Volume 42 No. 3 September 2011
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Abstract
In the new pneumatic caisson method (NPC), soil excavation and removal is completed remotely by workers on the ground. In 2007, this method was successfully applied in a tunnel shaft in Shanghai. Combined with the construction example, field monitoring and measuring has been conducted. Typical monitored results, such as the working pressure, lateral earth pressure, reaction pressure, and ground movements, were presented and analyzed. In addition, a numerical approach considering the soil disturbance during construction was proposed to predict the soil movements induced by the NPC construction. It was successfully implemented in the three-dimensional finite element method (FEM) codes. Calculated soil movements were examined and verified by the field measurements. In the meantime, these results were compared with the ones obtained from the two-dimensional approach proposed by the authors in the previous study. Results showed that, they agreed well with each other, and in general the three-dimensional analysis results approached the actual situation more closely.
Related papers
3D Modelling of Tunnel Excavation Using Pressurized Tunnel Boring Machine in Overconsolidated Soils
Studia Geotechnica et Mechanica, 2013
The construction of shallow tunnels in urban areas requires a prior assessment of their effects on the existing structures. In the case of shield tunnel boring machines (TBM), the various construction stages carried out constitute a highly three-dimensional problem of soil/structure interaction and are not easy to represent in a complete numerical simulation. Consequently, the tunnelling-induced soil movements are quite difficult to evaluate. A 3D simulation procedure, using a finite differences code, namely FLAC3D, taking into account, in an explicit manner, the main sources of movements in the soil mass is proposed in this paper. It is illustrated by the particular case of Toulouse Subway Line B for which experimental data are available and where the soil is saturated and highly overconsolidated. A comparison made between the numerical simulation results and the insitu measurements shows that the 3D procedure of simulation proposed is relevant, in particular regarding the adopted representation of the different operations performed by the tunnel boring machine (excavation, confining pressure, shield advancement, installation of the tunnel lining, grouting of the annular void, etc). Furthermore, a parametric study enabled a better understanding of the singular behaviour origin observed on the ground surface and within the solid soil mass, till now not mentioned in the literature.
Movements caused by the excavation of tunnels using face pressurized shields — Analysis of monitoring and numerical modeling results
Engineering Geology, 2013
In this paper, monitoring results of two cross tunnel sections are presented. This underground work has been realized for a subway in an urban area (Lyon, France). By comparison with measurements of other projects, it appears that the face instability and the annular gap identified after the shield release are the main sources of short-term settlements. These observations of vertical and horizontal movements during the tunnel excavation by a slurry pressurized tunnel boring machine are then compared with several numerical approaches. The 2D numerical approach uses the concept of volume loss and is applied to each excavation stage. It simulates approximately the observed movements but requires the use of empirical coefficients to represent in two dimensions the three-dimensional problem. The 3D approach considers more directly the physics of the problem and permits to take into account: the slurry pressure at the tunnel face, the shield conicity, the grout injection in the annular void and the grout consolidation. Three dimensional numerical calculations are the most accurate approaches to simulate all the physical processes occurring during tunneling. However this type of model assumes that all the parameters that control the movements induced by the excavation are well known. Due to the complexity of a tunneling boring machine, it is not necessarily the case.
Field observations and finite element 3-D analysis of soil displacements close to unsupported excavation
MATEC Web of Conferences
Soil displacements due to unsupported deep excavation may cause severe damages to the nearby structures and foundations systems. Such excavations affect the state of stresses and displacements field of the surrounding soil. In this study, the soil displacements at five observation points were continuously monitored for23 days, which was the time period of excavation of about 7 m deep open tunnel. The reference points were installed on the ground surface at horizontal distance ranging from 1.25 to 3.25 m from tunnel excavation edge. The construction work was related to the project of developing the Army Channel/ Zeyouna section in the Eastern part of Baghdad City. The field observations indicate upward vertical displacement and outward lateral displacements during the first 10 days when the excavation depth was within 4 m. After that, the displacement trend was generally reversed. The displacement values were within 12 mm and vary from one observation point to another depending on the lateral distance of each point from excavation edge. The finite element package PLAXIS 3D was used to simulate the problem after obtaining the required soil parameters by an extensive site investigation. The analysis results in general, compare well with the field observations in terms of soil displacements at the reference points, especially during the first 10 days. This finding may reveal the reliability of the analysis results at other locations in the surrounding soil.
