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.
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.
Stratigraphy. Subsoil conditions at the zone where the atypical deformations occurred on the tunnel’s primary lining are (Fig 1):
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.
SuperiorClayey Series (3 to 26m). These are clays and silts of high plasticity with thin lenses of volcanic ash and sandy silts.
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).
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 uaxis=145kN/m2, which is 65kN/m2 less than the hydrostatic pressure.
Figure 1. Stratigraphic section of the zone where atypical deformations were observed (1+000).
Cracking. Because the channel was built excavating the land, it is considered a zone of unloading. During construction of the tunnel’s shafts near the channel, the presence of subsoil cracking has been observed. In order to verify the existence of that unloading zone, and the presence of developed cracking given the low value of the shear resistance factor for Valley of Mexico clay (KIC≈1.9t/m3/2), the ko stress ratio at rest was determined at the site, and an exploration campaign was carried out with piezocones in zones near to and far from the channel. The stress ratio at rest for the superior clayey series was k0=0.19 for the zone near the channel, whereas at the zone away from the channel the value was k0=0.6. It is to be pointed out that the low k0 value measured for the superior clayey series at the channel zone is evidence of the state of decompression due to the channel’s influence, and vertical cracking presented by the superior clayey formation. One way of observing cracking on clayey soils is to measure point resistance and friction of the electric cone, because when a discontinuity crosses, the values of such resistance decrease. When comparing electric cone point resistances in soundings carried out near to and far from the channel, resistances are observed to descend at certain depths in the case of the cone near the channel, a condition not present in the far-away cone (Fig 2).
Figure 2. Soundings of piezocones carried out at the channel’s zone of influence and away from it.