Seismic fragility of buried steel natural gas pipelines due to axial compression at geotechnical discontinuities

This paper presents an extended set of numerical fragility functions for the structural assessment of buried steel natural gas (NG) pipelines subjected to axial compression caused by transient seismic ground deformations. The study focuses on NG pipelines crossing sites with a vertical geotechnical discontinuity, where high compression straining of a buried pipeline is expected to occur under seismic transient ground deformations. A de-coupled numerical framework is developed for this purpose, which includes a 3D finite element model of the pipe–trench system employed to evaluate rigorously the soil–pipe interaction effects on the pipeline axial response in a quasi-static manner. One-dimensional soil response analyses are used to determine critical ground deformation patterns at the vicinity of the geotechnical discontinuity, caused by the ground shaking. A comprehensive parametric analysis is performed by implementing the proposed analytical framework for an ensemble of 40 recorded earthquake ground motions. Crucial parameters that affect the seismic response and therefore the seismic vulnerability of buried steel NG pipelines namely, the diameter, wall thickness, burial depth and internal pressure of the pipeline, the backfill compaction level, the pipe–soil interface friction characteristics, the soil deposits characteristics, as well as initial geometric imperfections of the walls of the pipeline, are systematically considered. The analytical fragility functions are developed in terms of peak ground velocity at the ground surface, for four performance limit states, considering all the associated uncertainties. The study contributes towards a reliable quantitative risk assessment of buried steel NG pipelines, crossing similar sites, subjected to seismically-induced transient ground deformations

Physical modeling for the evaluation of the seismic behavior of square tunnels

The Chapter summarizes results from dynamic centrifuge tests performed on a rectangular tunnel model embedded in dry sand. The tests were carried out at the geotechnical centrifuge facility of the University of Cambridge, within the Transnational Access Task of the SERIES Research Project (Project: TUNNELSEIS). The experimental data is presented in terms of acceleration and displacement-time histories in the soil and on the tunnel, soil surface settlements, earth pressures on the side walls of the tunnel and internal forces of the tunnel lining. The goal of the experiment is twofold: to better understand the seismic behavior of these types of structures, and to use the high quality and perfectly constrained data to validate the numerical models which are commonly used for the design of rectangular embedded structures. The interpretation of the results reveals (i) rocking response of the tunnel model, (ii) existence of residual values on the earth pressures on the side walls and on the internal forces and (iii) important influence of the tunnel on the shear wave field. These issues are not well understood and are usually not taken into account in the simplified seismic analysis methods

Calibration of strain gauged square tunnels for centrifuge testing

A series of dynamic centrifuge tests were conducted on square aluminum model tunnels embedded in dry sand. The tests were carried out at the Schofield Centre of the Cambridge University Engineering Department, aiming to investigate the dynamic response of these types of structures. An extensive instrumentation scheme was employed to record the soil-tunnel system response, which comprised of miniature accelerometers, total earth pressures cells and position sensors. To record the lining forces, the model tunnels were strain gauged. The calibration of the strain gauges, the data from which was crucial to furthering our understanding on the seismic performance of box-type tunnels, was performed combining physical testing and numerical modelling. This technical note summarizes this calibration procedure, highlighting the importance of advanced numerical simulation in the calibration of complex construction models

