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Tunnels in Swelling Ground

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Tunnels in Swelling Ground Marco Barla Tunnels in Swelling Ground SIMULATION OF 3D STRESS PATHS BY TRIAXIAL LABORATORY TESTING Dottorato di Ricerca in Ingegneria Geotecnica Politecnico di Torino Politecnico di Milano Università degli Studi di Genova Università degli Studi di Padova Dottorato di Ricerca in Ingegneria Geotecnica (XII ciclo) Politecnico di Torino Politecnico di Milano Università degli Studi di Genova Università degli Studi di Padova November 1999 TUNNELS IN SWELLING GROUND SIMULATION OF 3D STRESS PATHS BY TRIAXIAL LABORATORY TESTING ………………………. Marco Barla Author ……………………….………. ………...….…………… ....…….………………… Prof. Michele Jamiolkowski Prof. Giovanni Barla Prof. Diego Lo Presti Supervisors ……………………….. Prof. Renato Lancellotta Head of the Ph.D. Programme in Geotechnical Engineering “Peace cannot be kept by force, it can only be achieved by understanding.” Albert Einstein SAN DONATO TUNNEL (FLORENCE, ITALY), 1986 SARMENTO TUNNEL (SINNI, ITALY), 1997 ABSTRACT I Abstract The present thesis is to contribute to the understanding of the swelling behaviour of tunnels with a major interest being placed on the stress and deformation response in the near vicinity of the advancing face, i.e. in three dimensional conditions. Following the introduction of the most recent developments, mostly based on contributions of the International Society for Rock Mechanics, the research examines the stress distribution around a circular tunnel by means of numerical methods. According to different stress conditions and stress-strain laws for the ground, the stress history of typical points around the tunnel (sidewalls, crown and invert) is described with the stress path method (Lambe 1967). This allows one to evidence how the three dimensional analyses results are necessary to describe the ground behaviour. In particular, it can be observed that the excavation is accompanied by a continuous variation of the mean normal stress even for an isotropic initial state of stress. This behaviour cannot be identified by the corresponding two dimensional solutions. With the stress paths computed, the thesis deals with the design, construction and calibration of a new triaxial apparatus developed with the intent to allow one to reproduce the proposed stress paths. This part of the thesis is intended to allow one to underline some peculiar aspects of the experimental programme: measurements of local deformations, ability to impose on the specimen the desired stress history, capability to assess different experimental quantities versus time. The thesis continues with the characterisation of the swelling soil used for the testing programme. This soil is a stiff clay (Caneva clay) with samples retrieved by means of a triple tube sampler and also by a cubic sample. Geotechnical characterisation is given in terms of deformability and strength as well as swelling properties by means of the Huder & Amberg oedometer test. Then the interest is moved to the innovative testing programme undertaken with the new triaxial apparatus and another triaxial cell having similar features. Specimens preparation and testing procedures are described in details. The specimens are submitted to the stress paths computed by numerical analyses with the intent to simulate, “at laboratory scale”, the ground behaviour around the tunnel during face advancement. The “undrained phase” is initially considered with “compression” (to simulate the sidewall behaviour) and “extension” tests (to simulate the crown/invert behaviour). Then the “drained phase” is reproduced with the intent to study the time dependent response, when the excavation is completed (the head of the tunnel is far away from the section under study) or during a standstill. II ABSTRACT The experimental testing allows one to draw some important conclusions on the excess pore pressure induced in the specimen. It has been shown how negative excess pore pressure can develop due to the “compression” stress paths and positive excess pore pressure due to the “extension” stress paths, for the stiff clay under study. This aspect is of a great importance from the engineering point of view, when the analysis is extended to simulate the swelling/consolidation phenomenon induced in the ground after tunnel excavation. The above observations were compared with similar experiments undertaken at the Massachusetts Institute of Technology on an anisotropic shale. It has been possible to verify the non unique response around the tunnel, with the necessity to undertake additional tests of the same kind as those described above, prior to deriving any conclusions on the design analysis methodology to be adopted. Some further recommendations are given at the end on the continuation of the research work