INTRODUCTION
General background
Vietnam's major cities are rapidly developing across various sectors, leading to an increased demand for infrastructure, particularly in transportation The Mass Rapid Transport (MRT) system, especially subway systems, is viewed as a crucial solution to current congestion issues and a means to foster future growth Since 2006, urban railway lines have been meticulously planned and gradually implemented in Hanoi and Ho Chi Minh City Due to limited land availability, portions of the railway network are being constructed underground, utilizing tunnel boring machines to minimize ground disruption This method is particularly effective in challenging geotechnical conditions, ensuring structural stability while addressing the complexities of varying soil types and hydrology.
Tunneling projects worldwide demonstrate the high efficiency of Tunnel Boring Machines (TBM) in underground construction; however, there are risks associated with ground displacement during and after the construction process Such displacement can lead to subsidence, adversely affecting both surface structures and areas along the tunnel axis Experiences from other countries indicate that shallower tunnels, which are not placed too deep, pose a greater risk of settlement and serious impacts Additionally, in Vietnam, the complexity of TBM construction technology remains a challenge, as it is still not widely understood by domestic engineers.
To enhance the development of TBM technology, it is essential to increase research efforts by Vietnamese scientists and engineers This will enable the creation of technically sound criteria for designing and producing effective TBM solutions, reducing reliance on foreign engineers.
Problem statement
The protection of existing buildings near tunnel construction is crucial in tunneling design and execution One effective solution is the Cut-off wall method, which utilizes a diaphragm wall system injected into the soil to minimize ground particle movement during Tunnel Boring Machine (TBM) operations This method involves creating a low-permeability barrier around the TBM excavation area to prevent groundwater seepage and soil displacement Depending on geological conditions, the wall system can be either a rigid steel wall or a pour-in-place concrete wall In Ho Chi Minh City, the successful application of this method, using Jet-grouting technology to form soil-cement piles, has effectively safeguarded the historic Opera House during the construction of urban railway No 1.
The cut-off wall system effectively separates tunnels from existing buildings, significantly reducing the development and expansion of the settlement trough above the tunnel This system also minimizes the volume of the settlement trough, as shown in Figure 1.1 The effectiveness of this separation is influenced by various factors, including geometric features like relative dimensions, layout depth, and wall thickness, as well as the hardness of the materials used and their interactions with the surrounding soil.
Figure 1.1 Cut-off wall method
The Cut-off wall method proves to be highly effective in enhancing and preserving ground stability at critical construction sites impacted by railway tunneling This research thesis examines the key parameters influencing the efficiency of this method, including geometrical aspects, material properties, and wall-soil interactions By integrating finite element simulations with experimental findings and observational data analysis, the study aims to refine the settlement trough equation for this protective approach.
Aims of the study
Ensuring tunnel safety during design and construction is crucial for both the tunneling process and the stability of nearby buildings This challenge involves various technical and technological hurdles The cut-off wall system has proven to be an effective protective method, successfully implemented in Vietnam and globally This thesis aims to enhance the urban railway system in Vietnam by focusing on the application of the cut-off wall system for ground improvement and building protection in soft soil conditions during tunneling.
In tunneling projects, selecting targeted soil parameters and protection structure dimensions is often guided by engineers' experiences and previous projects Soil stabilization and strengthening are critical aspects of engineering geology By utilizing numerical analysis through finite element modeling (FEM) and examining experimental and observed data, the study aims to validate geometry parameters, material characteristics, and wall-soil interactions These findings will enable designers to optimize parameters tailored to specific geological and construction conditions Additionally, this research will provide valuable design data for predicting ground settlement, enhancing the understanding of building protection via the cut-off wall method, and serve as a foundation for future studies.
Objectives of the study
This thesis aims to analyze and evaluate ground displacement amplitudes and the changes in settlement troughs on either side of a cut-off wall system affected by tunneling The research employs experimental results, which are verified and refined using finite element (FE) simulation models created with Plaxis software (version 8.6) alongside analytical equations.
