THREE DIMENSIONAL NUMERICAL INVESTIGATION OF BAGHDAD METRO PASSING UNDER TIGRIS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 10, October 2017, pp , Article ID: IJCIET_08_10_004 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed THREE DIMENSIONAL NUMERICAL INVESTIGATION OF BAGHDAD METRO PASSING UNDER TIGRIS Mr. Muammar H. Al-Taee Civil Engineering Department, Engineering Collage, Misan University, Misan, Iraq Dr. Aqeel Al-Adilli Building and Construction Engineering Department, University of Technology, Baghdad, Iraq Dr. Nagaratnam Sivakugan Science, Technology, and Engineering College, James Cook University, Townsville, Australia ABSTRACT Because of increased public concern regarding the impact of tunnelling on surface waterways under which tunnel passes and on surrounding environments, the effect of interaction between tunnelling and ground water on the soil around boring tunnel and ground surface has become an essential part of the planning, design, and construction of tunnelling project. In this paper, the commercially available finite element package, Abaqus 2016, is used to simulate the behaviour of the proposed Baghdad metro passing under Tigris before and after tunnel excavation by using a fully coupled three dimensional stress-pore pressure finite element model to realistically capture the mechanical and hydrological interaction between the tunnelling and ground water. It is found that maximum vertical displacement occurs at the crown with value equal to 4.25 m approximately to the downward after one year of liners installation. It is shown that there are significant differences in pore water pressures for soil surrounding around tunnel before, during, and after tunnel excavation. It is also shown that the head of water in river has significant difference before, during, and after tunnel excavation. Keywords: Baghdad metro; Tigris; Finite element method; Stress-pore pressure coupled analysis; Abaqus. Cite this Article: Mr. Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan, Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris, International Journal of Civil Engineering and Technology, 8(10), 2017, pp editor@iaeme.com

2 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris 1. INTRODUCTION Tunnelling beneath the ground water table causes changes in the state of stress and the porewater pressure distribution. In such tunnelling problems, there are three important issues that have to be addressed during design and construction including construction, stability, and environmental issues. First, water inflows during tunnelling significantly hamper the tunnelling works resulting in an increase in the construction costs. Second, as the stressstrain-strength characteristics of the surrounding ground are governed by the effective stress, the change in the pore water pressure distribution during the tunnelling process can affect the short- and long-term tunnel stability. Third, the direct environmental consequence of water inflows during tunnelling is the drawdown of groundwater level in the surrounding aquifer. The related ground subsidence occurring as a result of the reduction in water pressures in the soil layers can damage nearby structures or utilities. Proper control of the ground water during tunnelling requires a thorough understanding of interaction mechanisms between the tunnelling and the ground water within the context of the stress-pore water coupled effect. Despite the importance of understanding the stress-pore water coupled effect on tunnelling performance, studies concerning this subject are limited. Numerical methods have been used as primary tools in most of the available studies because of technical difficulties involved in physical modelling of the stress-pore pressure coupled behaviour in either small or large scale. Some of the available studies related to this subject performed numerical analyses with the steady-state seepage analysis or sequential seepage analysis and stress analysis which cannot accurately model the fully coupled interaction behaviour between the tunnelling and the ground water where much needs to be investigated to better understand thethree dimensional stress-pore pressure coupled interactionmechanism during tunnelling. This paper presents thefully coupled 3D finite element model and the simulation strategy results. 2. DESCRIPTION OF THE SITE The proposed Baghdad metro lies in Baghdad city. It has total length equal to 39 km including 42 stations. This proposed project comprises two lines that connect both sides of Baghdad city; Karkh and Rusafa. The central station in which the two lines of Baghdad metro encounter each other lies in Rusafa at Killani square on the Jamhurriya street as shown in Figure 1. Two routes for each line were proposed in the previous study in 1980 and the same proposition is adopted in the newest study by French firm Systra in 2014 (Mayoralty of Baghdad). The tunnel is circular in cross section with 6.3 m outer diameter and 0.3 m of concrete lining thickness. The vertical depth of tunnel is approximately in variation along its extension depending on the geological section of Baghdad city.an advanced rate of excavation must be allocated before according to the boring machine used in excavation, conditions of soil stratification, and other restrictions of urban area. The path of Baghdad metro passing under Tigris is considered in this study at location that its coordinates equal to m for X and m for Y. The two routes of tunnel at this location are excavated at the same depth from the bed of Tigris where the depth of the tunnel crown of the two routes is m with 45 m as a horizontal distance between the outer diameters of the two routes. The maximum depth to which the finite element model is built equal to 1.5 times of the outer tunnel diameter below the depth of the tunnel crown for the two routes because of the presence of boundary beyond them do not significantly influence in stress-strain-pore pressure in field (Yoo, 2005; Yoo et al., 2005; Yoo et al., 2007; and Yoo and Kim, 2008). Hence, the properties of soil layers from the ground surface to the lower boundary (Z mesh = D o =41.791m) in the three dimensional finite element model at this location becomes necessary to know. It is found in the previous site 19 editor@iaeme.com

