INTRODUCTION
General background
A landslide is defined as the movement of rock, debris, or earth down a slope, often caused by factors such as rising groundwater, soil mass imbalance, and various forms of mass wasting, including rock falls and mudslides Initiated by rainfall, earthquakes, volcanic activity, and human activities, landslides typically occur on slopes with an inclination of 5 to 20 degrees Key indicators of landslide mechanisms include surface deformations, scarps, and various types of cracks.
Figure 1.1 Schematic illustration of landslides (Varnes, 1978)
Landslides, classified into deep-seated and shallow types, pose significant hazards in mountainous and coastal regions, impacting both developed and developing countries Deep-seated landslides can reach depths of several meters to hundreds of meters, while shallow landslides are typically less than 3 meters deep These geological disasters are triggered by various environmental factors, including heavy rainfall, and are exacerbated by human activities like slope cutting during construction Such activities not only threaten human safety and infrastructure but also contribute to an increase in landslide occurrences, particularly in developing nations.
Landslides are categorized based on their movement and the materials involved, which can be either rock or soil Soil is further classified as earth, consisting mainly of sand-sized or finer particles, or debris, made up of coarser fragments The movement of a landslide mass can occur through various mechanisms, including fall, topple, slide, spread, or flow.
Table 1.1 Types of landslides, abbreviated version of Varnes' classification of slope movements (Varnes, 1978)
Figure 1.2 Schematic illustration of the major types of landslide movement
Between 1961 and 2010, Vietnam experienced an average of 12 typhoons annually, leading to significant rainfall and flooding (Cong et al., 2020) The country is notably prone to landslide hazards, particularly along key transportation routes such as the North-South railway and national highways, including the Ho Chi Minh route (Duc, 2013; Luong et al., 2017; M Nguyen & Tran, 2020) The catastrophic landslide in Quang Tri province in 2020 marked the deadliest event in Vietnam's history (Van Tien et al., 2021) On November 5, 2017, a landslide-induced tsunami-like wave struck the Truong River in Bac Tra My District, Quang Nam province (Duc et al., 2020) Additionally, a rapid rotational landslide occurred on October 13, 2020, in Phong Xuan commune, Thua Thien Hue province, triggered by heavy rainfall (Van Tien et al., 2021) Another significant rainfall-triggered landslide took place on December 15, 2005, in Van Canh district, Binh Dinh province (Duc, 2013) Lao Cai province has been identified as the area most frequently affected by landslides (Duan & Duc Vu; Le et al., 2021; L C Nguyen et al., 2023; M Nguyen & Tran, 2020; Van Thang et al., 2021).
Location and detail conditions of the study area
Vietnam, covering an area of 331,212 km², ranks fourth among Southeast Asian countries and features approximately 4,600 km of borders and 3,400 km of coastline It shares borders with China to the north, Laos and Thailand to the west, and the South China Sea to the east The country extends about 1,600 km from north to south and 600 km from east to west, presenting a long and narrow shape Geographically, Vietnam is divided into three regions: Northern, Central, and Southern My research focuses on two study areas within the Northern region, which features a gradual elevation decrease from northwest to southeast and is subdivided into three zones: Northeast, Northwest, and the Red River Delta While the Northeast and Northwest are predominantly mountainous, the Red River Delta consists of low-lying areas The Northern region comprises 23 provinces, with the Central region having 18 provinces and the Southern region containing 17 provinces.
Lao Cai is a province located in the northern region of Vietnam, covering an area of 6,283.9 km² It is bordered by Ha Giang, Yen Bai, and Lai Chau provinces, and shares a 203 km border with Yunnan province in China The province includes Lao Cai city and eight districts: Muong Khuong, Bat Xat, Bac Ha, Bao Thang, Sapa, Bao Yen, Van Ban, and Si Ma Cai.
My research focused on two landslide cases in Northern Vietnam, specifically in the Sapa district of Lao Cai Province, which spans 675.8 km² and ranges in elevation from 150 m to over 3000 m Known as a popular tourist destination, Sapa has faced an increase in landslide frequency and intensity in recent years, primarily due to rapid urbanization, construction activities, and agricultural practices Most landslide events in this region have been triggered by precipitation (Dang et al., 2018; Tien Bui et al., 2017).
Figure 1.3 Location of the study area: Northern region of Vietnam (Map of Vietnam)
1.2.1 Case study 1: Road No.155, near new Mong Sen bridge, Trung Chai commune, Sapa town, Lao Cai province
The study area is situated on Road No 155, between Km 12+667.85 and Km 12+711.57, near the new Mong Sen bridge in Trung Chai commune, Sapa town, Lao Cai province, Vietnam, at coordinates 22° 25′ 1.68″ N and 103° 54′ 18.03″ E, approximately 0.6 km from the bridge Trung Chai commune lies in the northeastern part of the Sapa district, connected by National Highway 4D, which links Lao Cai city and the Sapa district and was constructed in 2018 The region's mountainous terrain makes it susceptible to typhoons and landslides (Duan & Duc Vu; Le et al., 2021; L C Nguyen et al., 2023; M Nguyen & Tran, 2020; Van Thang et al., 2021).
Figure 1.4 Location of the case study 1 (Mong Sen)
1.2.2 Case study 2: Road No.152, near Muong Hoa valley, Cau May commune, Sapa town, Lao Cai province
The study area is situated along Road No 152, between Km 2+728.26 and Km 2+827.04, close to Muong Hoa valley and the Muong Hoa cultural project in Cau May commune, Sapa town, Lao Cai province, Northwestern Vietnam Geographically positioned at 22°19'11.95"N and 103°51'20.05"E, it lies approximately 2.3 km southeast of Sapa town and is adjacent to a crucial economic road corridor This region is characterized by its mountainous landscape and is recognized as a significant cultural and tourist destination.
Figure 1.5 Location of the case study 2 (Muong Hoa valley)
Research problem
Vietnam's diverse geography, with 75% of its mainland being mountainous and hilly, makes it susceptible to natural disasters such as landslides, soil erosion, and flash floods, particularly during the monsoon season The eastern coastal region of the Indochinese peninsula experiences an average of 12 typhoons annually, leading to significant rainfall and flooding Notably, landslide incidents frequently occur along major transportation routes, including the North-South railway and national highways, particularly in areas like the Ho Chi Minh route and the mountainous region of Lao Cai province, near Sapa town and the Sapa Ancient Rock Field.
Landslides occurred several times in 1990, 1994, 1996, 1998, 2000, 2001, 2002, 2010,
Between 2019 and 2021, the old Mong Sen bridge in Sapa town, Lao Cai province faced significant challenges due to heavy and continuous rainfall, slope cutting, and complex geological conditions (Do et al.; Duan & Duc Vu, 2011; L C Nguyen et al., 2023; Van Tien, Luong, Nhan, et al., 2021; Yamasaki et al., 2021) To mitigate risks of debris flow and landslides, the New Mong Sen bridge and the Noi Bai – Lao Cai highway were constructed in 2018 However, in October 2020, two deep-seated landslides occurred near the New Mong Sen bridge during excavation and construction activities (L C Nguyen et al., 2023) These landslides were classified into two zones, with significant soil erosion and water dissipation observed between them during heavy rains Consequently, a new deep-seated landslide emerged between these zones in the rainy season of 2021, although its sliding surface was smaller compared to the previous zones.
