1683
Effect of the subsoil conditions in the seismic interaction between two underground
stations connected by a circular section tunnel
Effet des conditions du sous-sol à l'interaction sismique entre deux stations de métro reliées
par un tunnel de section circulaire
Botero E., Ossa A., Ovando E., Sierra L., Giraldo V.
Instituto de Ingeniería, Universidad Nacional Autónoma de México
ABSTRACT: The new Line 12 of the Mexico City subway presents several special situations related to geotechnical and seismic
aspects. In this paper we analyze the influence of changes caused by variations in the properties and thickness of soil layers between
two underground stations connected by a circular tunnel. For this purpose we discuss the accelerations, stresses and deformations
generated in different elements of the underground structures and its variation along time. The results show that variations in the soil-
structure response must be considered in design of this type of structures located in Mexico City.
RÉSUMÉ : La nouvelle ligne 12 du métro de Mexico présente plusieurs situations particulières liées aux aspects géotechniques et
sismiques. Dans cet article, nous analysons l'influence des changements provoqués par les variations dans les propriétés et l'épaisseur
des couches de sol entre deux stations de métro reliés par un tunnel circulaire. A cet effet, nous discutons des accélérations, des
contraintes et des déformations générées dans les différents éléments des structures souterraines et sa variation au cours du temps. Les
résultats montrent que les variations de la réponse du sol-structure doit être pris en compte dans la conception de ce type de structures
situées dans la ville de Mexico.
KEYWORDS: Seismic interaction, subsidence phenomena, underground structures.
1 INTRODUCTION.
Certain areas of Mexico City are affected by regional
subsidence and the amplification of earthquake ground motions.
The first issue, caused by groundwater extraction from the
underlying aquifer, induces changes in soil properties over time
(Ovando et al., 2007). The consolidation process of soil strata
caused by ground water extraction also modifies the seismic
response of soil, the infrastructure works in the City (tunnels,
bridges and buildings) may become more vulnerable to seismic
events over time.
This study evaluates the evolution of seismic response a
tunnel and two underground subway stations, caused by
regional subsidence over a period of 50 years.
Analyses were performed using a three-dimensional finite
difference model which comprises two underground subway
stations, a tunnel section and the soil deposit. Acceleration,
stress and displacement histories were determined at the tunnel
station connections joints and those places where tunnel
changes direction.
2 PROBLEM DEFINITION.
The Metro Line 12 is 24.5 km in length, and it passes through
various geotechnical areas in which different geotechnical
problems are presented. Some of these are related to
stratigraphical variations along the line, regional subsidence and
variation of the seismic response during the structures’ life time.
Zapata and Parque de los Venados subway stations are
located in the middle-west of Mexico City in the so called
transition zone (NTC, 2004) where the thicknesses of soft clay
strata do not exceed 20m. Both stations were built at a depth of
22 m approximately and are connected to 9m diameter circular
tunnel. The studied area presents significant variations in the
clay layers thicknesses. Soil deposits underlying The Zapata
Station are formed by harder materials than those at the Parque
de los Venados Station which was built over 15m thick
compressible clay strata.
3 MATERIAL PROPERTIES
Soil properties of the studied area, were obtained from test on
undisturbed samples obtained from three borings 15 to 20 m
deep as well as from a cone penetration (CPT) and down-hole
tests (see Figure 1).
Shear wave velocities at the Zapata Station were directly
obtained from the down-hole test. Semiempirical correlations
(Eq. 1) that relate CPT strength with shear wave velocity of soil
(Ovando and Romo, 1991) were used for the characterization of
soils in Parque de los Venados Station.
Time (milliseconds) CPT strength, qc (kg/cm²)
Figure 1. a) Down.Hole test results, in Zapata station. b) Cone
penetration test results in Parque de los Venados station.