3 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan investigation reports conducted on Baghdad metro by the National Center for Construction Labs in Baghdad, that there are five soil layers different in properties at this location until Z mesh. The soil strata properties are shown in Table 1. The maximum depth of water above this location is considered equal to 6 m according to the study that was conducted by Nama in 2015 in estimating Tigris flow data by using the hydraulic model. This head of water leads to consider fully saturated initial conditions for all soil layers within the three dimensional finite element model of this location. Hydraulic boundary condition at the excavation step is also added due to existing this head of water. Figure 1 Layout of Baghdad metro plotting on satellite image of Baghdad city (60 cm error) editor@iaeme.com

4 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris Layer no. Table 1 Geotechnical properties of ground profile at location considered in this study Depth of layer (m) Thickness (m) From To Layer no. Soil description Dense grey-brown silty, in parts slightly silty, slightly to very fine and medium gravelly, fine and medium Sand and frequent coarse sand and gravel sized subangular to subrounded brick fragments Stiff, locally very stiff, brown and orange-brown mottled, in parts fine sandy, Clay with occasional pockets of silty fine and medium sand Brown and grey-brown and orange-brown mottled, becoming grey-brown below 16.4m, very silty fine, in parts fine and medium, Sand Grey-brown silty fine and medium gravelly to very gravelly fine and medium Sand with occasional clay pockets Grey-brown silty to very silty fine, in parts fine and medium, Sand ρ (t/m 3 ) Elasticity Plasticity ρ d ρ s E (KN/m 2 ) ν ϕ f (⁰) C (KN/m 2 ) K (m/s) e E E E E E Source: National Center for Construction Labs in Iraq. 3. THREE DIMENSIONAL FINITE ELEMENT MODEL The commercially available finite element package Abaqus/CAE 2016.HF4 is used for analysis of Baghdad metro behaviour passing under Tigris at location which its coordinates are m for X and m for Yby using three dimensional finite element model. In this study, Abaqus is selected so as to take advantage of its effectiveness in stresspore pressure coupled modelling as well as robustness in the numerical solution strategy for soil plasticity.the tunnel is assumed to be excavated full face. Figure 2 shows the finite element model adopted in this study consisting of nodes and elements. The finite element mesh extends to a vertical depth (Z-direction) of 1.5 times the outer tunnel diameter below the tunnel invert for the two routes, to a horizontal distance (Xdirection) of 8 times the outer diameter from the centerline of each route of tunnel (8D o to the left of the centerline of left route and 8D o to the right of the centerline of right route), and to a distance (in Y-direction) of 8 times of the outer tunnel diameter perpendicular to the vertical depth. The locations of these boundaries are selected so that the presence of boundary beyond them does not significantly effect in the stress-strain-pore pressure field in the domain (Yoo, 2005; Yoo et al., 2005; Yoo et al., 2007; and Yoo and Kim, 2008). The soil layers (saturated) are discretized using eight nodes trilinear displacement and pore pressure elements with reduced integration (C3D8RP) and the shotcrete liners are discretized using stressdisplacement eight nodes brick elements with reduced integration (C3D8R) editor@iaeme.com

