Consequences of slope instability and existing practices of mitigation in hydropower projects of Nepal

Abstract

Introduction

Nepal has witnessed several instances of slope instability associated with Hydroelectric Projects in the recent decades. Despite this, slope instability tends to receive less attention compared to other hazards.

Objectives

The objective of this study is to investigate fourteen hydroelectric projects with the aim of identifying the types and causes of slope failures. Additionally, it seeks to offer a comprehensive understanding of slope stability conditions and challenges encountered during construction at project sites.

Methods

To accomplish this, the study employs Key Informant Interviews with Questionnaires to delve into the slope stability concerns within Nepal’s hydropower projects. The findings are then validated through an extensive review of pertinent literature. To conduct a thorough assessment of slope stability, the study relies on on-site observations, measurements, investigations, and both in-situ and laboratory tests.

Results

It becomes evident that the careful selection of study sites, the application of geotechnical methods, and the establishment of regular monitoring are pivotal for ensuring favorable slope stability outcomes.

Conclusion

A majority of respondents concur that cutslope is the primary factor causing slope instability with 44.4% answering affirmatively. An independent t-test reveals there is no significant difference between the variables. Moreover, the correlation which is closed to 1 suggests that perception of respondents are interconnected and tend to vary in a synchronized manner. Participants in the study widely acknowledge numerical modeling methods as a means to overcome the limitations of slope stability studies.

Introduction

Background of Slope Stability

Slope instability occurs when an inclined mass of rock or soil is unable to withstand the force of gravity, resulting in the movement of rock and soil in the form of slides, flows, or falls (Dai et al. 2005). Conversely, a stable slope is characterized by the absence of movement in rock mass, soil mass, and debris down the slope (Dahal 2014). The factor of safety, representing the ratio of shear strength of soil/rock to the minimum shear strength required to prevent failure or the reduced shear strength at failure (Dawson et al. 1999; Duncan 2000), serves as a crucial criterion for determining slope stability. This factor is influenced by geomorphology, height, geology or geotechnical parameters of rock or soil mass, and pore water pressure (Dawson et al. 1999; Li et al. 2011; Giani 1992).

Natural factors such as high relief, toe cutting from river water flow, poor rock condition or loose soil mass, high rainfall, and seismic activity contribute to slope hazards (Pathak and Nilsen 2004; Deoja et al. 1991; Li et al. 2008; Pantelidis 2011). Anthropogenic factors like deforestation, improper land use, and construction activities (Sah and Mazari 1998) supplement these natural factors. Poorly designed and haphazardly constructed highways and infrastructures without proper planning exacerbate slope movement and landslides (Froude and Petley 2018).

Slope failures cause significant damage to irrigation canals, hydraulic structures of hydropower projects, and highways, impacting human life and property (Michael 2009). Historical dam failure events indicate that slope instability is a contributing factor to dam failures (MacDonald and Langridge-Monopolis 1984; Rico et al. 2008). The adverse effects of cut slopes in mountainous terrain can be analyzed through the geological and geotechnical investigation (Bonell et al. 2010) like detailed engineering geological mapping, geophysical survey, borehole drilling and laboratory tests of rock and soil samples. Therefore, construction activities should not proceed until geotechnical investigations are completed and slope stability is properly assessed.

A spatiotemporal analysis of a global dataset of fatal non-seismic landslides from January 2004 to December 2016 indicates that India, China, and Nepal, situated within the Himalayan arcs, are landslide-prone countries (Froude and Petley 2018). The study highlights that cut slopes, formed during construction, account for 40% of landslides, while those formed due to damming or river bank areas make up approximately 10%. Despite these findings, slope stability issues continue to be overlooked in civil structure construction. This article specifically focuses on slope stability issues associated with hydropower projects in Nepal. The primary objective is to address slope stability issues in hydropower projects, identify the types and causes of slope failures in different projects, and provide a generalization of slope stability conditions. The geotechnical investigations conducted by hydropower projects are reviewed and verified through a questionnaire survey.

