(Luận văn thạc sĩ) remedy solutions for deep seated landslides case studies in lao cai province, vietnam

INTRODUCTION

General background

A landslide is defined as the movement of a mass of rock, debris, or earth down a slope, often triggered by factors such as rainfall, earthquakes, volcanic activity, or human activities that disturb the slope This geological phenomenon occurs when the soil mass above a boundary deep in the ground shifts downward due to rising groundwater and an imbalance in the soil mass, leading to various types of movement, including rock falls, slides, and flows Landslides, categorized as "mass wasting," typically occur at relatively low inclination angles of about 5 to 20 degrees Key indicators of landslide mechanisms include small deformations, main scarps, and various types of cracks observed on the slope surface.

Figure 1.1 Schematic illustration of landslides (Varnes, 1978)

Landslides, categorized as deep-seated or shallow, pose significant geological hazards in mountainous and coastal regions, with depths ranging from less than 3 meters to several hundreds of meters These natural disasters threaten human safety, property, and infrastructure, impacting both developed and developing nations Various geological and environmental factors, including prolonged heavy rainfall and human activities such as slope alteration during construction, contribute to the occurrence of landslides In particular, construction practices in developing countries have exacerbated the frequency of these events, highlighting the urgent need for effective management and mitigation strategies.

Landslides are categorized based on their movement and the materials involved, which can be either rock or soil Soil is further classified as earth when it consists of sand-sized or finer particles, and as debris when made up of coarser fragments The movement type indicates the internal mechanics of the landslide, including fall, topple, slide, spread, or flow.

Table 1.1 Types of landslides, abbreviated version of Varnes' classification of slope movements (Varnes, 1978) h

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 the North-South railway and major highways, including the Ho Chi Minh route, affecting the northern mountainous and central regions (Duc, 2013; Luong et al., 2017; M Nguyen & Tran, 2020) A catastrophic rainfall-induced landslide in Quang Tri province in 2020 marked the deadliest event in Vietnam's history (Van Tien, Luong, Duc, et al., 2021) Additionally, on November 5, 2017, a landslide-triggered tsunami-like wave struck the Truong River in Bac Tra My District, Quang Nam province (Duc et al., 2020) On October 13, 2020, heavy rainfall caused a rapid rotational landslide in Phong Xuan commune, Thua Thien Hue province (Van Tien, Trinh, et al., 2021) Another significant landslide occurred on December 15, 2005, in Van Canh district, Binh Dinh province, triggered by rainfall (Duc, 2013) Among all regions, Lao Cai province is the most frequently impacted 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 main regions: Northern, Central, and Southern My research focuses on two study areas located in the Northern region, which is characterized by a gradual elevation decrease from northwest to southeast This region comprises three zones: the mountainous Northeast and Northwest, and the low-lying Red River Delta Overall, the Northern region consists of 23 provinces, while the Central and Southern regions contain 18 and 17 provinces, respectively.

Lao Cai is a northern province in 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, 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.

This study focuses on two landslide cases in Northern Vietnam, specifically in the Sapa district of Lao Cai Province, which spans 675.8 km² and features elevations ranging from 150 m to over 3000 m Known for its popularity as a tourist destination, Sapa has recently witnessed an increase in both the frequency and severity of landslides, primarily driven by rapid urbanization, construction activities, and agricultural practices Research indicates that most landslide events in this region are 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) h

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, close to 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 Sapa district, where the National Highway 4D connects Lao Cai city and Sapa This expressway, completed in 2018, traverses a predominantly mountainous region that is prone 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 the Muong Hoa valley and the Muong Hoa cultural project in Cau May commune, Sapa town, Lao Cai province, Northwestern Vietnam Geographically, it is positioned at 22°19'11.95"N and 103°51'20.05"E, approximately 2.3 km southeast of Sapa town This provincial road is adjacent to a significant economic corridor and lies within a mountainous region that is recognized as a key 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 land being mountainous and hilly, makes it susceptible to natural disasters such as landslides, soil erosion, and flash floods, particularly during the monsoon season The country, located on the eastern edge of the Indochinese peninsula, has experienced an average of 12 typhoons annually from 1961 to 2010, leading to significant rainfall and flooding Major landslide events frequently occur along critical transport routes, including the North-South railway and national highways, particularly in regions like the Northern mountainous area, Lao Cai province, and Sapa town, near 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, experienced significant geological challenges due to heavy and continuous rainfall, leading to landslides (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 the 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 slope excavation for the new highway (L C Nguyen et al., 2023) These landslides were categorized into two zones, with severe soil erosion and water dissipation noted between them during heavy rain, resulting in additional deep-seated landslides in the rainy season of 2021 While the previous landslide zones exhibited significant sliding surfaces, the newly affected area had a smaller sliding surface in comparison.

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 this area primarily included basic methods such as slope cutting, shotcrete application, and surface drainage Notably, a construction project was underway across from the landslide location, which is situated near a slope that had previously utilized shotcrete and surface drainage techniques It is important to note that the landslide in question is classified as a shallow landslide rather than a deep-seated one.

This research focuses on two study areas: the new highway road and a crucial economic road corridor Effective countermeasures are essential to address deep-seated landslides along the highway's roadside Despite numerous studies conducted over the years, slope stabilization methods for rainfall and earthquake-induced landslides in these regions continue 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 h

Outline and structure of the thesis

Figure 1.6 Flow chart shows the outline of the thesis (a) general framework of the research; (b) flow of the analysis steps h

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 covers a comprehensive literature review on landslides, exploring their causes and triggering mechanisms It discusses the classification of deep-seated landslides and examines various countermeasures Additionally, it delves into slope stability analysis and the methods used, including the formulation of the Limit Equilibrium Method and the Finite Element Method.

Chapter 3: Data Collection and Research Methodology

This chapter discusses the comprehensive data collection process, including topographical and geological investigations, rainfall and seismic data, field studies, and laboratory testing It outlines the procedures for numerical analysis, detailing the input parameters and numerical modeling techniques utilized in the 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 h

Findings and Research contributions

This study investigated the effects of rainfall infiltration and pseudo-static acceleration on saturated and unsaturated soil, as well as ground motion acceleration during earthquake conditions The research focused on both the initial slope and remedial solutions, employing numerical methods such as Limit Equilibrium Method (LEM) using GEO-SLOPE and Finite Element Method (FEM) with PLAXIS 2D.

This research demonstrates that initial slope stability aligns with real-world field results, providing effective countermeasures against landslides triggered by rainfall and earthquakes Additionally, the FEM (PLAXIS 2D) model proves to be a reliable tool for designing practical solutions for slope stabilization.

This research provides valuable insights into effective slope stabilization methods applicable to landslides, particularly in mountainous highway regions The findings can benefit similar sites globally, extending beyond the specific area of this study.

This research focuses on two study areas featuring distinct rock formations, allowing for the application of similar methods and models in comparable rock formation regions, such as Sapa town.

LITERATURE REVIEW

Literature review of case studies

This article emphasizes the necessity of reviewing and analyzing relevant studies for every research project, particularly focusing on landslide events and the historical context of earthquakes in Vietnam It discusses various triggering mechanisms of landslides, methods for slope stabilization, and techniques for slope stability analysis, including the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM) Additionally, it highlights previous studies on slope stability analysis conducted in the Lao Cai area using GEO-SLOPE and PLAXIS, as detailed in Section 2.7.

Recent studies in geotechnical engineering have focused on landslide prevention methods Notably, Zhang et al (2023) conducted research that integrated site investigations, deformation monitoring, laboratory tests, and theoretical calculations to examine the detailed evolution of a significant expressway roadside.