5 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan Figure 2 Three dimensional finite element model of Baghdad metro passing under Tigris at locations with coordinates m for X and m for Y. In this study, the initial conditions are classified as mechanical and hydraulic initial conditions. For the mechanical initial conditions, the vertical effective stress (geostatic stress) is defined for each layer within this model, from the ground surface to the lower vertical boundary, taking into account the active lateral pressure coefficient (K a ), as shown in Table 2. For the hydraulic initial conditions, no recharge at the ground surface during tunnelling is assumed for simplicity. The hydraulic initial condition for degree of saturation for soil layers is defined equal to 1. The hydraulic initial conditions are also defined for each layer within this model taking into account that initial pore water pressure at the upper surface of the model equal to 60 KN/m 2. For lining, there are no initial conditions for saturation and pore pressure.it is known that any material has permeable property, the initial condition of void ratio is also needed. Hence, the void ratio values viewed in Table 1 are considered as the initial values of the void ratio for each layer within this model. Table 2 Initial conditions of pore water pressures and effective stresses of three dimensional finite element model of Baghdad metro passing under Tigris at locations with coordinates m for X and m for Y. Layer no. Pore pressure (KN/m 2 ) Effective stress (KN/m 2 ) Top Bottom Top Bottom Lateral coefficient (K a) The boundary conditions are also classified as mechanical and hydraulic boundary conditions in this study. In terms of the displacement boundary conditions (mechanical boundary conditions), displacements perpendicular to the lateral boundaries (left, right, front, and back boundaries) are restrained while the vertical displacements perpendicular to the bottom boundary is restrained. The hydraulic boundary conditions are described as drainage boundary conditions. Drainage boundary conditions are defined before and after tunnel excavation as follows: before tunnel excavation, all surfaces are undrained; after excavation, free drainage is allowed for the excavated surface (soil faces ambient concrete liners) as well 22 editor@iaeme.com

6 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris as the inner faces of lining by assigning a zero pore water pressure flow boundary condition to allow for the water to occur during tunnel excavation. In the analysis, the soil layers are assumed to be an elastoplastic material conforming to the Mohr-Coulomb failure criterion together with the nonassociated flow rule proposed by Davis (1968), while the shotcrete lining is assumed to behave in a linear elastic manner. The time dependency of the strength and stiffness of the shotcrete lining after installation is not modelled in the analysis but rather an average value of Young's modulus representing green and hard shotcrete is employed. The mechanical properties of the shotcrete lining are shown in Table 3 and the mechanical and hydraulic properties of the soil layers for this location have been summarized in Table 1. Density (Kg/m 3 ) Table 3 The mechanical properties of the shotcrete liners Modulus (Ε, KN/m 2 ) Elasticity Poisson's Ratio (ν) SIMULATION PROCEDURE The actual tunnelling process of Baghdad metro consisting of a series of excavation and lining installation stages is closely simulated by the Model Change Method by using the three dimensional finite element model shown in Figure 2; this method is recommended in the Abaqus User's Manual. The Model Change Method simulates the actual tunnelling process by adding and removing corresponding elements at designated steps. After establishing the initial stress and pore pressure conditions with appropriate boundary conditions, the step by step tunnelling process is simulated.the steps of simulation are comprised the stages before, during, and after tunnel excavation. Hence, four steps of simulation are adopted in this model. These steps are geostatic, excavation, linings installation, and consolidation steps. The time period occupied to executed the excavation, linings installation, and consolidation steps are needed to find the field variables at each step. According to the geological conditions of the site considered in this study, it is assumed that 10 days and 30 Hours as sufficient time periods for the excavation and linings installation steps respectively. It is found that after one year of the linings installation, the effect of consolidation on the tunnel become constant. Therefore, one year is considered as a sufficient time period to study field variables at this step. 5. ANALYSIS OF FIELD VARIABLES OF BAGHDAD METRO After submitting the three dimensional finite element model shown in Figure 2in Abaqus, many output field variables from the deformed shape of this model can be considered in the analysis. The deformed shapes of this model submitted by Abaqus exhibit similar patterns with different scale factors through the historical progression for the excavation, installation linings, and consolidation steps. Figure 3 shows the deformed shape of this model for a typical increment of simulation step showing the observation points (nodes) at which the field variables are computed and the observation paths along them the field variables are computed.in this study, the following field variables are computed: 1. The vertical displacements of soil surrounding around Baghdad metro at the crown and invert pointsof the front and rear faces of each tunnel route during the excavation, linings installation, and consolidation steps respectively (time dependent values) editor@iaeme.com