In 2021, a landslide occurred near Muong Hoa Valley in Cau May commune, Sapa town, Lao Cai province, specifically on Road No 152 The existing countermeasures in the area include basic methods such as slope cutting, shotcrete application, and surface drainage Notably, a construction project is situated across from the landslide location, which is adjacent to a previously treated slope using shotcrete and surface drainage techniques This landslide is classified as a shallow landslide rather than a deep-seated one.
This research focuses on two key study areas: the new highway road and an important economic road corridor Effective countermeasures are essential to address deep-seated landslides along the highway roadside Despite numerous studies conducted over the past decades, developing slope stabilization methods for rainfall and earthquake-induced landslides in these regions continues to pose significant challenges in geotechnical engineering.
Research question
How should we select the appropriate remedy solutions against deep-seated landslides along the highway under heavy rainfall area and earthquake zone in Lao Cai province?
Research objectives
- To simulate the slope stability by using numerical analysis with LEM (GEO- SLOPE) and FEM (PLAXIS 2D)
- To find the effectiveness countermeasure for remedy solutions against landslide failure triggered by rainfall and earthquake along the highway in Loa Cao province.
Scope of the research
- Study the mechanism of landslides for different formation within the Lao Cai area
- Study about the effect of rainfall and earthquake induced landslide in this study area
- Study effect of several countermeasures for landslide area
- Recommend the suitable solutions for countermeasure methods in this area.
Outline and structure of the thesis
(b) Figure 1.6 Flow chart shows the outline of the thesis (a) general framework of the research; (b) flow of the analysis steps
In this thesis, structure of the thesis was organized as five chapters including introduction and conclusion parts
It provides a general background of landslides, and details location of the study area about the requirements of this research with its problems, objectives and scopes of the reseach work
This chapter encompasses a comprehensive literature review, explores the causes and triggering mechanisms of landslides, and discusses countermeasures for classifying deep-seated landslides It further delves into slope stability analysis and various methods, including the formulation of the Limit Equilibrium Method and the Finite Element Method.
Chapter 3: Data Collection and Research Methodology
This chapter outlines the data collection process, which includes topography and geology investigations, rainfall and seismic data, field investigations, and laboratory testing It also details the numerical analysis procedures, including the input parameters and numerical modeling steps essential for effective numerical analysis.
Chapter 4: Analysis Results and Discussions
This chapter presents all of the numerical analysis results based on the several condition, comparison of the results, and includes the discussion part compare with another research
This chapter highlights the facts that were found during the analysis and specific conclusions and recommendations on them.
Findings and Research contributions
This research investigates the effects of rainfall infiltration and seismic conditions on saturated and unsaturated soils, focusing on both initial and remedial slopes Utilizing numerical methods such as Limit Equilibrium Method (LEM) with GEO-SLOPE and Finite Element Method (FEM) with PLAXIS 2D, the study analyzes ground motion acceleration and pseudo-static acceleration during earthquakes.
This research demonstrates that the initial slope stability aligns with actual field results, providing effective countermeasures for landslides during rainfall and earthquakes Additionally, the FEM (PLAXIS 2D) model proves to be a reliable tool for designing practical slope stabilization solutions.
This research identifies effective slope stabilization methods applicable to landslide-prone areas, particularly in mountainous highway regions, offering valuable insights that can benefit similar locations globally.
This research focuses on two study areas featuring distinct rock formations, allowing for the application of similar methods and models to analogous rock formation regions in Sapa town.
LITERATURE REVIEW
Literature review of case studies
This chapter provides a comprehensive review of relevant literature on landslide events and the historical context of earthquakes in Vietnam, focusing on triggering mechanisms, slope stabilization methods, and stability analysis techniques It discusses the application of the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM) in slope stability analysis Additionally, Section 2.7 highlights previous studies conducted in the Lao Cai area using GEO-SLOPE and PLAXIS software, contributing to the understanding of slope stability in the region.
Recent studies in geotechnical engineering have focused on landslide prevention methods Zhang et al (2023) conducted a comprehensive analysis of a large expressway roadside by integrating site investigations, deformation monitoring, laboratory tests, and theoretical calculations to understand the evolution of the area.
In August 2018, a landslide occurred at the Banguang toll gate of the Huizhou – Shenzhen Coastal Expressway in the Guangdong – Hong Kong – Macao Greater Bay Area, South China, primarily due to mountain excavation for expressway construction combined with persistent extreme rainfall This study evaluates the slope safety factor, both without rainfall and under continuous rainfall conditions, utilizing theoretical calculations through an iterative method alongside laboratory test results.
Islam et al (2021) conducted a geotechnical investigation into the landslide disaster in the Chattogram Hill Tracts (CHT) of Bangladesh, which occurred on June 13, 2017, and is considered one of the deadliest landslides in the country The study identified excessive rainfall, inadequate drainage systems, and soft soil deposits as key triggering factors for the landslide Utilizing finite element modeling with PLAXIS 2D, the research revealed that many hill slopes are prone to failure following heavy rainfall The findings suggest that implementing bioengineering techniques, such as vegetation, and improving slope drainage can serve as effective and sustainable measures to prevent future landslide disasters.
Figure 2.2 Part of the road collapsed area of (Islam et al., 2021)
In August 2019, the Thae Phyu Kone landslide in Mon State, Myanmar, resulted in 75 fatalities and damaged 27 buildings, making it the largest landslide in the region Triggered by continuous heavy rainfall and insufficient slope stabilization along the mountainous highway, this disaster occurred in an area located 270 km southeast of Yangon, characterized by hilly mountain ranges and flat terrain The authors, Panday & Dong (2021), utilized remote sensing images, digital elevation models (DEM), and limited fieldwork to compile a landslide inventory, analyzing the topographic features of the landslides using ArcGIS.
Figure 2.3 Pre-event and Post-event of Thae Phyu Kone landslide (Panday & Dong,
A significant landslide occurred in the northern region of Vietnam, particularly along the Halong-Vandon expressway, triggered by heavy rainfall on July 21, 2018, and exacerbated by five consecutive days of rain This study investigates the causative factors, failure mechanisms, and characteristics of the landslide through comprehensive geological assessments, UAV surveys, and detailed analysis of rainfall data and the expressway project Utilizing the PLAXIS 2D model, researchers estimated the sliding surface across the slope Furthermore, the study proposes effective remedial solutions and slope stabilization methods to mitigate the risk of future landslides.
