INTRODUCTION
General background
Vietnam's major cities are experiencing rapid economic and social growth, leading to an increased demand for infrastructure, particularly in transportation The Mass Rapid Transport (MRT) system, especially subways, is seen as a solution to urban congestion and a catalyst for future development Since 2006, urban railway lines in Hanoi and Ho Chi Minh City have been planned and gradually implemented, with some sections designed as underground tunnels due to limited land usage This construction method employs Tunnel Boring Machines, which minimize soil displacement and maintain ground stability, making them suitable for complex geotechnical conditions, variable elevations, and unstable soil types.
Tunneling projects worldwide demonstrate the high efficiency of Tunnel Boring Machines (TBMs) in underground construction However, instances of ground displacement during and after construction can lead to subsidence, adversely affecting both surface structures and areas along the tunnel axis Experiences from other countries indicate that shallow tunnels, which are not deeply buried, are more prone to settlement issues that can have serious consequences Furthermore, in Vietnam, the complexity of TBM construction technology remains a challenge, as it is not yet well understood by local engineers, with many design and operational aspects still reliant on foreign expertise.
To enhance the development of Tunnel Boring Machine (TBM) technology in Vietnam, it is essential to increase the involvement of local scientists and engineers By conducting more comprehensive studies, they can establish technically sound criteria that align with the unique requirements of the region, reducing reliance on foreign expertise.
Problem statement
The protection of surrounding structures during tunneling is crucial in design and construction The Cut-off wall method, utilizing a diaphragm wall system injected into the soil, effectively minimizes ground movement during tunnel boring machine (TBM) operations This technique involves creating a low-permeability barrier around the excavation area, preventing groundwater seepage and soil displacement Depending on geological conditions, the wall can be a rigid steel system or a pour-in-place concrete wall In Ho Chi Minh City, the application of the Cut-off wall method, implemented through Jet-grouting technology to create soil-cement piles, successfully safeguarded the historic Opera House during the construction of urban railway No 1.
The cut-off wall system effectively mitigates the separation between tunnels and existing buildings, significantly reducing the development and expansion of the settlement trough above the tunnel This system minimizes the volume of the settlement trough, as demonstrated in Figure 1.1 The effectiveness of the separation is influenced by various factors, including geometric features such as 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 is highly effective in enhancing and preserving ground stability at critical construction sites impacted by railway tunneling This research investigates the key parameters influencing the method's efficiency, including geometrical factors, material properties, and wall-soil interaction By integrating finite element simulations with experimental findings and data analysis, the study aims to refine the settlement trough equation for improved protection outcomes.
Aims of the study
Ensuring tunnel safety during design and construction is crucial for both the tunneling process and the stability of nearby structures 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 soil parameters and protection structure dimensions relies heavily on engineers' experience and prior projects Soil stabilization and strengthening are critical aspects of engineering geology By employing numerical analysis through finite element modeling (FEM) alongside experimental and observational data, the study verifies key results related to geometry, material characteristics, and wall-soil interaction These findings enable designers to optimize parameters tailored to specific geological and construction conditions Additionally, this research provides essential design data for predicting ground settlement, understanding building protection through cut-off walls, and lays the groundwork for future studies.
Objectives of the study
This thesis aims to analyze and evaluate ground displacement amplitudes and settlement trough changes on either side of a cut-off wall system in areas impacted by tunneling The research incorporates experimental results, which are verified and refined through finite element (FE) simulation models using 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 n
LITERATURE REVIEW
Background
2.1.1 Overview of tunneling research in the world
In recent years, the construction of underground structures and railway tunnels has garnered significant attention from geological engineers Research indicates that while Tunnel Boring Machine (TBM) technology enhances construction efficiency, it also poses potential risks, particularly during the construction phase and in evaluating total settlement post-operation Consequently, there is a heightened focus 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 settlement and ground improvement in tunneling.