To study on effects of input parameters in cut-off wall system and geological condition (dimension, distribution, wall-soil interaction and soil parameters);
To simulate, verify, adjust the correlation among parameters to the effectiveness of the system;
To analyze and produce the equivalent equations for the change of the settlement curve with/without applying the cut-off wallsystem
LITERATURE REVIEW
Background
2.1.1 Overview of tunneling research in the world
In recent years, underground structures and railway tunnels have garnered significant attention from geological engineers, as research highlights the efficiency and potential risks associated with Tunnel Boring Machine (TBM) technology during construction and post-operation settlement assessments This focus has led to an increased emphasis on protecting nearby existing buildings Given the scientific and practical importance of this subject, numerous studies—both theoretical and empirical—have been conducted globally to analyze and assess ground settlement and improvement in tunneling projects.
The study of tunnel construction effects, particularly on soil behavior and ground settlement, was first illustrated by Peck (1969), who introduced the "Gaussian distribution settlement trough equation" to describe ground displacement above tunnels This foundational work paved the way for further research by Cording and Hansmire (1975) and Mair et al., contributing significantly to our understanding of tunnel-induced ground movements.
Since its introduction in 1993, Peck's settlement equation has remained a fundamental tool in the analysis and design of tunnel works, as confirmed by Ahmed and Iskander in 2010 The equation's relevance is underscored by the work of O'Reilly and New in 1982, who analyzed the relationship between horizontal ground displacement and the dimensions of the settlement trough caused by soil disturbances during construction, as noted by Mair et al in 1973.
In 1993, researchers conducted a study that established a correlation between cover depth and tunnel diameter, revealing their impact on the size of the settlement trough and the maximum settlement observed at the tunnel center This was achieved through comparative analyses of actual observed data and small-scale centrifuge experiments Additionally, the authors defined the inflection point of the settlement curve equation based on geological factors.
6 condition parameters at the tunnel construction site
Studies and assessment of the stability of the tunnel designed and constructed in soft ground were mentioned by the authors Broms and Bennermark
(1967) [11], Atkinson and Potts (1977) [12], Davis et al (1980) [13], Kimura and Mair (1981) [14], Leca and Dormieux (1990) [15], Anagnostou and Kovári (1994)
[16], Jancsecz and Steiner (1994) ) [17], Chambon and Corté (1994) [18], Broere
(2001) [19], Bezuijen and Van Seters (2005) [20] and Mollon et al (2009a) [21], Senet, S and Jimenez, R (2015) [45], Shiau, J and A1-Asadi, F (2020) [46], Pan,
Q and Dias, D., (2016) [47] However, these studies had not evaluated and mentioned the effects of the location of the tunnel located shallowly under the ground, conditions causing surface settlement and a very large settlement influence area In the studies of authors Vu Minh Ngan, Broere and Bosch and [22,23] had analyzed and evaluated the case of the tunnel lying shallow and gave suggestions on reducing the ratio of the tunneling depth to the tunnel diameter in order to ensure soil stability The study had considered the mechanism of the up-lift effect of the ground and the stability of the surface face under weak geological conditions
Ground stabilization and enhancement, particularly in underground projects, are crucial, prompting extensive global research Various soil improvement techniques, including altering geological structures with high-pressure and absorbent mortars, as well as soil cement, and maintaining geological integrity through compensating and compacting mortars, will be explored in Chapter 3 Notably, a research team from the University of Rome, led by Sebastiano Rampello, investigated the impact of tunneling on existing structures, assessing the effectiveness of diaphragm walls injected between tunnels and surface buildings.
2.1.2 Vietnamese authors gradually approach the research on tunnel issues
Regarding Vietnam research situation, studies on tunneling and ground improvement are not plenty but have also attracted the attention of a few scientists
7 and experts, but still at the level of understanding and analyzing specific problems
There are general studies such as research on tunneling technology, the adverse effects of hydro-geological conditions and selected protection solutions by authors
Recent studies by Le Trung Hien and Nguyen Hong Duong, as well as Nguyen Van Toan, have utilized finite element models to analyze stress distribution around tunnels and calculate the spacing between them Additionally, Tran Quy Duc has conducted simulations to assess how different tunnel layouts and sizes impact volume loss and ground surface settlement under the geological conditions of Ho Chi Minh City.