Slope Stability issues in Nepal

In recent decades, countries like Nepal in the Himalayas have seen a rise in landslide disasters (Froude and Petley 2018). Some presumed the linkage between climate change, precipitation and other drivers for landslide hazards (Muñoz-Torrero Manchado et al. 2021). Jure landslide 2014 and Melamchi flash flood 2021 can’t be attributed to single cause but these are examples of compound hazard and cascading events of high precipitation, damming due to landslides and flash flood. Although, Nepal’s mountainous terrain which makes it a prime location for hydropower projects, such steepy terrain also poses a significant risk of landslides. Nepal has witnessed several instances of slope instability associated with Hydroelectric Projects (HEP). Notably, Kulekhani plants (I & II) faced a flood disaster coupled with slope instability in 1984 and 1986 due to heavy rains. Another significant flood occurred as a result of intense rainfall, causing extensive damage to the steel penstock, headworks, and disrupting power generation at an unspecified location (Dhital 2003). In 2014, the sudden damming of the Sunkoshi River caused by the Jure Landslide resulted in the inundation of the Sunkoshi Power Plant (Chattoraj 2014). The 2015 earthquake led to rock falls rupturing the surface penstock pipe of the Upper Bhote Koshi Project, flooding the downstream surface powerhouse (Reynolds 2018). Similarly, the Mailung Project experienced severe damage to the diversion weir, surface Settling Basin, and Headrace Penstock Pipe due to slides and rock falls (Sunuwar 2018). These incidents underscore the substantial consequences of slope failures, ranging from large landslides to debris flows, which cannot be overlooked. Despite the attention given to large-scale failures, medium-scale and small-scale incidents with localized impacts are still being neglected.In the context of Nepal, rainfall-induced slope failures are prevalent, causing extensive damage to lives, property, infrastructure, and the environment, particularly during the monsoon season (Dahal 2012). Given Nepal’s status as an earthquake-prone country in the South Asian Region, earthquake-induced slope failures result in significant loss of life and property. The situation exacerbates when earthquakes, climate change, and unplanned road construction converge (Dahal 2015; Jigyasu 2002; McAdoo et al. 2018; Tuladhar et al. 2015). Slope stability issues are commonly observed in areas with weak rock masses, major geological structures like faults and thrust passes. It is noted that the hanging wall of the thrust zone is more susceptible to the distribution of medium to large-scale landslides (Bhandari and Dhakal 2020). Geomorphological conditions also play a pivotal role in slope stability. For instance, the northern region of Nepal experiences frequent failures due to higher relief, steep topography, fragile geological conditions, rainfall, seismic activity, and unregulated civil development works (Hewitt et al. 2008). The construction of cut slopes for roads can trigger or activate landslides, further catalyzed by river toe-cutting and debris deposition (Froude and Petley 2018; Devkota et al. 2013; Kavzoglu et al. 2014). In some cases, thrust faults contribute to the formation of thick deposits of loose, weathered rock material, forming a heavily shattered zone during intense weathering (Chen et al. 2015; Regmi et al. 2013).

Overall, Slope stability is paramount for safe and sustainable hydropower development in Nepal. The main reason is the safety as Landslides can cause dam breaches, flooding downstream communities and causing loss of life and property. Ensuring slope stability mitigates these risks. Project longevity can’t be ascertained without stable slopes because unstable slope can damage project infrastructure like tunnels and penstocks. Hence, slope stability should be addressed to extend the project’s lifespan and reduce maintenance costs.