7 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan 2. The pore water pressures of soil surrounding around Baghdad metro at the front crown, front invert, rear crown, and rear invert points of each tunnel route during the excavation, linings installation, and consolidation steps respectively (time dependent values) as well as the pore water pressures at the upper surface (Tigris bed) computed at the upper front left, middle, and right nodes and upper rear left, middle, and right points during the same simulation steps respectively (time dependent values). Figure 3 Deformed shape of the three dimensional finite element model of Baghdad metro passing under Tigris showing layout of the observation points and paths at which the field variables computed. Figure 4 shows the change curves of the vertical displacement of Baghdad metro under Tigris at location its coordinates are m for X and m for Yat the crown and invert points of the front and rear faces of the left and right routes during10 days for the excavation, 30 hours for the linings installation, and 1 year for the consolidation steps (i.e. total time is days).the negative sign of vertical displacement in the change curves of the vertical displacements refers to occur displacement in the downward direction while the positive sign refers to occur displacement in the upward direction editor@iaeme.com

8 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris (A) Excavation step (10 days) (B) Linings installation step (30 hours or 1.25 days) (C) Consolidation step (1 year) Figure 4 Change curves of the vertical displacements (U3) for the front and rear faces of the left and right routes of Baghdad metro under Tigris during excavation (10 days), linings installation (30 hours), and consolidation (1 year) steps editor@iaeme.com

9 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan The pore water pressures of Baghdad metro under Tigris at this location computed at the crown and invert points of the front and rear faces of the left and right routes during the time period of the excavation, linings installation, and consolidation step are viewed for each step separately as follows: i. During the excavation step: Graphical comparisons between the change curves of the pore water pressures at the crown and invert points in the front and rear faces of the left and right routes of Baghdad metro under Tigris at this location during the excavation step (with time) are given in Figure5 which shows very clearly that the change curves of the pore water pressures at the front and rear crown points of the left and right routes during the time period of this step are quite coincided throughout the period with root mean square errors (RMSE) equal to KN/m 2 and the coincidence is also satisfied for the change curves of the pore water pressures at the front and rear invert points of the left and right routes with RMSE equal to KN/m 2. Then, graphical comparisons between the change curves of the pore water pressures at six points in Tigris bed above Baghdad metro (Figure 3) during the excavation step (with time) are also given in Figure 6 which shows very clearly that the change curves of the pore water pressures at the upper front left and right points and upper rear left and right points during the time period of this step are quite coincided throughout the period with RMSE equal to KN/m 2 and the coincidence is also satisfied for the change curves of the pore water pressures at the upper front and upper rear middle points with RMSE equal to KN/m 2. With regarding to the initial pore water pressures for points above which hydrostatic pressures exist, it is found that the computed values (= KN/m 2 ) are 4.643% larger than actual values (=60 KN/m 2 ) where the Abaqus User's Manual mentioned that the computed values may reach up to 5%. Figure 5 Graphical comparison between the change curves of PWPs for the front and rear faces of the left and right routes of Baghdad metro under Tigris during the excavation step period (10 days) 26 editor@iaeme.com