Research by Vu Minh Ngan and colleagues at Delft University of Technology has examined the volume loss associated with tunneling and its impact on the settlement trough's size and width, particularly focusing on the ratio of depth to tunnel diameter Additionally, their theoretical study explores the area affected by settlement resulting from tunnel construction.
Tunnel construction in Vietnam is a relatively recent development, leading to a limited number of studies in this field There has been insufficient focus on ground improvement techniques and the protection of surface structures during tunnel construction Notably, a recent study conducted by author Phan Sy addresses these critical issues.
In 2016, Liem conducted an analysis on ground improvement techniques to safeguard the Ho Chi Minh City Opera House, utilizing high-pressure grouting technology, specifically jet grouting The study also provided insights into the thickness and strength of the wall system.
The authors focus on legacy research in Vietnam regarding the cut-off wall method, also known as the diaphragm wall, aimed at reducing settlement caused by tunneling Vu et al (2020) highlight the application of the jet-grouting technique in tunneling, specifically in the protection of the Saigon Opera House during the Ho Chi Minh Metro Line 1 project.
Overview of settlements induced by tunneling
2.2.1 The principle of settlements induced by tunneling
2.2.1.1 Ground displacements surrounding the tunnel
The application of the TBM (tunnel boring machine) is proven to bring a high efficiency and a good applicability to many types of geological conditions
Comment [NVM1]: Cite baif bao co ten em
Excavation inevitably leads to ground surface settlement due to the rebalancing of mass-stress states that displace soil particles This risk escalates when shallow tunnels are excavated through weak, soft geology near significant structures The displacement of soil masses surrounding the tunnel directly affects the excavated area, as evidenced by point-displacement vectors observed in centrifugation tests conducted by Mair (1979).
Clay (Mair, 1979) Dense sand (Potts, 1976)
Figure 2.1 Underground transition vectors of soil surrounding tunnel
During tunnel excavation, radial displacements occur due to mass stress equilibrium, alongside volume loss at the tunnel surface, which leads to ground displacement towards the surface, commonly referred to as "volume loss." The total volume loss, which encompasses all components of volume loss during tunnel construction, is critical for understanding the impact of excavation activities Research by Attewell and Farmer (1974), Cording and Hansmire (1975), and Mair and Taylor (1999) provides detailed insights into these displacement components.
Figure 2.2 Volume lost components along the shield [25]
Volume loss at the tunnel face occurs as soil particles are displaced due to disturbances and stress release This phenomenon can be managed through effective pressure balance methods during tunneling and by utilizing an adequate number of monitors positioned ahead of the tunnel face.
Volume loss around the tunnel, known as radial loss, occurs due to soil particles shifting into the space between the shield and the ground This phenomenon varies depending on the extent of excavation and the designed shape of the shield.
Volume loss occurs at the gap between the shield and tunnel segments during the fabrication of in-situ segments The movement of the shield creates a space gap with the surrounding ground, necessitating the injection of high-grade mortar to prevent soil spread The effectiveness of this process relies on the pressure, volume, and quality of the mortar, which are carefully monitored and controlled.
Component 4 refers to the volume loss in the tunnel caused by soil consolidation, which occurs in the small spaces between soil particles after the shield is installed This process, influenced by the formation of the motard layer and mass-stress, contributes to both short-term and long-term ground consolidation above the tunnel Nevertheless, the impact of this component is relatively minor compared to the other factors involved.
Total Volume loss ( ) is combined as equation as:
Based on years of experience in TBM operation and analysis of site monitors from similar tunnel projects worldwide, engineers typically estimate the expected volume loss percentage for initial assessments and deployment plans.
The extent of ground volume loss during construction is influenced by various subjective factors related to the construction process and objective geological conditions Therefore, insights and data from previous projects serve as valuable references A thorough assessment of geological conditions and the careful selection of appropriate construction and support methods are crucial for successful tunnel construction.