Study area

The study areas have been strategically chosen to address the slope stability conditions throughout Nepal, focusing on both historical and recent landslides. The areas under investigation include regions along the Arun, Kaligandaki, Trishuli, Sunkoshi, Tamakoshi, Raghuganga, Seti, Kulekhani, Bheri, and Karnali rivers, encompassing the vicinity of Upper Arun HEP, Kaligandaki A HEP, Trishuli 3 A HEP, Sunkoshi HEP, Tamakoshi V HEP, Rahughat HEP, Chainpur Seti HEP, Kulekhani Reservoir HEP, Jagadulla Storage HEP, Betan Karnali, and Phukot Karnali HEP as shown in Figs. 1 and 2. This selection ensures coverage across a significant portion of Nepal’s drainage system. The study is designed to explore and address slope stability issues in various hydropower projects situated throughout the country.

Fig. 1

Location of study area on drainage map of Nepal

Fig. 2

Regional Geological Map of Nepal (modified after Dhital 2015)

Practices and mitigation measures

Conventional slope stability analysis traditionally was relied on accumulating experience and information to understand landslide mechanisms. Limit equilibrium analysis, particularly with vertical interface slides, faced limitations regarding the reliable simulation of rock mass discontinuities (Tuckey 2012; Goodman 1976). Advances in technology, such as powerful computers and versatile simulation software, have now enabled the processing of large datasets and simulation in complex, jointed rock masses. Table 1 outlines the typical problems and methods of analysis in this context.

Table 1 Typical problems and Method of Analysis

Current practices involve combined approach of site investigation and selection, proper stabilization techniques and installing landslide monitoring systems. Thorough geological surveys and comprehensive stability assessments are crucial (Chen 1995; Porter and Morgenstern 2013). Nepal is emphasizing these to identify unstable slopes and avoid building in high-risk zones. The proper stabilization techniques can include bioengineering (planting vegetation), mechanical support structures (retaining walls), and drainage systems to control water infiltration. In case of landslide monitoring, early detection is vital. There are practices of using tools like inclinometers, extensometers, and remote sensing to monitor slopes for movement.

In regards to Slope Stability study, Department of Electricity Development (DoED 2018) had defined the scope of the study that varies based on the hydropower project’s capacity, head, and scheme type, with specific reference to the level of study, such as Pre-Feasibility, Feasibility, and Detailed Design. These stages cater to hydropower projects of different capacities and schemes, covering a spectrum of technical, economic, financial, environmental, and other relevant aspects. DoED had specified reservoir and PROR project for the comprehensive slope stability investigation, analysis and monitoring system. This includes comprehensive engineering assessment of the slopes surrounding the reservoir to identify potential failure mechanisms and slope monitoring works by installing instruments like inclinometers, piezometers, and extensometers to measure slope movement. Only one reservoir of Nepal is Kulekhani I hydropower Plant which was commissioned in 1982 had not installed such time dependent monitoring system. The Kaligandaki Hydropower Project which was commissioned in 2002 and is the second largest hydropower project of Nepal, had installed piezometer in Dam-site. Similarly, Upper Tamakoshi Hydropower Project of 456 MW and was commissioned in 2021 has installed rock fall barrier system in Powerhouse area. However, hydropower project of Nepal still lags behind neighboring countries China and India for installing real time dependent slope monitoring system (Zou et al. 2023) and (Xue et al. 2024).

Geology of the project area

The geological composition of the Nepal Himalaya can be classified into five major tectonic units, arranged from North to South: Tibetan Tethys Himalaya, Higher Himalaya, Lesser Himalaya, Sub-Himalaya (Siwalik range), and Terai plain (Indo-Gangetic plain). These units are demarcated by four significant tectonic structures, namely the South Tibetan Detachment Fault System (STDS), Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT) as outlined by Hagen (1969) and Upreti (1999).

The geological features of the Tibetan Tethys Himalaya include deep-sea sediments ranging from Cambro-Ordovician to Eocene, with a thickness of up to 10 km (Stöcklin 1980). The predominant lithology in this region consists of fossiliferous rocks, with major rock types including limestone, marble, quartzite, dolomite, schist, shales, slates, and conglomerate.

