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Assessment of Empirical Method Used to Study Tunnel System Performance
Évaluation de la méthode empirique utilisée pour étudier la performance du système de
tunnel
Mazek S.A.
Civil Engineering Department, Military Technical College, Cairo, Egypt
El Ghamrawy M.K.
Civil Engineering Department, El-Azhar University, Cairo, Egypt
ABSTRACT: Tunnels are major projects of buried structures in civil purposes. The Greater Cairo metro and El-Azhar road tunnels have
been constructed as major projects of buried structures in Cairo City. Tunneling causes movements in surrounding soil. Tunnel construction
in cohesionless soil is a sophisticated process leading to cause potential damage to exist surface and subsurface structures. The finite element
analysis (FEA) is used to predict surface displacement due to tunnelling. Surface displacement equation (SDE) proposed by Peck and
Schmidt (1969) is also used to calculate surface displacement due to tunnelling.
For assessing the reliability of the FEA, a case history along the Greater Cairo Metro tunnel is considered. A comparison between the field
measurements and those obtained by the FEA and the SDE is made. The surface settlements obtained by the SDE are examined with those
obtained by the FEA at different sand soil densities. However, the SDE does not consider impact of different sand soil types. The surface
settlement profile computed by the SDE is in poor agreement with those obtained by the FEA in loose to medium sandy soil. The surface
settlement profiles computed by the SDE agree well with those calculated by the FEA in dense and very dense sandy soil.
RÉSUMÉ :
Tunnels et ouvrages souterrains sont grands projets de structures enterrées dans des fins civiles et militaires. Le Grand métro du
Caire et El-Azhar tunnels routiers ont été construits comme des grands projets de structures enterrées au Caire City. Construction souterraine
profonde provoque des mouvements dans le sol environnant. Mouvement du sol est un facteur important lors de la construction de tunnels.
La construction de tunnels dans le sol sans cohésion est un processus complexe conduisant à potentiel de causer des dommages à l'existence
des structures superficielles et souterraines. Les mouvements de terrain dépendent des propriétés du sol, la profondeur du tunnel, le diamètre
du tunnel, et la méthode de construction.
Dans la présente étude, l'analyse par éléments finis (FEA) est utilisé pour prédire le déplacement de surface en raison de processus de
tunneling basé sur l'étude de cas. Équation de déplacement en surface (SDE) proposé par Peck et Schmidt (1969) est également utilisé pour
calculer le déplacement de surface en raison de processus de la structure de la construction enterrée. Pour évaluer la fiabilité de l'analyse par
éléments finis, une histoire de cas le long de la ligne Grand Caire tunnel de métro 2 est envisagée.
KEYWORDS: Tunneling, settlement, numerical modeling and analysis, displacement, empirical method.
1 INTRODUCTION
Tunnel construction causes movements in surrounding soil.
Ground movement is an important factor during design phase and
selection of appropriate method of tunnel construction. The state of
stress and the soil displacement around a tunnel system are affected
by construction process of tunnel (Ahmed, 1994; El-Nahhass, 1986;
Mazek and El-Tehawy, 2008). The influence zone due to tunneling
depends on many parameters such as type of subsoil properties,
tunnel depth, tunnel geometry, and tunneling method. The tunnel
excavation can be modeled by finite element method under
different soil conditions, different tunnel geometries, and different
construction procedures (Ahmed, 1994; El-Nahhass, 1986; El-
Nahhass, 1999; Mazek and El-Tehawy, 2008; Ezzeldine, 1999).
In this paper, a 2-D finite element model (FEM) is proposed to
predict surface displacement due to tunneling. The modeling of
such a problem should include details of tunnel construction phases
and associated changes of stresses around the tunnels. A nonlinear
stress-strain constitutive model is adopted for soil media
surrounding the tunnel. A case study on the Greater Cairo Metro
tunnel Line 2 (Fig. 1) is conducted to assess the accuracy of the
finite element analysis (FEA). The computed surface settlements
are compared with the field measurements. A good agreement is
found. The main objective of this paper is to examine surface
displacement obtained by both the FEA and the surface
displacement equation (SDE) developed by Peck and Schmidt
(1969). The 2-D finite element analysis considers parameters of
different sand soil densities. However, the SDE does not consider
influence of different sand soil types on surface settlement due to
tunneling.
The subsurface soil profile along the Greater Cairo Metro is shown
in Fig. 1 (Compo and Richards, 1998; El-Nahhass et al., 1994; EL-
Nahhass, 1999; NAT, 1993, 1999, 2010). The Geotechnical
properties of soil in central Cairo city are presented in Table 1
(Abdel-Salam, 1998; EL-Nahhass, 1999; NAT, 1993, 1999, 2010).
2 FINITE ELEMENT MODEL
The finite element code Plaxis-V8.2 is used to model tunnel system
performance. Analyses of displacement and stress around tunnel
system are carried out using a 2-D plane strain finite element taking
into account behavior of tunnel lining and soil media. The soil, the
tunnel lining, and the interface medium are simulated using appropriate
finite elements model as shown in Fig. 2. Numerical modeling of tunnel
system reflects the ground continuum and the tunnel lining. In addition,
the compatibility and equilibrium condition at the interface between the
soil and the tunnel system are idealized in the numerical model. Six
nodes triangular 2-D plane strain element is used to model soil
media. 2-D beam element is used to model the tunnel lining.
A nonlinear stress-strain constitutive model is adopted for soil around
tunnel system. A yield function of the Mohr-Coulomb and a plastic
potential function of the Drucker-Prager are employed. Linear elastic
behavior is assumed for the tunnel liner.
Boundary conditions are defined to provide stability of tunnel
system. The vertical boundaries of the 2-D finite elements model
are restrained by roller supports to prevent a movement normal to
the boundaries. The horizontal plane at the bottom of the mesh
represents a rigid bedrock layer and the movement at this plane is
restrained in two directions. The movement at upper horizontal
plane is free to simulate a free ground surface.
3 PROPERTIES OF MODEL MATERIALS
The geological formations along Greater Cairo Metro line 2
projects are typical Cairo Nile Alluvial deposits (NAT, 1993, 1999;
2010). The tunnel was mostly bored in slightly sand. The ground
water table varies between 2 m and 4 m from ground surface.
