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Concrete tunnel segments with combined traditional and fiber reinforcement: optimization of the structural behavior and design aspects
No full textUnderground infrastructures have assumed a relevant role in the modern society: the continuous need to improve the public transport and to satisfy the growing traffic demands make these structures very attractive.
Among the consistent numbers of technical problems related to their design in urban environments, it is worth mentioning the low mechanical characteristics of soils, often soft and considerably heterogeneous. New technologies were recently developed to make the construction process faster and, if possible, as mechanized as possible. In addition, there is now a greater emphasis on bored tunnel in order to reduce the hindrance to the infrastructure above the ground and to the environment.
Special machines (Tunnel Boring Machines; TBMs), allowing for a complete excavation, were introduced in order to promptly address the project needs and function effectively under most actual geotechnical-geological situations. These machines require that precast tunnel segments be put in place, in order to make the construction process faster with respect to ordinary linings cast in place. These segments are generally made of ordinary reinforced concrete; however, the addition of fibers is gaining considerable attention among designers and producers due to the enhanced mechanical properties, especially after cracking. Fibrous reinforcement allows firstly a partial substitution of the traditional reinforcement placed in the aforementioned elements and, secondly, an improvement in the quality and reliability of such members.
This research work intends to be a contribution to the knowledge on fiber reinforcement for use in tunnel segments. In order to evaluate the opportunities offered by Steel Fiber Reinforced Concrete (SFRC), especially in combination with traditional reinforcement, for these precast elements, the principal mechanisms governing the tunnel behavior are investigated and listed in the following:
• the thrust jack phase is studied by means of numerical analyses;
• the lining embedded in ground is analyzed with a parametric study at the Ultimate Limit State (ULS) and with an analytical approach at the Serviceability Limit State (SLS);
• the grouting process is examined through a numerical simplified model.Le infrastrutture sotterranee hanno assunto un ruolo di rilievo nella società moderna: la necessità continua di migliorare il trasporto pubblico e di soddisfare le richieste di un traffico sempre crescente rende queste strutture oggetto di notevole interesse.
Le problematiche più importanti nella realizzazione delle infrastrutture sotterranee sono legate alle esigenze di lavorare, spesso, in ambito urbano, in terreni sciolti o con deboli resistenze meccaniche. Ulteriori difficoltà possono sorgere in seguito alla presenza di condizioni del terreno particolarmente eterogenee, con delle differenze notevoli tra i materiali incontrati durante lo scavo.
Le esigenze, sempre crescenti, di minimizzare o eliminare del tutto le interferenze con le attività di superficie, nonché quelle di ridurre i tempi di esecuzione ed i costi di realizzazione delle opere in sotterraneo, sono state perseguite dall’industria mediante l’introduzione di nuove tecnologie improntate sulla meccanizzazione dello scavo.
Tale tendenza ha portato all’introduzione di macchine a scavo integrale (Tunnel Boring Machines; TBMs), in grado di affrontare e superare qualsiasi difficoltà geologico-geotecnica ed alla necessità di disporre di rivestimenti in conci prefabbricati la cui messa in opera fosse compatibile con tempi di scavo più rapidi. Tali conci sono normalmente realizzati in calcestruzzo armato tradizionale. Le recenti esigenze di miglioramento della qualità e dell’affidabilità di tali rivestimenti hanno spostato l’interesse dei progettisti e dei costruttori verso l’uso di materiali innovativi quali i calcestruzzi fibrorinforzati, ponendo l’attenzione sulla possibilità di sostituire parte dell’armatura tradizionale con fibre, garantendo, in tal modo, una riduzione dei costi.
Il presente lavoro di ricerca intende, pertanto, contribuire alla conoscenza del calcestruzzo fibrorinforzato per la realizzazione di rivestimenti in conci prefabbricati. A tale scopo, si valutano le opportunità offerte dal calcestruzzo rinforzato con fibre d'acciaio (Steel Fiber Reinforced Concrete - SFRC), in combinazione con le barre tradizionali d’armatura, riferendosi ai principali meccanismi che governano il comportamento ed il dimensionamento di tali strutture.