Beneath the Tibetan Tethys Himalaya lies the Higher Himalayan unit, characterized by pelitic, arenaceous, calcareous paragneiss, granitic gneiss, migmatite, and leucogranite. This unit, considered the infrastructural basement complex, serves as the central axis for orogeny and the origin of crystalline nappes. The rocks in the Higher Himalayan unit, found abundantly in the Upper Arun Hydroelectric Project and Phukot Karnali Peak Run of River HEP, are of Precambrian age. The separation between the Higher Himalayan unit and the Lesser Himalayan unit is marked by the Main Central Thrust (MCT).

The Precambrian rocks in the Lesser Himalaya exhibit two distinct geological settings: Allochthons and Autochthons. The Bhimphedi Group is categorized as Allochthons, while the Nawakot Group is considered Autochthons in terms of rock succession, as identified by Stöcklin (1980). The rocks within the Nawakot Group are present in various hydropower projects such as Andhikhola Storage HEP, Betan Karnali HEP, Chainpur Seti HEP, Jagadulla PROR HEP, Kaligandaki A HEP, Rahughat HEP, Sunkoshi HEP, and Uttarganga Storage HEP. The predominant rock types in this unit include quartzite, dolomite, marble, phyllite, schist, met sandstone, phyllite, limestone, and slate. Notably, the rock types of the Kulekhani Reservoir Hydropower Plant belong to the Bhimphedi Group.

The Sub-Himalaya (Siwalik range) is demarcated by the Main Boundary Thrust (MBT). This unit comprises fossiliferous rock molasses from the Middle Miocene to Early Pleistocene. The Siwaliks series is further subdivided into Upper Siwalik, Middle Siwalik, and Lower Siwalik, with the main rock types being conglomerate, salt-pepper-sized sandstone, mudstone, and mudstone/siltstonerespectively.The separation between the Sub-Himalaya and the Indo-Gangetic plain is marked by the Main Frontal Thrust (MFT). The primary constituent of the Sub-Himalaya unit is clastic material derived from the Higher and Lesser Himalaya. Table 2 below provides a summary of the rock types and geological structures in the project area.

Table 2 Rock types, geomorphology and location of the selected hydropower projects

Geomorphology of selected project sites

Approximately 83% of Nepal’s total area is enveloped by mountainous regions. The country is delineated into eight well-known geomorphic zones from North to South, including (i) Terai or Indo-Gangetic plain, (ii) Siwalik (Churia) Range, (iii) Dun valley, (iv) Mahabharat Range, (v) Midlands, (vi) Fore-Himalaya, (vii) Higher Himalaya, and (viii) Inner and Trans-Himalayan Valleys (Upreti 1999). Each of these physiographic units exhibits distinct relief, climate variations, and elevations.

The selected project areas are physiographically situated in the Mahabharat Range, Midlands, Fore-Himalayas, and Higher Himalayas. The altitude of the study area ranges from 400 m above sea level (amsl) to 4000 m amsl. The valley shapes vary from steep V-shaped valleys to U-shaped valleys. The river flow patterns are predominantly meandering, with some exhibiting braided characteristics. The geomorphological conditions significantly influence slope stability issues, with higher relief and steep topography in the northern parts leading to an increased susceptibility to slope instability compared to the southern parts.

The selected hydropower projects are strategically situated within four distinct geomorphic divisions: Higher Himalayas, Fore-Himalayas, Midlands, and Mahabharat.

In the Higher Himalayas zone, the Phulot Karnali PROR HEP is situated. The topography of the project area is characterized by a V-shaped valley along the NW to SE trending perennial Karnali River. The Sanigad area widens the valley, but it gradually narrows downstream, forming a deep gorge. The hillslope is steep, contributing to a rugged topography.