The evaluation of volumetric ground loss and prediction of surface settlement during tunnel construction typically relies on finite element modeling, alongside experimental results and established formulas Notably, Peck's 1969 method posits that the curvilinear shape of the settlement trough closely aligns with actual observed outcomes from construction projects Further details on this methodology are provided in the subsequent section.
Evaluating the effects of tunnel construction on nearby structures is crucial during the design phase Key factors in this assessment include monitoring ground displacement, measuring surface subsidence, and considering the proximity of existing buildings to the tunnel.
Tunnel construction can significantly impact surrounding structures, necessitating thorough analysis of the affected areas to mitigate these effects Research by Vu Minh Ngan (2017) has identified the boundaries of these areas based on allowable settlement values and slope angles, as illustrated in Figure 3.3 This analysis enables engineers to assess potential risks and damages to nearby buildings, facilitating informed decisions on monitoring and implementing soil reinforcement methods to minimize settlement impacts.
Figure 2.3 Affected area assessed by the displacement of the ground [24]
Analyzing the relationship between soil parameters and the effects of tunnel depth and diameter on ground displacement reveals that reinforcing the soil mass around the tunnel—whether by altering soil properties or maintaining them—significantly reduces settlement in nearby buildings By maintaining a specific distance from the tunnel axis, it is possible to achieve settlement levels well below permissible limits.
Numerous methods for enhancing soil mass resistance have been refined to suit practical applications Each technique offers unique advantages and is tailored for specific geological and construction conditions.
2.2.3 Ground strengthening methods in tunneling
2.2.3.1 Strengthening by changing soil properties methods
Permeation grouting, the oldest grouting technique dating back to 1802, involves injecting grout into highly permeable granular soil to fill voids without altering the soil structure This method effectively saturates and cements soil particles, creating a stabilized zone ideal for tunneling applications.
Figure 2.4 Permeation grouting in tunneling [37]
This technique involves pumping grout from the surface or directly from the tunnel section, either ahead of the excavation face or through dedicated grouting galleries using sleeved pipes (tube à manchette, or TAM) Initially, a coarse injection grout is applied, followed by a fine injection grout The TAM method allows for the injection of different grouts at various times within the same hole It is crucial to maintain the injection pressure below the threshold defined by the formula αγh, where γh represents the overburden pressure and α is an empirical factor ranging from 0.3 to 3.
13 depending on soils Permeation grouting technique is suitable for sands and gravels
In tunneling, permeation grouting has been applied in many projects, such as Turin Railway Interchange, Roma and Napoli metro projects
ANALYSIS METHODOLOGY
Methodology
This research employs an empirical method integrated with a numerical approach using finite element modeling to analyze ground displacement above a tunnel By comparing experimental results and actual data from construction sites with the Gaussian equation, the study establishes equivalent equations with initial parameters Finite element analysis models, alongside analytical equations, are used to investigate the relationship between input parameters and the effects of the cut-off wall system on ground displacement changes Key input parameters, such as geometric dimensions and soil-wall interactions, are adjusted to identify the optimal combination for specific geological conditions This methodology enables the assessment of displacement and settlement trough equations, ultimately evaluating ground stability and the geological impact behind the cut-off wall system.
Mitigating measure selection
In tunnel design and construction, selecting appropriate mitigating measures is influenced by project costs, work speed, design-construction uncertainties, and safety The choice of soil improvement methods is crucial, focusing on flexibility, feasibility, durability, and work efficiency Tunneling in challenging conditions like peat and soft clay can lead to significant volume loss at various points, necessitating the use of ground improvement and reinforcement techniques Therefore, meticulous control during tunneling operations is essential to ensure safety and project success.
Mitigating measures to enhance soil properties are typically implemented prior to tunneling, with laboratory estimates determining the necessary grout quantity to achieve desired soil parameters In contrast, compensation measures for settlement, which do not alter soil properties, are employed to address settlement caused by tunneling Cavity expansion methods can be utilized to estimate the required grout quantity for these compensation measures Consequently, the selection process in this study will be guided by specific requirements.