In August 2018, a significant landslide occurred at the Banguang toll gate of the Huizhou – Shenzhen Coastal Expressway in the Guangdong – Hong Kong – Macao Greater Bay Area, South China The primary cause of the landslide was attributed to mountain excavation for expressway construction, compounded by 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 that struck the Chattogram Hill Tracts (CHT) region of Bangladesh on June 13, 2017, which is noted as one of the deadliest landslides in the country The study identified excessive rainfall, inadequate drainage systems, and soft soil deposits on slopes as key triggering factors for the landslide Utilizing finite element modeling through PLAXIS 2D, the research revealed that many hill slopes were prone to failure following heavy rainfall Additionally, the study proposed bioengineering techniques, particularly the use of vegetation, as sustainable solutions to mitigate landslide risks, while emphasizing slope drainage as an effective measure to enhance safety.

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 inadequate slope stabilization along the mountainous highway, this disaster occurred in an area 270 km southeast of Yangon, characterized by hilly terrain and a crucial connecting road from Mawlamyine to Yangon The study utilized remote sensing images, Digital Elevation Models (DEM), and limited fieldwork to compile a landslide inventory, with topographic features analyzed using ArcGIS.

Figure 2.3 Pre-event and Post-event of Thae Phyu Kone landslide (Panday & Dong,

Research indicates that the northern region of Vietnam is particularly prone to landslides, with a significant deep-seated landslide occurring on July 21, 2018, triggered by heavy rainfall along the Halong-Vandon expressway in Quang Ninh province This event was exacerbated by five consecutive days of rainfall, primarily during slope cutting The study conducted detailed geological investigations, UAV surveys, and data analysis—including geology, geomorphology, and rainfall events—using the PLAXIS 2D model to assess the sliding surface of the slope The findings led to the proposal of comprehensive remedial solutions and slope stabilization methods to mitigate future landslide risks.

Figure 2.4 Landslide body on Halong-Vandon new expressway

In their 2021 study, Tran, Pham, et al examined 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 aimed to assess the stability of cut-slopes made of both low and high hydraulic conductivity soils under varying rainfall conditions To analyze the rainfall-induced slope failure process, the study utilized the coupled modules SEEP/W and SLOPE/W.

Figure 2.5 Sliding failure along the Noi Bai – Lao Cai highway

Recent research has explored a non-linear, time-variant method to simulate rainfall-induced slope failures in unsaturated soil using the TRIGRS program and SLOPE/W (Tran et al., 2021; Van Thang et al., 2021) The study focused on a landslide that occurred on August 5, 2019, along Provincial Road No 152 at Km 9+100, near the border of Hau Thao, Ta Van, and Su Pan communes in Sapa district, Lao Cai province, which resulted in one fatality and significant road blockage T V Tran et al (2021) investigated future landslide prediction and updated topographic conditions using Scoops 3D, GIS, and the limit equilibrium method Additionally, the landslide susceptibility map was developed through 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 data; however, it is situated near significant fault lines, indicating a potential for seismic activity in northern Vietnam As illustrated in Figure 2.8, this region has a history of serious earthquakes Notably, since 1900, several significant earthquake events have been recorded in Vietnam.

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, significant construction projects took place across various locations in Vietnam, including Dien Bien, Lai Chau, Vinh, and Hanoi, which were established in 1990 Subsequent developments occurred in Son La and Hue during 1994-1995, followed by the construction of Chua Tram, Tam Dao, Doi Son, Ba Vi, Met, and Yen Tu from 1996 to 2002 The final phase of this expansion included Tuan Giao, Tram Tau, Song Ma, Lang Chanh, Thanh Hoa, and Moc Chau in 2003, as illustrated in Figure 2.7.

Figure 2.7 Seismic network of Vietnam (L M Nguyen et al., 2012)

Between January 2006 and December 2009, the northern region of Vietnam experienced significant seismic activity, with 53 shallow earthquakes recorded by 14 broadband stations Notably, the two strongest earthquakes of the last century occurred in 1935 and 1983, alongside the Dien Bien earthquake, which are highlighted by three 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 a combination of geological, morphological, physical, and human factors Geological causes include weak or weathered soil materials and structural weaknesses such as shearing and fissuring Morphological factors involve tectonic activity, slope erosion, gradient, shape, vegetation removal, and freeze-thaw cycles Physical triggers encompass intense rainfall, rapid water level changes, earthquakes, and volcanic eruptions Human activities contributing to landslides include slope excavation, loading, irrigation, deforestation, mining, artificial vibrations, water leaks, and infrastructure development like highways and railroads.

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, and fluctuations in groundwater levels Additionally, changes in water levels along coastlines, earth dams, and the banks of lakes and rivers 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 rainfall and rising groundwater levels.

Mountainous areas susceptible to landslides are often affected by seismic activity Earthquakes in these steep regions 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 regulating activities in hazard zones, communities can minimize exposure to these dangers Additionally, the public can enhance their safety by researching the historical hazard data of specific sites and consulting with local planning and engineering departments Engaging professional services from engineering geologists, geotechnical engineers, or civil engineers is crucial for accurately assessing and implementing effective mitigation strategies against landslide hazards.

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 numerous landslides primarily due to heavy and continuous rainfall, as noted in studies by Do et al (2011), Duan & Duc Vu (2011), L C Nguyen et al (2023), Van Tien et al (2021), and Yamasaki et al (2021) Although there is no precise earthquake data for Lao Cai, its proximity to significant past earthquakes in 1935 and 1983, along with major fault lines, indicates potential seismic risk (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, close to Lao Cai (Vu, 2021) This research utilizes recorded data from Dien Bien to enhance the understanding of geological hazards in the region.

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 countermeasures is to improve the safety factor; a safety factor below 1.00 indicates slope instability To address this issue, various countermeasures should be evaluated, and integrated solutions may be proposed to elevate the safety factor.

Table 2.1 categorizes typical landslide countermeasures into two main types: prevention and control Landslide prevention methods include slope cutting with shotcrete, retaining walls, ground anchor systems, pile and anchor work, steel pile installations, shaft work, and spray crib techniques, often combined with reinforced earth methods In contrast, landslide control measures involve the use 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 an effective and straightforward method for landslide prevention, particularly on irregular slopes, as it does not require specialized equipment This technique involves spraying a mixture of mortar and concrete onto the slope but is not suitable for erodible sandy soils, weathered soft rocks, or colluvial deposits, as rainwater can lead to slouching or failure Adequate drainage is essential when spring water is present, and a wire mesh must be anchored on the slope before application It is crucial to avoid spraying during heavy rain and windy conditions to prevent the cement from being washed away Additionally, this method offers significantly lower construction costs compared to other landslide prevention techniques.

Retaining walls are essential structures designed to support cuts or embankments and prevent instability They 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 workforce 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 soil backfilling during construction Economically, this method is considered cost-effective compared to other stabilization techniques.

Ground anchors, which include ground anchors, soil nails, and rock bolts, utilize high-intensity steel materials for their pulling elements, offering significant resistance to sliding forces and stabilizing large-scale slope failures They can penetrate deep into the soil, addressing both shallow and deeply seated potential failure surfaces Specialist design and installation are essential, with rigorous quality control necessary to ensure anchorage capacity While effective for medium to small-scale landslides, ground anchors are best combined with other structural methods, such as concrete beam frames and reinforced walls Maintenance involves double anti-corrosion treatment to enhance durability Ground anchors can reach lengths of up to 40 meters, with typical usage at 20 to 30 meters, but are relatively expensive at approximately $200 per meter In contrast, soil nails are limited to shallow slip-resistant surfaces with a maximum length of 12 meters, making them unsuitable for larger failures, yet they are more cost-effective at around $100 per meter.

The installation of ground anchors and steel piles, known as pile with anchor work, is essential for stabilizing landslides, particularly those occurring near roadways This method utilizes H-steel for horizontal coupling, ensuring effective support and safety in vulnerable areas.