Evaluation of the Importance of Gradually Releasing Stress Around Excavation Regions in Soil Media and the Effect of Liners Installation Time on Tunneling
Geotechnical and Geological Engineering, 2019
Tunnels are structures which have vital roles in the development of societies. In the numerical models of underground cavities, such as tunnels, loading due to zone elimination is induced instantaneously in the soil mass, and it might cause a disturbance in the stress state especially around the excavation area. However, this is not compatible with the principles of elastoplastic constitutive models used in soil behavior simulations. Besides, the predicted load on the tunnel liner will be larger than the actual value in this kind of modeling. In other words, it causes the so-called overestimated design. Using an appropriate constitutive model could lead the numerical analyses to accurate results. In this research, loading increment in the simulation of soil behavior is evaluated according to experimental data. Next, a correct way for numerical simulation related to underground excavation is described according to gradually eliminating (incremental) stress around tunnels based on the numerical modeling in the finite-difference code called FLAC. Hence, the effect of releasing the stress on the results is illustrated by the stress paths and deformations around a tunnel. Finally, the installation time of the tunnel liner and its impact on the numerical results are considered based on some experimental and field data. It is concluded that the use of software default in modeling the tunnel issues might lead to extreme oscillations in the stress paths, and it could affect the numerical results. Therefore, it is reasonable to utilize a proper way to release the stress around the excavation area gradually.
Movements Induced by Tunneling With an Epb Machine in over Consolidated Soils: Compans Monitoring Section of Toulouse Subway Line B
2008
A part of Toulouse's underground line B has been excavated by a 7.8 meters diameter earth-pressure balance machine. A comprehensive monitoring section has been installed in the Compans garden at a point where the tunnel cover is 12.8-meter and is running through overconsolidated clayey molasses underlying a 6.8 meters layer of made ground and gravels. The monitoring devices give full information on the ground movements above the tunnel and on deformations of the tunnel lining. Vertical movements are measured by 5 multipoint borehole extensometers with automatic data acquisition (every 5 seconds when the TBM crosses the monitoring section). At the end of each excavation phase, a high precision levelling of the borehole extensometer heads is performed in order to determine the total vertical displacements. Horizontal movements are measured by 3 inclinometers, one in the axis of the tunnel and the others on its sides, 2.3 meters away from the tunnel extrados. One of these inclinometers is 42 m long, i.e. 21.8 m below the level of the tunnel invert. The induced horizontal strains at ground level are measured by an invar thread in both longitudinal and transverse directions. Measurements show a specific behaviour of the soil due to its overconsolidated character (K 0 greater than 1.5): even though the magnitude of the displacements is limited to a few millimeters, horizontal convergent movements are observed at the level of the tunnel axis while 3 to 4 times smaller heave is measured at ground surface (associated with horizontal extensions). Excavation parameters are automatically recorded and give information on the variation of the torque on the cutting wheel, the advancement rate, the confining pressure, the injection of annular void between soil and the concrete tunnel lining… This paper presents the results of the different monitoring devices and the possible correlations with the tunnel excavation parameters recorded by the TBM.
INFLUENCE OF APPLYING DIFFERENT MODEL PARAMETERS ON TUNNELS INDUCED DEEP EXCAVATION IN LAYERED CLAY SOIL
Research, 2021
The applications of constitutive model for underground structures may be not assessed completely. Several researchers proposed various constitutive models to present details of different aspects of the soil performance. Therefore, the current study attempts to present 2D convenient finite element model for adopting a comparison study between different model parameters in soft soils to understand the effects of deep excavation on lateral movement and on the ground surface with tunnel presence. The tunnel was simulated at buried depths (C = 3, 6, and 9 m), with two different shapes (circular and horseshoe) and with diameter (D = 6 m). The analysis was by Hardening Soil (HS) and Soft Soil (SS) models in case of tunnel presence once before deep excavation beside and other once under the effect of close deep excavation. In the case of only tunnel presence, and the mass of soil unloading due to excavated tunnel, explicit heave revealed in the soil surface above tunnel body. In the case of close deep excavation where (He = 12 m), significant draw down for the soil surface above the zone of buried tunnels. The suitable tunnel depth without substantially disrupting the surface above may be with increasing the tunnel depth more than 4D.