It is not necessary to improve a large area of land around the tunnel;
Ensure separating the area of important building from the influence zone of the tunnel while excavating;
The technology has been applied and dealt with geological conditions in Vietnam;
To ensure the stability of significant buildings located above tunnels and to minimize the impact of tunneling on adjacent structures, implementing a Cut-off wall system is an effective solution Additionally, the Jet-grouting method is suitable for urban construction projects that require these considerations.
Figure 3.2 Combined method is the Cut-off wall by Jet-grouting
Figure 3.3 Three typical methods of Jet-grouting method
3.2.1 Using cut-off wall method to mitigate the settlement
20 a) Normal settlement trough b) After cut-off wall applying Figure 3.4 Settlement trough is changed in both shape and depth [35]
The Cut-off wall or diaphragm wall method is highly effective for controlling the quality of wall systems prior to installation in the ground, as demonstrated by its ability to limit settlement displacement (see Figure 3.10) This study evaluates the effectiveness of the wall system not only through parameters like length, thickness, and depth but also by considering the hardness and surface interaction properties with the surrounding ground.
The length is divided into long wall (2.5 times the thickness of the soil cover) and short wall (1.5 times the thickness of the soil cover);
Thickness is classified into hard wall and soft wall;
Roughness or surface interaction is divided into rough wall (usually constructed by grouting technology) and smooth wall (diaphragm wall or in- situ wall types);
Experimental results with laboratory scale [45], simulate the influence of the above parameters are shown as follows (Figure 3.11, Figure 3.12)
Figure 3.5 Verify the results of the reference experiment (without using the wall) with the theoretical result (Gaussian equation) a) Rough wall b) Smooth wall
Figure 3.6 Horizontal displacement and settlement of cut-off wall used in cases of different variable parameters
The surface interaction of the cut-off wall system significantly influences ground displacement changes, as demonstrated by its roughness and other properties.
22 sides Details are described as following section
3.2.1.1 Effect of rough wall system on ground displacement
Experimental results show that under the same support pressure, the volume of the settlement trough is significantly larger when employing the rough wall system This system, characterized by its roughness, size, and ability to bear large loads, necessitates increased supporting pressure during tunnel construction, leading to a notable rise in volume loss While the rough wall system has a minimal effect on reducing total settlement and its form, it significantly influences settlement distribution by concentrating it around the construction area of the wall system.
3.2.1.2 Effect of smooth wall system on ground displacement
The interaction between the smooth wall system and the ground significantly reduces ground displacement around the tunnel, affecting not only the wall system area but also altering the settlement trough, which is primarily concentrated on the tunnel side Notably, there is a marked discontinuity in ground displacement adjacent to the wall system on both sides Additionally, the settlement displacement behind the wall system is distinct from the settlement trough on the tunnel side, indicating that the construction of the tunnel imposes limitations on the surrounding works.
The finite element method analysis of various cut-off wall system scenarios revealed consistent findings, highlighting the distinction between ground displacement development and the original settlement trough equation These results are illustrated in Figure 3.13.
Figure 3.7 Results of analysis by using FEM method to calculate cases of different types of wall system [35]
To evaluate the effectiveness of the method, use the quantitative equation (5) considering the settlement at locations close to the wall system illustrated in
Figure 3-14 With = 1 wall system is considered to be absolutely effective
Figure 3.8 Effective evaluation of cut-off wall system application
However, in practice with complex geological and construction conditions and many risks, it is difficult to achieve this result Therefore, before deciding to
24 implement, the stage of analysis and selection of wall structure and interaction with the ground so that the value of achieved is the highest
Sbw: Settlement to the ground right behind the wall system;
Sw: Settlement of the wall system itself (consider at the top);
Sref: The settlement of the ground when not using the wall system at the same location.