Steel pile installation is a technique used to stabilize landslide-prone areas by inserting a steel pipe into a drilled 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 regions with gentle lower slopes where traditional anchor work may be challenging to implement.

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 perform similarly to steel piles This method is typically employed in the lower sections of landslides and is associated with higher construction costs compared to alternative techniques.

Spray crib work is an effective countermeasure for slope cutting, aimed at enhancing surface stability by mitigating erosion and weathering This structure features a core of reinforcing bars arranged in a lattice formation, complemented by a continuous application of concrete with a rectangular cross-section Its primary purpose is to stabilize the slope face, ensuring long-lasting protection and durability.

Landslide control measures, such as catchment well and lateral boring work, effectively manage groundwater drainage by lowering underground water levels Earth removal, typically conducted at the head of the landslide area rather than the tail, is a reliable method often employed for medium and small-scale landslides Additionally, counterweight embankment work involves adding earth to the lower portion of the landslide area to stabilize the slope through counterbalancing.

Slope stability analysis and methods

Slope stability is a critical issue in geotechnical engineering, focusing on evaluating the safety factor of slopes Effective slope stability analyses ensure that resisting forces exceed failure forces, assessing the safety of structures, understanding failure surface shapes, and simulating stability based on geological and climatic factors Key elements such as soil stratigraphy, strength parameters, and groundwater variations are essential for accurate analysis The evolution of slope stability methods has been driven by technological advancements, improved instrumentation, and new soil behavior theories Today, various methods, including the limit equilibrium and finite element methods, are employed, supported by extensive research in the field.

Limit Equilibrium Method (LEM)

The Limit Equilibrium Method (LEM) is a widely used technique for evaluating the safety factor of natural slopes and embankments, having been a cornerstone of slope stability analysis since the early 20th century This method has maintained its popularity in geotechnical engineering due to its simplicity and accuracy in assessing earth slope stability LEM involves defining a proposed slip surface, which is then analyzed to determine the factor of safety Additionally, limit equilibrium formulations based on the method of slices are increasingly applied to the stability analysis of structures like tie-back walls and reinforced slopes, as well as evaluating sliding stability under high horizontal loads Various solution techniques exist within the limit equilibrium framework, distinguished by the statics equations they satisfy and the relationships between intercolumn shear and normal forces.

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 accounts for stress-strain relationships and soil deformation, the limit equilibrium method (LEM) focuses on stability analysis without these considerations In 1916, Petterson introduced the stability analysis of Stigberg Quay in Gothenburg, Sweden, utilizing a circular slip surface and dividing the sliding mass into slices This approach was further refined by researchers such as Fellenius, Janbu, and Bishop over the following decades 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 methods, including those proposed by Bishop and Fellenius, emerged based on different assumptions regarding slice interactions and equilibrium equations Today, numerous slope stability calculation methods using LEM are in development, and this study employs a limit equilibrium slope stability analysis program based on the Bishop method for effective comparison with finite element analysis.

Modern limit equilibrium software programs for LEM slope stability analyses include GEO-SLOPE, SLIDE 2D and 3D, GEO5, and Oasys Slope, all of which leverage advancements in geotechnical engineering.

In the 1950s, Professor Bishop at Imperial College London introduced a method that considered interslice normal forces while neglecting interslice shear forces, leading to the formulation of an equation for normal forces at the slice base by summing vertical slice forces Bishop’s Simplified Method is generally accurate for most conditions, although it may yield different safety factors compared to the ordinary method of slices due to numerical issues This method is particularly effective for analyzing circular slip surfaces and demonstrates greater accuracy than the Ordinary Method of Slices, especially in effective stress analyses with high pore-water pressures While originally designed for circular slip surfaces, the assumptions of Bishop’s Simplified Method can also be applied to noncircular slip surfaces In scenarios without pore-water pressure, the simplified factor of safety equation is expressed as: tan sin tan 1 sin c W c.

The factor of safety (FS) is present on both sides of the equation, resembling the standard factor of safety equation, with the exception of the m α term This term is 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 h

The permeability coefficient function can be estimated using the volumetric moisture content function proposed by Fredlund & Xing, which relies on the saturated volumetric moisture content of the soil This method is governed by a specific equation that links these parameters effectively.

The permeability coefficient (k w) relates to negative pore water pressure, while the saturated permeability coefficient (k s) indicates the maximum water flow capacity The volumetric moisture content (θ s) is essential for understanding soil moisture levels In this context, 'e' is the natural number, and 'y' serves as a dummy variable representing the logarithm of negative pore-water pressure The interval (i) ranges from the least negative pore-water pressure (j) to the maximum (N) described by the final function Additionally, 'φ' denotes the suction corresponding to 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 (2.6) describes the relationship between shear strength (𝜏) and various factors influencing it, including effective cohesion (𝑐) and net normal stress on the failure plane, represented as (σ n - u a) Here, σ n denotes total normal stress, while u a and u w represent air pore pressure and pore water pressure, respectively The difference between air pore pressure and pore water pressure, (u a - u w), indicates matric suction, which, along with the friction angle (φ) and the angle (φ b) that connects the increase in shear strength to matric suction, plays a crucial role in understanding soil behavior under stress.

The GEO-SLOPE program offers powerful tools for slope stability analysis through SLOPE/W and saturated/unsaturated seepage analysis via SEEP/W These tools effectively derive pore-water pressure and volumetric water content distributions from seepage data To examine rainfall infiltration patterns in an infinite slope slice, one-dimensional infiltration analyses can be conducted using the SEEP/W program, enabling a comprehensive understanding of water movement in slope stability assessments.

The Finite Element Method (FEM) is a powerful computational tool in engineering, essential for obtaining reliable and accurate results in complex engineering problems 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 same input parameters required for limit equilibrium analyses are adequate for FEM in evaluating slope stability Known for estimating realistic deformations and safety factors of slopes and embankments, FEM simplifies the computation of slope safety factors.

In finite element method (FEM) analysis of slope stability, the strength reduction method is utilized to determine the safety factor by systematically decreasing the soil's cohesion (c) and the tangent of the friction angle (tan φ) This reduction simulates the conditions leading to a landslide in natural slopes and embankments The strength reduction factor (∑ MsF) starts at 1 and increases until failure occurs The global safety factor is defined as the total multipliers (∑ MsF) at the failure point, representing the ratio of the initial strength parameters to the reduced strength parameters.

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 essential for determining the stress parameters of soil in slope stability analysis Throughout the calculation process, the overall 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, as soil can be effectively modeled as an elastic-perfectly plastic material following Mohr-Coulomb failure criteria This model assumes constant soil parameters throughout loading and unloading stages, requiring six key inputs: friction angle (θ), cohesion (c), dilation angle (ψ), deformation modulus (E), Poisson’s ratio (ν), and unit weight (γ) In this context, a dilation angle of ψ = 0 is utilized, reflecting a non-associated flow rule based on findings from Griffiths (1999).

With advancements in computer performance, the use of Finite Element Method (FEM) in geotechnical analysis has gained popularity Leading 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 soil failure Various failure criteria exist, with some focusing on strain levels, while the majority assess shear stress relative to shear strength Among these, the Mohr-Coulomb failure criterion stands out as the most prevalent in soil mechanics, 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

Recent studies have established a correlation between Limit Equilibrium Method (LEM) and Finite Element Method (FEM) slope stability analyses in the Lao Cai area Researchers utilized advanced software such as GEO-SLOPE and PLAXIS to assess slope stability, predict landslide blocks, and analyze rainfall-induced failures Tran, Pham, et al (2021) examined the stability of cut slopes with varying hydraulic conductivity under different rainfall scenarios using SLOPE/W and SEEP/W, highlighting the significant influence of soil hydraulic properties on unsaturated cut-slope stability Similarly, Hung et al (2021) simulated rainfall-induced slope failures in unsaturated soils with TRIGRS and SLOPE/W, validating their predictions against actual slope failures, thus demonstrating the effectiveness of their approach in forecasting landslide occurrences Additionally, Do et al conducted seepage, stress deformation, and stability analyses using PLAXIS 2D, while L C Nguyen et al (2023) investigated the impact of road construction on sloping soil masses and proposed remedial solutions for deep-seated landslides.