PREDICTION OF FIELD RESPONSE OF SOIL-SUPPORT SYSTEMS IN DEEP EXCAVATIONS
Fourth Geo-China International Conference July 25–27, 2016 | Shandong, China © 2016 American Society of Civil Engineers, 2016
The task of predicting the performance of deep excavations is challenging due to the existence of many influencing factors. Strength and deformation parameters of soil, type, stiffness of the support system, and sequence of excavation and installation of support elements are very important factors in studying the performance. Performance of deep excavation support systems is related to both stability and deformation. Ground deformation around the excavations can damage surrounding buildings, streets and utilities. Therefore understanding the factors affecting the performance of deep excavations and the ability to predict the behavior of the support system and the associated ground deformations is an important issue for geotechnical engineers. In this thesis, two case studies with well documented data of field measurements are analyzed numerically. The analyses are performed using the finite element program “PLAXIS-2D”. Two material soil models are implemented in the analyses: the hardening soil model and Mohr Coulomb model. In both case studies analyzed, good agreement between the field measured displacements and the calculated displacement using the hardening soil model is obtained. Results of analysis showed also that the hardening soil model is superior to Mohr Coulomb model in predicting the displacements in both case studies. After calibrating the numerical model, a parametric study is performed to investigate the effect of anchor-pretension-force on the resulting deformation for case study (A) and the effect of strut stiffness on the resulting deformation for case study (B). Results of analysis showed that both factors have important effects on the resulting excavation-support-system deformation.
Numerical Analyses of Soil Deformations Around Deep Excavations
2013
In this study, the deep excavation of Cincin Station located along the Bağcılar-Otogar metro line which is currently under construction in Istanbul is modeled numerically. The excavation (depth 32.5 m) of the station is carried out with a surrounding slurry trench diaphragm wall and top-down construction method. The six slabs of the station building and the foundation mat are used as support elements. Lateral soil displacements are measured with inclinometers placed in the wall and nearby soil layers. The results of numerical analysis using soil profile and geotechnical parameters obtained from conventional field and laboratory tests and measured lateral displacements are compared. Then using the same soil model, soil displacements expected to occur if some other alternative excavation support systems were used is investigated. As alternative support systems use of steel pipes as internal bracing and a piled wall with pre-stressed tie-backs are considered. The calculated soil displacements for different support systems are compared with each other and the measured values. The effects of certain design parameters such as the rigidity of internal bracing elements, the pile diameters and the pre-stressing level of tie-backs are investigated through numerical analysis.
Ability of three different soil constitutive models to predict a tunnel’s response to 3 basement excavation
2015
Many constitutive models are available nowadays to predict soil-structure interaction 30 problems. It is sometimes not very easier for engineers to select a suitable soil model to carry 31 out their design analyses in terms of complexity versus accuracy. This paper describes the 32 application of three constitutive models to back-analyse a well-instrumented centrifuge model 33 test, in which the effect of basement excavation on an existing tunnel was simulated. These 34 three models include a linear elastic-perfectly plastic model with the Mohr-Coulomb failure 35 criterion (called MC model), a nonlinear elastic Duncan-Chang model (DC) and a 36 hypoplastic model (HP), the last of which can capture the state-, strain-and path-dependent 37 soil stiffness even at small strains and path-and state-dependent soil strength. By comparing 38 with measured data from the centrifuge model test, it is found that the HP model yielded the 39 best predictions of tunnel heave among the three models. Not only the gradient but also the 40 magnitude of tunnel heave is predicted well by this HP model. This can be explained by the 41 fact that the HP model can capture the state-, strain-and path-dependent soil stiffness even at 42 small strains and path-and state-dependent soil strength but not the MC and DC models. 43
Related papers
Field Observation of Soil Displacements Resulting Due Unsupported Excavation and Its Effects on Proposed Adjacent Piles
Journal of Engineering
Soil movement resulting due unsupported excavation nearby axially loaded piles imposes significant structural troubles on geotechnical engineers especially for piles that are not designed to account for loss of lateral confinement. In this study the field excavation works of 7.0 m deep open tunnel was continuously followed up by the authors. The work is related to the project of developing the Army canal in the east of Baghdad city in Iraq. A number of selected points around the field excavation are installed on the ground surface at different horizontal distance. The elevation and coordinates of points are recorded during 23 days with excavation progress period. The field excavation process was numerically simulated by using the finite element package PLAXIS 3D foundation. The obtained analysis results regarding the displacements of the selected points are compared with the field observation for verification purpose. Moreover, finite element analysis of axially loaded piles that ar...