DATA ANALYSIS AND DISCUSSION
Location and scope
In this report, the author has chosen to study the underground tunnel in Metro Line 1 from Ben Thanh to Suoi Tien constructed by shield excavator or TBM
The EPB design document from 2010, prepared by the Management Board of Urban Rail, outlines the underground section that traverses key landmarks in Ho Chi Minh City's inner city, including the historically significant Opera House area It emphasizes the importance of protecting this iconic building while ensuring that any settlement remains within acceptable limits.
10 mm Therefore, it is necessary to study solutions to reduce surface settlement for the above area
The research was conducted in the underground passage and City Theater area, as illustrated in Figures 4.1 and 4.2, which depict the typical cross-section and geological conditions of the site.
Figure 4.1 Research location of tunnels
Figure 4.2 General section at the location
Analyze the settlement due to tunneling
Geological conditions were taken at U-150 Bore hole and shown as below
Figure 4.3 Location of U-150 bore hole in project Table 4.1 Soil layers at borehole/ location of research
Backfill Mainly composed of a mixture of clay, sand, rocks, organic materials Ac2 Mainly gray-brown clay, very soft hard soil
Ac3 Mainly fine sandy clay, soft to hard Seen as an intermediate layer separating As1 and As2
As1 Mainly very liquid clayey sand with medium density, reddish brown As2 Mainly fine to medium grain sand, reddish brown to golden brown
Dc Mainly brown to light gray clay, medium to very hard hardness
Ds Mainly composed of light gray mixed sand, the density is very high
The geological input parameters used for the Mohr-Coulomb model are shown at table below
Figure 4.4 Soil layers at the location
Table 4.3 Design input parameter of TBM
Table 4.4 Design input parameter of TBM shield
4.2.2 Calculate settlement based on semi-empirical method
Figure 4.5 Settlement trough as Gaussian distribution curve [1]
Peck (1969) hypothesized that the curve in the cross-section of the settlement is described as Figure 4.5, according to the equation of the Gaussian distribution curve as follows:
x: Distance from tunnel axis to settlement calculation point in the horizontal direction of tunnel;
i: Distance from tunnel axis to the bend point according to tunnel settlement deformation line;
zo: Depth from ground surface to center of tunnel;
K: The dimensionless coefficient depends on the soil type;
0.2-0.3: granular soil above the groundwater level;
Smax: Maximum deformation above the tunnel;
Since the tunnel shapes are usually designed as circular shape, this variable can be defined as following equation [7]:
4.2.3 Calculate settlement based on FEM method
The railway tunnel, categorized as a stretching section along an axis, allows for the application of 2D analysis to assess the total settlement upon operation This approach streamlines the calculation process while maintaining reliable analysis results For modeling the tunnel design, Plaxis 2D software version 8.6 is utilized, incorporating the volume loss process that occurs during construction.
The Mohr-Coulomb model is selected to apply the calculation in the drainage problem with the main material is sandy soil layers By not taking into
31 account the types of loads acting on the surface, the correlation comparison with the research results by the empirical formula ensures the reasonableness
In this study, the impact of existing loads from stable structures, which have been in use for many years, is disregarded Instead, the focus is on calculating settlement effects caused solely by tunneling in a green field scenario It is assumed that the surrounding works exert a load of 60 kN/m², while the neighboring park, including vehicle loads, contributes an additional 10 kN/m² These loads are treated as evenly distributed across the ground surface.
Figure 4.6 The model is simulated using Plaxis 2D software
Figure 4.7 Compare the results of the two calculation methods
The comparison of two calculation methods reveals that their results are relatively similar, with the semi-empirical method indicating a larger settlement at the tunnel axis and a narrower settlement trough compared to the finite element method This discrepancy arises because the semi-empirical method does not fully account for critical factors such as soil characteristics, tunnel structure parameters, and the interaction between closely constructed tunnels Consequently, the semi-empirical approach, particularly the New & O'Reilly method, is typically employed for initial settlement forecasting before proceeding to a detailed design using numerical methods like Plaxis 2D software.
To effectively address the significant settlement issues resulting from tunnel construction, we will utilize the finite element method for our analysis The subsequent sections will detail the analysis process and present the findings.