Within the Fore-Himalayan zone, three projects are located: Chainpur-Seti HEP, Jagadulla PROR HEP, and Upper Arun Storage HEP. The general altitude of this zone exceeds 3000 m. In Chainpur Seti HEP, the dam site is positioned on steep cliffs along both banks of the Seti River, while the powerhouse area is situated in a relatively moderate to gently sloping terrain. The tributaries of Seti River exhibit deep incisions with parallel to sub-parallel drainage patterns. Jagadulla Storage’s headworks area features a slightly gentle and widened valley, transitioning to steep topography and a V-shaped valley along the headrace tunnel and powerhouse areas. In Upper Arun, the river valley at the headworks forms a deep, steep, narrow gorge, which widens at the powerhouse area with a gentle slope. The highest peak in this region is Makpalung Dada, with an elevation of 2700 m.

The Midlands, encompassing low hills, river valleys, and tectonic basins, form the most significant physiographic province in Nepal (Upreti 1999). This region displays a mature landscape, with major rivers flowing through it exhibiting very low gradients and forming extensive Quaternary terraces along their courses. Several hydropower projects, including Andhi Khola Storage HEP, Betan Karnali PROR HEP, Rahughat HEP, Sunkoshi HEP, Tamakoshi V HEP, Tamor Storage HEP, Trishuli 3 A HEP, and Uttarganga Storage HEP, are situated within the Midlands section.

The Kulekhani Storage HEP is located on the Mahabharat Range, which is distinguished by its geomorphological features, including a sudden increase in height, rugged topography, sharp crest, and steep southern slopes. Rivers intersecting this range flow to the south, and the area is characterized by a population residing among the ridges, as well as degraded forests and pasture lands.

Distinct soil types are associated with each geomorphic range. In the Higher Himalayas, talus and colluvial deposits are found below steep hillslopes, with glacio-fluvial deposits and residual soil. In Fore-Himalaya and Midlands, residual soil, thicker alluvial deposits, and glacio-fluvial deposits are observed. The alluvial deposits along riverbanks are more sorted and contain both coarse and fine sediments. In the Mahabharat Range, colluvial deposits are distributed along hillslopes, while thick alluvial deposits dominate along riverbanks.

Methodology

Research overview

The research is designed to achieve specific study objectives. To facilitate this, a curated list of hydropower projects was selected, and a comprehensive questionnaire was formulated to address the necessary requirements. The review of slope stability issues and data acquisition was conducted with a focus on understanding slope stability problems, the methodologies employed for investigation, and the outcomes derived from these investigations. The questionnaire survey was structured to capture relevant information pertaining to slope stability in the selected hydropower projects.

Data collection

For the Key Informant Interview survey, a questionnaire data sheet was created (Fig. 3), and a total of 44 Key Informants with diverse academic and professional backgrounds, varying experiences, and age groups were interviewed. The main purpose of the division of KI is to realize the similarities and variation in perception and understanding due to different fields namely Civil Engineer and Geologist, due to different years of experience and due to different background namely professional and academicians. The rationale behind division of more than 20 years and less than 20 years is to identify the variation of respondent’s perception due to technological advancement and capacity building of human resources engaged in hydropower power sector within past two decades. In addition, accuracy of response was managed by conducting KII Questionnaire so that respondent’s error and biasness could be removed. The Key Informants who were participated in the survey represent equals to half of the professional and academicians who had worked both in slope stability and hydropower projects in Nepal. The distribution of Key Informants included 65% from Geoscience backgrounds and 35% from Civil Engineering backgrounds. Among the participants, 39% had over 20 years of experience, while 61% had less than 20 years of experience. The majority of Key Informants (86%) had a professional background, while 14% were from academic or research backgrounds. The survey criteria were adopted from established standards and suggestions found in the literature (Tuladhar et al. 2015; Pantelidis 2011; Arya 1993; Kuroiwa 1993; Ronan 1997; Tanaka 2005; Shiwaku et al. 2007; Ronan et al. 2010).