NUMERICAL ANALYSIS OF INFLUENCE OF DEEP EXCAVATION ON NEARBY EXISTING TUNNEL
The constantly increasing urbanization of the metropolises requires more and more realization of a fast and efficient transportation network. Underground transport lines often constitute an ideal solution in term of efficiency and low nuisance for the environment. The technical advancements brought to the underground excavation methods allow the achievement of projects in difficult environmental conditions, especially in urban areas. Providing space for parking, public amenities, etc in multi-storey buildings at town centers has created a need to go deep excavations into ground. The construction of deep excavations in the urban environment is a technically challenging problem. Underground construction may be in the vicinity of already existing tunnels. Deep excavations may have impact on the adjacent tunnels. Such excavation will impose significant influence on the tunnel linings, including changes of stress and deformation. Such effects are observed due to the "space effect" of excavation. The study involves the analysis of influence of deep excavation on the existing tunnel, structural impact on the tunnel lining and measures for controlling the soil and tunnel deformation using finite element software Plaxis (2D). In this study the influence of deep excavation on the existing tunnel with varying position is analyzed. Analysis is carried out by considering tunnel just below the excavation and just beside the excavation. Effect of excavation on tunnel lining and displacement of tunnel due to excavation is observed. Also Analysis involves soil improvement technique to reduce impacts of excavation on tunnel. Grouting method is adopted for the analysis and its role in controlling the effects on tunnel is analyzed.
Analysis of Deformation Characteristics of Foundation-Pit Excavation and Circular Wall
Sustainability
The surrounding ground settlement and displacement control of an underground diaphragm wall during the excavation of a foundation pit are the main challenges for engineering safety. These factors are also an obstacle to the controllable and sustainable development of foundation-pit projects. In this study, monitoring data were analyzed to identify the deformation law and other characteristics of the support structure. A three-dimensional numerical simulation of the foundation-pit excavation process was performed in Midas/GTS NX. To overcome the theoretical shortcomings of parameter selection for finite-element simulation, a key data self-verification method was used. Results showed that the settlement of the surface surrounding the circular underground continuous wall was mainly affected by the depth of the foundation-pit excavation. In addition, wall deformation for each working condition showed linearity with clear staged characteristics. In particular, the deformation curve had o...
Numerical Simulation of the Mechanical Response of the Tunnels in the Saturated Soils by Plaxis
The forecast of settlement and movements caused by tunneling represents a significant challenge of technology. The evaluation of these movements is indeed of primary importance in order to prevent them.
Parametric Investigation of Interaction between Soil-Surface Structure and Twin Tunnel Excavation: A Comprehensive 2D Numerical Study
Infrastructures
The growing demand for transportation tunnels in densely populated urban areas has led to the widespread adoption of twin tunnel configurations in contemporary infrastructure projects. This research focuses on investigating the complex interaction between soil, structures, and the excavation of twin tunnels. The study employs the tunnel boring machine (TBM) method and utilizes two-dimensional numerical modeling based on the finite element method (FEM). The numerical model is validated by comparing its results with field measurements obtained from a twin tunnel project in Italy, specifically the New Milan Metro Line 5. A comprehensive parametric study is conducted to analyze various parameters that influence soil–structure interaction during tunnel excavation. These parameters include the positioning of the tunnels in relation to each other, the spacing between them, the presence of structures above the tunnels, eccentricity between the structure axis and tunnel axis, and tunnel dept...
Research on the Influence of Excavation and Loading on Z-Direction Displacement in Surrounding Soil Mass
Archives of Mining Sciences
To probe into the pattern in which the excavation and loading process have on such factors as stress and displacement in neighboring regions of deep open pits, a mechanical unloading model in coal mining process and another model for the loading process are set up respectively. Besides, FLAC3D software is used to simulate dynamic excavating and loading process in open pits and record such data as the unbalanced stress, unloading strength and displacement fluctuations, which further serve as basis for studying the functional relationship about different mining heights and scope of influence using fitting method. The research results indicate that the unloading strength enhances with increasing mining depth in a linear fashion. In addition, a noticeable displacement circle takes shape around the stope, which would also extends with growing mining depth. As to waste loading, it brings about large-scale surface subsidence in neighboring regions, which follows a logarithm function conver...
