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Design of tunnel lining in consolidating soft soils
Conception du revêtement des tunnels dans des sols mous en processus de consolidation
Rodríguez-Rebolledo J.F., Auvinet G., Vázquez F.
Instituto de Ingeniería, Universidad Nacional Autónoma de México
ABSTRACT: In this paper, a detailed description of the methodology employed for analysis and design of the final lining of a tunnel
that will be part of Mexico City drainage system is presented. The tunnel crosses soft clayey soils of the lake zone of Mexico Valley.
These clays are submitted to an on-going subsidence process associated to intense pumping of water from the aquifer in the urban
area. The obtained results allow a better understanding of the soil-tunnel interaction as the medium is submitted to the effects of a
double process of consolidation: firstly due to changes in effective stresses generated by the tunnel excavation and, secondly, due to
the long term piezometric drawdown in the soil. It is shown that Finite Element Method (MEF) is a powerful tool for the analysis and
design of tunnels in these difficult conditions. FEM allows considering different constitutive models, and representing soil
consolidation due to tunnel excavation, piezometric drawdown and interaction between the tunnel lining and the surrounding soil.
RÉSUMÉ : Cette communication décrit la méthode suivie pour la conception du revêtement définitif d’un tunnel qui fera partie du
système d’assainissement de la ville de Mexico. Le tunnel traverse les argiles molles de la zone lacustre de la vallée de Mexico. Ces
sols sont soumis à un processus de consolidation dû au pompage d’eau intensif de l’aquifère de la zone urbaine. Les résultats obtenus
permettent de mieux comprendre l’interaction entre sol et tunnel sous l’effet d’un double processus de consolidation associé aux
changements de contraintes effectives causés par le creusement du tunnel puis aux abattements piézométriques dus au pompage dans
le sol à long terme. On montre que la méthode des éléments finis (MEF) est un outil puissant pour la conception des tunnels dans ces
conditions difficiles. La MEF permet d’utiliser divers modèles de comportement du sol et de considérer la consolidation due au
creusement, les abattements de pression ainsi que l’interaction entre le tunnel et le sol environnant.
KEYWORDS: tunnel lining, soft soils, piezometric drawdown, Finite Element Method.
1 INTRODUCTION
In the lake zone of Mexico City valley, tunnels liners are
subjected to a double consolidation process. A first process is
due to an effective stress change generated by the excavation
itself and primary liner installation (Kirsch 1898, Morgan 1961,
Wood 1975, Curtis 1976, Alberro 1983, Bobet 2001, Auvinet
and Rodríguez-Rebolledo 2010, Zaldívar
et al
. 2012), whereas a
second process is due to the piezometric drawdown originated
by dewatering of the deep aquifer (Alberro and Hernández
1989, Farjeat and Delgado 1988, Equihua 2000, Flores 2010). It
is known that the first process only affects the primary liner and
that the excess of pore pressure dissipates sometime after tunnel
excavation (Gutiérrez and Schmitter, 2010). The second process
acts in a permanent fashion (long term) on both liners over the
serviceability period of the tunnel.
Hence, in order to design the secondary liner, long term
mechanical properties of the liners and of the soils have to be
considered.
This paper presents a description of the methodology
employed for geotechnical design of the permanent liner of a 62
km long tunnel that will be part of the drainage system of
Mexico City (“Túnel Emisor Oriente” TEO), in a 2.8 km stretch
that crosses clayey soils of the lake zone of Mexico City valley.
The inner diameter of the tunnel is 7m. The primary liner is a
35cm thick ring of prefabricated segments and the secondary
liner is a
35cm thick
cast-in-place concrete continuous ring.
2 THE GEOTECHNICAL MODEL
2.1 Soil profile
The stratigraphic sequence in the area of interest mainly
consists of the following layers:
Layer CS.
Superficial crust, 0 to 3m thick, average specific
weight of 14kN/m
3
and mean water content of 33%.
Layer B
.
Highly plastic soft clays and silts with microfossils,
22 to 24m thick, average specific weight of 11.3kN/m
3
and
mean water content of 293% (
e
0
=6.6)
Layer C.
Silts interbedded with sandy silts, 1.5 to 2.5m
thick, average specific weight of 15kN/m
3
and mean water
content of 56%.
Layer D.
Highly plastic soft clays and silts, 9.5 to 13.5m
thick, average specific weight of 12kN/m
3
and mean water
content of 165% (
e
0
=3.9).
Layer E.
Stiff silts interbedded with sandy silts (hard layer),
5 to 7.5m thick, average specific weight of 16kN/m
3
and mean
water content of 40%.
Layer F.
Highly plastic soft clays and silts interbedded with
volcanic ashes, average specific weight of 13.2kN/m
3
and mean
water content of 115% (
e
0
=2.9).
2.2 Piezometric conditions
The initial pore pressure distribution was obtained from
piezometric stations installed in the area. A typical pore
pressure profile is presented on Figure 1. A significant
drawdown with respect to the hydrostatic condition is observed