Additionally, a desk study of articles related to hydropower projects was conducted to understand slope stability issues. Feasibility reports, detailed design reports, and project reports for fourteen hydropower projects in Nepal were reviewed. Data related to slope failures of soil and rock, methods of investigation, and outcomes were meticulously collected for further analysis.

The survey questionnaire focused on two main aspects:

Related to Slope Stability issues

Nine questions were designed to gather information on slope stability issues.

Ordinal categorical variables were used, with numeric coding (e.g., 2 = Yes, 0 = No, and 1 = I don’t know) to treat these variables as numeric variables for analysis.

Related to methods

Seven questions were formulated to inquire about the methods employed in geotechnical or geological investigations.

An additional category “Others” was included in some questions, along with the common options Yes, No, and I don’t know. These were coded accordingly for analysis using statistical tools.

Fig. 3

Types of Questionnaire prepared for the survey

Related to output from study

A set of four questions was posed regarding the output of commonly used slope stability methods. Three of these questions were objective, featuring categories such as Yes, No, and I don’t know, which were coded for subsequent analysis using statistical tools. The fourth question was subjective in nature, allowing for more qualitative responses.

Method of analysis

The primary scope of this study is to identify the types and causes of slope failures in various hydropower projects and to generalize the slope stability conditions in Nepal. To achieve this, pie charts, independent sample t-tests, and Pearson correlation analysis were conducted to investigate the relationship between the actual site conditions and the results obtained from the questionnaire survey. These statistical analyses were employed to provide a comprehensive understanding of the factors influencing slope stability in the context of hydropower projects in Nepal.

Result and analysis

The slope stability issues in fourteen hydropower projects were systematically reviewed and categorized based on causative factors such as the nature and intensity of slope failure, geological structures, damming-induced inundation, construction behavior, and slope failures triggered by earthquakes. The responses from the questionnaire survey of key informants, totaling forty-four participants, were also incorporated, providing analytical insights. Participants included both geologists and civil engineers, as well as professionals and academicians from the fields of civil engineering or geology. The survey considered individuals with varying levels of experience, distinguishing between highly experienced professionals and those with less experience.

Figures 4 and 5 depict the percentage of “Yes” responses in relation to specific fields, backgrounds, and experiences, distinguishing between geologists and civil engineers, professionals or academicians in civil engineering or geology, and highly experienced professionals versus those with less experience. These figures serve to visually represent the survey responses and provide a clearer understanding of the perspectives within the key informant groups.

Fig. 4

Positive Response of KII informants of questionnaire survey

The results of the questionnaire survey align closely with the review of hydropower projects, indicating a shared concern among all informants regarding slope stability issues in such projects. The ongoing construction and study phases of larger-capacity hydropower projects, particularly Peak Run of the River (PROR) and Reservoir projects, are anticipated to introduce greater complexity to slope stability challenges in the future.

A majority of informants highlight cut slope-induced failures as the main causative factor, with some variation in responses regarding factors such as pore pressure, toe cutting, and geotechnical parameters. The consensus among participants is that proper design and construction play crucial roles in mitigating slope instability. However, there is a mixed response regarding the adequacy of the current procedures for slope investigation, suggesting the need for standardized guidelines that can assist in field surveys.

The survey underscores that soil slope instability is more concerning than rock slope stability, and there is a general agreement among participants about noticeable changes in slope instability before and after construction. Numerical modeling is widely supported as the preferred method for analyzing slope instability and implementing effective mitigation measures.

Another noteworthy finding is the consensus among participants that limited budgets allocated for slope stability works hinder proper investigation. This insight emphasizes the importance of allocating sufficient resources to ensure thorough and effective slope stability assessments. Overall, the survey results provide valuable insights into the perceptions and concerns of experts in the field, offering a foundation for addressing slope stability challenges in hydropower projects.