10 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris Figure 6 Graphical comparison between the change curves of PWPs for Tigris bed above Baghdad metro during the excavation step period (10 days). ii. During the linings installation step:graphical comparisons between the change curves of the pore water pressures at the crown and invert points in the front and rear faces of the left and right routes of Baghdad metro under Tigris at this location during the excavation step (with time) are given in Figure 7 which shows very clearly that the change curves of the pore water pressures at the front and rear crown points of the left and right routes during the time period of this step are quite coincided throughout the period with root mean square errors (RMSE) equal to KN/m 2 and the coincidence is also satisfied for the change curves of the pore water pressures at the front and rear invert points of the left and right routes with RMSE equal to KN/m 2. Then, graphical comparisons between the change curves of the pore water pressures at six points in Tigris bed above Baghdad metro (Figure 3) during the excavation step (with time) are also given in Figure 8 which shows very clearly that the change curves of the pore water pressures at the upper front left and right points and upper rear left and right points during the time period of this step are quite coincided throughout the period with RMSE equal to KN/m 2 and the coinciding is also satisfied for the change curves of the pore water pressures at the upper front and upper rear middle points of this bed with RMSE equal to KN/m 2. Figure 7 Graphical comparison between the change curves of PWPs for the front and rear faces of the left and right routes of Baghdad metro under Tigris during the linings installation step (30 hours) editor@iaeme.com

11 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan Figure 8 Graphical comparison between the change curves of PWPs for Tigris bed above Baghdad metro during the linings installation step period (30 hours). iii. During the consolidation step: Graphical comparisons between the change curves of the pore water pressures at the crown and invert points in the front and rear faces of the left and right routes of Baghdad metro under Tigris at this location during the excavation step (with time) are given in Figure 9 which shows very clearly that the change curves of the pore water pressures at the front and rear crown points of the left and right routes during the time period of this step are quite coincided throughout the period with root mean square errors (RMSE) equal to KN/m 2 and the coincidence is also satisfied for the change curves of the pore water pressures at the front and rear invert points of the left and right routes with RMSE equal to KN/m 2. Then, graphical comparisons between the change curves of pore water pressures at six points in Tigris bed above Baghdad metro (Figure 3) during the excavation step (with time) are also given in Figure 10 which shows very clearly that the change curves of the pore water pressures at the upper front and upper rear left and right points during the time period of this step are quite coincided throughout the period with RMSE equal to KN/m 2 and the coinciding is also satisfied for the change curves of the pore water pressures at the upper front and upper rear middle points of this bed with RMSE equal to KN/m 2. It also noticed obviously that change curves of the pore water pressures at these six points tend to be very coincided approximately after 80 days from the end of the linings installation step to the end of this step editor@iaeme.com

