This thesis analyses the design methodology and response of two tunnel adits constructed in London as part of the Canterbury Works Ventilation Shaft for the High Speed 2 (HS2) project. These tunnels, i.e. the central adit and one of the TID adits, are excavated in cohesive soil condition and supported by means of sprayed concrete lining (SCL). The soil at the shaft location mostly consists of an overconsolidated fissured clay, namely London Clay. To predict the behaviour of such clay, both the traditional Mohr-Coulomb (MC) and the advanced BRICK hardening constitutive model are implemented for finite element (FE) modelling in Plaxis. The results of Plaxis 2D and 3D models confirm that the Mohr-Coulomb model has limited applicability in London Clay. The BRICK model is therefore adopted to capture the small strain stiffness behaviour and the anisotropic characteristics, thus defining the expected soil conditions for tunnelling. To determine the expected load on the lining, accounting for its installation at some distance from the face of the excavation, it is necessary to determine the level of soil relaxation. These loads are related to the radial deformation of the soil caused by stress redistribution at the tunnel perimeter due to the excavation. The magnitude of soil convergence is estimated with the Convergence-Confinement Method (CCM). Two design solutions are implemented for supporting the top-heading excavation, showcasing how the construction sequence should be flexible in response to critical design challenges. The first solution consists in mounting elephant feet supports at the bottom of the top heading. In this case, the excavation of the invert is immediate. The alternative solution features the introduction of a temporary invert, which is used in SCL tunnels to reduce the excavation area and provide support for an extended period. The introduction of such structural element modifies the construction sequence from ‘subsequent’ to ‘staged’. The key objective of the thesis is to analyse the different strategies for the sprayed concrete lining of the aforementioned adits, and to validate the FE modelling outcomes through a comparison with the monitoring data provided by the site team. The best outcome in terms of matching real data is seen when the structure is modelled in Plaxis 3D and BRICK constitutive model is implemented to capture the soil behaviour.
Questa tesi analizza la metodologia di progettazione e la risposta di due gallerie di accesso costruite a Londra nell'ambito del pozzo di ventilazione Canterbury Works per il progetto High Speed 2 (HS2). Queste gallerie, ovvero l’adito centrale e uno degli aditi TID, sono scavate in terreno coesivo e supportate mediante rivestimento in calcestruzzo spruzzato (SCL). Il terreno nella zona del pozzo è costituito principalmente da un'argilla fessurata sovraconsolidata, nota come London Clay. Per prevedere il comportamento dell’argilla, vengono implementati sia il modello costitutivo tradizionale di Mohr-Coulomb (MC) sia il modello avanzato di incrudimento BRICK per la modellazione agli elementi finiti (FE) in Plaxis. I risultati dei modelli Plaxis 2D e 3D confermano che il modello di Mohr-Coulomb ha applicabilità limitata alla London Clay. Il modello BRICK viene adottato per catturare il comportamento a piccole deformazioni e le caratteristiche anisotrope, definendo così le condizioni di terreno attese per lo scavo. Per determinare il carico sul rivestimento, considerando la sua installazione a una certa distanza dal fronte di scavo, è necessario valutare il rilassamento del terreno. Questi carichi sono correlati alla deformazione radiale causata dalla ridistribuzione degli sforzi al perimetro della galleria. L'entità della convergenza del terreno viene stimata con il metodo Convergence-Confinement (CCM). Due soluzioni progettuali vengono implementate per il supporto dello scavo della calotta, dimostrando come la sequenza costruttiva debba essere flessibile in risposta a sfide critiche. La prima soluzione prevede l’installazione di supporti a "piedi d’elefante" alla base della calotta, permettendo lo scavo immediato dell’arco rovescio. L’alternativa introduce un arco rovescio provvisorio, utilizzato nelle gallerie SCL per ridurre l'area di scavo e fornire supporto più a lungo. Questo elemento modifica la sequenza costruttiva da "successiva" a "per fasi". L'obiettivo principale della tesi è analizzare le diverse strategie per il rivestimento in calcestruzzo spruzzato delle gallerie di accesso e validare i risultati della modellazione FE confrontandoli con i dati di monitoraggio forniti dal team di cantiere. Il miglior risultato rispetto ai dati reali si ottiene quando la struttura è modellata con un modello geometrico 3D e il modello costitutivo BRICK viene adottato per simulare il comportamento del terreno.
Displacement induced by tunnelling in London Clay: monitoring data and modelling of the steel fibre reinforced concrete lining
MERDARI, MARA ANDREEA
2023/2024
Abstract
This thesis analyses the design methodology and response of two tunnel adits constructed in London as part of the Canterbury Works Ventilation Shaft for the High Speed 2 (HS2) project. These tunnels, i.e. the central adit and one of the TID adits, are excavated in cohesive soil condition and supported by means of sprayed concrete lining (SCL). The soil at the shaft location mostly consists of an overconsolidated fissured clay, namely London Clay. To predict the behaviour of such clay, both the traditional Mohr-Coulomb (MC) and the advanced BRICK hardening constitutive model are implemented for finite element (FE) modelling in Plaxis. The results of Plaxis 2D and 3D models confirm that the Mohr-Coulomb model has limited applicability in London Clay. The BRICK model is therefore adopted to capture the small strain stiffness behaviour and the anisotropic characteristics, thus defining the expected soil conditions for tunnelling. To determine the expected load on the lining, accounting for its installation at some distance from the face of the excavation, it is necessary to determine the level of soil relaxation. These loads are related to the radial deformation of the soil caused by stress redistribution at the tunnel perimeter due to the excavation. The magnitude of soil convergence is estimated with the Convergence-Confinement Method (CCM). Two design solutions are implemented for supporting the top-heading excavation, showcasing how the construction sequence should be flexible in response to critical design challenges. The first solution consists in mounting elephant feet supports at the bottom of the top heading. In this case, the excavation of the invert is immediate. The alternative solution features the introduction of a temporary invert, which is used in SCL tunnels to reduce the excavation area and provide support for an extended period. The introduction of such structural element modifies the construction sequence from ‘subsequent’ to ‘staged’. The key objective of the thesis is to analyse the different strategies for the sprayed concrete lining of the aforementioned adits, and to validate the FE modelling outcomes through a comparison with the monitoring data provided by the site team. The best outcome in terms of matching real data is seen when the structure is modelled in Plaxis 3D and BRICK constitutive model is implemented to capture the soil behaviour.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/234787