Fig. 5

Response of KII informants for questionnaire

Effect of response due to background

An independent t-test (refer to Table 3) indicates that there is no significant difference in the perception of Geologists and Civil Engineers concerning the output and methods of investigation. This conclusion is drawn from the fact that the significance values of the t-tests are greater than 0.05 (two-tailed). The results suggest that Geologists and Civil Engineers share common perspectives on the output and results, emphasizing the need for improvement by introducing more scientifically updated methods to achieve more realistic and reliable results.

Table 3 Impact of field on slope stability questionnaire (SSQ)

Nevertheless, there is a variation in perception regarding slope stability issues and certain slope stability methods, as indicated by the significance of the t-test results being less than 0.05. The observed differences in slope stability issues might be attributed to the specific field of expertise. Figure 6 illustrates that there is no significant effect on the output or outcomes related to slope stability due to the field effect, as all the output-related questionnaires are statistically significant. On the other hand, 25.0% of the method of investigation-related key issues exhibit statistical significance, indicating some variation in this aspect. The most pronounced variation is observed in slope stability issues, where 18.7% of the questionnaires related to slope stability issues are affected by the field, suggesting differing perspectives based on the specific expertise of Geologists and Civil Engineers.

Fig. 6

Percentage of statistical significant and not statistical significant issues, methods and output due to the effect of field

Effect of response due to experience

The participants were divided into two groups based on their experience levels: those with more than 20 years of experience and those with less than 20 years of experience. The responses from the participants exhibited a mixed pattern regarding slope stability issues, methods, and output. Notably, there appears to be statistical significance in most of the slope stability issues, as indicated by t-values greater than the significance level. This suggests that the perspectives on slope stability issues vary significantly between participants with more and less than 20 years of experience.

Table 4 Effect of response due to experience

There appears to be an effect of experience on slope stability methods and output. Specifically, only 37.5% of the questionnaires related to slope stability issues exhibit statistical significance, while a lower percentage, 6.3%, of the questionnaires related to output show statistical significance (refer to Fig. 7). This suggests that the participants’ experience levels play a more substantial role in influencing their perspectives on slope stability methods compared to their views on the outcomes.

Fig. 7

Percentage of Statistical significance for effect of response due to field

Correlation analysis of response

The correlation between variables demonstrates a strong positive correlation, as indicated in Table 5. All correlation values are close to 1, and a value closer to 1 suggests a stronger correlation. The variables, including Civil Engineer or Geologists, Experience (more than 20 years or less than 20 years), and Professional or Academicians, show responses that align closely with one another. This correlation analysis suggests that these variables are interconnected and tend to vary in a synchronized manner among the participants.

Table 5 Correlation matrix based on the responses of KII informants

Discussion

The review of literature was fruitful for analyzing the mode of failures, mechanism of failure and mode of investigation applied for the study of hydropower projects (Fig. 8). The methods applied for slope stability study is kinematic analysis, geological mapping, geophysical survey and simulation which depends upon severity and the requirement for the hydropower projects. The wedge failure is common mode of slope failure (Fig. 8). The guidelines need to be improvised depending on the advance geotechnical and geological study. In addition, simulation of the slope should be in common practice for the realistic output. The literature review and questionnaire survey provide few essential information about slope stability issues. The cut slope prepared during construction which are mentioned in the report makes the slope vulnerable to failure. Rock failure and soil slope failure due to pore water pressure, toe cutting due to river are common causes of failure in all hydropower projects. In case of output in regards to slope stability, slope stability analysis is still need to be improvised with sufficient budget for the geotechnical investigation and analysis. Similarly, development for new thumb rules based on the rock-types, geomorphology and pore water pressure would be beneficial. Most of KII informants suggest manpower should be skilled, numerical modelling to be introduced during slope stability research.

Fig. 8

Mode of investigation, mechanism and mode of failure observed in selected hydropower project

The combined approach of literature review and KII Questionnaire survey has been applied for the present study to avoid the limitations of literature review and KII Questionnaire Survey separately. KII Questionnaire Survey skips the potential error due to incomplete knowledge of respondents hopefully and removes the biasness of project report due to information gap. However, integrations challenges while combining qualitative data from Questionnaire Survey and Quantitative data from literature review and potential confirmation biasness of survey data with reviewed data from literature might be possible. Hence, data triangulation with survey data from with other sources like geotechnical investigations and historical records is recommended to get a more complete picture especially for the future avenues.

