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Performance of a pioneer foundation of the skirt type for the Metro-Line 12 overpass
on the Mexico City soft clay
Comportement d'un nouveau type de fondations de type radier à jupe, utilisé pour les tronçons en
viaduc de la ligne 12 du Métro fondés sur les argiles molles de Mexico
Mendoza M.J., Rufiar M., Ibarra E., Mendoza S.A.
Instituto de Ingeniería, Universidad Nacional Autónoma de México, 04510 Mexico City
ABSTRACT: Given the extension and importance of Mexico City’s Metro-Line 12, it was considered relevant to monitor the
behavior of the foundation of one of the supports for the overpass solution, which is laid on very soft clayey soils. This pioneer
foundation, a footing or foundation slab with long skirts, locally known as structural cell or “inverted glass” type, was used for the
first time in the city. Geotechnical sensors and accelerometers were included in the foundation to follow its behavior, not only during
its construction and long term operation, but to record significant variables during strong earthquakes. From measurements made to
date (two years after the beginning of construction) it stands out the effective coefficient K of earth pressure, reaching values close to
the unit, at the soil-concrete walls (cast in place) contact.
RÉSUMÉ : Au vu des dimensions et de l’importance de la Ligne 12 du Métro de la ville de Mexico, il a été jugé nécessaire de
procéder à l’auscultation des fondations de l’un des appuis du tronçon de la ligne en viaduc, implantées dans des sols lacustres très
mous. Les fondations, d’un type nouveau à Mexico, sont constituées par un radier avec une jupe de parois moulées et sont connues
localement comme cellule structurée ou « verre renversé ». Des jauges géotechniques et accéléromètriques ont été insérées dans les
fondations afin non seulement de suivre leur comportement durant la construction et à long terme mais aussi d’enregistrer certaines
variables importantes durant les séismes intenses. Les mesures réalisées à ce jour mettent en évidence un coefficient de poussée des
terres K effectif de l’ordre de l’unité au contact entre le sol et les parois de béton.
KEYWORDS: Foundations, Static and seismic performance, geotechnical instrumentation, soft clayey soils.
1 INTRODUCTION.
Construction of Mexico City’s Metro-Line 12 has involved
tunnel, semi-deep box, surface and overpass solutions along 25
km of development, connecting the southeast and southwest
zones of the country’s capital. Stratigraphic conditions along the
layout have demanded novel solutions, mainly at the portion
that runs into Lake zone subsoil consisting of very compressible
and low strenght clayey lacustrine deposits. Such is the case of a
pioneering type of foundation in Mexico City, known locally as
structural cell or “inverted glass” which was used at the
supports of an overpass almost 1.7 km long located near the
Tláhuac terminal station. This reinforced concrete foundation
consists of four perimeter walls, each 60 cm thick and 10.5 m
long, cast in place, that constitute the skirting, Figure 1. The
precast footing-column unit is integrated to the four perimeter
walls by casting in place the rest of the footing’s four sides, thus
forming what is known locally as an “inverted glass”.
The instrumented support is named ZP16, and is located
between the Zapotitlán and Nopalera stations. The foundation’s
behavior is exposed based on vertical pressures measured under
the footing, lateral pressures on the walls, and pore water
pressures at the contact between clayey soil and walls, for
which total pressure cells, push-in pressure cells, and
piezometers were used, respectively. With the objective to
improve our knowledge on the behavior since the construction
process, in the long term and during an earthquake, the adopted
instrumentation required long life trustworthy ad hoc equipment
with sufficient precision and immediate dynamic response
during seismic events. The latter required the adoption of an
automatic recording system for these geotechnical data and the
accelerations measured at the footing.
This paper describes the type of foundation, its construction
process, the geotechnical and accelerographic instrumentation
that was integrated into the foundation, and the monitoring
carried out to learn about its geotechnical behavior, not only
during construction and long term operation, but to record
significant variables in the foundation precisely during a strong
earthquake. The monitoring time period covers the constructive
process and its evolution over almost two years. It also includes
static measurements before and after two earthquakes of
moderate intensity.
Solar
panel
Recording
room
Fill
Soft clayey
soil
Figure 1. Overpass for Metro-Line 12. Support ZP-16.
