1763
Performance of the tunnel lining subjected to decompression effects on very soft
clay deposits
Performance du revêtement du tunnel soumis à des effets de décompression sur les dépôts
d'argile très mous
Rangel-Núñez J.L
Universidad Autónoma Metropolitana, Azcapotzalco
Aguilar-Tellez M.A.
Ingenieros Civiles Asociados, Construcción especializada
Ibarra-Razo E., Paniagua W.
Ingeum
ABSTRACT: Superficial channels on very soft clay deposits undergoing consolidation processes can generate tension zones that
potentially can induce semi-vertical cracking. During construction of any underground works, such as tunnels, these cracks can be
reactivated, especially if the construction process causes significant changes in the initial stress state of the ground, and then generates
important deformation of the tunnel lining from confining loss around the tunnel, especially if dowels rings are used as lining. On the
other hand, it is also possible to generate significant lining deformations if there are changes in the state of stress in the ground’s
surface due to the dredging of channels. This paper presents a case history about the behavior and numerical modeling of the primary
tunnel lining during and after tunneling with an EPB machine in Mexico City soft clay deposits subjected to decompression stresses
caused by the dredging of channels. Total displacements induced during tunneling under superficial channels were high but less than
1% of the tunnel diameter. After dredging, such channels’ additional deformations were induced in the lining because of a reactivation
of pre-existent cracks in the clay deposit. Numerical modeling was carried out to study the optimal solution. Based on numerical
results, two solutions were applied: lining reinforcement and soil improvement.
RÉSUMÉ : Canaux superficiels sur les dépôts d'argile très douces en cours de processus de consolidation peut générer des zones de
tension qui peut potentiellement induire des semi-verticale fissuration. Lors de la construction des ouvrages souterrains, tels que les
tunnels, ces fissures peuvent être réactivés, surtout si le processus de construction entraîne des changements importants dans l'état
initial des contraintes du sol, puis génère une déformation importante du revêtement du tunnel de la perte de confinement autour du
tunnel, surtout si les chevilles des anneaux sont utilisés comme doublure. Cet article présente une étude de cas sur le comportement et
la modélisation numérique du revêtement du tunnel principal pendant et après un tunnel avec une machine EPB dans les dépôts de
Mexico argile molle soumis à une décompression contraintes provoquées par le dragage des chenaux. Déplacements totaux induits
lors des tunnels sous canaux superficiels étaient élevés, mais moins de 1% du diamètre du tunnel. Après dragage ont été produites
déplacement supplémentaire relance revêtement se craquelle. Les modèles numériques ont été utilisés pour étudier ces facteurs et
déterminer la solution optimale. Avec ces résultats, nous proposons deux solutions: augmenter le revêtement et l'amélioration des sols.
KEYWORDS: tunneling in soft soils, soil fracture, decompression stresses, Mexico city tunnels.
1 INTRODUCTION
The Túnel Emisor Oriente (TEO, Spanish acronym for Eastern
Emitter Tunnel) will be the new drainage system for Mexico
City. It is located to the north of the city and it is a circular
tunnel 62 km long, of 7 m inner diameter, set at variable depths
between 30 and 155 m. It crosses all types of soils along 97% of
its length, from very soft to hard, with the rest of the length
crossing volcanic rock. For its construction, Earth Pressure
Balance (EPB) tunnel boring machines are used, with a primary
lining formed by dowels rings with sections 0.35 and 0.40m
thick (COMISSA 2010). Almost the entire tunnel is under the
groundwater level, with pore pressures of up to 0.8MPa.
The project’s first trajectory, approximately 8 km long, is
located at a zone of very compressible clays with low shear
resistance, with water content in the order of 300%, running
parallel to a surface channel. A particular aspect of this section
is that on land near the channel surface cracks have been
observed, and in the zone where the tunnel crosses under the
channel (1+032 to 1+300) it has been observed that before the
crossing (0+920 to 1+032) important primary lining
deformations have occurred, with a tendency to their
stabilization. This anomalous behavior of the tunnel has been
caused by a diversity of factors, among which stand out the
channel’s dredging and the presence of intense fracturing at the
zone of that channel.
The objective of this work is to evaluate the effects on the
tunnels of the unloading induced by dredging surface channels
located on cracked clayey deposits, and as a particular case the
TEO project is presented.
2 GEOTECHNICAL CONDITIONS
Stratigraphy. Subsoil conditions at the zone where the atypical
deformations occurred on the tunnel’s primary lining are (Fig
1):
i.
Superficial Crust
(0 to 3m). It is a stratum formed by
interspersions of sandy silts and hard silty sands, and on
occasions fills up to 2m thick.
ii.
Superior Clayey Series
(3 to 26m). These are clays and silts
of high plasticity with thin lenses of volcanic ash and sandy
silts.
iii.
Hard Layer
(26 to 28 m). These are interspersions of sandy
silts and silty sands (tunnel is located at the inferior part of
the Superior Clayey Series resting on the Hard Layer).
iv.
Inferior Clayey Series
(28 and 42 m). It is a very
compressible clayey deposit.
Conditions of subterranean water. At this zone the groundwater
level is located at 3m depth, and the pore pressure measured at
the tunnel’s axis is in the order of
u
axis
=145kN/m
2
, which is
65kN/m
2
less than the hydrostatic pressure.
