Assessing the Role of Unsaturated Slopes in Landslide Events and Its Impact on Infrastructure Sustainability

Background and Rationale

Several variables, including water content, slope gradient, and soil composition, determine and influence slope stability. The movement of rock, soil, and debris down a slope due to gravity is known as a landslip. Conventional models frequently concentrate on saturated situations where water seeps into the soil, raising pore pressure and lowering friction resistance. Conversely, unsaturated slopes, where the soil is less wet, exhibit a distinct dynamic. One major geological hazard that puts infrastructure and human lives in danger is landslides. Saturated slopes, where water content is critical to slope stability, have been the focus of most traditional landslip research. However, new research suggests that unsaturated slopes, where the soil lacks complete saturation with water, could also significantly contribute to landslip incidents. Despite their potential to impact infrastructure and slope stability, we often overlook these unsaturated conditions. It is imperative to comprehend the significance of unsaturated slopes in landslip incidents, especially in areas where climate change is causing disparities in precipitation patterns.

By evaluating how unsaturated circumstances contribute to landslides and how these occurrences impact the durability of infrastructure, this research seeks to close this gap.

Let us investigate the effect of unsaturated conditions on slope stability and land-slip occurrences, which is the first of this research's main goals.

Assess the effect of landslides on the sustainability of infrastructure.

Design proper mitigation plans to lessen the danger of landslides on unsaturated slopes.

Stability of Slopes and Landslides

The composition of the soil, the slope gradient, and the water content are some of the factors that affect slope stability in landslides. Conventional model environments frequently concentrate on saturated states, in which water seeps into the soil, raising pore pressure and lowering friction resistance. Conversely, unsaturated slopes exhibit a different dynamic due to the reduced water content in the soil.

Soil Mechanical Unsaturation

Researching unsaturated soils is essential to understanding slope stability in non-saturated situations. The study of soil falls under the category of unsaturated soil mechanics.

Methods of Research

Subject Matter The study will concentrate on areas with various meteorological variables and a high frequency of landslides. The selection of case studies will be based on slope saturation levels, historical data, and the availability of current infrastructure. The Himalayan mountain ranges, coastal regions vulnerable to seasonal rainfall, and locations impacted by drought and its aftermath are all possible study areas.

Rainfall infiltrations that deepen the wetting band reduce the matric suction, which is the primary cause of landslides. During an intense downpour, unsaturated slopes collapsed in this instance. Basic research on the mechanism and origin of landslides reveals several interesting technical findings, including the downward movement of the wetting band, the failure mechanism of a soil depth, the impact of groundwater flow, and the rise in groundwater level. Based on these findings, we perform an integrated analysis to determine the hydraulic characteristics of unsaturated soil, which includes the field matric suction. We demonstrate that the field matric suction regulates the rate of change in water infiltration for the landslip analysis, based on antecedent rainfall.

Concern over the threats that climate change poses to industrialized and developing nations' real estate has grown in recent years. Climate variability and associated extremes, along with policies implemented to reduce greenhouse gas emissions, are likely to impact the property industry. To help stakeholders take the required action to lessen adverse effects, this study will examine what is currently known about future climate change and its potential effects on Malaysia's real estate market.

The study predicts that Peninsular Malaysia's temperature will rise by 1.1 to 3.6°C during the northeast monsoon, leading to changes in rainfall patterns and potential increases in river discharge of up to 43% in some river basins.

In general, the study will help direct the operational reactions of different authorities, particularly with regard to those activities meant to reduce the danger of climate change in Malaysia's real estate market.

The embankments and cuttings known as earthworks enable a railroad to keep a specific line, grade, and level throughout the terrain. Engineers use an engineered bank of earth as an earth embankment to elevate a railway above the surrounding terrain. The railway passes through ground that has a natural level above the railway line via a cutting. The meticulous design of earthworks built after the 1960s ensures their optimal functioning. Conversely, earthworks constructed over a century ago, without the use of automated machinery, power many railroads.

