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Effects of groundwater seepage in steady-state conditions in deep tunnel shafts
Norma P. López-Acosta, Gabriel Auvinet & Marco A. Pérez Instituto de Ingeniería, UNAM, México.

ABSTRACT The aim of this paper is to study the effects of groundwater seepage in steady-state conditions in the excavation of a 12m diameter and 90m deep tunnel shaft. Several of the soil strata located in the excavationarea of the tunnel shaft are constituted by medium to high hydraulic conductivity materials, which generate important flow velocities and flow rates. Additionally, the presence of lower hydraulic conductivity strata interbedded between more permeable layers suggests the occurrence of significant hydraulic gradients, and therefore assessment of the uplift pressure in certain stages of the tunnelshaft excavation becomes necessary. A methodology to carry out this type of analyses by two-dimensional numerical modeling based on the finite element method is exposed. The results presented refer to the hydraulic head field, velocity vectors, among others, with emphasis on exit hydraulic gradients and flow rate at different stages of the tunnel shaft excavation. In particular, graphs related tothe exit hydraulic gradient, exit seepage velocity, and flow rate as a function of the excavation depth are provided. Finally, conclusions of the analyses performed and recommendations in order to control the groundwater seepage and to mitigate uplift problems into the tunnel shaft are provided. RÉSUMÉ Le but de cet article est d'étudier les effets d'écoulement des eaux souterraines en régimepermanent dans l'excavation d'un puits de 12m de diamètre et 90m de profondeur. Plusieurs strates du sol situé dans la zone d'excavation du puits sont constituées par des matériaux à moyenne et haute conductivité hydraulique, qui génèrent des vitesses d'écoulement et des débits importants. En outre, la présence de strates moins perméables intercalées entre des strates très perméables conduit àd'importants gradients hydrauliques, et par conséquent l'évaluation de la sous-pression dans certaines étapes d'excavation du puits devient indispensable. On expose une méthodologie pour effectuer ce type d'analyses par modélisation numérique à deux dimensions basée sur la méthode des éléments finis. Les résultats présentés se rapportent au domaine de la charge hydraulique, vecteurs de vitesse, entreautres, en mettant l'accent sur des gradients de sortie hydraulique et le débit à différentes étapes de l'excavation du puits. En particulier, on présente des graphiques liés au gradient de sortie hydraulique, vitesse de sortie et débit en fonction de la profondeur d'excavation. Enfin, on présente les conclusions des analyses effectuées et des recommandations en vue de contrôler l'écoulement des eauxsouterraines et atténuer les problèmes de sous-pression dans le puits.

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INTRODUCTION

The aim of this paper is to show the importance of performing groundwater seepage analyses through soils in geotechnical engineering practice projects. This type of analyses is not usually carried out, not even in projects of great geotechnical importance. The tunnel shaft analyzed in this work is partof a deep drainage system. The study of this structure was chosen firstly because it is a tunnel shaft of a great depth (90m), this feature will cause a great hydraulic head loss between the original water table and groundwater level at the bottom of the excavation, which will generate important hydraulic gradients and seepage forces; and secondly, because this tunnel shaft is located in a veryheterogeneous layered medium with contrasting hydraulic conductivities from one layer to another, which will cause variations in velocities, hydraulic gradients and flow rates in different stages of the tunnel shaft excavation. Based on the aforesaid, this paper focuses on evaluating the results calculated by steady-state flow analyses in different stages of a tunnel shaft excavation. These results...