Research on the solution of the wall system built by Jet-grouting
Calculation of surface settlement when the diaphragm wall system has been constructed by jet-grouting technology is based on the theory of settlement by
The 33-layering method enhances the soil volume around the tunnel area using high-pressure grout, altering parameters such as elastic modulus, specific gravity, friction angle, and adhesive force compared to the original soil Due to the complexity and time-consuming nature of this problem, Plaxis 2D software, which utilizes the finite element method, is selected for analysis This approach evaluates the settlement reduction effect of the diaphragm wall system, ensuring it achieves adequate strength prior to tunnel construction.
Parameters of the diaphragm wall system after constructed by jet-grouting technology to reinforce the ground are shown in below table
Table 4.5 Design input parameter of cut-off wall system
Unsaturated specific gravity (γ unsat ) 20 kN/m 3
Saturated specific gravity (γsat) 22 kN/m 3
Elastic Modulus (E) 2 nd Variable kN/m 2
Horizontal permeability coefficient (Kx) 0.5 m/day
The model calculated using Plaxis 2D program with spray mortar treatment
34 is shown in the following figure
Figure 4.8 Model simulates the reinforced wall system with Plaxis 2D
The study examines variations in the wall system's thickness and elastic modulus while maintaining a consistent distance of 5 meters from the tunnel center to the inner wall These modifications are crucial for understanding the structural integrity and performance of the tunnel system under different conditions.
- Elastic modulus changed as follows: 10 MPa, 30 MPa, 50 MPa, 100 MPa;
300 MPa; 500 MPa, 750 MPa, 1000 MPa; 1500 MPa, 2000 MPa.
Initial calculation with values of modulus of variation from 100 MPa to
This study examines the impact of limiting settlement based on an elastic modulus of 5000 MPa, using a fixed δ value of 1.5m, which corresponds to the typical thickness of walls constructed through jet-grouting The findings of the calculations are presented below.
Figure 4.9 Settlement change according to modulus values of wall system
Figure 4.10 Relationship between surface settlement and modulus of wall system
From the above analysis results indicate that:
As the elastic modulus (E) of the wall system rises to 50 MPa, there is a noticeable increase in settlement at the survey site This occurs because the original soil, with a specific gravity of 19.5 kN/m³, is replaced by a heavier soil mixed with spray mortar, which has a specific gravity of 22 kN/m³ The increased self-weight of the subsoil, combined with the wall system's insufficient modulus to counteract soil movement from tunneling volume loss, contributes to this settlement phenomenon.
(2) When the value of modulus E of the wall system increases from 70 MPa to 750 MPa, the surface settlement tends to decrease rapidly From 11.2 mm down to 6.7 mm for surface settlement
(3) When the value of modulus E of the wall system increases from 750 MPa to
As the compressive strength reaches 2000 MPa, the settlement diminishes gradually, albeit at a slow pace until it becomes negligible This gradual reduction poses challenges in the construction of high-strength mortar mixes, highlighting the importance of factoring this aspect into the design phase.
The cut-off wall system functions like a single pile under lateral loads caused by volume loss during impact tunneling This allows for the application of the beam-on-horizontal-elastic-foundation hypothesis in the analysis Increasing the thickness of the wall system enhances its modulus of resistance, which reduces horizontal deformation Consequently, the settlement of the soil behind the wall system is expected to improve theoretically.
In the simulation analysis, the diaphragm wall system is fixed with an elastic modulus of 100 MPa, a typical value for sandy soil The study examines variations in the thickness of spray mortar, ranging from 0.5m to 3.5m, to evaluate its effectiveness in restricting settlement The results of these calculations are illustrated in the accompanying graph.
The analysis of the graph indicates a nearly linear relationship between wall thickness and settlement when the elastic modulus E is set at 100 MPa, confirming the initial hypothesis regarding settlement reduction efficiency.
Figure 4.11 Relationship between surface settlement and thickness of wall system
The δ value serves as a key reference for understanding ground settlement in relation to the elastic modulus E Additionally, wall thickness achieved through jet-grouting technology typically ranges from 0.5m to 3.0m, depending on the construction equipment and machinery utilized.