Design, procedure, and strategy

In this work, we have employed a variety of tactics to tackle different threats. We gathered both quantitative and qualitative data during fieldwork, which included information on the size and shape of valleys, river channels, and valley sides, as well as vegetation density, sediment closet measurement, slope angle, sediment character evaluation, floodplain stratigraphy, and vulnerable element identification. We integrated these data with satellite image analysis to characterize the characteristics of river catchments and their vulnerability to flooding, using Collier and Fox's 2007 technique of Flash flood forecasting. We also examined catchment connectivity, landslip scars, and general terrain analysis. Literature reviews and queries to seismic databases helped us put together information on the potential catchment floodwater amounts, the epicentre, magnitude, depth, and date of seismic events, as well as current theories on the return period of major Himalayan earthquakes.

Literature Review

To determine the present level of knowledge on the behaviour of unsaturated soil slopes and their significance in landslip events, a thorough study of the literature will be carried out. The specifically mentioned areas will remain the prime focus of further review: Processes of soil-water interaction in unsaturated soils: In unsaturated soils, complex dynamics, including water content, suction, and soil structure, interact to affect the mechanical behaviour of the soil and are involved in the soil-water interaction processes. These interactions, which are especially important in slope environments, have an impact on the strength and stability of the soil during cycles of soaking and drying.

Soil properties' impact on the stability of a slope: Because of the direct effects of permeability, plasticity, and grain size on the soil's shear strength and moisture-change response, soil parameters have a major impact on slope stability. Because variations in these characteristics might produce distinct stability outcomes under comparable environmental circumstances, reliable slope stability assessments depend on the right description of the soil.

The impact of rainfall on the stability of unsaturated soil slopes is significant. Rainfall has a significant impact on the stability of unsaturated soil slopes because it can diminish matric suction, which lowers shear strength and increases failure risk. Rapid variations in pore water pressure caused by rainfall infiltration can further destabilize slopes that remain stable under dry conditions.

Techniques for analysing slope stability in unsaturated soils: These techniques, which take into consideration the peculiar behaviour of unsaturated soils, include numerical modelling techniques and limit equilibrium techniques. By taking into account how suction and moisture content affect soil strength, these techniques aid in the prediction of failure processes and offer crucial information for engineering applications.?

Landslip effects on the durability of infrastructure: Because landslides can seriously harm buildings, roads, and other vital infrastructure, they have a huge effect on the sustainability of infrastructure. Landslides cause social unrest and financial losses; therefore, enhancing the resilience of impacted communities requires efficient risk management and mitigation techniques.

Field Investigations

At a few chosen study locations, fieldwork will be done to gather information on the environmental factors, slope geometry, and soil characteristics. There will be the following activities carried out:

To find out about soil qualities, including permeability, plasticity, and grain size distribution, laboratory testing and soil sampling are used.

Cone penetration tests (CPT) and dilatometer tests (DMT) are examples of in-situ testing methods used to assess the stiffness and strength of the soil.

Installing equipment, such as piezometers and tension meters, to track variations in pore water pressure and soil suction during rainfall events.

To describe the geometry of the slope and pinpoint possible failure surfaces, topographic surveys and slope stability evaluations are used.

Numerical Modelling

To simulate the behaviour of unsaturated soil slopes under different rainfall situations, numerical modelling will be employed. There will be the following steps to take:

Creation of a conceptual model that depicts the mechanisms behind soil-water interaction in unsaturated soils.

Field and lab data are used for the numerical model's calibration and validation.

Research using parametric analysis to look into how environmental variables, slope geometry, and soil characteristics affect slope stability.

To measure the degree of uncertainty related to the inputs and outputs of the model, use probabilistic analysis.

Risk Assessment Framework

To estimate the danger of landslides on unsaturated soil slopes, a thorough risk assessment framework will be constructed based on the findings of the field research and numerical modelling. The following elements will be incorporated into the framework:

Hazards and possible failure situations are identified.

Application of reliability-based techniques to estimate the likelihood of slope collapse.

Evaluation of the effects of slope failure, including potential fatalities and infrastructure damage.

Creation of risk-reduction tactics, such as early warning systems and slope stabilisation techniques.

