1681
Diametric deformations in the concrete segment lining of a tunnel excavated in soft
soils. Criteria for their evaluation and mitigation actions for their control
Déformations diamétrales dans le secteur du béton revêtement d'un tunnel creusé dans les sols
mous. Critères de leur évaluation et des mesures d'atténuation pour leur contrôle
Aguilar M.A., Valencia J.D., Schmitter J.J.
Ingenieros Civiles Asociados
Auvinet-Guichard G.
Instituto de Ingeniería, UNAM
Rangel-Núñez J.L.
Universidad Autónoma Metropolitana, Azcapotzalco
ABSTRACT: A criterion is presented for the evaluation of diametric deformation that occurred at the initial support of a tunnel
formed by pre-cast concrete segmented rings, placed in a very soft and high compressibility clay medium. This criterion is presented
on a working graph, which involves the time factor with respect to the deformation rate, for several percentages of the diametric
deformation, with respect to the initial diameter of the concrete segment lining. With available information it is possible to draw a line
on the working graph and, depending on its particular location, it is possible to define the recommended mitigation action to be
followed: only a continuous topographic monitoring if diametric deformations are minimal, additional grouting stages around the
concrete segmented rings if they are more pronounced, or additional internal reinforcement structural systems if they seem dangerous
for the lining’s stability.
RÉSUMÉ : On propose un critère d’évaluation de la déformation diamétrale du revêtement primaire d’un tunnel constitué de
voussoirs préfabriqués et construit dans un sol mou et compressible. Ce critère est représenté sur un graphique de travail qui fait
intervenir le taux de déformation pour et les pourcentages de déformation diamétrale par rapport au diamètre initial du revêtement
primaire en voussoirs de béton. Les données disponibles permettent de tracer une ligne sur le graphique et en fonction de sa position
de recommander les mesures de contrôle à prendre : un simple suivi topographique si les déformations sont faibles, des injections
complémentaires autour du revêtement primaire si elles sont plus importantes ou un renforcement interne au moyen de structures
adaptées si les déformations semblent indiquer un danger pour la stabilité du revêtement.
KEYWORDS: tunnel deformation, control strategy, lining’s stability, soft soil, tunnelling.
1 INTRODUCTION
The case history is presented of a tunnel for the transport of
residual water from Mexico City, with an excavation diameter
of 8.70m. Its initial support consists of concrete segmented
rings with exterior and interior diameter of 8.4 and 7.7m
respectively, and 1.5m wide (6 pieces plus wedge).
During construction of the tunnel with an earth pressure
balance (EPB) tunnel boring machine in very soft and highly
deformable clayey soils of the Valley of Mexico, the periodic
measurement over time was carried out of the interior horizontal
and vertical diameters of each ring, recording their differences
regarding their initial reading, at different dates, observing that
the deformational behavior of their initial support depends
substantially on the geotechnical properties of the materials it
goes through, on the natural actions that occur in its
neighborhood, such as soil “drying” and posterior “saturation”
due to the effects of the absence or abundance of rain, and
mainly due to the medium’s own compression. Therefore, it is
considered that the stability of the segmented rings that form the
tunnel’s initial support depends basically on the compression it
receives from the neighboring soil where it is being built, and in
particular on the differences in the value of the compression’s
components, horizontal and vertical. If for reasons inherent to
the subsoil itself the difference between horizontal and vertical
pressures is critical, for example when the horizontal
component tends to cero, notable deformations are induced on
the lining, which start with sudden increments of the
deformation speed. Therefore, in case the increments are
observed on the lining’s diametric deformations, it is necessary
to adopt control and mitigation measures, to thus avoid
problems in the tunnel’s structural stability.
A graphic criterion is presented in this document, to evaluate
the deformational behavior shown by the segmented rings of the
tunnel’s initial support, and the corrective measures that were
applied to achieve its stabilization.
2 DEFORMATIONAL BEHAVIOR OF THE SEGMENTS
The initial support of a tunnel excavated with an EPB machine
in clayey soil of very soft consistency, and lined with
segmented rings of reinforced concrete, is a classic example of
“soil/structure” interaction, because without the segmented
rings the hollow excavated by the tunnel boring machine would
not be kept open and without the compression provided by the
adjacent soil, the mentioned rings would not even support their
own weight. The relative flexibility the segmented rings have in
principle allows them to adjust conveniently to horizontal and
vertical pressures induced on them by the neighboring terrain,
with acting pressures being redistributed, the horizontal and
vertical adjustment of their diameters as their answer, which in
general tends to stabilization, without any major transcendence.
As part of the segmented ring’s deformational control, a
periodic horizontal and vertical measurement of the diameters
of each ring is carried out, the first reading being the one
recorded when the ring is recently placed within the boring
machine’s shield, which is what the tunnel is excavated with. As
result of these periodic measurements, it has been observed that
the tunnel’s initial support has various deformational behaviors
that are linked mainly to the type of terrain it is in, and to the
extraordinary events that occur in its neighborhood. It is
important to point out that the largest segmented ring
deformation occurs just when the ring leaves the shield, because
it enters into contact with the natural terrain and receives the
injection of mortar on its outer surface.
