Professional judgement is the basis for many of the decisions taken by geotechnical engineers to make progress in the design, execution and works supervision. Judgment is a mandatory component of any engineering achievement, essential to assess the various uncertainties that inevitably affect engineering practice. Confidence in such judgements can result in small to big consequences, not only for the engineer itself, but also for others, sometimes with the risk of human loss and significant damage. The definition and the development of judgment in geotechnical engineering is discussed. The bases of the judgement are analysed in detail and the heuristics and bias, responsible for failures in the judgment, are identified. The importance of experts’ judgement and codification are highlighted and ways to improve judgment are also described. The lessons learned in a case study of one accident and two incidents that have occurred during the execution of the Lisbon Terreiro do Paço metro station construction works are presented to highlight the importance of an informed decision making informed through the engineering judgement.
The paper presents a solution executed for the connection of a pre-existing metro tunnel made with a tunnel-boring machine to the structure of a station constructed by the cut-and-cover method. The geotechnical conditions of the site were very difficult: 26 m of man-made fill and alluvial soft silty clay and loose sands, with the water level close to the surface. The tunnel was completely covered by these soft soils. The solution consisted mainly in extending the retaining wall of the station beyond the planes of the portals, in order to confine the tunnel in the stretches adjacent to the station, and creating a volume of treated soil by jet grouting limited by the tunnel lining, the stiff layer underlying the alluvial soft ground, the lateral confining walls and the station wall. The paper presents details of the execution and of the performance of the system. Some conclusions are extracted, emphasising the main advantages of the solution. Cet article présente l'exécution d'une solution destinée à connecter un tunnel de métro préexistant creusé au moyen d'un appareil de forage de tunnels à la structure d'une station construite par la méthode de déblai-remblai. Les conditions géotechniques sur le site étaient particulièrement difficiles: 26 m de remblais, de sédiments d'argile silteuse molle et de sables meubles, avec un niveau d'eau proche de la surface. Le tunnel était complètement envahi par ces sols mous. La solution apportée a consisté essentiellement à étendre la paroi de soutènement de la station au-delà des plans des portails, afin de confiner le tunnel dans les extensions adjacentes à la station et à créer un volume de sol, traité par injection par jet, délimité par le revêtement du tunnel, la couche rigide sous-jacente au sol meuble de sédiments, les parois latérales de confinement et le mur de la station. Cet article présente les détails de la mise en oeuvre et les performances du système. Plusieurs conclusions sont avancées, qui soulignent les principaux avantages de la solution apportée.
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