Le condizioni di carico analizzate sono:
• la fase di spinta dei martinetti idraulici della TBM, studiata attraverso simulazioni numeriche in ambito non-lineare;
• la condizione finale in cui il rivestimento è caricato dall'azione del terreno che è stata analizzata tramite uno studio parametrico allo Stato Limite Ultimo (SLU) e mediante un opportuno approccio analitico allo Stato Limite di Esercizio (SLE);
• la fase di “grouting”, in cui il vuoto lasciato dallo scudo della TBM viene riempito tramite l’iniezione di una boiacca (grout); tale transitorio è stato indagato mediante un modello numerico semplificato
A structural application of Fiber Reinforced Concrete: precast tunnel segments
No full textThe paper addresses precast tunnel segments for the Line T02 of Saronno-Malpensa railway (Italy). After the experimental determination of the material properties, the structural behavior of the lining is simulated with FE analyses based on Non Linear Fracture Mechanics (NLFM). The numerical analyses presented herein will be mainly focused on the force exerted by hydraulic jacks on the ring during the excavation process. The numerical analyses were carried out considering different loading configurations and atypical boundary conditions in order to investigate the advantages coming from the use of Steel Fiber Reinforced Concrete (SFRC) for a better crack control and for enhancing the bearing capacity. An optimized reinforcement based on the combination of rebars and steel fibers, is proposed
Il calcestruzzo fibrorinforzato per i conci prefabbricati delle gallerie
No full textIl calcestruzzo fibrorinforzato (FRC) è un composito caratterizzato da una matrice di calcestruzzo rinforzata con fibre discontinue di varia natura. Tra le applicazioni del FRC che stanno suscitando particolare interesse ci sono certamente i conci prefabbricati per galleria. Nel presente articolo vengono presentati i risultati di uno studio numerico su conci prefabbricati in FRC per la realizzazione della nuova Linea 9 della Metropolitana di Barcellona (Spagna). In base a proprietà dei materiali determinate sperimentalmente, le analisi numeriche hanno permesso di ottimizzare l'armatura dei conci utilizzando sia barre d'armatura tradizionale che fibre di acciaio
Fiber reinforced concrete combined with traditional reinforcement for use in precast tunnel segments
No full textThe paper focuses on the advances in precast tunnel segments with particular reference to the reinforcement for use in these elements. The behavior of segmental tunnel lining of Brescia Metro is investigated at the final state, thus under the embedded soil load condition. A parametric study was carried out to assess the influence of the stiffness of longitudinal and ring joints on the internal forces acting on the lining.
Furthermore, different tunnel depths were considered in order to get a better understanding of the sectional response of the tunnel lining. The study was developed by means of numerical analyses based on a 2 1⁄2 dimensional model. Finally, after the evaluation of the forces applied on the lining, the interest was focused on the optimization of the reinforcement (fiber and rebars) with respect to the embedded-soil load-condition.
Therefore, the tunnel lining behavior at ULS was considered
Concrete tunnel segments with combined traditional and fiber reinforcement: optimization of the structural behavior and design aspects
No full textThis research work concerns shield driven tunnels made in ground conditions. The reinforcement of precast tunnel segments is designed with respect to several mechanics, temporary and permanently loading conditions. These elements are generally made of ordinary reinforced concrete. Fiber Reinforced Concretes (FRC) are composite materials with a cementitious matrix and a discontinuous reinforcement, the fibers, that may be made of metal, glass, synthetic or natural materials.
Fiber Reinforced Concrete (FRC) could be a competitive design alternative for tunnel segments as it would allow time reduction in handling and placing the curved reinforcement that has to be used in ordinary RC elements. FRC represents a minimum reinforcement spread out everywhere into the segment, especially in the concrete cover, which in the ordinary segments need to be often considerably thick for the fulfillment of the fire protection and durability requirements.