This study examines how the elastic modulus (E) values, ranging from 10 MPa to 2000 MPa, correlate with variations in layer thickness of the mortar-reinforced wall system in response to ground surface settlement at the survey site The composite values are illustrated as curves in the accompanying chart.
Figure 4.12 The relationship between two parameter and effective range
The chart indicates that the settlement reduction efficiency of the design wall system is optimal when the elastic modulus ranges between 100 MPa and 300 MPa, achieving a settlement reduction of 10mm below the allowable limit Beyond this range, the effectiveness diminishes gradually, becoming negligible at approximately 1000 MPa Consequently, it is essential to consider construction and geological conditions to prioritize the technical parameters that will ensure the appropriate thickness or modulus for optimal performance.
4.3.3 Verify the effectiveness with site location monitoring data
The subway tunnels in Ho Chi Minh City extend from the Opera House station to the Ba Son shipyard, traversing a central urban corridor This project may affect nearby structures and underground utilities due to tunneling and excavation activities.
Various protective methods have been employed along the alignment to safeguard buildings from the impacts of underground construction The selection of these methods is influenced by the significance and risk levels associated with the affected structures.
The study aims to compare the performance of sections reinforced by the cut-off wall method using jet-grouting with the results from the analytical model Detailed monitoring data is presented in Table 4.6.
Table 4.6 Monitoring settlement data at reinforced locations
*HCM CITY urban railway construction project (LINE 1)
To ensure safety during TBM excavation near significant structures, engineers implemented a high-safety-factor protective solution Specifically, cut-off wall systems were utilized around the HCM Opera House, featuring a substantial thickness and high elastic modulus These parameters fall within the effective ranges determined in this study (Figure 4.13).
*HCM CITY urban railway construction project (LINE 1)
Operahouse (JET) 6.30 1/7353 10.00 1/1000 OK km 0+861 (JET) 11.10 1/3279 10.00 1/1000 OK km 0+775 19.20 1.4/1000 20.00 2/1000 OK km 0+755 19.70 0.88/1000 20.00 2/1000 OK km 0+735 16.80 0.42/1000 20.00 2/1000 OK km 0+695 12.70 - 20.00 2/1000 OK km 0+625 19.30 1.72/1000 20.00 2/1000 OK km 0+615 19.80 1.76/1000 20.00 2/1000 OK
Figure 4.13 The efficiency of the HCM application is in the effective range
CONCLUSION & DISCUSSION
Conclusion
The report examines the implementation of a cut-off wall system using jet-grouting technology in Ho Chi Minh City to mitigate the effects of double tunnel construction Key aspects analyzed include the deformation modulus and the thickness of the diaphragm wall system.
The analysis conducted using the finite element model with Plaxis 2D software reveals a significant correlation between the elastic modulus and the wall system's thickness.
For wall systems with a thickness of less than 1.5 meters, it is essential to utilize a low-capacity spray mortar system to create a module that accommodates limited settlement Employing high-strength mortar can complicate the spraying process into the soil.
When utilizing wall systems thicker than 3.0m, challenges arise due to limited space and narrow construction areas in urban settings However, employing wall systems with an elastic modulus ranging from 180 MPa to 250 MPa and a thickness between 1.5m and 3.0m ensures that ground surface settlement remains within permissible limits while maintaining adequate bearing capacity.
Discussion
The author's research has highlighted two key parameters of the diaphragm wall system—elastic modulus and thickness—that can help mitigate settlement effects caused by tunnel construction However, to achieve a more precise understanding of settlement reduction, it is crucial to consider additional factors, including the interaction between ground soil and the pile system, the roughness of the wall system, the distance from the tunnel, and the relationship between wall system deformation and ground deformation These aspects will be explored further in the author's upcoming studies.
Because of the short research time and the author's experience with
The analysis of the 42 underground works, including a railway tunnel built using TBM technology, may yield insufficient results at various stages I welcome any comments and further discussion on this topic.
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