undertaken with the present thesis. Considering the number of underground infrastructures in difficult conditions, that are nowadays at the design stage or under construction in Italy and around Europe, this continuation is highly desirable. SOMMARIO III Sommario La Tesi affronta un importante problema, di interesse applicativo nel settore dello scavo di gallerie, ricorrendo a metodi teorici e sperimentali propri dell’Ingegneria Geotecnica. Si tratta dell’analisi del comportamento di gallerie in terreni rigonfianti, durante lo scavo e nel lungo termine, ad opera ormai completata. Dopo un primo inquadramento del problema, alla luce delle conoscenze disponibili e dei più recenti sviluppi sul tema, soprattutto a cura della International Society for Rock Mechanics, lo studio esamina per via teorica (mediante metodi numerici della meccanica del continuo) la distribuzione dello stato di sforzo nell’intorno di una galleria circolare. Per diverse condizioni tensionali originarie e leggi sforzo-deformazione del terreno in cui avviene lo scavo, lo sviluppo dello stato di sforzo in punti caratteristici nell’immediato intorno della galleria (calotta, piedritti ed arco rovescio) viene rappresentato con il metodo dello stress path (Lambe 1967). Questa procedura consente di evidenziare come i risultati delle analisi numeriche tridimensionali siano indispensabili al fine di descrivere congiuntamente l’evoluzione delle componenti di sforzo durante l’avanzamento del fronte di scavo. Si osserva come, anche nelle condizioni di stato tensionale originario di tipo isotropo, lo scavo sia accompagnato da una variazione continua del primo invariante degli sforzi, durante l’avanzamento del fronte, in contrasto con quanto consentono di prevedere le corrispondenti soluzioni bidimensionali. In particolare il primo invariante degli sforzi aumenta mentre il fronte della galleria si avvicina alla sezione di studio e subisce una consistente diminuzione subito dopo il suo passaggio, per poi aumentare nuovamente ritornando al valore iniziale. Nella condizione di stato tensionale iniziale non isotropo (Ko = 2), per ciò che concerne la simulazione del comportamento dell’elemento sul piedritto, si osserva una diminuzione del primo invariante delle tensioni. In corrispondenza invece dell’arco rovescio si ha un aumento. In entrambi i casi, i percorsi di sollecitazione derivanti da un’analisi bidimensionale sono lineari mentre i risultati delle analisi tridimensionali descrivono andamenti diversi. Introducendo nei modelli una legge di comportamento di tipo elasto-plastico rammollente, i percorsi di sollecitazione risultano più complessi e di difficile interpretazione. In entrambi i casi di condizione tensionale iniziale (Ko = 1 o Ko = 2), e per entrambe le situazioni (piedritto e arco rovescio) il valore del primo invariante delle tensioni diminuisce durante lo scavo, per effetto della plasticizzazione che si sviluppa sul contorno della galleria. Stabilite in tal modo le condizioni di base per il successivo sviluppo della ricerca, la tesi affronta la progettazione, costruzione e taratura di una nuova apparecchiatura triassiale, con la quale riprodurre sperimentalmente gli stress path precedentemente calcolati. Si tratta di una parte IV SOMMARIO molto importante del lavoro, che consente di porre in luce alcuni aspetti rilevanti dell’attività sperimentale svolta: la misura delle deformazioni locali; la capacità di imporre al provino, in modo attento e rigoroso, la storia tensionale scelta; la possibilità di controllo preciso delle diverse grandezze sperimentali nel tempo. La nuova attrezzatura triassiale consente di raggiungere pressioni in cella di 2 MPa e carichi verticali di 50 kN. Può ospitare campioni di 50 e 70 mm di diametro. Il piedistallo su cui poggia il campione è costituito da un meccanismo di scivolamento che ne permette il libero movimento, virtualmente senza attrito, su di un piano orizzontale e si presta ad evitare gli effetti negativi dovuti alle bande di taglio durante la rottura. La cella è dotata di trasduttori di pressione per la misura delle pressioni interstiziali alla base e confinamento, di LVDT per la misura dello spostamento assiale esterno, di una cella di carico interna, di misuratori di deformazione locale assiale e radiale. La tesi prosegue quindi con la caratterizzazione del terreno rigonfiante preso a riferimento per l’esecuzione delle prove in condizioni di stress path controllato. Si tratta di un’argilla consistente (argilla di Caneva), i cui campioni sono stati prelevati sia a mezzo di sondaggi stratigrafico- geotecnici accurati che con campione cubico in galleria. Oltre alla caratterizzazione geotecnica dello stesso materiale, in termini di deformabilità e resistenza, sono presentati i risultati di prove di rigonfiamento (in particolare, la prova di Huder & Amberg). Si passa
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