Case Studies

The below mentioned results can be drawn from these research:

A deeper comprehension of how unsaturated soil characteristics affect slope stability in different rainfall scenarios.

A thorough methodology for risk assessment that is designed to analyse the likelihood of landslides on slopes with unsaturated soil.

Suggestions for practical mitigating techniques to lessen the influence of landslides on the long-term viability of infrastructure.

Increased public and local government understanding of the dangers of unsaturated soil slopes and the value of proactive risk management among stakeholders, including managers of infrastructure and the general public.

Expected Outcomes

It is anticipated that the planned study would produce the following results:

Improved knowledge of the function of unsaturated soil characteristics in slope stability under different rainfall scenarios.

A thorough risk assessment methodology for assessing the possibility of landslides on slopes with unsaturated soil.

Strategies for mitigating the effects of landslides that are effective in reducing their negative effects on the sustainability of infrastructure.

Increased understanding of the dangers of unsaturated soil slopes and the value of proactive risk management among stakeholders, including the public, infrastructure managers, and local government officials.

Timeline and Budget

It is anticipated that the proposed research will be finished in three years, according to the schedule below:

Review of literature, fieldwork, and initial numerical modelling in the first year.

Year 2: Ongoing field research, numerical modelling, and framework building for risk assessment.

Year 3: Research findings are disseminated, case studies are conducted, and the risk assessment system is improved.

The planned research is expected to cost $500,000, which covers the following costs:

$145,000 for both laboratory testing and field research

Software licenses and numerical modelling: $105,000

Pay for students and research personnel: $210,000

Attendance at conferences and travel: $55,000

Conclusion

Analysing the contribution of unsaturated soil slopes to landslip occurrences and how this affects the durability of infrastructure is an important field of study with important applications. The goal of the proposed study is to create a thorough framework for assessing the risk of landslides on slopes with unsaturated soil and recommending practical mitigation measures. The anticipated results of this study will help to lessen the negative effects of landslides on the sustainability of infrastructure and increase our understanding of geotechnical engineering.

Findings

The study concludes that there is a considerable danger of landslips, floods, and seismic hazards in SECMOL and a large portion of the Ladakh region. In the Himalayas, high-magnitude earthquakes can cause significant amounts of damage and have return times ranging from hundreds to approximately a millennium. Whenever possible, it is wise to maximize earthquake engineering. The 2010 Leh floods demonstrated high levels of devastation, which, if storms were to center nearby, could seriously harm the SECMOL complex. This study demonstrates the interconnection of catchments at various elevations and potential interactions between nearby catchments. Evacuation plans must be created for the college. SECMOL's northern ridges have the potential to bury portions of the campus in the event of an earthquake or heavy rain. It is possible to reduce slope angles and big boulders [10].

Realistic justification

Three separate strata make up the geology from the drilling survey: a 1.5–4.5 m layer of colluvium, a 0.5–9.9 m layer of heavily weathered rock, and new rock (Figure 1). The colluvium soils are defined as silty sand and comprise a broad variety of particle sizes. In colluvium deposits, gravel, cobbles, and pebbles of different sizes are found. They seem to have sub-angular to angular forms. The colluvium's standard penetration number (N-value) is roughly 4/30-5/30. N-values less than 10 typically indicate very soft and loose soil. It follows that the colluvium deposit provides the source material for debris flows. Weathered rock's N-value varies greatly from 12/30 to 50/10, depending on the degree of weathering. Based on the results of the constant head permeability and shear tests in boreholes, the permeability of the colluvium is approximately 10-4 m/s, and the permeability of the weathered rock is on the order of 10-5 m/s.

Here is a summary of the physical characteristics of soil samples taken from the colluvium at depths ranging from 0.15 to 0.5 m. The water contents, which range from 14.1 to 32.1%, show that the soils are highly saturated and that there is moisture in the superficial layer. The soils have plastic and liquid limits between 20.9 and 23.8% and 30.2 and 42.1%, respectively. Colluvium deposits have fines (clay and silty) that range from 28.8 to 55.7%. These fines support high porous water pressure in the debris flow and help it move (Rocscience, 2021). Ellen and Fleming [5] looked at the amount of clay in soil samples from colluvium slopes and found that slope failure is strongly linked to a clay content of 8 to 25%.