Figure 2.4 Landslide body on Halong-Vandon new expressway
In a study by Tran, Pham, et al (2021), the focus was on a sliding failure that occurred along the Noi Bai – Lao Cai highway in Yen Bai province, northern Vietnam, during the rainy season of 2018 The research evaluated the stability of cut-slopes made of low and high hydraulic conductivity soil under varying rainfall conditions Utilizing the coupling modules SEEP/W and SLOPE/W, the study analyzed the process of rainfall-induced slope failure.
Figure 2.5 Sliding failure along the Noi Bai – Lao Cai highway
Recent research has focused on the non-linear, time-variant simulation of rainfall-induced slope failures in unsaturated soil using the Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Analysis (TRIGRS) program and SLOPE/W (Tran, Hung, et al., 2021; Van Thang et al., 2021) The study, conducted along Provincial Road No.152 at Km 9+100 in the Sapa district of Lao Cai province, investigated a landslide that occurred on August 5, 2019, causing one fatality and significant road obstruction T V Tran et al (2021) aimed to predict future landslide locations and updated topographic conditions using Scoops 3D, GIS, and the limit equilibrium method Additionally, a landslide susceptibility map was developed utilizing the Analytical Hierarchy Process (AHP) model (Le et al., 2021).
Figure 2.6 Location of the landslide site
2.1.2 Historical background of earthquake in Vietnam
Lao Cai province lacks precise earthquake records; however, the northern region of Vietnam has a history of seismic activity As illustrated in Figure 2.8, Lao Cai is situated near a major fault line, making it susceptible to significant earthquakes Notably, since 1900, Vietnam has experienced several notable earthquake events.
1) Earthquake occurrence in Dien Bien area (1935), M = 6.8
2) Earthquake in the Luc Yen (Yen Bai), 1953 and 1954, M = 5.4
3) Earthquake in the Bac Giang, (1961) M = 5.6
4) Earthquake in the Cau river, Nghia Binh province, (1970 and 1972), M = 5.3
Seismic network of Vietnam includes 24 stations: Phu Lien (1924), Nha Trang (1957), Sapa (1961), Bac Giang (1967), Hoa Binh (1972), Tuyen Quang (1973), Da Lat
Between 1980 and 2003, several significant developments occurred in Vietnam, including the construction of various locations such as Dien Bien, Lai Chau, Vinh, and Hanoi in 1990 Subsequent projects included Chua Tram, Tam Dao, Doi Son, Ba Vi, Met, and Yen Tu from 1996 to 2002, followed by Tuan Giao, Tram Tau, Song Ma, Lang Chanh, Thanh Hoa, and Moc Chau in 2003, as shown in Figure 2.7.
Figure 2.7 Seismic network of Vietnam (L M Nguyen et al., 2012)
Between January 2006 and December 2009, a significant series of earthquakes occurred in northern Vietnam, as evidenced by data from 14 broadband stations monitoring 53 shallow earthquakes Notably, the two most powerful earthquakes recorded in the last century took place in 1935 and 1983, along with the Dien Bien earthquake, all marked by open stars labeled as Nos 1, 2, and 3 in Figure 2.8.
Figure 2.8 Map of the recorded earthquake in North of Vietnam
Landslides causes and triggering mechanisms of Lao Cai province, Vietnam
Landslides are triggered by various factors, which can be categorized into four main causes: geological factors, including weak or weathered soil materials and structural weaknesses; morphological factors, such as tectonic activity, slope erosion, gradient, and vegetation removal; physical factors, including intense rainfall, earthquakes, and volcanic eruptions; and human factors, which involve activities like excavation, deforestation, mining, and construction that destabilize slopes Understanding these causes is crucial for effective landslide prevention and management.
Although there are several types of causes of landslides, the following causes are most of the damaging landslides:
Landslides primarily occur due to the saturation of slopes with water, which can result from intense rainfall, snowmelts, or fluctuations in groundwater levels Changes in water levels along coastlines, earth dams, and various water bodies also contribute to this phenomenon The relationship between landslides and flooding is significant, as both are influenced by precipitation, runoff, and ground saturation Indirect causes of landslides include increased rainfall and rising groundwater levels.
Mountainous regions prone to landslides are often affected by seismic activity Earthquakes in these steep areas can trigger landslides as the shaking causes soil materials to dilate, facilitating rapid infiltration of underground water.
Landslide hazards pose significant risks to human lives and infrastructure, but their impact can be mitigated through effective land-use policies and regulations implemented by local governments By restricting or prohibiting activities in hazard zones, communities can minimize exposure to these dangers Additionally, individuals can educate themselves about the historical hazard data of their locations and consult with local planning and engineering departments Engaging the expertise of professionals such as engineering geologists, geotechnical engineers, or civil engineers is essential for evaluating appropriate mitigation strategies against landslide risks.
According to the literature review, causal factors of landslide in Vietnam are as follows:
- Continuous rainfall (especially during the rainy season)
- Heavy rainfall (especially during the rainy season)
- Slope gradient, Steep slope, Vegetation removal
- Construction of new highway road
Literature review of causes by rainfall and earthquake in Lao Cai province
Lao Cai province has experienced significant landslides primarily due to heavy and continuous rainfall, as documented in various studies (Do et al.; Duan & Duc Vu, 2011; L C Nguyen et al., 2023; Van Tien et al., 2021; Yamasaki et al., 2021) Although specific earthquake data for Lao Cai is lacking, its proximity to historical earthquake events in 1935 and 1983, as well as major fault lines, raises concerns (L M Nguyen et al., 2012) Additionally, ground motion data from the Dien Bien earthquake on February 19, 2001, provides relevant insights, as Dien Bien province is also located in the Northwest region of Vietnam, adjacent to Lao Cai Consequently, this research utilizes recorded data from Dien Bien to enhance understanding of seismic risks in Lao Cai province.
Classifications of countermeasure for deep-seated landslides
In steep slope and highway roadside areas, it is crucial to identify effective countermeasures for deep-seated landslides to enhance safety The primary goal of these measures is to improve the safety factor, which indicates slope stability; a factor of safety below 1.00 signifies instability A range of countermeasures should be evaluated, and integrated solutions may be proposed to effectively raise the safety factor.
Landslide mitigation strategies can be categorized into two primary types: prevention and control Prevention measures include slope cutting with shotcrete, retaining walls, ground anchors, pile and anchor work, steel pile installations, shaft work, and spray crib techniques, often used in conjunction with reinforced earth methods On the other hand, control measures consist of catchment wells, lateral boring, earth removal, and counterweight embankments.
Table 2.1 Classification of countermeasure for deep-seated landslide (N.C.Koei &
No Control works Schematic illustration
Slope cutting with shotcrete is a straightforward and effective method for landslide prevention on irregular surfaces, requiring no specialized equipment This technique involves spraying a mixture of mortar and concrete onto the slope; however, it is not suitable for erodible sandy soils, weathered soft rocks, or colluvial deposits, as rainwater can lead to slouching or failure Adequate drainage facilities are essential when spring water is present, and a wire mesh must be anchored to the slope before spraying Additionally, spraying should be avoided during heavy rain and windy conditions to prevent the cement from being washed away Overall, shotcrete construction costs are significantly lower compared to other landslide prevention methods.