Evaluation of a Braced Excavation by Numerical Method: A Case Study
Civil Engineering and Architecture, 2023
Approximately 18 by 22 by 100 meters in size, a braced excavation operation at the Mahallati station on Tehran’s Metro Line 7 took over eleven consecutive phases. Due to the significant depth-to-width ratio, a PLAXIS plane-strain finite element analysis was carried out. The lateral wall of the braced cut excavation was supported with three types of struts in four different rows. Due to the excavation of the soil, the tension condition was changed and caused some displacements and instabilities; therefore, the horizontal and vertical displacement of the excavation was studied. The maximum horizontal displacements of 35.32 mm occurred in the lateral wall at the excavation surface, whereas the maximum vertical displacements of 35.00 mm occurred at the excavation’s base. In all stages, the highest lateral wall deflection values were between 0.00018 and 0.0016 of the depth. The maximum ground surface settlement near the excavation was 22.41mm, approximately 0.67 times the maximum subsequent wall deflection. In each phase, the maximum ground surface settlement distance from the wall was almost equivalent to 0.4 times the excavation depth. The numerical modeling shows that Plaxis2D is an effective software for analyzing the excavation of a braced cut.
A hybrid model for estimation of ground movements due to mechanized tunnel excavation
Computer-Aided Civil and Infrastructure Engineering, 2019
To provide realistic predictions of mechanized tunnel excavation-induced ground movements, this research develops an innovative simulation technique called hybrid modeling that combines a detailed process-oriented finite element (FE) simulation (submodel) with the computational efficiency of metamodel (or surrogate model). This hybrid modeling approach has three levels. In Level 1, a small scale submodel is cut out from the global model and the continuous simulations are conducted in this submodel. Level 2 deals with identification of uncertain soil parameters based on the measurements (e.g., surface settlements) during tunnel excavation. In Level 3, the tunneling process parameters (e.g., grouting pressure) can be optimized to control tunneling-induced ground movements or building deformations according to the design criterion. The proposed hybrid modeling approach is validated via a 3D numerical simulation of mechanized tunnel excavation. The results show the capability of the proposed approach to provide reliable model responses in the near field around the tunnel with reduced computational costs.
The use of model tests to investigate the ground displacements associated with multiple tunnel construction in soil
Tunnelling and Underground Space Technology, 2006
Understanding, and hence predicting, the ground displacements associated with multiple tunnel constructions in soft ground (soil rather than rock) is particularly important in urban areas. It has been shown by a number of previous authors that the effect of previously strained soil above the first tunnel has an effect on the ground displacements observed above subsequent tunnels constructed in close proximity to the first. This paper describes the design, development and preliminary test results for a purpose built test tank for modelling, at approximately 1/50 scale, multiple tunnels constructed in soft clay. The tank is 1.80m long, 0.60m wide and 0.45m high (this height is doubled when the consolidation section of the tank is added). The soil is consolidated from slurry to a prescribed strength by specifying the moisture content. A water bag arrangement is used to maintain a surcharge at the soil surface after consolidation in order to produce a realistic over-consolidation ratio. The tunnels (0.08m diameter) are constructed using an augering technique, with a shield and lining arrangement that allows a consistent 'volume loss' to be produced during construction. The tank has side faces made of clear Perspex, which enables the movement of the soil close to these faces to be observed. The soil displacements are recorded using digital cameras and analysed by computer. The paper describes some of the promising initial test data and the trends observed.
Ground Movements Caused by Caisson Installation at the Lurie Excavation Project
Geotechnical Engineering for Transportation Projects, 2004
Lateral deformations caused by the installation of 0.76 m to 1.5 m diameter, caissons were measured by inclinometers placed at the excavation for the Lurie Research Center in Chicago, IL. The bells of the caissons were founded at an average elevation of -23.8 m Chicago City Datum (CCD). Subsurface conditions at the site and construction techniques used to install the caissons are described. Eight inclinometers were placed around the perimeter of the site and one inclinometer was placed in an ad hoc test section consisting of two caissons located in the interior of the site. The ad hoc test section and production caisson inclinometer responses are compared. The times needed to construct the production caissons around the exterior of the site and the test section caissons were very similar so that the time-dependent responses of the clays squeezing into the caisson did not cause differences in the observed responses. Larger ground movements were observed adjacent to the production caissons located near the perimeter of the temporary wall than adjacent to the two caissons located at the test section in the interior of the site. The performance data show a correlation between lateral movements caused by installing caissons and the stability number. A finite element simulation models the stress differences between the production and test section caissons. An empirical means to estimate the movements associated with caisson construction is provided.
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