Numerous studies have focused on landslides in the Lao Cai area, addressing aspects such as landslide prediction, characteristics of large-scale landslides, mitigation strategies, and the impact of rainfall on the stability of unsaturated cut-slopes While methods like PLAXIS and GEO-SLOPE have been utilized for slope stability simulation, there is a notable gap in research concerning the mechanisms of landslides and the effectiveness of countermeasures during rainfall and seismic events Specifically, the application of Finite Element Method (FEM) using PLAXIS to assess the initial and remedial slope stability under earthquake conditions remains underexplored Thus, further investigation into slope stability, incorporating both Limit Equilibrium Method (LEM) and FEM under varying rainfall and earthquake scenarios, is essential for the Lao Cai region.

DATA COLLECTION AND RESEARCH METHODOLOGY

Research methodology

This research investigates slope stability across various rock formations in the same province, focusing on conditions with and without remedial solutions The study is structured into three main parts: assessing normal slope conditions, evaluating rainfall-induced impacts, and analyzing earthquake-induced scenarios, all considering remedial solutions Three countermeasures are selected to mitigate deep-seated landslides, with their effectiveness evaluated based on design, site conditions, construction duration, and cost, as detailed in the Appendix Utilizing Limit Equilibrium Method (LEM) and Finite Element Method (FEM) for stability analysis, this thesis aims to determine safety factors and potential slip failure surfaces, comparing results from both methods under the specified conditions Ultimately, the research identifies the most effective countermeasure based on comprehensive analysis, facilitating practical application in slope stability management.

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-induced and earthquake-induced conditions 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 h

Figure 3.2 Numerical analysis flow chart h

Data collection

This section outlines the field investigation conducted for two case studies, focusing on topography and geological assessments, geological structures derived from geological maps, and the collection of meteorological and seismic data It includes laboratory testing and the input parameters necessary for simulating numerical analyses Additionally, numerical modeling is performed using Limit Equilibrium Method (LEM) and Finite Element Method (FEM) for the selected case studies The data utilized is sourced from design documents related to the projects addressing landslides on 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) h

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 follows National Highway 4D, connecting Lao Cai city to Sapa town, with a V-shaped cross-section topography that includes the Dum and Mong Sen streams 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 survey route runs along the left mountainside of the valley, and as indicated in the geological map, the region's geological composition belongs to the Posen formation, comprising intrusive rocks such as diorite, granodiorite, and granite.

Figure 3.5 Geological map of case study 1 (Mong Sen)

The Po Sen Complex (δγPZ1ps) is characterized by granitoid blocks that range in composition from diorite to granodiorite and biotite-amphibolite granite, primarily located in the eastern region of the 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 The majority of these rocks exhibit signs of crushing and milonitization, displaying a banded, patchy gneiss structure, with the degree of migmatization and milonitization increasing from the edges towards 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,

Legend: Old Mong Sen bridge New Mong Sen bridge

The Fault Road River study area is characterized by the Posen formation, which is divided into upper and lower sections, and features a complex network of rivers and streams across its valleys Covering over 250 square miles, the natural land cover constitutes more than 90% of the region, with predominant slopes oriented towards the east and south The Sapa area is notable for its high mountain peaks, where the abrupt transition from low-lying regions to elevated terrain creates steep, dissected slopes This results in narrow horizontal strips of land, with steep and potentially hazardous inclines A geological map of this study area is provided 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)

In the study areas, a tectonic fault system oriented Northwest-Southeast significantly contributes to the landslide process This faulting leads to intense crushing of soil and rock, which 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 h

The geology of the area 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, which consists of sericite quartz schist and marble; and the Cambrian–Ordovician Cam Duong formation, primarily composed of conglomerate, gritstone, shale, lime, and apatite Additionally, the Po Sen complex formation includes diorite, granodiorite, and granite, while the Siluric–Devonian Ban Nguon formation features shale and limestone-rich sandstone.

Pap formation(D 1-2 bp), which contains thin-layered limestone and clay-limestone The

The Ye Yen Sun complex formation (γEys), representing the youngest rock formation of the Paleogene Period, consists of granite and alkaline grano-syenite This formation is underlain by Quaternary (dp) sediments primarily composed 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 consists of 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 three bored holes were utilized to conduct Standard Penetration Tests (SPTs) and collect soil samples for laboratory analysis.

The bored hole layout cross section, illustrated in Figure 3.8, examines four bored 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 collected 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, located in the Northwest, features a mountainous climate characterized by cold winters and a cool, rainy summer from May to October The region experiences significant rainfall, with approximately 80% of the annual precipitation occurring during the rainy season Monthly rainfall averages around 220 mm, peaking at 460 mm in October, while the lowest recorded monthly rainfall was 60 mm in 2021 In 2022, the average rainfall decreased to 150 mm, with maximums of 350 mm in May and August, and a minimum of 70 mm Months with lower rainfall typically see averages of 50-100 mm Additionally, hail events are common in February, March, and April, as evidenced 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 h

In the numerical simulation using LEM and FEM, acceleration data was collected, specifically from the Sapa station This data incorporated the seismic coefficient to facilitate a comprehensive analysis of earthquake-induced slope stability.

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 conduct a numerical simulation (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) h

Laboratory testing

Laboratory testing conducted from Standard Penetration Tests (SPTs) provided insights into the stratigraphy and physical mechanical properties of soil layers in the landslide study areas 1 and 2, as illustrated in Figure 3.11 In study area 1, three bored holes revealed five distinct layers The first layer consists of yellow/brown medium sandy clay with 40% boulder content, varying in thickness from 5 m to 15 m and exhibiting an uncorrected SPT value (Nspt) between 15 and 50 The fourth layer, also yellow/brown medium sandy clay mixed with gravel, ranges from 4 m to 10 m in thickness with Nspt values of 12 to 20 The fifth layer, characterized as yellow/dark medium to hard sandy clay with gravel, has a thickness of 5 m to 15 m and Nspt values of 20 to 50 Additionally, the ninth layer comprises gray strongly weathered rock with a Total Core Recovery (TCR) of 40% to 50% and a Rock Mass Quality (RQD) of 30% to 40%, varying in thickness from 3 m to 10 m The tenth layer, identified as gray weathered rock, demonstrates a TCR of 60% to 70% and RQD of 50% to 60%, with considerable thickness found at depths of 10 m, 30 m, and 15 m, respectively.

In the study area 2, as depicted in Figure 3.11 (b), two boreholes (BH-2 and BH-4) reveal distinct soil layers BH-2 consists of two layers: Layer 1 features a golden brown-grey soft and hard clay mixed with gravel, measuring 5.7 meters thick, with an uncorrected Standard Penetration Test (SPT) value of N SPT = 7 to 8 Layer 2 comprises strongly weathered grayish-white limestone, 1.8 meters thick, exhibiting a Total Core Recovery (TCR) of 0% to 65% and Rock Quality Designation (RQD) of 0% to 48% BH-4 presents similar soil layer characteristics as 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%

Sieving analysis and hydrometer tests were conducted to gather grain size distribution data, while laboratory tests provided soil properties for each layer The friction angle and cohesion of the soil were assessed through direct shear tests under both natural and saturated conditions Additionally, compressive strength tests were performed on weathered rock layers Detailed laboratory testing results can be found 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

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

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 used in the analysis of the Limit Equilibrium Method (LEM) and Finite Element Method (FEM) for Case Study 1 Additionally, Tables 3.6 and 3.7 provide the necessary input parameters for soil layers in the analysis of the LEM and FEM for 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)

Cohesion at natural state (kPa), c

Friction angle at natural state (), 

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)

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)

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)

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 analyze the LEM (GEO-SLOPE) model across two case studies, while Table 3.9 details the input parameters for countermeasures applied in the FEM (PLAXIS) model for the same case studies.