Retaining walls provide essential support for cuts or embankments, preventing instability and can be categorized into five main types: gabion walls, stone masonry walls, crib retaining walls, gravity retaining walls, and supported type retaining walls This conventional construction method is straightforward and does not necessitate specialized equipment; however, it requires a significant labor force and has a height limitation of approximately 6 meters, making it unsuitable for soft soil due to potential bearing failure Additionally, more space is needed for machinery mobilization and backfilling soil behind the wall during construction Economically, this method is more cost-effective compared to other stabilization techniques.
Ground anchors, including ground anchors, soil nails, and rock bolts, utilize high-strength steel materials for effective stabilization against sliding forces, making them suitable for large-scale slope failures Their design allows for deep penetration into the soil, addressing both shallow and deeply seated failure surfaces Expert installation and stringent quality control are essential to ensure anchorage capacity, particularly for medium to small-scale landslides This method is most effective when combined with other structural elements, such as concrete beam frames and reinforced walls, and requires double anti-corrosion treatment for durability Ground anchors can reach lengths of up to 40 meters, with 20 to 30 meters commonly used, though they are relatively costly at approximately $200 per meter In contrast, soil nails are limited to a maximum length of 12 meters and are suited for shallow slip-resistant surfaces, costing about $100 per meter, making them less effective for larger slip failures.
The installation of ground anchors and steel piles, known as pile with anchor work, involves the use of H-steel for horizontal coupling This technique is particularly effective for addressing landslides that are located close to roadways.
Steel pile work is a technique used to stabilize landslides by inserting a steel pipe into a large-diameter vertical hole Concrete is then packed both inside and outside the steel pile, anchoring it securely to the base ground layer This method is particularly effective in areas with gentle lower slopes where traditional anchor work may be challenging to implement Its successful application in various landslide-prone regions highlights its importance in enhancing ground stability.
Shaft work entails excavating a vertical pit with a diameter ranging from 2.5 to 6.5 meters, which is subsequently filled with reinforced concrete to serve a similar purpose as steel piles This method is typically employed in the lower sections of landslides, although it tends to incur higher construction costs compared to alternative techniques.
Spray crib work is an effective solution for enhancing slope stability by mitigating erosion and weathering This method involves the use of a lattice core reinforced with bars and the continuous application of concrete with a rectangular cross-section, which serves to stabilize the slope's surface.
Landslide control measures include groundwater drainage methods such as catchment wells and lateral boring, which help lower underground water levels Earth removal work typically involves excavating soil from the head of the landslide area, rather than the tail, unless in special circumstances This technique is highly effective and commonly employed for medium and small-scale landslides Additionally, counterweight embankment work involves adding earth to the lower part of the landslide area to enhance slope stability through counterbalancing.
Slope stability analysis and methods
Slope stability is a critical issue in geotechnical engineering, focusing on assessing the safety factor of slopes by ensuring that resisting forces exceed failure forces Stability analysis evaluates structural safety, identifies failure surface shapes, simulates stability under various geological and climatic conditions, monitors slope movement, and determines remedial measures Key factors like soil stratigraphy, strength parameters, and groundwater variations are essential for accurate analyses While traditional methods have evolved with advancements in technology and soil behavior theories, numerous contemporary techniques and extensive research, including discussions on the limit equilibrium method and finite element method, are available for assessing slope stability.
Limit Equilibrium Method (LEM)
The Limit Equilibrium Method (LEM) is a widely used technique for determining the safety factor of natural slopes and embankments, having been a staple in slope stability analysis since the early 20th century Its enduring popularity in geotechnical engineering stems from its simplicity and accuracy in assessing earth slope stability LEM involves defining a proposed slip surface, which is then analyzed to calculate the factor of safety This method is increasingly applied to the stability analysis of various structures, including tie-back walls and fabric-reinforced slopes, as well as evaluating sliding stability under high horizontal loads Different solution techniques for LEM are characterized by the equations of statics they satisfy and the relationships between intercolumn shear and normal forces, as illustrated in typical models of sliding masses discretized into slices, where normal and shear forces act on both the base and sides of each slice.
Figure 2.9 Selection flow chart of countermeasure
In LEM, the factor of safety against global failure (F.S G ) is defined as the ratio of the resisting force and driving force along the surface resistingforces
Unlike the finite element method, which factors in stress-strain relationships and soil deformation, the limit equilibrium method (LEM) focuses solely on stability analysis Pioneered by Petterson in 1916 for the Stigberg Quay in Gothenburg, Sweden, this method involved dividing the sliding mass into circular slices Over the decades, researchers such as Fellenius, Janbu, and Bishop refined the slice advancement calculations The 1960s saw the advent of advanced computer calculations, leading to the development of mathematical formulas by Morgenstern and Prince, as well as Spencer Various alternatives emerged based on the assumptions regarding inter-slice forces and equilibrium equations, including the Bishop and Fellenius methods Today, numerous slope stability calculation methods utilizing LEM are under development, and this study specifically employs a limit equilibrium slope stability analysis program based on the Bishop method for effective comparison with finite element analysis.
Modern limit equilibrium methods for slope stability analysis can be effectively conducted using various geotechnical engineering software programs Notable examples include GEO-SLOPE, SLIDE 2D and 3D, GEO5, and Oasys Slope, which are widely used in the industry.
In the 1950s, Professor Bishop from Imperial College, London, introduced a method that accounted for interslice normal forces while neglecting interslice shear forces, leading to the development of an equation for normal forces at the slice base by summing vertical slice forces Bishop’s Simplified Method is generally accurate for circular slip surface analysis, although it may yield different safety factors compared to the ordinary method due to numerical issues This method proves to be more precise than the Ordinary Method of Slices, particularly in effective stress analyses with elevated pore-water pressures While initially designed for circular slip surfaces, the underlying assumptions of Bishop’s Simplified Method can also be applied to noncircular slip surfaces The basic equation for the Bishop’s Simplified factor of safety, without considering pore-water pressure, is: tan sin tan 1 sin c W c.
The factor of safety (FS) is integral to both sides of the equation, resembling the traditional factor of safety equation However, it uniquely incorporates the term m α, defined as sin tan cos m α FS.
Figure 2.10 Bishop’s Simplified factor of safety (Calgary, 2020)
The Fredlund and Xing model was proposed in 1994 soil water characteristics curve The governing equation is as follows:
Where w = volumetric moisture content; s = saturated volumetric moisture content; e
= natural number; C () = correction factor; C () = 1; = negative pore water pressure; a, n, m = curve fitting parameter
The permeability coefficient of soil can be estimated using the volumetric moisture content function developed by Fredlund & Xing, which relies on the saturated volumetric moisture content The key governing equation for this estimation method is fundamental to understanding soil permeability.
