analysis of behaviour of soil surrounding … · tunneling beneath the ground water table causes...
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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 10, October 2017, pp. 31–40, Article ID: IJCIET_08_10_005
Available online at http://http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication Scopus Indexed
ANALYSIS OF BEHAVIOUR OF SOIL
SURROUNDING AROUND BAGHDAD METRO
AT BAGHDAD CITY CENTER DURING AND
AFTER TUNNEL EXCAVATION USING THE
FINITE ELEMENT METHOD
Mr. Muammar H. Al-Taee
Civil Engineering Department, Engineering College, 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
Tunnel excavation disturbs the initial stresses balance and causes stresses
redistribution in soil around it. It is significant to study the characteristic of
displacements of soil surrounding around tunnel with and without lining. The finite
element analysis is used world widely in tunnelling to obtain the soil displacements
caused by tunnel excavation. Based on numerical simulation method, the vertical
displacements of soil surrounding around Baghdad metro at Baghdad city center,
under Al-Tairan Square, are predicted in this paper using the commercially available
finite element package, Abaqus 2016. Aseries of actual tunnelling processare
simulated 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. The vertical displacements of soil
surrounding around Baghdad metro passing under Al-Tairan Square are computed
through the time periods of three sequential simulation steps. These steps are named
as the excavation, linings installation, and consolidation steps. It is found that
maximum vertical displacement occurs at the crown to the downward with value equal
to 28.5 m approximately after linings installation immediately. Also highlighted is the
importance of the stress-pore pressure coupled analysis in the numerical prediction
tunnel behaviour.
Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at Baghdad City Center During
and After Tunnel Excavation using the Finite Element Method
http://www.iaeme.com/IJCIET/index.asp 32 [email protected]
Keywords: Baghdad metro; vertical displacements, Finite element method; Stress-pore
pressure coupled analysis;Abaqus.
Cite this Article: Muammar H. Al-Taee, Dr. Aqeel Al-Adilli, and Dr. Nagaratnam
Sivakugan, Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at
Baghdad City Center During and After Tunnel Excavation using the Finite Element
Method, International Journal of Civil Engineering and Technology, 8(10), 2017,
pp. 31–40
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=10
1. INTRODUCTION Tunneling beneath the ground water table causes changes in the state of stress and the pore-
water pressure distribution. In such tunneling 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 tunneling significantly hamper the tunneling
works resulting in an increase in the construction costs. Second, as the stress-strain-strength
characteristics of the surrounding ground are governed by the effective stress, the change in
the pore water pressure distribution during the tunneling process can affect the short- and
long-term tunnel stability. Third, the direct environmental consequence of water inflows
during tunneling 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.
Despite the importance of understanding the stress-pore water coupled effect on tunneling
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 tunneling and
the ground water where much needs to be investigated to better understand the three
dimensional stress-pore pressure coupled interaction mechanism during tunneling. 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.
Baghdad metro at Baghdad city passing under Al-Tairan Square is considered in this
study at coordinates equal to 445383m for X and3688176.563m 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 15.415 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
Muammar H. Al-Taee, Dr. Aqeel Al-Adilli, and Dr. Nagaratnam Sivakugan
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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 (Zmesh=15.415+6.3+1.5Do=31.165m) in the three dimensional finite element
model at this location becomes necessary to know. It is found in the previous site
investigation reports conducted on Baghdad metro by the National Center for Construction
Labs in Baghdad, that there are nine soil layers different in properties at this location until
Zmesh. The soil strata properties are shown in Table 1. The average ground water level at this
location is predicted from spatial interpolation techniques conducted on 49 wells within
Baghdad city by using ArcGIS 10.5 equal to 1.95 m which it is obtained equal to 1.95 m.
Hence, the initial ground water table is considered equal to this level in the analysis.
Figure 1 Layout of Baghdad metro plotting on satellite image of Baghdad city (60 cm error).
Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at Baghdad City Center During
and After Tunnel Excavation using the Finite Element Method
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Table 1 Geotechnical properties of ground profile at location considered in this study
Layer no. †
Depth of layer (m)
† Thickness
(m) † Soil description †
From To
Above
W.T. 1 0 1.95 1.95 Fill (Sandy silty clay with pieces of brick)
Below
W.T.