Table 3.8 Input parameters of countermeasure to input GEO-SLOPE

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

Weathered Rock Saturated/Unsatu rated

Table 3.11 Input parameters of Hydraulic Conductivity in SEEP/W for case study 1

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)

Table 3.13 Input parameters of Hydraulic Conductivity in SEEP/W for case study 2

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

Table 3.16 Ground water flow parameters of soil material for case study 2 (Muong

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 various slope geometries and stabilization methods to combat deep-seated landslides Initially, the slope geometry serves as the baseline condition Three primary stabilization techniques are proposed to mitigate future landslide risks: the first method involves slope cutting with shotcrete; the second includes the construction of a concrete retaining wall, with two options—one without bored piles at the slope's toe and another with bored piles; finally, the third method utilizes ground anchors and soil nails integrated with a concrete beam frame and reinforced concrete wall Detailed cross-sectional representations of the slope geometries are provided in the appendix.

3.5.2 Model geometry of case study 2 (Muong Hoa)

This case study examines various slope geometries and stabilization methods to address deep-seated landslides Initially, the slope geometry is assessed as the baseline condition Three slope stabilization techniques are proposed to mitigate future landslide risks: the first method involves slope cutting with shotcrete to enhance stability; the second method utilizes a concrete retaining wall without bored piles at the toe of the slope, as installation is hindered by proximity to the rock layer; and the third method incorporates ground anchors and soil nails, complemented by 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 conducted using the limit equilibrium (LE) numerical method through the GeoStudio software, specifically focusing on two case studies detailed in the appendix The research investigates the impact of various countermeasures on slope stability under three scenarios: normal conditions, conditions with rainfall infiltration, and conditions influenced by seismic activity The SLOPE/W software was utilized to predict potential slope failure surfaces and assess stability, employing the Mohr-Coulomb shear strength criterion for geotechnical materials Rainfall-induced effects were analyzed using SEEP/W, which evaluates pore water pressure and saturation levels at steady state, while earthquake impacts were simulated through pseudo-static analysis, a method that incorporates seismic forces into limit equilibrium calculations Additionally, the initial groundwater level was accounted for via a piezometric line, and the LEM program required specific input for slip surfaces to determine trial slip conditions Detailed geotechnical parameters are outlined in Section 3.4, with Figures 3.16 and 3.17 illustrating the numerical model for the initial slope, including remedial solutions and rainfall boundaries.

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 PLAXIS 2D computer program to simulate slope stability analysis through the finite element (FE) numerical method, examining three conditions: normal slope stability, slope stability with rainfall infiltration, and slope stability under pseudo-static and ground motion acceleration A plain strain model was implemented, assuming zero displacement and strains in the z-direction, while employing a non-homogeneous, two-dimensional plane strain soil material model The elastic-perfectly plastic Mohr-Coulomb model, which approximates soil and rock behavior, was applied to both materials, allowing for efficient computation of deformation estimates For earthquake scenarios, dynamic multipliers and pseudo-static options were used to model dynamic forces, with pseudo-static analysis simulating horizontal forces that alter equilibrium without actual rotation Rainfall-induced slope stability was analyzed using steady-state groundwater flow calculations, incorporating seepage and infiltration boundary conditions The study aimed to determine the factor of safety (M sf) and identify slip surfaces, with detailed soil properties outlined in Section 3.4, and numerical model representations provided 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

This article presents a numerical model for assessing the initial slope stability, incorporating 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, while SEEP/W analyzes both saturated and unsaturated soil conditions during rainfall infiltration The model establishes the initial groundwater level through a piezometric line positioned above the rock layer Stability analysis is performed with SLOPE/W to identify both entry and existing slip surfaces, facilitating the evaluation 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), focuses on determining the M sf value and slip surface while incorporating remedy solutions and rainfall boundaries In this model, groundwater levels are influenced by seepage above the rock layer, and rainfall effects are represented by surface infiltration and countermeasures Additionally, under earthquake conditions, horizontal acceleration and ground motion are applied through pseudo-static and dynamic multipliers to assess slope stability.

Figure 3.22 Model geometry (a) initial slope; (b) with remedy solution 1; (c) with remedy solution 2; (d) with remedy solution 3 h

Figure 3.23 Model geometry of rainfall condition by infiltration boundary

ANALYSIS RESULTS AND DISCUSSIONS

Analysis results of Case Study 1 (Mong Sen) based on LEM and FEM

This case study examines slope stability under various conditions, including normal (case 1), rainfall (case 2), and earthquake scenarios (case 3), utilizing Limit Equilibrium Method (LEM) with GEO-SLOPE and Finite Element Method (FEM) with PLAXIS 2D Additionally, it assesses ground motion effects during earthquakes through FEM (case 4) to identify effective remedial solutions.

4.1.1 Case 1: Normal condition of initial and remedy solutions slope stability result

The analysis results of Limit Equilibrium Method (LEM) and Finite Element Method (FEM) reveal critical findings regarding slope stability Figure 4.1 depicts the initial slip failure surface and the corresponding factor of safety (F.S.) for the initial slope stability In Figure 4.2, the slip failure surface and critical F.S for Remedy Solution 1 are presented, showcasing its effectiveness in enhancing slope stability Figure 4.3 illustrates the results for Remedy Solution 2 (Option 1), while Figure 4.4 provides insights into Remedy Solution 2 (Option 2), both highlighting their respective slip failure surfaces and critical F.S outcomes Finally, Figure 4.5 demonstrates the slip failure surface and critical F.S result for Remedy Solution 3, further contributing to the understanding of slope stability through LEM and FEM analysis.

Figure 4.1 illustrates the critical factor of safety (F.S.) results from Geo-Slope and the incremental displacement from PLAXIS, highlighting the changes in nodal displacement during stability analysis An increase in the M sf coefficient or a reduction in soil shear strength is indicated by the incremental displacement The slip failure surface is identified through the shading in the slip plane plot, revealing that both the limit equilibrium method (LEM) and finite element method (FEM) show similar slip failure surfaces occurring within the soil layer The F.S results are 0.995 for Geo-Slope and 1.047 for PLAXIS, both falling short of the Vietnamese standard requirement of 1.2 (TCVN 13346, 2021), indicating slope instability.

The slope stability analysis results for remedy solution 1, depicted in Figure 4.2, indicate that the failure surfaces are similar but exhibit smaller slip surface shades compared to the results in Figure 4.1 The factor of safety (F.S) for Figure 4.2 (a) is 1.18, while Figure 4.2 (b) shows an F.S of 1.203, both of which are higher than the initial results This analysis confirms that the soil instability has been addressed by removing the unstable soil and applying a shotcrete cutting slope.

The results of the slope stability analysis for remedy solution 2 (option 1), illustrated in Figure 4.3, indicate that the slip failure surfaces are similar yet exhibit smaller shades compared to remedy solution 1 The factor of safety (F.S.) for Figure 4.3 (a) is 1.213, while Figure 4.3 (b) shows an F.S of 1.229, both exceeding the F.S of remedy solution 1 Additionally, these F.S values surpass the required threshold of 1.2 as per Vietnamese standard TCVN 13346 (2021) This analysis confirms that the instability of the soil has been addressed by removing and cutting the slope, followed by the application of shotcrete and a retaining wall at the slope's toe.