The permeability coefficient (k w) relates to negative pore water pressure, while the saturated permeability coefficient (k s) and volumetric moisture content (θ s) are also important factors The variable 'y' serves as a dummy variable of integration, representing the logarithm of negative pore-water pressure The range of integration is defined by 'i', which spans from the least negative pore-water pressure (j) to the maximum negative pore-water pressure (N) Additionally, 'φ' denotes the suction for the j-th interval, and 'θ 0' represents the initial value of the equation.
For the stability analysis of unsaturated soil slope, the shear strength formula of unsaturated soil proposed by Fredlund et al (1978) proposed a linear relationship that is written as:
The equation for shear strength (𝜏) incorporates effective cohesion (𝑐) and net normal stress on the failure plane, represented as ( n - u a), where n is the total normal stress and u a is the air pore-air pressure Additionally, it considers pore water pressure (u w) and matric suction, defined as (u a - u w) The friction angle () and the angle ( b) that correlates the increase in shear strength with matric suction are also integral to the equation.
The GEO-SLOPE program offers powerful tools for slope stability and seepage analysis, with SLOPE/W analyzing slope stability and SEEP/W handling both saturated and unsaturated seepage Through seepage analysis, it provides essential data on pore-water pressure and volumetric water content distributions Additionally, one-dimensional infiltration analyses can be conducted using SEEP/W to explore rainfall infiltration patterns in an infinite slope slice.
The Finite Element Method (FEM) is a powerful computational tool in engineering, particularly for complex problems requiring reliable and accurate results Recently, it has been utilized to calculate the factor of safety, similar to limit equilibrium analysis (Griffiths, 1999) Unlike the limit equilibrium method, FEM accounts for both linear and non-linear stress-strain behavior of soil when assessing shear stress The input parameters for limit equilibrium analyses are also applicable for finite element analysis in evaluating slope stability FEM is renowned for accurately estimating realistic deformations and safety factors for slopes and embankments, making it significantly easier to compute the factor of safety for a slope.
In Finite Element Method (FEM) analysis of slopes, the strength reduction method is utilized to determine the safety factor by decreasing the soil's cohesion (c) and tangent of the friction angle (tan φ) As landslides occur, these parameters are diminished, and the strength reduction factor (∑ Msf) begins at 1 and increases until failure The global safety factor is defined as the total multipliers (∑ Msf) at the failure point, represented as the ratio of the initial strength parameters to the reduced ones.
tan tan input input sf reduced reduced
Where M sf = reduction factor of calculation; tan input and c input = soil parameters in accordance by the original conditions; tan reduced and c reduced = reduction parameters during the calculation process
The total value of ΣM sf is crucial for determining the stress parameters of soil in slope stability analysis calculations Throughout the analysis process, the safety factor is derived using a specific formula.
The advantages of the finite element analysis over conventional limit equilibrium analysis (Griffiths, 1999):
1) It is not necessary to divide the domain into vertical slices
2) Since there are no slices, no assumptions are required for side forces between slices
3) The FEM determines the locations of failure zones by calculation of stresses, without the search for a critical slip surface that is required in limit equilibrium analyses
For slope stability analysis, complex soil stress–strain models are unnecessary; instead, soil is modeled as an elastic-perfectly plastic material following Mohr–Coulomb failure criteria This model assumes constant soil parameters throughout loading and unloading stages and requires six key inputs: friction angle (θ), cohesion (c), dilation angle (ψ), deformation modulus (E), Poisson’s ratio (ν), and unit weight (γ) In this analysis, a dilation angle of ψ = 0 is utilized, reflecting a non-associated flow rule as indicated by Griffiths (1999).
The modernization of computer performance has led to a rise in the use of Finite Element Method (FEM) for geotechnical analysis Popular software for computer-based FEM slope stability analysis includes PLAXIS 2D and 3D, Rocscience Rs2 and Rs3, GEOTECH 2D and 3D, and MIDAS Geotech.
The Mohr-Coulomb failure model is extensively utilized in geotechnical applications, serving as a key criterion for defining failure Various criteria exist to determine failure, with some focusing on strain levels, while the majority assess shear stress relative to shear strength The Mohr-Coulomb failure criterion stands out as the most prevalent in soil mechanics, defined by the relationship f = c + σ tan(φ), as outlined in "Principles of Geotechnical Engineering" (Das & Sobhan, 2014, 8th Edition).
Where η f shear strength at failure, c' is effective cohesion, ζ f is effective stress at failure, and θ ' is the effective angle of friction
The basic idea of an elastic perfectly plastic model is to decompose the strains and strain rates into an elastic and a plastic part: e p e p
(2.10) where ε e and ε e are the elastic strain and strain rate, whilst ε p and ε p are the plastic strain and strain rate Figure 2.11 shows the decomposition of strains in a stress-strain graph
Figure 2.11 Elastic perfectly plastic model concept
Previous studies of slope stability using LEM (GEO-SLOPE) and FEM (PLAXIS)
(PLAXIS) for Lao Cai area
Numerous studies have examined the correlation between Limit Equilibrium Method (LEM) and Finite Element Method (FEM) in slope stability analyses within the Lao Cai area Researchers utilized advanced software like GEO-SLOPE and PLAXIS to assess slope stability, predict landslide blocks, and analyze rainfall-induced failures and safety factors Tran, Pham, et al (2021) evaluated the stability of cut slopes with varying hydraulic conductivities under different rainfall scenarios using SLOPE/W and SEEP/W, revealing the significant influence of soil hydraulic conductivity and rainfall on unsaturated cut-slope stability Hung et al (2021) simulated rainfall-induced slope failures in unsaturated soils with TRIGRS and SLOPE/W, validating their predictions against actual slope failures, which demonstrated the effectiveness of their approach in forecasting landslide occurrences Additionally, Do et al conducted seepage, stress deformation, and stability analyses for slow-moving landslides using PLAXIS 2D and field measurements Other research highlighted the impact of construction-related cutting activities on slope stability and proposed remedial solutions for deep-seated landslides through PLAXIS 2D (L C Nguyen et al., 2023).
Numerous studies have been conducted on landslides in the Lao Cai area, focusing on predicting landslide blocks, analyzing large-scale landslide features, and exploring mitigation solutions related to rainfall impacts on unsaturated cut-slopes While methods like PLAXIS and GEO-SLOPE have been utilized for slope stability simulations, there is a notable gap in research regarding the mechanisms of landslides and the effectiveness of countermeasures during rainfall and earthquake events Specifically, the application of Finite Element Method (FEM) using PLAXIS to assess initial and remedial slope stability in the context of ground motion from earthquakes remains underexplored Thus, further investigation is essential to enhance understanding of slope stability and develop effective solutions against landslides, employing both Limit Equilibrium Method (LEM) and FEM under varying rainfall and earthquake conditions.
DATA COLLECTION AND RESEARCH METHODOLOGY
Research methodology
This research investigates slope stability in various rock formations within the same province, focusing on conditions with and without remedy solutions The methodology encompasses three scenarios: normal conditions, rainfall-induced conditions, and earthquake-induced conditions for slopes To mitigate deep-seated landslides, three countermeasures were selected, considering factors such as design, construction site, timeline, and cost The study employs Limit Equilibrium Method (LEM) and Finite Element Method (FEM) for slope stability analysis, enabling the calculation of safety factors and slip failure surfaces By comparing results from both methods across the three conditions, the thesis aims to identify the most effective countermeasure for practical application.