12 Three Dimensional Numerical Investigation of Baghdad Metro Passing Under Tigris Figure 9 Graphical comparison between the change curves of PWPs for the front and rear faces of the left and right routes of Baghdad metro under Tigris during the consolidation step (1 year). Figure 10 Graphical comparison between the change curves of PWPs for Tigris bed above Baghdad metro during the consolidation step period (1 year). 6. CONCLUSIONS A fully coupled three dimensional stress-pore pressure finite element model is used to realistically capture the mechanical and hydrological interaction between the tunnelling and ground water. The analysis of boring tunnel behaviour before and after tunnel excavation enables us to take into account the impact of tunnelling on surrounding environments. In this paper, the commercially available finite element package Abaqus/CAE 2016.HF4 is used to analyze Baghdad metro behaviourpassing under Tigris by using three dimensional finite element model. The actual tunnelling process consisting of a series of excavation and lining installation stages is closely simulated by the Model Change Method. Before and after tunnel excavation, the vertical displacements and the pore water pressures of the soil surrounding around two tunnel routes are computed and the pore water pressures at the Tigris bed are also computed. Hence, it is obtained the following findings: 1. Downward displacements occur at the crowns of the left and right routes of Baghdad metro while upward displacements occur at the inverts of the left and right routes of tunnel. It is found that the RMSE between the vertical displacements at the crowns of the left and right routes of tunnel through the time periods of the excavation, linings installation, and consolidation steps are m, m, and m respectively which indicate quite coincidences between the values of vertical displacements at the crowns of the left and right routes of tunnel through the time periods of these three steps. It is also found that the RMSE between the vertical displacements at the inverts of the left and right routes of tunnel through the time periods of the excavation, linings installation, and consolidation steps are m, m, and m respectively which indicate quite coincidences between the values of vertical displacements at the inverts of the left and right routes of tunnel through the time periods of these three steps. 2. Negative pore water pressures occur around the left and right routes of Baghdad metro at the end of excavation step while positive pore water pressures occur around the two tunnel routes at the end of linings installation step and beyond. It is found that the values of pore water pressures around the left and right routes of tunnel rebound approximately to the geostatic step after one year from linings installation editor@iaeme.com

13 Muammar H. Al-Taee, Dr. Aqeel Al-Adilli and Dr. Nagaratnam Sivakugan 3. It is found that the hydrostatic pressured at the Tigris bed above the left and right routes of tunnel rebound approximately to the case before tunnel excavation after one year from linings installation. REFERENCES [1] Abaqus Inc., Abaqus User's Manual, Version 2016.HF4, Hibbit, Karlson, and Sorensen Inc, Pawtucket, Providence, R.I., USA. [2] Davis, E. H., Theories of Plasticity and Failure of Soil Masses, In Soil mechanics: selected topics. (ed. I. K. Lee), PP New York, NY, USA: Elsevier. [3] Mayoralty of Baghdad, Baghdad, Iraq. [4] National Center for Construction Labs, Baghdad, Iraq. [5] Yoo, C. and Kim, S. B., Three Dimensional Numerical Investigation of Multifaced Tunnelling in Water-Bearing Soft Ground, Canadian Geotechnical Journal, 45: , NRC Research Press. [6] Yoo, C., Interaction Between Tunnelling and Ground Water-Numerical Investigation Using Three Dimensional Stress-Pore Pressure Coupled Analysis, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 131, No. 2, PP , ASCE. [7] Yoo, C., Kim, S. B., Lee, Y. J., Kim, S. H., and Kim, H. T., Interaction Between Tunnelling and Groundwater-Its impact on tunnel behaviour and Ground Settlement, Underground Space the 4 th Dimension of Metropolises Barták, Hrdina, Romancov and Zlámal (eds), PP , Taylor and Francis Group, London. [8] Yoo, C., Kim, S. B., Shin, H. C., and Baek, S. C., Effect of Tunnelling and GroundWaterInteraction on Ground and Lining Responses, Underground Space Use: Analysis of the Past and Lessons for the Future Erdem and Solak (eds), PP , Taylor and Francis Group, London. [9] Ciro Caliendo and Maria Luisa De Guglielmo, Simplified Method For Risk Evaluation In Unidirectional Road Tunnels Related To Dangerous Goods Vehicles, International Journal of Civil Engineering and Technology, 8(6), 2017, pp [10] Nouaman Tafraouti, Rhali Benamar and Nouzha Lamdouar, Study of The Effect of Fluctuation of The Fundamental Soil Parameters On Ground Movements Induced by Tunnel Construction. International Journal of Civil Engineering and Technology, 8(1), 2017, pp editor@iaeme.com