marl beds, heightening instability within sensitive marl formations and amplifying the risk of landslides. Moreover, the recent landslide-induced cracks, expanding the waterway network, predominantly perpendicular to their direction, impede surface water flow. Consequently, these cracks exacerbate surface water infiltration into the ground, leading to aquifer formation on marl layers, providing ideal conditions for slip occurrences due to their low resistance against water infiltration.
The region exhibits a rich presence of groundwater, notably influencing recent landslides and potentially affecting future occurrences. Laboratory analyses on alluvial materials and marl beds confirm a significant decline in resistance attributed to groundwater, particularly pronounced in marl layers. This diminished resistance contributes to landslide occurrences, especially during seismic events and heavy rainfall. The fracture system within the region directs water flow towards lower beds, raising groundwater levels, resulting in the emergence of springs and limited vegetation development. Furthermore, laboratory results indicate a notable decrease in resistance, especially in marl beds, due to water presence, further exacerbating landslide risks in the area. Tectonic movements and earthquakes compromising lower beds' shear resistance and changes in pore water pressure sans precipitation escalate landslide probabilities. The combination of created gaps and preserved waterways amplifies landslide risks during heavy rainfalls or seismic activities akin to the 1918 earthquake. Consequently, this intricate interplay of geological factors and water dynamics accentuates the area's vulnerability to landslide occurrences. "
 
Conclusion
"The presence of dense and brittle limestone-dolomite beds, coupled with faults, joints, and fractures within these rock formations, facilitates water infiltration and downward movement into lower beds. Consequently, these waters accumulate on the impermeable marl layers, creating conditions conducive to forming aquifers. A visible manifestation of this process is the emergence of springs and water seepage across the region. The influence of these waters extends to diminishing the resistance of the beds, particularly the marl beds. Laboratory tests assessing Atterberg limits and soil shear strength from both marl and alluvial layers reveal a notable trend as the beds exhibit decreased resistance as moisture content increases. Additionally, with heightened slope angles, adhesion decreases while the angle of internal friction increases. These combined conditions create a favorable environment for landslide occurrences."
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.