ANALYSIS OF BEHAVIOUR OF SOIL SURROUNDING … · Tunneling beneath the ground water table causes changes in the state of stress and ... Numerical methods have been used ... Abaqus

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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


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


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).




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.



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




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



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-




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.


routes of Baghdad metro under Al-Tairan Square during the linings installation step (30 hours; 1.25

day).



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.


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




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.

REFERENCES

[1] Abaqus Inc., 2016. Abaqus User's Manual, Version 2016.HF4, Hibbit, Karlson, and

Sorensen Inc., Pawtucket, Providence, R.I., USA.

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selected topics. (ed. I. K. Lee), PP. 341–354. 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., 2008. Three Dimensional Numerical Investigation of Multifaced

Tunnelling in Water-Bearing Soft Ground, Canadian Geotechnical Journal, 45: 1467-

1486, NRC Research Press.

[6] Yoo, C., 2005. Interaction Between Tunnelling and Groundwater-Numerical Investigation

Using Three Dimensional Stress-Pore Pressure Coupled Analysis, Journal of Geotechnical

and Geoenvironmental Engineering, Vol. 131, No. 2, PP 240-250, ASCE.

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Underground Space–the 4th Dimension of Metropolises–Barták, Hrdina, Romancov and

Zlámal (eds), PP 939-944, Taylor and Francis Group, London.

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[9] Ciro Caliendo and Maria Luisa De Guglielmo, Simplified Method For Risk Evaluation In

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Documents

ANALYSIS OF BEHAVIOUR OF SOIL SURROUNDING … · Tunneling beneath the ground water table causes changes in the state of stress and ... Numerical methods have been used ... Abaqus