This thesis utilizes the SLOPE/W and SEEP/W programs from GeoStudio 2018 to conduct a limit equilibrium slope stability analysis, focusing on the effects of rainfall and earthquakes on slope instability and stability.
FE slope stability analysis, PLAXIS 2D (V21.01) (PLAXIS CONNECT Edition V21.01 PLAXIS 2D-Tutorial Manual.) was utilized to simulate the slope stability and instability with rainfall-induced and earthquake-induced conditions
Figure 3.2 Numerical analysis flow chart
Data collection
This section outlines the field investigation for two case studies, focusing on topography and geological assessments It includes an analysis of geological structures based on geological maps, along with the collection of meteorological and seismic data Additionally, laboratory testing is conducted to determine input parameters essential for simulating numerical analyses The numerical modeling employs Limit Equilibrium Method (LEM) and Finite Element Method (FEM) techniques Data is sourced from design documents for projects addressing landslides along Provincial Road 155 and Road No 152 near Muong Hoa Cultural Park.
A field investigation was conducted to assess the topography, perform geological drilling, and collect soil samples from the landslide area to determine its geotechnical properties, as illustrated in Figures 3.3 and 3.4.
Figure 3.3 Slope collapsed area for case study 1 (Mong Sen)
Figure 3.4 Slope collapsed area for case study 2 (Muong Hoa)
Topography and geology setting for case study 1 (Mong Sen)
Case study 1 is situated at coordinates 22°25'1.68"N and 103°54'18.03"E, approximately 0.6 km from the new Mong Sen bridge The area features steep terrain characterized by heavily weathered soil and rock layers The survey route along National Highway 4D connects Lao Cai city to Sapa town, with a V-shaped topography that includes the Dum and Mong Sen streams at the valley's base The Dum stream originates from Sapa village, merges with the Mong Sen stream at the Mong Sen bridge, and ultimately flows into the Red River in Lao Cai city The geological composition of this region, as depicted in Figure 3.5, belongs to the Posen formation and consists of intrusive rocks such as diorite, granodiorite, and granite.
Figure 3.5 Geological map of case study 1 (Mong Sen)
The Po Sen Complex (δγPZ1ps) features granitoid blocks ranging from diorite to granodiorite and biotite-amphibolite granite, predominantly located in the eastern Phan Si Pan zone The largest block, Po Sen, spans an area of 250 km², while smaller blocks can be found in Lung Po, Ngoi Bo, and the northeast of Lech village Most rocks within this complex exhibit signs of crushing and milonitization, characterized by a banded and patchy gneiss structure, with the degree of migmatization and milonitization increasing from the edges toward the center of the block.
Quaternary undivided: Sedimentary slopes - accumulative (dpQ) distributed at the foot of low hills Sediment is chips, pebbles, grit mixed with powder, mixed clay
Topography and geology setting for case study 2 (Muong Hoa valley)
Sapa has the typical topography of the northern region, with steeps ranging from 35-
40 0 on average, some places with slopes above, rugged terrain, and complicated sections The study area has diverse and complex topography High hills, steep slopes,
The study area, encompassing the Old Mong Sen bridge and the New Mong Sen bridge, is characterized by the Posen formation, divided by a complex system of rivers and streams within the valleys Covering over 250 square miles, the natural land cover constitutes more than 90% of the total area, with predominant slopes directed east and south The Sapa region features numerous high mountain peaks, where the abrupt transition from low-lying areas to steep terrains creates a narrow horizontal strip with dangerously steep slopes The geological map of this study area is illustrated in Figure 3.6.
Da Dinh formation with the base rock composition being marble
Figure 3.6 Geological map of case study 2 (Muong Hoa)
The study area features a tectonic fault system oriented Northwest to Southeast, which significantly contributes to the landslide process This faulting leads to intense crushing of soil and rock, accelerates weathering, and deteriorates the physical and mechanical properties of the soil.
Ban Nguon formation Ban Pap formation Cam Duong formation Da Dinh formation Posen formation
Quatemary Sapa formation Sinh Quyen formation Suoi
Chieng formation Ya Yen Sun Complex
The geology of the region features a diverse range of lithological units spanning from the Proterozoic Era to the Quaternary Period Key formations include the Proterozoic Da Dinh formation, characterized by marble, dolomite, and tremolite-rich marble; the Sa Pa formation, consisting of sericite quartz schist and marble; and the Cambrian–Ordovician Cam Duong formation, which primarily contains conglomerate, gritstone, shale, lime, and apatite Additionally, the Po Sen complex formation is noted for its diorite, granodiorite, and granite, while the Siluric–Devonian Ban Nguon formation features shale and sandstone with lime content.
Pap formation(D 1-2 bp), which contains thin-layered limestone and clay-limestone The
The Ye Yen Sun complex formation (γEys) features granite and alkaline grano-syenite, marking it as the youngest rock formation of the Paleogene Period This formation is succeeded by Quaternary (dp) sediments, primarily consisting of cobbles, gravel, sand, and clay.
There are 9 bored holes in overview of the bored hole layout cross section Figure 3.7
The study area includes three cross sections, each containing three bored holes: BH-1, BH-2, and BH-3 in cross section 1-1; BH-4, BH-5, and BH-6 in cross section 2-2; and BH-7, BH-8, and BH-9 in cross section 3-3 Notably, the Zone 3 landslide area is located near cross section 3-3 In study area 1, the investigation involved conducting Standard Penetration Tests (SPTs) and collecting soil samples for laboratory analysis.
The bored hole layout cross section, illustrated in Figure 3.8, includes four drilled holes: BH-1, BH-2, BH-3, and BH-4 The landslide area of study area 2 is located near BH-2 and BH-4, as shown in cross section 1-1 Standard Penetration Tests (SPTs) were conducted to analyze soil samples obtained from each bored hole.
Figure 3.7 Geological bored hole layout cross section for case study 1 (Mong Sen)
Figure 3.8 Geological bored hole layout cross section for case study 2 (Muong Hoa)
Sapa town experiences a mountainous climate characterized by two distinct seasons: a cool, rainy summer from May to October and a cold winter The region's rainfall varies with altitude, with the rainy season contributing approximately 80% of the annual precipitation Average monthly rainfall is around 220 mm, peaking at 460 mm in October, while 2022 recorded an average of 150 mm per month, with maximums of 350 mm in May and August Months with lower rainfall typically see averages between 50-100 mm Additionally, hail is common in February, March, and April, as indicated by data from the Sapa Meteorological Station.