Figure 4.4 presents the findings from the slope stability analysis of remedy solution 2, indicating that the slip failure surfaces are smaller compared to those in remedy solutions 1 and 2 The factor of safety (F.S.) results show values of 1.219 in Figure 4.4 (a) and 1.284 in Figure 4.4 (b), both exceeding the F.S of remedy solutions 1 and 2 Additionally, these F.S values surpass the required minimum of 1.2 as per Vietnamese standard TCVN 13346 (2021) The analysis demonstrates that soil instability has been effectively addressed through slope cutting, shotcrete application, and the installation of a retaining wall with bored piles at the toe of the slope.

Figure 4.5 presents the findings of the slope stability analysis for remedy solution 3, showcasing similar slip failure surfaces with the smallest shades compared to other results The factor of safety (F.S.) for Figure 4.5 (a) is 1.267, while Figure 4.5 (b) shows an F.S of 1.302, both of which are the highest among the evaluated remedy solutions Additionally, ground anchors effectively penetrated the slip failure zone.

(a) (b) Figure 4.1 Slope stability analysis results of initial slope (a) LEM; (b) FEM

(a) (b) Figure 4.2 Slope stability analysis results of remedy solution 1 (a) LEM; (b) FEM

(a) (b) Figure 4.3 Slope stability analysis results of remedy solution 2 (option 1) (a) LEM; (b)

Figure 4.4 Slope stability analysis results of remedy solution 2 (option 2) (a) LEM; (b)

In summary, the Limit Equilibrium Method (LEM) results are consistently lower than those of the Finite Element Method (FEM) in both initial and remedial solution scenarios Figure 4.6 illustrates that the initial conditions yield the lowest factor of safety (F.S.) for both methods, aligning with real-world landslide occurrences Among the remedial solutions, Solution 3, which combines ground anchors and soil nails with a concrete beam frame, demonstrates the highest F.S and the least pronounced slip failure surface, making it a logical choice for enhancing slope stability.

Figure 4.6 Normal condition of initial and remedy solution slope stability results based on LEM and FEM

4.1.2 Case 2: Rainfall condition of initial and remedy solutions slope stability result

This article analyzes the slip failure surfaces and critical factor of safety (F.S.) results for various rainfall-induced slope stability scenarios using Limit Equilibrium Method (LEM) and Finite Element Method (FEM) Figures 4.7 to 4.11 present the findings for the initial slope stability and different remedy solutions Specifically, Figure 4.7 depicts the initial slope stability under rainfall infiltration, while Figure 4.8 showcases the results for remedy solution 1 Figures 4.9 and 4.10 illustrate the outcomes for remedy solution 2, with option 1 and option 2 respectively Finally, Figure 4.11 highlights the results for remedy solution 3, all analyzed through LEM and FEM methods.

Figure 4.7 illustrates the critical Factor of Safety (F.S.) results in Geo-Slope (a) and the incremental displacement in PLAXIS (b), highlighting the increase in nodal displacement during stability analysis An increase in the M sf coefficient or a decrease in soil shear strength indicates potential slip failure surfaces, which can be identified through the shading in the slip plane plot Both LEM and FEM analyses show similar slip failure surfaces occurring within the soil layer The F.S results are 0.92 for Geo-Slope and 1.019 for PLAXIS, both falling below the Vietnamese standard requirement of 1.2 (TCVN 13346, 2021), suggesting slope instability following rainfall events.

Figure 4.8 illustrates the outcomes of remedy solution 1 in the context of rainfall-induced slope stability analysis The analysis reveals that the slip failure surface areas are reduced compared to the initial rainfall-induced slope stability results The factor of safety (F.S.) for Figure 4.8 (a) is 1.123, while Figure 4.8 (b) shows an F.S of 1.171 Although these F.S values exceed the initial results, they fall short of the required threshold of 1.2, indicating that remedy solution 1 remains ineffective against rainfall infiltration in this slope scenario.

Figure 4.9 illustrates the outcomes of remedy solution 2 (option 1) in the context of rainfall-induced slope stability analysis The slip failure surfaces observed in this plot are similar, with smaller shades compared to remedy solution 1 The factor of safety (F.S.) for Figure 4.9 (a) is 1.191, while Figure 4.9 (b) shows an F.S of 1.204, both exceeding the values of remedy solution 1 and approaching the required threshold of 1.2 However, it is important to note that these results decrease under rainfall conditions.

Figure 4.10 presents the findings of remedy solution 2 in the context of rainfall-induced slope stability analysis, revealing that the slip failure surfaces are similar and exhibit smaller shades compared to remedy solutions 1 and 2 The factor of safety (F.S.) results indicate values of 1.211 for Figure 4.10 (a) and 1.265 for Figure 4.10 (b), both exceeding those of the previous remedy solutions Additionally, these F.S results surpass the required threshold of 1.2 established by the Vietnamese standard (TCVN 13346, 2021), confirming that the slopes remain stable under rainfall conditions.

Figure 4.11 presents the outcomes of remedy solution 3 in the context of rainfall-induced slope stability analysis The slip failure surfaces depicted are notably similar, with the smallest shades observed compared to other figures The factor of safety (F.S.) results for this solution are 1.241 in Figure 4.11 (a) and 1.276 in Figure 4.11 (b), both of which are the highest among the various remedy solutions analyzed The presence of ground anchors that extend through the slip failure surfaces contributes to the slope's stability under rainfall conditions.

Figure 4.9 Slope stability analysis results of remedy solution 2 (option 1) (a) LEM; (b)

Figure 4.10 Slope stability analysis results of remedy solution 2 (option 2) (a) LEM;

The results indicate that the Limit Equilibrium Method (LEM) yields lower Factor of Safety (F.S.) values compared to the Finite Element Method (FEM) in both initial and remedial scenarios Additionally, rainfall-induced slope stability results are less favorable than those under normal conditions As illustrated in Figure 4.12, both LEM and FEM show the lowest F.S for the initial condition, aligning with the occurrence of actual landslides Among the remedial solutions, Solution 3, which combines ground anchors and soil nails with a concrete beam frame, demonstrates the highest F.S.

F.S = 1.241 h value and the smallest shade of slip failure surface compared with the other results of LEM and FEM That’s an acceptable result

Figure 4.12 Rainfall condition of initial and remedy solutions slope stability results based on LEM and FEM

4.1.3 Case 3: Earthquake condition of initial and remedy solutions slope stability result

Analysis results of Case Study 2 (Muong Hoa) based on LEM and FEM

This case study presents the findings on slope stability across three scenarios: normal conditions (case 1), rainfall conditions (case 2), and earthquake conditions (case 3) The analysis utilizes Limit Equilibrium Method (LEM) via GEO-SLOPE and Finite Element Method (FEM) through PLAXIS 2D to assess the stability and propose remedial solutions Additionally, ground motion effects during earthquakes are evaluated using FEM techniques.

4.2.1 Case 1: Normal condition of initial and remedy solutions slope stability result

The analysis results of the Limit Equilibrium Method (LEM) and Finite Element Method (FEM) reveal critical findings regarding slope stability Figure 4.20 illustrates the slip failure surface and the critical factor of safety (F.S.) for the initial slope Subsequent analyses for remedy solution 1 are depicted in Figure 4.21, while Figure 4.22 showcases the results for remedy solution 2 Finally, Figure 4.23 presents the slip failure surface and critical F.S for remedy solution 3, highlighting the effectiveness of each solution in enhancing slope stability.