1 1.95 9.8 7.85 Fill (Sandy silty clay with pieces of brick)
2 9.8 10.8 1 Stiff to hard brown clay to clayey silt ( CL to CH)
3 10.8 12.6 1.8 Stiff to hard grey sandy silty clay (CL)
4 12.6 13.5 0.9 Dense to very dense silty fine sand (SM to SP)
5 13.5 16.4 2.9 Sandy silty clay to clayey fine sand (CL to SC)
6 16.4 22.5 6.1 Dense silty sand (SM)
7 22.5 25.1 2.6 Hard brown sandy silt to sandy silty clay (ML to
CL)
8 25.1 30 4.9 Dense to very dense silty fine sand (SM to SP)
9 30 31.17 1.17 Very dense grey gravelly silty sand (SM)
Layer no. †
ρ (t/m3) † Elasticity † Plasticity †
K (m/s) † e † ρd ρs E (KN/m
2) ν ϕf(⁰)
C
(KN/m2)
Above
W.T. 1 1.585
3233.3 0.3 20.3 180 5.00E-10 0.599
Below
W.T.
1
1.948 3233.3 0.3 20.3 180 5.00E-10 0.599
2 1.98 67500 0.425 10 98 5.00E-09 0.7
3 1.97 3233.3 0.3 0 65 5.00E-10 0.8
4 1.95 21000 0.32 7 67 6.00E-06 0.75
5 1.96 7750 0.25 6.9 70 5.00E-09 0.975
6 2.4 13000 0.28 14 35 1.00E-08 0.7
7 1.91 7500 0.31 11 110 1.00E-09 0.6
8 1.95 21000 0.32 7 67 6.00E-06 0.75
9 2.5 15000 0.35 32 13 1.00E-07 0.78
† 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 behaviour of soil surrounding around Baghdad metro passing under Al-Tairan
Square which its coordinates are 445383m for X and3688176.563m for Yby using three
dimensional finite element model. In this study, Abaqus is selected so as to take advantage of
its effectiveness in stress-pore 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 62286nodes and
54100elements.
Muammar H. Al-Taee, Dr. Aqeel Al-Adilli, and Dr. Nagaratnam Sivakugan
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Figure 2 Three dimensional finite element model of Baghdad metro under Al-Tairan Square with
coordinates 445383 m for X and3688176.563 m for Y.
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 (X-
direction) of 8 times the outer diameter from the centerline of each route of tunnel (8Do to the
left of the centerline of left route and 8Do 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).Eight nodes trilinear
displacement and pore pressure elements with reduced integration (C3D8RP) are used for
discretizing the soil layers below the initial ground water table and the shotcrete liners and the
soil layer above the initial ground water table are discretized using stress-displacement eight
nodes brick elements with reduced integration (C3D8R).
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 (Ka), as shown in Table
2.For the hydraulic initial conditions, no recharge at the ground surface during tunnelling is
assumed for simplicity although there may be near-surface recharge from leaking water pipes
in urban situations. Then, it is assumed that the soil layer above the initial ground water level
is to be a fully dray layer. The initial pore water pressures are also for each layer within this
model. For the linings of tunnel, 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 under Tairan Square with coordinates 445383 m for X
and3688176.563 m for Y.
Layer no. Pore pressure (KN/m
2) Effective stress (KN/m
2) Lateral
coefficient (Ka) Top Bottom Top Bottom
Above W.T. 1 0.000 0.000 30.908 0.485
Below W.T. 1 0.000 78.500 30.908 105.326 0.485
Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at Baghdad City Center During
and After Tunnel Excavation using the Finite Element Method
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2 78.500 88.500 105.326 115.126 0.704
3 88.500 106.500 115.126 132.586 1.000
4 106.500 115.500 132.586 141.136 0.783
5 115.500 144.500 141.136 168.976 0.785
6 144.500 205.500 168.976 254.376 0.610
7 205.500 231.500 254.376 278.036 0.679
8 231.500 280.500 278.036 324.586 0.783
9 280.500 292.200 324.586 342.136 0.307
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
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.
Table 3 The mechanical properties of the shotcrete liners
Density (Kg/m3)
Elasticity
Modulus (Ε, KN/m2) Poisson's Ratio (ν)
2500 30000000 0.2
4. 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 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 vertical displacements of soil surrounding around Baghdad metro under Al-
Tairan Square at each step. With respect to the underground conditions of the site considered
in this study, it is assumed those 10 days and 30 Hours as sufficient time periods for the
excavation and linings installation steps respectively. It is found that after 10 days of the
Muammar H. Al-Taee, Dr. Aqeel Al-Adilli, and Dr. Nagaratnam Sivakugan
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linings installation, the effect of consolidation on the tunnel become constant. Therefore, 10
days are considered as a sufficient time period to predict the vertical displacements at this
step.