As the town of Sapa is mountainous, it experiences the tropical monsoon climate, with a dry, cold season from October to March and a rainy season lasting from April to September
(a) (b) Figure 3.9 Monthly rainfall and accumulative rainfall data (a) 2021; (b) 2022
Acceleration data collected from the Sapa station was utilized in the numerical simulation of Limit Equilibrium Method (LEM) and Finite Element Method (FEM) to analyze earthquake-induced slope stability, incorporating the seismic coefficient for accurate numerical analysis.
Table 3.1 Acceleration data from Vietnamese Standard (TCVN 9386-2012)
This thesis utilizes data from the Dien Bien earthquake recorded on February 19, 2001, at the Dien Bien station to perform numerical simulations (FEM) for dynamic slope stability analysis The findings from the Geophysical Research Institute, illustrated in Figure 3.10, indicate that the input ground acceleration can be applied to areas with similar earthquake source structures and conditions.
Record, Dien Bien earthquake in 19/02/2001, M s = 5.3; R hyp = 12 km, Dien Bien station, R rup = 19 km, PGA = 88.4 cm/s 2
Figure 3.10 Ground motion recorded from Dien Bien earthquake (2001)
Laboratory testing
Standard Penetration Tests (SPTs) were conducted to analyze the stratigraphy and mechanical properties of soil layers in the landslide areas of study areas 1 and 2, as illustrated in Figure 3.11 In study area 1, three bored holes reveal five distinct layers: the first layer consists of yellow/brown medium sandy clay with 40% boulder, varying in thickness from 5 to 15 meters and an uncorrected SPT value (Nspt) of 15 to 50 The fourth layer is similar but mixed with gravel, ranging from 4 to 10 meters in thickness and an Nspt of 12 to 20 The fifth layer is yellow/dark medium to hard sandy clay with gravel, exhibiting a thickness of 5 to 15 meters and an Nspt of 20 to 50 The ninth layer is gray, strongly weathered rock with a Total Core Recovery (TCR) of 40% to 50% and Rock Mass Quality (RQD) of 30% to 40%, with thicknesses between 3 to 10 meters Lastly, the tenth layer consists of gray weathered rock, showing a TCR of 60% to 70% and RQD of 50% to 60%, with significant thickness found at depths of 10, 30, and 15 meters, respectively.
In study area 2, two boreholes (BH-2 and BH-4) were analyzed, each containing two distinct layers In BH-2, Layer 1 consists of a golden brown-grey and brown soft to hard clay mixed with gravel, measuring 5.7 meters in thickness and exhibiting an uncorrected SPT value of N SPT = 7 ~ 8 Layer 2 is characterized by grayish-white, strongly weathered limestone, with a thickness of 1.8 meters, TCR ranging from 0% to 65%, and RQD between 0% and 48% The soil layers in BH-4 are similar to those found in BH-2.
2 Layer 1 thickness is 15.7 m with an uncorrected SPT value of N SPT = 5 ~ 13 Layer
2 thickness is 12.0 m with TCR = 0% ~ 65% and RQD = 0% ~ 48%
Grain size distribution data was collected through sieving analysis and hydrometer tests, while soil properties for each layer were assessed via laboratory tests The friction angle and cohesion of the soil were determined using direct shear tests under both natural and saturated conditions Additionally, compressive strength tests were conducted on weathered rock layers Detailed results of these laboratory tests are presented in Section 3.3.1.
Figure 3.11 Geological distribution of bored hole cross section (a) case study 1 (Mong
Sen); (b) case study 2 (Muong Hoa)
Table 3.2 and Table 3.3 show the physical and mechanical properties of each soil layer tested from laboratory Table 3.2 represents to analyze for slope stability of case study
1 Table 3.3 represents to analyze for slope stability of case study 2 In this Table, 9 th layer and 10 th layer are rock layers Grain size distribution curve of soil for both two case studies describe in Appendix
Table 3.2 Physical and mechanical properties of soil layer and rock layer
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Coefficient of compressibility (kPa), av 1-2
Cohesion at natural state (kPa), c
Friction angle at natural state (),
Cohesion at saturated state (kPa), c
Friction angle at saturated state (),
Dry compressive strength (kg/cm 2 ), R dry
Saturated compressive strength (kg/cm 2 ),
Table 3.3 Physical and mechanical properties of soil layer and rock layer
Soil Type Clay mixed with gravel
Soil Type Clay mixed with gravel
Coefficient of compressibility (kPa), av 1-2 4.609 -
Cohesion at natural state (kPa), c 18.4 -
Friction angle at natural state (), 13.35 -
Cohesion at saturated state (kPa), c 22.6 -
Friction angle at saturated state (), 17.28 -
Dry compressive strength (kg/cm 2 ), R dry - 246.4
Saturated compressive strength (kg/cm 2 ),
Input parameters for LEM and FEM model of two case studies
Geological input parameters ( , c, , E) for two case studies
Tables 3.4 and 3.5 outline the input parameters for soil layers utilized in the analysis of the Limit Equilibrium Method (LEM) and Finite Element Method (FEM) models for Case Study 1 Additionally, Tables 3.6 and 3.7 detail the soil layer input parameters necessary for the assessment of the LEM and FEM models in the subsequent case study.
2 Rock layer input parameters of both case studies 1 and 2 were taken from RocLab software
Table 3.4 Input parameters of soil material to input GEO-SLOPE (Case Study 1)
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Cohesion at natural state (kPa), c
Friction angle at natural state (),
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Cohesion at saturated state (kPa), c
Friction angle at saturated state (),
Table 3.5 Input parameters of soil material to input PLAXIS 2D (Case Study 1)
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Drainage Type Undrained Undrained Undrained Drained Undrained
Saturated unit weight (kN/m 3 ), sat
Friction angle at saturated state (),
Table 3.6 Input parameters of soil material to input GEO-SLOPE (Case Study 2)
Soil Type Clay mixed with gravel
Soil Type Clay mixed with gravel
Cohesion at natural state (kPa), c 18.40 150.0
Friction angle at natural state (), 13.35 30.0
Saturated unit weight (kN/m 3 ), sat 19.10 27.0
Cohesion at saturated state (kPa), c 22.60 150.0
Friction angle at saturated state (), 17.28 30.0
Table 3.7 Input parameters of soil material to input PLAXIS 2D (Case Study 2)
Soil Type Clay mixed with gravel
Unsaturated unit weight (kN/m 3 ), unsat 17.70 26.5
Saturated unit weight (kN/m 3 ), sat 19.10 27.0
Cohesion at saturated state (kPa), c 19.1 150.0
Friction angle at saturated state (), 17.28 30.0
Countermeasure input parameters for two case studies
Table 3.8 outlines the input parameters for countermeasures used to evaluate the LEM (GEO-SLOPE) model in two case studies, while Table 3.9 details the input parameters for countermeasures applied to the FEM (PLAXIS) model in the same two case studies.