As per the USCS Classification System of Unified Soil, the colluvium soils are designated as silty sand (SM) and SC-CL sand clayey, having several fines.

Consequences for practice

Hazards are not common, yet they have extremely high to very high potential effects. Among the steps taken to mitigate hazards are engineering solutions for steep inclines, earthquake-proofing of buildings, and large-scale flood evacuation plans.

Uniqueness and worth

These research methodologies have been thoroughly evaluated. The interdisciplinary connections are ambitious and somewhat novel. It is innovative to apply this work to a particular college centre location that can quickly implement suggestions. In this region of Ladakh, catchment interconnectedness has never been identified before. The originality value of this work is increased by complementing with research that addresses the community components of this investigation.

The flux boundary conditions, which are regulated at the slope's surface, were determined using rainfall data obtained at the Seocho station. To replicate a realistic rainfall event, the 850 hours (35.4 days) of rain that fell between June 22 and July 27 were broken up into multiple parts. Furthermore, a non-ponding boundary condition was implemented to avoid an excessive build-up of precipitation on the slope surface. To replicate the no flow zone, the slope's base and sides were treated to a nodal flux value of zero. The boundary condition of the analysis and the pore water pressure distribution findings from the infiltration study are shown in Figure 2. The hydraulic boundary condition was implemented with the intensity of the rainfall in real-time.

To look into slope instabilities and failure zones, a crucial slip surface about the infiltration characteristics of the soil slope was assessed.

Critical slip surfaces in the Raemian and Dukwooam watersheds are depicted in Figure 3.

Throughout rainstorm episodes, the critical slip surface gradually deepens towards the bedrock. At the top of the Raemian and Dukwooam watershed, which is the most hazardous area, Figure 3 illustrates the fluctuations in the factor of safety. With time, the slope safety factor for both watersheds dropped, reaching its lowest value at the conclusion of the rainfall events. Because of the various slope angles and soil depths, the Raemian watershed has a bigger initial factor of safety (1.64) during the driest period than the Dukwooam watershed (1.47). When the rainstorm episodes first started, the factor of safety dramatically dropped because of the impact of a decrease in matric The factor of safety dramatically dropped at the start of the rainstorm events. Rainwater seeping into the soil layer caused the matrix suction to rapidly drop, which in turn caused the factor of safety to drop. The intensity of the rainfall has an impact on the factor of safety as well as its magnitude and pace of decline. The longitudinal profile before and after the landslip is shown in Figure 4. According to the LiDAR scan, the Raemian watershed under study has a channel length of 632 m, a width of 30–40 m, and an average incised depth of 1.6 m.

The findings suggest that the temporal and spatial distributions of groundwater fluctuations serve as the primary triggers for landslides.

As previously established, deficient matrix suction is one of the most crucial factors in the analysis of landslides caused by rain. Previous rainfall affects the initial matric suction, changing the rainfall-infiltration depth, which is known as the main mechanism causing landslides. Assessing a rainfall-induced landslip on unsaturated soil slopes is challenging due to the need to consider numerous factors such as groundwater level, initial matric suction, and rainfall. Landslides that happen across watersheds show a strong numerical method that combines hydro-geotechnical analysis for both shallow and deep-seated slope failure. The results of the investigation closely align with the simulated results, indicating that the applied method is appropriate for simulating rainfall-induced landslides at the watershed scale.

Conclusion

Using the 2011 Umyeonsan landslide as an example, the main goal of this investigative study is to investigate rainfall-induced landslides on slopes with partially saturated soil. An integrated investigation of landslides brought on by rainfall used lab testing, field testing, and numerical calculations. To ascertain the wetting band depth for both shallow and deep-seated slope failure in watershed-scale landslides, particular attention was paid to rainfall penetration studies.

Written by

SHYAM GOKARN

India

M:?+91-9481010670

Email:?[email protected]