The study of tunnel construction effects was notably advanced by Peck (1969), who examined soil behavior and the total ground settlement above tunnel structures He introduced the "Gaussian distribution settlement trough equation," which describes the settlement distribution of ground displacement above tunnels This foundational work paved the way for further research by Cording and Hansmire (1975) and Mair et al.
Since its introduction, Peck's settlement equation has remained a fundamental tool in the analysis and design of tunnel works, as confirmed by Ahmed and Iskander (2010) O'Reilly and New (1982) further contributed to this field by analyzing the relationship between horizontal ground displacement and the dimensions of the settlement trough, which is influenced by soil disturbance during construction.
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's center This was achieved through comparative analyses of actual observed data and small-scale centrifuge experiments The authors also 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 strengthening, particularly in underground projects, are crucial, prompting extensive global research Various soil improvement techniques exist, including altering geological structures through high-pressure mortar and soil cement, as well as maintaining structures with compensating mortar and wall systems, which will be elaborated in Chapter 3 Notably, a research team from the University of Rome, led by Sebastiano Rampello, investigated the effects of tunneling on existing buildings and assessed the effectiveness of diaphragm wall parameters injected between the tunnel and the above-ground structures.
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 n
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 have utilized finite element models to analyze various aspects of tunnel construction Notably, Nguyen Van Toan examined the stress distribution surrounding tunnels and calculated the spacing between them Additionally, Tran Quy Duc conducted simulations to assess how different tunnel layouts and dimensions impact volume loss and ground surface settlement, particularly under the geological conditions present in Ho Chi Minh City.
Research conducted by Vu Minh Ngan and colleagues at Delft University of Technology has examined the volume loss associated with tunneling and the impact of the depth-to-diameter ratio on the dimensions of the settlement trough Additionally, their theoretical study investigates 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 on the subject There has been insufficient focus on ground improvement techniques in tunnel construction and the protection of surface structures Recently, research conducted by author Phan Sy has begun to address these gaps.
In 2016, Liem conducted an analysis on ground improvement techniques to safeguard the Ho Chi Minh City Opera House, focusing on the application of high-pressure grouting technology, specifically jet grouting The study emphasized the importance of wall system thickness and strength in enhancing structural stability.
The authors focus on legacy research in Vietnam regarding the cut-off wall method, also known as the diaphragm wall, to reduce settlement caused by tunneling Additionally, Vu et al (2020) highlight the applications of the jet-grouting technique in tunneling, specifically in the preservation of the Saigon Opera House during the Hochiminh 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 n
Excavation activities inevitably lead to ground surface settlement due to the rebalancing of mass-stress states, particularly when tunneling through weak and soft geological formations near significant structures This settlement poses a heightened risk, as soil particle displacements occur in the areas directly above and surrounding the tunnel The point-displacement vectors illustrated in Mair's (1979) centrifugation test results further demonstrate the impact of these displacements on the excavated tunnel zone.
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 state 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 is the cumulative sum of all volume loss components, is assessed after tunnel construction Studies by Attewell and Farmer (1974), Cording and Hansmire (1975), and Mair and Taylor (1999) provide detailed descriptions of these displacement components.
Figure 2.2 Volume lost components along the shield [25]
Volume loss at the tunnel face occurs as soil particles shift 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 monitoring devices positioned in front of the tunnel.
Volume loss around the shield occurs due to the movement of soil particles into the gap between the shield and the ground, a phenomenon influenced by the extent of excavation and the shield's design shape.
Volume loss can occur in the gap between 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 use of high-grade mortar to fill this void and prevent soil spread The effectiveness of this process relies on the pressure, volume, and grade of the mortar, which are closely monitored to ensure optimal results.
Component 4 addresses the volume loss in the tunnel caused by soil consolidation in the spaces between soil particles after the shield This process results in the formation of a motard layer and mass-stress, contributing to both short-term and long-term ground consolidation above the tunnel However, it is important to note that this component has a relatively minor impact compared to other factors.
Total Volume loss ( ) is combined as equation as:
Based on extensive experience in Tunnel Boring Machine (TBM) operations and analysis of site monitors from similar global tunnel projects, engineers typically estimate the expected volume loss percentage for initial assessments and deployment strategies.