Table 3.8 Input parameters of countermeasure to input GEO-SLOPE
Type of countermeasure Unit weight
Type of countermeasure Unit weight
Type of countermeasure Item Value Unit
Table 3.9 Input parameters of countermeasure to input PLAXIS 2D
Rainfall and Earthquake input parameters in LEM
Table 3.10 Input parameters of SWCC in SEEP/W for case study 1 (Mong Sen)
Soil type Material model Parameter Value Unit
Sandy Clay Saturated/Unsatu rated
Soil type Material model Parameter Value Unit
Weathered Rock Saturated/Unsatu rated
Table 3.11 Input parameters of Hydraulic Conductivity in SEEP/W for case study 1
Soil type Material model Parameter Value Unit
Sandy Clay Saturated/Unsatu rated
Weathered Rock Saturated/Unsatu rated
Table 3.12 Input parameters of SWCC in SEEP/W for case study 2 (Muong Hoa)
Soil type Material model Parameter Value Unit
Table 3.13 Input parameters of Hydraulic Conductivity in SEEP/W for case study 2
Soil type Material model Parameter Value Unit
Soil type Material model Parameter Value Unit
Figure 3.12 Input parameters window for SWCC and Hydraulic Conductivity Table 3.14 Rainfall parameters and Earthquake coefficient for GEO-SLOPE
Parameters Model condition Boundary condition
Rainfall, q Saturated/Unsaturated Hydraulic 1.28E-6 m/sec SEEP/W
Rainfall and Earthquake input parameters in FEM
Table 3.15 Ground water flow parameters of soil material for case study 1 (Mong
Properties 1 st layer 4 th layer 5 th layer 9 th layer 10 th layer
Table 3.16 Ground water flow parameters of soil material for case study 2 (Muong
Soil Type Clay mixed with gravel
Figure 3.14 Input parameters window for ground water flow
Table 3.17 Rainfall parameters and Earthquake coefficient for PLAXIS
Infiltration 0.1106 m/day Steady state ground water flow
Figure 3.15 Input parameters window for pseudo-static and ground motion earthquake
Model geometry to analyze slope stability by LEM and FEM
3.5.1 Model geometry of case study 1 (Mong Sen)
This article analyzes the initial slope geometry and countermeasure slope stability in relation to deep-seated landslides It identifies three slope stabilization methods to mitigate future landslide risks The first method involves slope cutting with shotcrete, while the second method offers two options for concrete retaining walls: one without bored piles at the slope's toe and another with bored piles The third method combines ground anchors and soil nails with a concrete beam frame and a reinforced concrete wall Detailed cross-sectional geometry is provided in the appendix.
3.5.2 Model geometry of case study 2 (Muong Hoa)
This case study analyzes initial slope geometry and countermeasure solutions for deep-seated landslide stability It identifies three slope stabilization methods aimed at enhancing shear strength and minimizing sliding actions along the slip surface The first remedy involves slope cutting with shotcrete, while the second solution features a concrete retaining wall without bored piles at the slope's toe, as installation is hindered by proximity to the rock layer The third remedy combines ground anchors and soil nails with a concrete beam frame and reinforced concrete wall Detailed cross sections of the slope geometry can be found in the Appendix.
Numerical modelling of case study 1 (Mong Sen) based on LEM and FEM
This thesis presents a slope stability analysis utilizing the limit equilibrium (LE) numerical method through the GeoStudio software, with model geometry outlined in the appendix The study investigates the effects of various countermeasures on slope stability across three scenarios: normal conditions, conditions with rainfall infiltration, and conditions with seismic influence The SLOPE/W software was employed to predict potential slope failure surfaces, applying the Mohr-Coulomb shear strength criterion Rainfall-induced effects were analyzed using SEEP/W to evaluate pore water pressure and saturation levels, while earthquake impacts were simulated through a pseudo-static analysis method This approach models seismic forces as a permanent body force in the limit equilibrium analysis, calculating the necessary horizontal seismic coefficient to adjust the Factor of Safety (F.S.) of the trial slip surface Additionally, the initial groundwater level was factored in through pore water pressure represented by a piezometric line The LEM program requires the specification of slip surfaces for trial surface determination, contrasting with the FEM approach used in PLAXIS Detailed geotechnical parameters are elaborated in Section 3.4, with Figures 3.16 and 3.17 illustrating the numerical model of the initial slope and the remedy solutions under rainfall conditions.
Figure 3.16 Model geometry (a) initial slope; (b) remedy solution 1; (c) remedy solution 2 (option 1); (d) remedy solution 2 (option 2); (e) remedy solution 3
Figure 3.17 Model geometry of SEEP/W by hydraulic boundary
This research utilized the FE numerical method through the PLAXIS 2D program to conduct slope stability analysis under three conditions: normal, rainfall infiltration, and pseudo-static with ground motion acceleration A plain strain model was employed, assuming zero displacement in the z-direction, while a non-homogeneous, two-dimensional plane strain soil material model was utilized The elastic-perfectly plastic Mohr-Coulomb model, based on isotropic elasticity, was applied to both soil and rock materials for initial analysis, allowing for fast computations and first estimates of deformations For earthquake analysis, both dynamic multiplier and pseudo-static options were implemented, with pseudo-static forces simulating horizontal movements and dynamic loads defined by multipliers Rainfall-induced slope stability was analyzed using steady-state groundwater flow calculations, where seepage and infiltration were modeled as flow boundary conditions The study aimed to determine the M sf value and slip surface, with detailed soil parameter properties outlined in Section 3.4, supported by numerical models illustrated in Figures 3.18 and 3.19.
Figure 3.18 Model geometry (a) initial slope; (b) remedy solution 1; (c) remedy solution 2 (option 1); (d) remedy solution 2 (option 2); (e) remedy solution 3
Figure 3.19 Model geometry of rainfall condition by infiltration boundary
Numerical modelling of case study 2 (Muong Hoa) based on LEM and FEM
The numerical model of the initial slope incorporates remedy solutions and rainfall boundaries using LEM (SLOPE/W and SEEP/W) SLOPE/W is utilized to determine the critical factor of safety (F.S.) value and assess slope stability, while SEEP/W analyzes soil conditions under both saturated and unsaturated states due to rainfall infiltration In this model, the initial groundwater level is represented by a piezometric line positioned above the rock layer Stability analysis is performed with SLOPE/W to identify entry and existing slip surfaces, facilitating the determination of trial slip surfaces.
Figure 3.20 Model geometry (a) initial slope; (b) with remedy solution 1; (c) with remedy solution 2; (d) with remedy solution 3
Figure 3.21 Model geometry of SEEP/W by hydraulic boundary
The numerical model of the initial slope, developed using FEM (PLAXIS 2D), aims to determine the M sf value and identify the slip surface while incorporating rainfall boundaries and remedy solutions In this model, groundwater levels are influenced by seepage above the rock layer, and rainfall effects are represented by surface infiltration and countermeasures Additionally, for earthquake conditions, horizontal acceleration and ground motion are applied through pseudo-static and dynamic multipliers.
(c) (d) Figure 3.22 Model geometry (a) initial slope; (b) with remedy solution 1; (c) with remedy solution 2; (d) with remedy solution 3
Figure 3.23 Model geometry of rainfall condition by infiltration boundary