STABILITY ANALYSIS OF SLOPES IN EASTERN GHATS, ODISHA, INDIA

Abstract

The Eastern Ghats Mobile belt is aligned parallel to the Eastern coast of India and belongs to the Archean-Proterozoic era. With time, the rocks have undergone a high grade of Amphibolite type of metamorphism. It results in the Khondalite and Charnokite series of rocks, suggesting the granular texture within the rocks. However, the tropical climate and favorable geomorphological aspects allow the formation to witness laterization process. Therefore, the weathering action is very prominent in the region. The weathering altered the mineralogical assemblage in the formation and reduces the geomechanical properties of the rocks. It causes instability of slopes (natural and excavated) for the region. Several major landslides are reported in different parts of the Eastern Ghats every year particularly during monsoon and cyclonic seasons. The debris slide, rockfall, and rolling out of boulders are the type of failure events noticed along the excavated slopes. As lateralization is more dominant in the region, debris slide is frequently observed than rockfall and rolling out of boulders from the cut-slopes. Also, with the increase in high-intensity rainfall events in a year, landslide frequency is increased. The Koraput-Rayagada railway line, passing through the Eastern Ghats, is severely affected due to landslides along the railway cut slopes. It is known as the lifeline in the region for communication and transportation. However, due to frequent landslides, railway accidents and halting of traffic are very common. Also, landslide dams are formed due to the failure of natural slopes along the narrow mountainous channel of the region, which further triggered flash floods in the downstream area. Therefore, systematic investigation is required to study the vulnerable region and advice for the remedial measures. Also, real-time monitoring and prediction are equally essential to reduce socio-economic losses. In the present study, the landslide investigation was carried out along the Koraput Rayagada railway track. Also, a case study of a landslide dam from the Eastern Ghats has been studied. Before the field investigation, a detailed desk study has been carried out with a thorough literature review and data collection of past landslide history for the region. Infield investigation, demarcation of vulnerable slopes was the first priority, followed by each slope's structural and geotechnical mapping. Further representative samples were collected to determine the geotechnical properties of the slope masses. Different testing approaches were adopted in the laboratory according to the type of material and the requirement of parameters. Integration of field observation and laboratory data was used in the numerical model to determine the failure pattern and displacement of material from the slope. In slope stability analysis of rock slopes, identifying the mechanism behind slope failure is the main focus of the study. Rock mass characterization and Kinematics analysis, like geotechnical investigation methods, are not infallible all the time for the weathered slopes. Different grades of weathered mass on the slope profile act independently to influence the overall failure pattern. Numerical analysis by Finite Element Method (FEM) is evolved as a primary slope stability analysis tool by overcoming the limitations of the stratified weathered materials on the slope. The study focuses on the stability analysis of railway-cut laterite slopes of the Eastern Ghats of India. The medium to high-grade metamorphism, followed by Physio Chemical weathering, favors the region for more prone to failure. The Force Equilibrium Model and Moment Equilibrium Model have been considered to determine the possibility of independent failure events for the probability analysis. Monte Carlo’s probability simulation method has delivered a better result for the weathered slopes by taking many variables for the studied region. A compression study has been made between the outcomes of slope stability analysis methods and the probability analysis models, which provides a comprehensive list of factors to understand the failure mechanism of weathered slopes. Inherent geomechanical properties of earth materials are primarily controlled by several extraneous factors, among which weathering processes and their end products play an essential role. Due to intense rainfall and high temperature, the chemical weathering of rocks/debris is relatively more profound in tropical and sub-tropical regions. Hence, it is vital to understand the impacts of the end products of weathering, such as swelling and non-swelling clay minerals, on the geomaterials' geotechnical properties. The present study highlights the effect of these end products on the geotechnical or geomechanical properties of earth materials. Eighteen vulnerable railway cut debris slopes in India's tropical eastern coastal region were selected for the detailed geotechnical study. Recognizing the importance of distinctive geographical location, meteorological, environmental conditions, the shear strength reduction factor was determined under the dry as well as wet conditions. It has been found that all studied slopes are vulnerable to failure (under soggy conditions) and may fail shortly. The detailed insights of controlling parameters have been discussed in the article. Various geotechnical tests, including sieve analysis, atterberg limits, direct shear tests, were conducted on the representative soil/debris samples. The geotechnical parameters have been correlated with the geochemical results derived from the X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. Quantification of weathering has been further used to describe the behavior of geotechnical properties in a saturated condition. ii The analysis chronicles the landslide dam burst event from the Eastern Ghats region of India during the Titli cyclone on 12th October 2018. The event has been marked as one of the most destructive events in the region due to the abrupt increase in rainfall intensity and duration. The highly weathered profile of the Eastern Ghats favors such landslide dam triggered flash flood incidents, mainly during the monsoon and cyclone. Geological and Geotechnical investigation of the region followed by numerical modeling has been done to study the cause and mechanism of the incident. The failure has been simulated from the landslide initiation to the dam formation and afterward in the Optum G2-based finite element model. The boulder matrix layer of steep laterite cliff is identified as the primary concern for the geotechnical analysis in the Eastern Ghats mountains. Real-time monitoring of natural dams is essential to minimize the amount of risk in the region from the further occurrence of such landslide-related terminal hazards. Rainfall-induced landslides have always been one of the most frequent natural hazards for India. External factors like climate and topography can be blamed for their occurrence. The prevailing sub-tropical climate of the Indian sub-continent is probably the more favorable element behind the abrupt increase in the frequency of rainfall-induced landslides. The maximum number of landslides is observed during the month of June-September, i.e., monsoon season, followed by post-monsoon season (October-December). North-western Himalaya, North-Eastern Himalaya, Western Ghats, and the Eastern Ghats are the four major zones, demarcated as the most landslide-prone regions of India. The present study has been focused on the derivation of rainfall thresholds for different landslide-prone regions of India to trigger a landslide. The Rainfall Intensity-Duration (ID) thresholds and the antecedent rainfall thresholds were estimated by using the TRMM rainfall data. The “Frequentist Statistic Method” has been applied to derive the Rainfall Intensity-Duration (ID) thresholds for all the landslide-prone regions of India by taking the past landslide data of 2007-2016. Further, the Rainfall Intensity-Duration (ID) thresholds were validated with the landslide data of 2017 2019. Cumulative rainfall of 2, 3, 4, 5, 10, 15, 20, 25, and 30 days had been compared with the total rainfall on the day of failure to derive the antecedent rainfall thresholds. The Ghats regions of India required heavier rain to trigger landslides as compared to the Himalayan regions. Antecedent rainfall of 5 days in the Himalayan region has anticipated the failure of slopes; however, in the Eastern and Western Ghats, the desired antecedent rainfall days is 10.