The extent of ground volume loss during construction is influenced by both subjective factors, such as the construction process, and objective factors, like geological conditions Consequently, 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 critical for successful tunnel construction.
The evaluation of volumetric ground loss and prediction of surface settlement during tunnel construction typically relies on finite element modeling, combined with established formulas and experimental findings Notably, the Peck method from 1969 posits that the curvilinear shape of the settlement trough closely aligns with observed outcomes from actual projects Further details of this methodology will be discussed in the following section.
Evaluating the effects of tunnel construction on nearby structures is crucial during the design phase This assessment involves analyzing ground displacement, surface subsidence, and the proximity of buildings to the tunnel to ensure their stability.
Tunnel construction can significantly impact surrounding structures, necessitating a thorough analysis of the affected areas to minimize adverse effects Research by Vu Minh Ngan (2017) identified specific boundaries of these areas based on allowable settlement values and slope angles, as illustrated in Figure 3.3 This analysis enables engineers to evaluate risks and potential damage to nearby buildings, guiding them in making informed decisions regarding monitoring and implementing timely soil reinforcement and improvement methods to mitigate 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—regardless of whether soil properties are altered—can significantly reduce settlement in nearby buildings By maintaining a specific distance from the tunnel axis, it is possible to achieve settlement levels well below the allowable limits.
Numerous methods for enhancing soil mass resistance have been refined for practical applications, each offering unique advantages tailored to various 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 first applied in 1802, involves filling soil voids with injection grout while preserving the soil structure This method is particularly effective in high permeable, granular soils, where grout is pumped to saturate and bond soil particles, creating a stabilized zone ideal for tunneling projects.
Figure 2.4 Permeation grouting in tunneling [37]
This technique involves pumping grout from the surface or the tunnel section, ahead of the excavation face, or through dedicated grouting galleries using sleeved pipes (tube à manchette, or TAM) Initially, a coarse injection grout should be applied, followed by a fine injection grout The TAM method allows for the injection of different grouts into the same hole at various times It is crucial to ensure that the pressure used in this technique does not exceed the value of 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 study employs an empirical method alongside a finite element model to analyze ground displacement above tunnels By utilizing experimental results and real data from construction sites, we compare these findings to the Gaussian equation to establish equivalent equations with initial parameters Through finite element analysis and analytical equations, we explore the relationship between input parameters—such as geometrical dimensions and soil-wall interactions—and their effects on ground displacement due to the cut-off wall system The research aims to identify optimal parameter combinations tailored to specific geological conditions, ultimately deriving displacement and settlement equations while assessing ground stability and the impact on the geological area behind the cut-off wall system.
Mitigating measure selection
In tunnel design and construction, the selection of mitigating measures is influenced by project costs, work speed, design-construction uncertainties, and safety considerations Choosing an appropriate soil improvement method is crucial, focusing on flexibility, feasibility, durability, and work efficiency Particularly in tunneling through peat and soft clay, significant volume loss can occur at the tunneling face, along the shield, and during consolidation Therefore, it is advisable to implement ground improvement methods alongside reinforcement techniques for the tunneling face, along with careful monitoring throughout the tunneling process.
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, methods to compensate for settlement without altering soil properties are employed to address tunneling-induced settlement Cavity expansion techniques can help estimate the grout quantity needed for these compensatory measures Consequently, the selection process in this study will be based on 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; n
The technology has been applied and dealt with geological conditions in Vietnam;
To ensure the stability of crucial buildings located above a tunnel while minimizing the impact of tunneling on adjacent structures, implementing a Cut-off wall system is an effective solution Additionally, Jet-grouting serves as a suitable method 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 n
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 an effective technique for controlling the quality of wall systems prior to their installation in the ground This approach significantly limits ground settlement displacement, as illustrated in Figure 3.10 The effectiveness of the wall system is influenced not only by characteristics like length, thickness, and depth but also by 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) n
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 cutoff wall system significantly affects ground displacement changes due to 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 indicate that under consistent support pressure, the volume of the settlement trough is significantly larger when utilizing the rough wall system This system, characterized by its roughness, size, and capacity for heavy loads, necessitates increased supporting pressure during tunnel construction, which in turn leads to a notable rise in volume loss While the rough wall system has minimal impact on reducing total settlement and its form, it significantly alters the settlement distribution, concentrating it around the construction area of the wall system.