The questionnaire survey reveals the importance attributed to slope stability study and investigation. A significant number of participants express the need for the introduction of new geotechnical methods and analytical tools in the field. Dissatisfaction with current investigation techniques, methods, and guidelines is prevalent among the participants. The acceptance of numerical modeling methods by a majority of participants is notable, as these methods are seen as a means to overcome the limitations of slope stability studies.

Several important suggestions emerge from the survey responses. Participants highlight the strategies like long term slope monitoring landslide monitoring and maintenance programs and advanced modelling of landslides that should be formulated to predict landslides occurrence and assess potential impacts. They also suggested areas of improvements such as strengthening regulations, installation of early warning system and capacity building of manpowers engaged in hydropower sectors. They had common understanding on improving data quality, ensuring data acquisition by competent and skilled professionals, selecting proper sites and designing them appropriately, monitoring slopes effectively, and accurately calculating shear strength parameters of soil and rock i.e. cohesion and friction angle. Additionally, there was a consensus on the necessity of utilizing more real test data for analysis rather than relying solely on assumed data or calculations from empirical relations. These insights underscored the demand for advancements in both methods and approaches within the field of slope stability study and investigation.

Conclusion

In this research, the stability concerns of slopes in various hydropower projects, the credibility of geological and geotechnical methodologies, and findings from the investigations have been validated using responses obtained through a questionnaire survey involving Key Informant Groups. The aim of this study is to emphasize the challenges related to Slope Stability in Hydropower Projects, discern the types and causes of slope failures in diverse hydropower projects, and establish generalizations about slope stability conditions. The resulting conclusions of this study can be summarized as follows:

• According to an independent t-test (Table 3), the perception of Geologists and Civil Engineers regarding the output and investigation methods does not differ significantly, as the significance of the t-tests exceeds 0.05 (two-tailed). However, a notable difference in perception is observed concerning slope stability issues and methods. As indicated by an independent t-test (Table 4), the responses of participants with over 20 years of experience and those with less than 20 years differ significantly. The participants’ responses were diverse when it came to slope stability concerns, methodologies, and output.

• The correlation among variables such as Civil Engineer or Geologists, experience exceeding 20 years or less than 20 years, and Professional or Academicians of Civil Engineer or Geologists revealed consistent responses that align with each other.

• The primary triggers for landslides are the vulnerable geological conditions, precipitation, and toe cutting caused by rivers or streams. Nevertheless, inadequate study and planning in local road construction and other human settlement activities contribute to an elevated occurrence of landslides. Additionally, earthquakes have induced fresh landslides or reactivated existing ones in various regions. Due to study limitations, the specific effects of slope instability resulting from earthquakes could not be thoroughly elucidated. Notably, a significant number of landslides were initiated by variations in pore pressure during the operation of hydropower projects.

• A majority of respondents concur that cutslope is the primary factor causing slope instability, with 44.4% answering affirmatively. This aligns with the findings of a global study on fatal non-seismic landslides, which reported a similar figure of 40%. Furthermore, this agreement resonates with slope stability cases observed in hydropower projects, where the percentage is recorded at 38.5%.

• A consensus among most respondents indicates a belief that guidelines for hydropower projects in Nepal should be tailored separately for the Study phase, Construction phase, and the phase involved in electricity generation. Regarding slope stability analysis (as inferred from responses to Q. 3 and Q. 20), it is suggested that probabilistic analysis and the deterministic approach should be corroborated by a simulation approach. Additionally, there is a unanimous agreement on the necessity of implementing an effective monitoring system for an enhanced slope stability approach (Q. 20).