Figure 4.20 illustrates the critical factor of safety (F.S.) results from Geo-Slope and incremental displacement from PLAXIS during stability analysis The incremental displacement reflects changes in nodal displacement, indicating either an increase in the M sf coefficient or a reduction in soil shear strength The slip failure surface is identified through the shading in the slip plane plot, revealing that both Limit Equilibrium Method (LEM) and Finite Element Method (FEM) indicate similar slip failure surfaces within the soil layer The F.S results are 0.764 for Geo-Slope and 0.7706 for PLAXIS, both falling below the Vietnamese standard requirement of 1.2 (TCVN 13346, 2021), which signifies slope instability.

The slope stability analysis results for remedy solution 1, as depicted in Figure 4.21, indicate a factor of safety (F.S.) of 0.927 in Figure 4.21 (a) and 0.9581 in Figure 4.21 (b) While these F.S values are higher than the initial results, they remain below the required threshold of 1.2, suggesting that further improvements may be necessary for optimal stability.

The slope stability analysis for remedy solution 2, as illustrated in Figure 4.22, shows that the slip failure surfaces are similar but smaller than those observed in remedy solution 1 The factor of safety (F.S.) results are 1.226 for Figure 4.22 (a) and 1.254 for Figure 4.22 (b), both exceeding the F.S of remedy solution 1 and the required value of 1.2 set by the Vietnamese standard (TCVN 13346, 2021) This improvement in stability is attributed to the removal of unstable soil, the slope cut, and the application of shotcrete along with a retaining wall at the toe of the slope, indicating that this remedy solution effectively enhances slope stability.

Figure 4.23 presents the findings from the slope stability analysis of remedy solution 3, showing that the slip failure surfaces are similar and exhibit the smallest shades compared to other results The factor of safety (F.S.) for Figure 4.23 (a) is 1.472, while Figure 4.23 (b) shows an F.S of 1.482, both of which are the highest among the other remedy solutions Additionally, ground anchors have been installed through the slip failure surfaces in this slope.

In summary, the Limit Equilibrium Method (LEM) results are consistently lower than the Finite Element Method (FEM) results for both the initial and remedial solution scenarios Figure 4.24 indicates that the initial condition yields the lowest Factor of Safety (F.S.) for both methods, as subsequent results reflect the application of remedial solutions This initial slope stability finding aligns with real-world landslide occurrences, with Solution 3 among the most effective remedy options.

M sf = 1.482 F.S = 1.472 h value and smallest shade of slip failure surface compared with the other results of LEM and FEM That’s a reliable result

Figure 4.24 Normal condition of initial and remedy solution slope stability results based on LEM and FEM

4.2.2 Case 2: Rainfall condition of initial and remedy solutions slope stability result

In this section, we analyze the slip failure surfaces and critical factor of safety (F.S.) results for initial slope stability under rainfall infiltration using Limit Equilibrium Method (LEM) and Finite Element Method (FEM) Figures 4.25 to 4.28 illustrate the outcomes for various remedy solutions, showcasing the slip failure surfaces and critical F.S for rainfall-induced slope stability across all analyses Each figure provides insights into the effectiveness of the different remedial approaches in enhancing slope stability under adverse weather conditions.

Figure 4.25 illustrates the critical factor of safety (F.S.) results from Geo-Slope and the incremental displacement from PLAXIS, highlighting the nodal displacement during stability analysis An increase in the M sf coefficient or a decrease in soil shear strength indicates potential slip failure surfaces, which can be identified through the shading of the slip plane plot The results from both Limit Equilibrium Method (LEM) and Finite Element Method (FEM) demonstrate that the slip failure surfaces are consistent, confirming that slip failures are occurring within the soil layer.

The results of 0.741 and 0.7562, as shown in Figure 4.25 (b), fall short of the Vietnamese standard requirement of 1.2 (TCVN 13346, 2021), indicating potential instability of the slope following rainfall.

Figure 4.26 illustrates the outcomes of remedy solution 1 in the context of rainfall-induced slope stability analysis The analysis reveals that the slip failure surface areas are smaller compared to the initial rainfall-induced slope stability results The factor of safety (F.S) for Figure 4.26 (a) is 0.902, while for Figure 4.26 (b) it is 0.942 Although these F.S values are higher than the initial results, they remain below the required threshold of 1.2, indicating that remedy solution 1 is still inadequate to prevent collapse under rainfall infiltration.

Figure 4.27 illustrates the outcomes of remedy solution 2 in the context of rainfall-induced slope stability analysis The slip failure surfaces observed are comparable, with the shades indicating smaller slip failure surfaces than those in remedy solution 1 The factor of safety (F.S.) for Figure 4.27 (a) is 1.208, while Figure 4.27 (b) shows an F.S of 1.218 Both values exceed the F.S of remedy solution 1 and surpass the minimum required threshold of 1.2 However, it is important to note that these rainfall-induced results are diminishing under wet conditions.

Figure 4.28 presents the findings from remedy solution 3 in the context of rainfall-induced slope stability analysis The slip failure surfaces depicted are consistent, with the smallest shades observed compared to other results The factor of safety (F.S.) for Figure 4.28 (a) is 1.365, while Figure 4.28 (b) shows an F.S of 1.399, both of which are the highest among the various remedy solutions The presence of ground anchors traversing the slip failure surfaces contributes to the stability of this slope under rainfall conditions.

(a) (b) Figure 4.25 Slope stability analysis results of original terrain (a) LEM; (b) FEM

The analysis reveals that all Limit Equilibrium Method (LEM) results are lower than those obtained from the Finite Element Method (FEM) in both initial and remedial scenarios Additionally, rainfall-induced slope stability results are less favorable compared to normal conditions As illustrated in Figure 4.29, both LEM and FEM yield the lowest Factor of Safety (F.S.) under initial conditions, aligning with actual landslide occurrences Among the remedial solutions, Solution 3, which combines ground anchors and soil nails with a concrete beam frame, demonstrates the highest F.S value and the least slip failure surface shading, indicating its effectiveness.

Figure 4.29 Rainfall condition of initial and remedy solution slope stability results based on LEM and FEM

4.2.3 Case 3: Earthquake condition of initial and remedy solutions slope stability result

In Case 3, the analysis results from Limit Equilibrium Method (LEM) and Finite Element Method (FEM) reveal the slip failure surface and critical Factor of Safety (F.S.) for initial slope stability under earthquake horizontal loading acceleration, as shown in Figure 4.30 Figures 4.31, 4.32, and 4.33 illustrate the slip failure surfaces and critical F.S results for remedy solutions 1, 2, and 3, respectively, highlighting their effectiveness in enhancing earthquake-induced slope stability through LEM and FEM analysis.

Discussions

A field investigation of two case studies in Lao Cai province revealed significant slope failures triggered by heavy rainfall in October 2021 The first case, near the new Mong Sen bridge, showed deep-seated failure during the construction of a retaining structure for the Lao Cai – Sapa 4D highway, with severe water runoff and soil erosion observed Similarly, the second case in Muong Hoa valley experienced slope failure during the rainy season, with high rainfall intensity identified as a primary trigger for landslides Geological assessments indicated that both sites featured thick sandy clay and clay mixed with gravel, underlain by weathered materials that are soft and susceptible to erosion The steep geological structure further exacerbated the risk of landslides by promoting rainwater infiltration and reducing soil shear strength Both case studies involved different rock formations—Posen formation (diorite, granodiorite, granite) and Da Dinh formation (marble)—with stability analysis using Limit Equilibrium Method (LEM) and Finite Element Method (FEM) revealing that the factor of safety (F.S.) values were below the required threshold, indicating slope instability Historical records also show a pattern of landslides in this region, underscoring the ongoing risk.