3.2.1.2 Effect of smooth wall system on ground displacement
The experiments demonstrated that the smooth wall system significantly reduces ground displacement around the tunnel, affecting not only the immediate area of the wall but also altering the settlement trough, with displacement primarily concentrated on the tunnel side A notable discontinuity in ground displacement was observed adjacent to the wall system on both sides, indicating a major change Additionally, the settlement displacement behind the wall system is distinct from the tunnel side's settlement trough, highlighting the constraints imposed by tunnel construction on the surrounding area.
The finite element method was employed to analyze various cases of the cut-off wall system, revealing consistent findings that differentiate ground displacement development from 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 n
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. n
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, created 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 these vital structures and stipulates that any ground settlement should be minimized to ensure their preservation.
10 mm Therefore, it is necessary to study solutions to reduce surface settlement for the above area
The research focuses on the underground passage and the City Theater area, with typical cross-sections illustrated in Figures 4.1 and 4.2 Additionally, the geological conditions relevant to this location are discussed in the following section.
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 n
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: n
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
For the railway tunnel, which features stretching sections along an axis, a 2D analysis is appropriate to assess the final total settlement upon operation This approach streamlines the calculation process while maintaining the reliability of the results The analysis utilizes Plaxis 2D software version 8.6, leveraging the tunnel design module and incorporating the volume loss process 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 n
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 works on the survey site, which have stabilized over years of use, is disregarded Instead, the focus is on calculating settlement due to tunneling in a green field scenario The analysis assumes a surrounding load of 60 kN/m² and a vehicle load from a neighboring park of 10 kN/m², both of which are considered to be 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 results from the two calculation methods show relative similarity, with the semi-empirical method indicating a larger settlement at the larger tunnel axis and a narrower settlement trough compared to the finite element method This discrepancy arises because the semi-empirical method does not adequately consider factors such as soil characteristics (using the conversion coefficient K), tunnel structure parameters, and the interactions between closely constructed tunnels Consequently, the semi-empirical method, particularly the New & O'Reilly approach, is primarily utilized for initial settlement forecasting before transitioning to a detailed design using numerical methods, specifically through simulations with Plaxis 2D software.
To effectively analyze the significant settlement resulting from tunnel construction, it is advisable to utilize the finite element method in the subsequent steps The following 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 n
The 33-layering method enhances the soil volume around the tunnel area using high-pressure grout, which alters parameters such as elastic modulus, specific gravity, friction angle, and adhesive force compared to the original soil This complexity necessitates a time-consuming analysis, leading to the selection of Plaxis 2D software, which employs the finite element method to address the issue The software is utilized to assess the effectiveness of the diaphragm wall system in reducing settlement after achieving 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 n
34 is shown in the following figure
Figure 4.8 Model simulates the reinforced wall system with Plaxis 2D
The distance from the tunnel center to the inner wall is consistently maintained at 5 meters, while variations in the wall system's thickness and elastic modulus are implemented.
- 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 constant δ value of 1.5m, which is a standard thickness for 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 n
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, an increase in settlement at the survey site is observed This occurs because the original soil, with a specific gravity of 19.5 kN/m³, is replaced by a soil mixture with spray mortar, resulting in a higher specific gravity of 22 kN/m³ This change elevates the self-weight of the subsoil, while the wall system's modulus is insufficient to counteract soil movement caused by the loss of tunneling volume.