5. BEHAVIOUR OF SOIL SURROUNDING AROUND BAGHDAD
METRO AT BAGHDAD CITY CENTER After submitting the three dimensional finite element model shown in Figure 2in Abaqus, the
behaviour of soil surrounding around Baghdad metro under Al-Tairan Square can be studied
from the deformed shape of this model. 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. The vertical displacements of
soil surrounding around Baghdad metro at the crown and invert points of the front face of
each tunnel route through the excavation, linings installation, and consolidation steps
respectively are predicted. The negative sign of vertical displacement refers to occur
displacement in the downward direction while the positive sign refers to occur displacement
in the upward direction. Figure 3 shows the deformed shape of this model for a typical
increment of simulation step showing the observation points (nodes) at which the vertical
displacements are computed.
Figure 3 Deformed shape of the three dimensional finite element model of Baghdad metro under Al-
Tairan Square showing layout of the observation points at which the vertical displacements computed.
The vertical displacements of soil surrounding around Baghdad metro under Al-Tairan
Square computed at the crown and invert points of the front 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: The change curves of the vertical displacements at the crown
and invert points in the front faces of the left and right routes of Baghdad metro under Al-
Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at Baghdad City Center During
and After Tunnel Excavation using the Finite Element Method
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Tairan Square during the excavation step are given in Figure 4 which shows very clearly that
the change curves of the vertical displacements at the front 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 0.00046 m and the coincidence is also satisfied for the
change curves of the vertical displacements at the front invert points of the left and right
routes with RMSE equal to 0.00304 m.
Figure 4 Change curves of the vertical displacements (U3) for the front faces of the left and right
routes of Baghdad metro under Al-Tairan Square during the excavation step (10 days).
ii. During the linings installation step: The change curves of the vertical displacements at the
crown and invert points in the front faces of the left and right routes of Baghdad metro under
Al-Tairan Square during the linings installation step are given in Figure 5 which shows very
clearly that the change curves of the vertical displacements at the front crown points of the
left and right routes during the time period of this step are quite coincided throughout the
period with RMSE equal to 0.00158mand the coincidence is also satisfied for the change
curves of the vertical displacements at the front invert points of the left and right routes with
RMSE equal to 0.0044m.
Figure 5 Change curves of the vertical displacements (U3) for the front faces of the left and right
routes of Baghdad metro under Al-Tairan Square during the linings installation step (30 hours; 1.25
day).
Muammar H. Al-Taee, Dr. Aqeel Al-Adilli, and Dr. Nagaratnam Sivakugan
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iii. During the consolidation step: The change curves of the vertical displacements at the
crown and invert points in the front faces of the left and right routes of Baghdad metro under
Al-Tairan Square after linings installation complete are given in Figure 6. It is noted that after
completing the linings installation the vertical displacements at the crown and invert points in
the front faces of the two tunnel routes have the same values for long time which are
approximately equal to -28.4 m for the crown points and 11.62 m for the invert points. Hence,
10 days are selected as a time period for the consolidation step to compute the vertical
displacement during them. It is also noted very clearly that the change curves of the vertical
displacements at the front crown points of the left and right routes during the time period of
this step are quite coincided throughout the period with RMSE equal to 0.00158 m and the
coincidence is also satisfied for the change curves of the vertical displacements at the front
invert points of the left and right routes with RMSE equal to 0.0044 m.
Figure 6 Change curves of the vertical displacements (U3) for the front faces of the left and right
routes of Baghdad metro under Al-Tairan Square during the consolidation step (10 days).
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 prediction of behaviour of soil surrounding around boring tunnel behaviour
during the sequential actual tunnelling processes enables us to take into consideration the
impact of surrounding soil on the tunnel lining response when the project becomes under
servicing and the probable solutions have to be followed. In this paper, the commercially
available finite element package Abaqus/CAE 2016.HF4 is used to analyze behaviour of soil
surrounding around Baghdad metro at Baghdad city center passing under Al-Tairan Square 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. During the time periods of the excavation and linings installation steps, the vertical
displacements at the significant circumferential points around the two routes of Baghdad
metro under Al-Tairan Square are computed. Then, the time after linings installation complete
at which those displacements have inconsiderable changes is considered as a sufficient time to
predict the mentioned displacements.
It is also obtained the following the downward displacements occur at the crowns of the
left and right routes of Baghdad metro while upward displacements occur at the inverts of the
Analysis of Behaviour of Soil Surrounding Around Baghdad Metro at Baghdad City Center During
and After Tunnel Excavation using the Finite Element Method
http://www.iaeme.com/IJCIET/index.asp 40 [email protected]
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 0.00046m, 0.00158m, and 0.00158m
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 0.00304 m, 0.0044 m, and 0.00443 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.
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