Effect of countermeasure on safety factor of slope

This research investigates effective remedy solutions for slope stabilization and landslide prevention based on two case studies Slope stability analyses were performed using LEM (GEO-SLOPE) and FEM (PLAXIS 2D) models The first proposed solution involved slope cutting with shotcrete to mitigate weathered soil mass The second solution utilized retaining methods, with case study 1 (Mong Sen) employing bored piles, while case study 2 (Muong Hoa) used a retaining wall due to bedrock conditions Finally, an anchoring method combining ground anchors and soil nails with a concrete beam frame was implemented, with specific configurations varying between the two case studies based on differing rock formations The findings indicate that these methods effectively address the predicted failure surfaces, highlighting the need for tailored approaches in landslide remediation in Vietnam.

L C Nguyen et al., 2020; Pham et al., 2020; L C Nguyen et al., 2023) h

Effect of rainfall-induced and earthquake-induced landslide

Sapa town in Lao Cai province is situated in a mountainous and monsoon-prone region, characterized by seismic activity and proximity to major fault lines This study investigates the impact of rainfall and earthquake-induced landslides, highlighting that during rainy periods, the risk of landslides escalates Case study 1 (Mong Sen) showed a 3% reduction in the factor of safety (F.S) from normal to rainy conditions, and an 8% decrease from normal to earthquake conditions Similarly, case study 2 (Muong Hoa) experienced a 4% drop from normal to rainy conditions and an 11% decrease from normal to earthquake conditions Existing research on slope stabilization remedies for Lao Cai province is limited Additionally, finite element method (FEM) analyses reveal significant decreases in F.S values under various countermeasures when considering dynamic ground motion Thus, it is crucial to incorporate ground motion factors when assessing slope stability and selecting appropriate remedial solutions.

This research evaluates three countermeasure methods for landslide remediation in two case studies The slope cutting method yields Factor of Safety (F.S.) values below the required threshold (F.S = 1.2 during rainfall and F.S = 1.08 during earthquakes), while both the retaining and anchoring methods exceed this requirement under similar conditions The study finds that Finite Element Method (FEM) results in higher F.S values compared to Limit Equilibrium Method (LEM), aligning with previous research In terms of construction feasibility, the slope cutting method is noted for its durability, ease of maintenance, cost-effectiveness, and safety The retaining method shares similar advantages, while the anchoring method excels in durability, maintenance, construction ease, and safety, albeit at a higher cost Ultimately, the anchoring method emerges as the most effective solution for addressing landslide risks associated with heavy rainfall and earthquakes.

Figure 4.36 Summarize of F.S results under normal, rainfall, and earthquake conditions (a) LEM; (b) FEM

Figure 4.37 Summarize of F.S results under normal, rainfall, and earthquake conditions (a) LEM; (b) FEM h

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

This research investigates the mechanisms and countermeasures for deep-seated landslides in the Lao Cai area, focusing on two main objectives: simulating slope stability using numerical analyses through Limit Equilibrium Method (GEO-SLOPE) and Finite Element Method (PLAXIS), and identifying effective countermeasures to mitigate landslide failures triggered by heavy rainfall and earthquakes along the highway in Lao Cai province The study combines field investigation results with numerical analysis to elucidate the landslide mechanisms, both with and without remedial solutions The findings aim to inform appropriate slope stabilization methods applicable to similar landslide-prone areas, particularly in mountainous highway regions Key conclusions highlight the importance of tailored countermeasures for effective landslide management.

In this study, numerical analyses using Limit Equilibrium Method (LEM) and Finite Element Method (FEM) revealed that the factor of safety (F.S.) values for both rainfall (F.S = 1.2) and earthquake (F.S = 1.08) conditions were below the required thresholds, indicating slope instability consistent with field observations The analyses demonstrated that FEM provided higher F.S values compared to LEM across various conditions due to its detailed parameter input and lack of assumptions regarding slip surface range While LEM (GEO-SLOPE) efficiently calculated F.S values, FEM (PLAXIS 2D) offered a comprehensive analysis, including displacement and anchor load data The ability of PLAXIS to incorporate detailed input parameters, such as prestressed forces in anchor systems, makes it more suitable for complex slope stability designs Overall, this study underscores the effectiveness of both LEM and FEM in understanding landslide mechanisms triggered by rainfall and earthquakes.

This research identifies heavy rainfall as a key factor contributing to slope stability issues in two case studies, highlighting its impact on the factor of safety Additionally, the study examines the influence of earthquakes on slopes to mitigate future landslides A numerical analysis using GEO-SLOPE and PLAXIS 2D models evaluated the sliding plane and slope behavior with three countermeasures The slope cutting method revealed ongoing sliding in the soil layer, resulting in low factor of safety (F.S.) values under both rainfall and earthquake conditions In contrast, the retaining method demonstrated a sliding plane above the retaining wall, achieving F.S values that meet Vietnamese standards The anchoring method exhibited the smallest sliding surface and the highest F.S values, proving to be the most effective solution despite its higher cost Ultimately, the anchoring method is recommended as a durable countermeasure against landslides triggered by heavy rainfall and earthquakes in Lao Cai province.

Recommendations

As of this research, some recommendations are as follows:

- For the practical design of slope stabilization works, FE analysis (PLAXIS) should use to access the accurate results

In regions characterized by steep slopes, complex geology, heavy rainfall, and seismic activity, it is essential to incorporate both saturated and unsaturated soil conditions into rainfall-induced slope stability models Additionally, earthquake-induced slope stability models should account for not only the earthquake coefficient but also ground motion acceleration to ensure accurate assessments.

To construct an effective rainfall infiltration model, it is essential to analyze the grain size distribution curve under both saturated and unsaturated soil conditions Therefore, soil samples must be collected, and laboratory tests should be conducted to accurately determine the properties of unsaturated soil.

- When designing anchoring countermeasure method, the surface drainage systems to run-off the surface water and horizontal drainage pipes to reduce the ground water table should consider

For effective slope stabilization against deep-seated landslides caused by rainfall and earthquakes, the combination of ground anchors and soil nails is recommended This method enhances the strength of the ground and can be installed at the top of the slope without the need for extensive soil excavation.

Future research should focus on examining the impact of countermeasures when altering the bond length of ground anchors Additionally, further investigation is needed to understand the effects of ground motion during earthquakes, as relying on a single input parameter is inadequate for comprehensive analysis.

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Figure 1 Criteria for selection of countermeasure method

Figure 2 Grain size distribution curve of soil for case study 1 (a) 1st layer; (b) 4th layer; (c) 5th layer

Figure 3 Grain size distribution curve of soil for case study 2 of layer 1

SWCC and hydraulic conductivity of soil and rock layer in SEEP/W

Figure 4 Volumetric Water Content for soil and rock layer

Figure 5 Hydraulic Conductivity for soil and rock layer Table 1 Technical specification of applied countermeasures

Design force (Pre-load) 290 kN

Soil nail Material Steel SD 400

Table 2 Application of countermeasures for landslides (N.C.Koei & JICA, 2007)

Drainage ditches Horizontal drain holes Slope

Note: = Very good or very easy, = Good or easy, = Good or easy in some cases h

Model geometry of case study 1 (Mong Sen)

Remedy solution 2 (Option 1) Remedy solution 2 (Option 2)

Model geometry of case study 2 (Muong Hoa)

Details of countermeasure slope for case study 1 (Mong Sen)

Tiêu đề	Remedy Solutions For Deep-Seated Landslides: Case Studies In Lao Cai Province, Vietnam
Tác giả	Thu Zar Aung
Người hướng dẫn	Dr. Nguyen Chau Lan
Trường học	Vietnam National University, Hanoi Vietnam Japan University
Chuyên ngành	Infrastructure Engineering
Thể loại	master’s thesis
Năm xuất bản	2023
Thành phố	Hanoi

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