(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
At 2000 MPa, the settlement of high strength mortar mixes decreases gradually, albeit at a slow rate until it becomes negligible This gradual reduction in settlement presents construction challenges, highlighting the importance of incorporating these factors during 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-elastic-foundation hypothesis in the analysis Increasing the thickness of the wall system enhances its modulus of resistance, resulting in reduced horizontal deformation Consequently, the settlement of the soil behind the wall system is theoretically improved.
In simulation analysis, the diaphragm wall system is fixed with an elastic modulus of 100 MPa, a standard value for sandy soil The study examines the impact of varying the thickness of spray mortar from 0.5m to 3.5m to assess its effectiveness in restricting settlement The results of these calculations are illustrated in the graph below.
The analysis of the graph indicates a nearly linear relationship between wall thickness and settlement when the elastic modulus E is fixed at 100 MPa, confirming the initial hypothesis regarding settlement reduction efficiency.
Figure 4.11 Relationship between surface settlement and thickness of wall system
The value of δ serves as a key reference for assessing ground settlement in conjunction with the elastic modulus E Additionally, wall thickness achieved through jet-grouting technology typically ranges from 0.5m to 3.0m, depending on the standard construction equipment and machinery utilized.
This study examines how the elastic modulus (E) ranges from 10 MPa to 2000 MPa, influenced by the thickness of the mortar-reinforced wall system and the settlement of the ground surface 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 design wall system's settlement reduction efficiency is optimal when the elastic modulus ranges from 100 MPa to 300 MPa, resulting in a settlement reduction of 10mm below the allowable limit As the elastic modulus increases towards 1000 MPa, the effectiveness diminishes significantly until it becomes negligible Consequently, it is essential to consider construction and geological conditions to prioritize the technical parameters that will yield the most suitable thickness or modulus for the project.
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 and cut & cover area, traversing a central urban corridor This project has the potential to affect surrounding structures and underground utilities due to tunneling and excavation activities.
Various protective methods are employed along the alignment, each impacting buildings differently The selection of these methods is based on the significance and risk levels associated with buildings and structures affected by underground construction.
This study aims to compare the performance of sections reinforced by the jet-grouting cut-off wall method with the outcomes from the analysis model, as detailed in Table 4.6.
Table 4.6 Monitoring settlement data at reinforced locations
*HCM CITY urban railway construction project (LINE 1)
To address potential risks associated with Tunnel Boring Machine (TBM) excavation near significant structures, project engineers implemented a protective solution with a high safety factor Specifically, they utilized cut-off wall systems around the HCM Opera House, designed with substantial thickness and elastic modulus values These parameters fall within the effective ranges determined in this study, as illustrated in 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 n
CONCLUSION & DISCUSSION
Conclusion
The report examines the implementation of a cut-off wall system, utilizing jet-grouting technology, to mitigate the effects of double tunnel construction in Ho Chi Minh City's geological environment Key aspects of the diaphragm wall system analyzed in the study include the deformation modulus and the wall thickness.
The analysis conducted using the finite element model in Plaxis 2D software demonstrates a strong correlation between the elastic modulus and the thickness of the wall system.
For wall systems with a thickness under 1.5 meters, it is essential to utilize a small-capacity spray mortar system to create a module that accommodates limited settlement Additionally, employing high-strength mortar can pose challenges when spraying into the soil.
When utilizing a wall system thicker than 3.0m, larger capacity machines encounter challenges due to restricted space and narrow construction areas in urban environments However, employing a wall system 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, thereby maintaining adequate bearing capacity.
Discussion
The author's research highlights the importance of two key parameters, elastic modulus and thickness, in the diaphragm wall system to mitigate settlement effects during tunnel construction However, for a more comprehensive understanding of settlement reduction, it is crucial to consider additional factors, including the interaction between the ground soil and the pile system, wall system roughness, the distance from the tunnel, and the relationship between wall system deformation and ground deformation These aspects will be further explored in the author's upcoming studies.
Because of the short research time and the author's experience with n
The analysis of the 42 underground works, including a railway tunnel constructed using TBM technology, may yield insufficient results at various stages of the process I welcome any comments and further discussion on this topic.
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