The principal aim was to evaluate the opportunities offered by SFRC (Steel Fiber Reinforced Concrete) in precast tunnel elements. It is of main interest the combination of traditional and fiber reinforcement in order to achieve a good lining structural response with respect to the principal mechanisms governing the design process of tunnel segments
Fiber Reinforced Concrete for tunnel linings
No full textThe proposed paper concerns the optimized design of FRC tunnel linings with special emphasis to precast tunnel elements.
FRC represents a reinforcement spread out into the segment, including the concrete cover that, in tunnel segments, is often considerably thick for the sake of fire protection requirements. Moreover, FRC improves the resistance to impact loading, due to its toughness, and allows a better control of cracking, with advantages in terms of durability of the members.
The optimized reinforcement is based on to use of both traditional reinforcement (longitudinal rebars), adequate for localized stress, and of FRC, adequate for diffusive stress, that allows a better crack control and a significantly reduction of the amount of transverse reinforcement. A proper combination of diffused (fibers) and localized (rebars) reinforcement guarantees the performance requirements at the lowest costs.
FRC tunnel segments are better designed by taking into account the toughness offered by fibers, which generally provide a significant residual strength to the cross section
Parametric study on tunnel linings in fiber reinforced concrete combined with traditional reinforcement
No full textIn the present paper, the structural behavior of tunnel segments has been investigated by means of a 21⁄2 D model which enables to take into account the influence of longitudinal and ring joints on lining internal forces (M,V). The research aims to study the grouting phase (during construction) as well as the embedded ground load condition (final state) in order to evidence the most critical resistant mechanism.
The results refer to a broad parametric study developed in order to get a better understanding on the actions occurring in segmental lining during the final state and the grouting process (construction stage). Two different lining geometries were considered: the first one corresponds to the Brescia metro line, which is a single deck metro with an internal diameter (Dint) of 8,15 m; the second one represents a highway tunnel having a larger diameter (14,90 m). Several tunnel depth projections and ground conditions were considered
Tunnel linings made by precast concrete segments
No full textTunnel segments are generally reinforced with conventional rebars that are placed in r.c. elements to resist the tensile stresses both at Serviceability (SLS) and Ultimate (ULS) Limit States. As far as the service conditions are concerned, in recent years durability issues have became of paramount importance. Durability design gen-erally requires rebars protection against corrosion that can be achieved by reducing concrete porosity and crack width. The former can be obtained by using a matrix with a low water/cement ratio while the latter can be achieved by using a diffused reinforcement; to this aim, fiber reinforcement may represent an optimal solu-tion since it is diffused in the concrete matrix. As far as the ultimate conditions are concerned, localized stresses (due to bending actions) are better contrasted by localized reinforcement (rebars) while diffused stresses are better resisted by diffused reinforcement as fibers. For the reasons mentioned above, since both localized and diffused stresses are present, an optimized reinforcement for tunnel linings can be obtained by using a combination of conventional (rebars) and fiber reinforcement. In this Section, the results of a research program on concrete tunnel segments are presented. After the experimental determination of the material properties, the structural behaviour of the lining was simulated with FE nonlinear analyses based on fracture mechanics. Some of the main issues concerning the design process of segmental tunnel linings are pointed out and a the attention is devoted to the beneficial effects of fiber reinforcement. The load cases mainly concern the force exerted by TBM pressure on the ring during the excavation process (the so-called thrust jack phase)
Steel fibers reinforced concrete for precast tunnel segments
No full textThe paper concerns precast tunnel segments for the Line 1 of the Valencia Metro (Venezuela) where the conventional reinforcement (rebars) may be partially substituted by steel fibers. After the experimental determination of the material properties, nonlinear analyses allowed to study the structural behavior of the tunnel segments with different types of reinforcement, according to different loading conditions. Also an optimized reinforcement, based on a combination of rebars and steel fibers, is adopted
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