Σεισμική συμπεριφορά και σχεδιασμός σηράγγων

This thesis investigates several aspects of the transversal seismic response of circular and rectangular tunnels subjected to ground shaking. In particular, a series of dynamic centrifuge tests that were carried out on a circular tunnel embedded in dry sand were numerically analyzed by means of full dynamic time history analysis. Additional analyses were conducted to investigate several crucial parameters on the response. The calibrated numerical models were then used for the validation of existing simplified analysis methods. Among the most interesting findings were the effects of the tunnel flexibility and the soil-tunnel interface characteristics on the soil yielding response around the tunnel that caused post-earthquake residual values on the dynamic lining forces. The simplified analyses either overestimated or underestimated the response compared to the dynamic analysis. As regards the rectangular tunnels, a series of dynamic centrifuge tests were performed on rectangular model tunnels in dry sand. Representative tests were numerically simulated, while additional analyses were performed to investigate the effects of crucial parameters. The calibrated numerical models were used for the validation of simplified analysis methods. The results revealed a racking-rocking deformation mode for the tunnels during shaking and a post-earthquake residual response for the soil-tunnel system due to the soil yielding that was amplified with the increase of the flexibility of the tunnel. Generally, the simplified analysis methods underestimated the response compared to the dynamic analysis. To extend the investigation of the response of rectangular tunnels, a numerical parametric study was also conducted. The effects of salient parameters on the complex deformation modes of tunnels were revealed, while several aspects of the response were further evaluated. Representative cases of this parametric study were implemented to the evaluation of analytical solutions that are used for the computation of the soil springs for tunnels. This procedure revealed that the solutions results may differ significantly from the actual soil stiffness. Finally, the effects of above ground structures on the dynamic response of shallow tunnels were investigated through a numerical parametric study. For the investigated cases, the results indicated an increase of the tunnel response, when the above ground structures were considered that was generally higher for shallow and stiff tunnels.Σκοπός της διατριβής είναι η μελέτη της συμπεριφοράς κυκλικών και ορθογωνικών σηράγγων υπό σεισμική ταλάντωση στην εγκάρσια έννοια. Αναφορικά με τις κυκλικές σήραγγες, πραγματοποιήθηκε η αριθμητική ανάλυση μιας σειράς δυναμικών πειραμάτων φυγοκεντριστή σε δοκίμιο κυκλικής σήραγγας σε άμμο, ενώ εκτελέσθηκαν επιπρόσθετες αναλύσεις, ώστε να διερευνηθεί η επιρροή κρίσιμων παραμέτρων στη συμπεριφορά. Τα βαθμονομημένα προσομοιώματα χρησιμοποιήθηκαν για τη αξιολόγηση απλοποιημένων μεθόδων ανάλυσης. Μεταξύ άλλων, αναδείχθηκε η επιρροή της ευκαμψίας της σήραγγας, αλλά και των χαρακτηριστικών της διεπιφάνειας στην πλαστικοποίηση του εδάφους περιμετρικά της σήραγγας, η οποία οδήγησε σε παραμένουσες τιμές για τα εντατικά μεγέθη της σήραγγας. Οι απλοποιημένες μεθοδολογίες, άλλοτε υποεκτίμησαν και άλλοτε υπερεκτίμησαν την απόκριση σε σχέση με τη δυναμική ανάλυση. Σε ότι αφορά τις ορθογωνικές σήραγγες: εκτελέσθηκε μια σειρά πειραμάτων φυγοκεντριστή σε δοκίμια ορθογωνικών σηράγγων σε στεγνή άμμο. Αντιπροσωπευτικές περιπτώσεις αναλύθηκαν αριθμητικά, ενώ αξιολογήθηκαν και απλοποιημένες μεθοδολογίες. Τα αποτελέσματα ανέδειξαν μια σύνθετη διατμητική-λικνιστική παραμόρφωση των ορθογωνικών σηράγγων κατά την σεισμική ταλάντωση, καθώς επίσης και μια παραμένουσα συμπεριφορά του συστήματος σήραγγα-έδαφος, λόγω της πλαστικοποίησης του εδάφους, η οποία εμφανίστηκε πιο έντονα σε εύκαμπτες σήραγγες. Οι απλοποιημένες μεθοδολογίες, άλλοτε υποεκτίμησαν και άλλοτε υπερεκτίμησαν την απόκριση σε σχέση με τη δυναμική ανάλυση. Η σεισμική συμπεριφορά των ορθογωνικών σηράγγων διερευνήθηκε πιο διεξοδικά στο πλαίσιο μιας παραμετρικής αριθμητικής ανάλυσης. Τα αποτελέσματα ανέδειξαν την επιρροή κρίσιμών παραμέτρων στις σύνθετες μορφές παραμόρφωσης, καθώς επίσης και σε διάφορα χαρακτηριστικά της συμπεριφοράς. Αντιπροσωπευτικές περιπτώσεις της παραμετρικής ανάλυσης χρησιμοποιήθηκαν για την αξιολόγηση αναλυτικών λύσεων για τον προσδιορισμό της δυσκαμψίας εδαφικών ελατηρίων στην περίπτωση σηράγγων. Η διαδικασία, έδειξε ότι οι αναλυτικές λύσεις δύνανται να δώσουν αποτελέσματα αρκετά διαφοροποιημένα σε σχέση με την πραγματική εδαφική δυσκαμψία. Τέλος, διερευνήθηκε η επιρροή των υπέργειων κατασκευών στη σεισμική συμπεριφορά αβαθών σηράγγων, μέσω μια δεύτερης αριθμητικής παραμετρικής ανάλυσης. Για τις εξεταζόμενες περιπτώσεις, η θεώρηση των υπέργειων κατασκευών οδήγησε σε αύξηση της σεισμικής καταπόνησης των σηράγγων, με την επιρροή να είναι μεγαλύτερη στις περιπτώσεις αβαθών και δύσκαμπτων σηράγγων

On the response of integral abutment bridges under a sequence of thermal loading and ground seismic shaking

This article investigates the response of Integral Abutment Bridges (IAB) when subjected to a sequence of seasonal thermal loading of the deck followed by ground seismic shaking in the longitudinal direction. Particular emphasis is placed on the effect of pre-seismic thermal Soil-Structure Interaction (SSI) on the seismic performance of the IAB, as well as on the ability of various backfills configurations, to minimize the unfavorable SSI effects. A series of two-dimensional numerical analyses were performed for this purpose, on a complete backfill-integral bridge-foundation soil system, subjected to seasonal cyclic thermal loading of the deck, followed by ground seismic shaking, employing ABAQUS. Various backfill configurations were investigated, including conventional dense cohesionless backfills, mechanically stabilized backfills and backfills isolated by means of compressive inclusions. The responses of the investigated configurations, in terms of backfill deformations and earth pressures, and bridge resultants and displacements, were compared with each other, as well as with relevant predictions from analyses, where the pre-seismic thermal SSI effects were neglected. The effects of pre-seismic thermal SSI on the seismic response of the coupled IAB-soil system were more evident in cases of conventional backfills, while they were almost negligible in case of IAB with mechanically stabilized backfills and isolated abutments. Along these lines, reasonable assumptions should be made in the seismic analysis of IAB with conventional sand backfills, to account for pre-seismic thermal SSI effects. On the contrary, the analysis of the SSI effects, caused by thermal and seismic loading, can be disaggregated in cases of IAB with isolated backfills

Developments in Seismic Vulnerability Assessment of Tunnels and Underground Structures

Underground structures are being constructed at an increasing rate in seismic prone areas, to facilitate the expanding needs of societies. Considering the vital role of this infrastructure in densely populated urban areas and interurban transportation networks, as well as the significant losses associated with potential seismically induced damage, its assessment against seismic hazard is of great importance for stakeholders, operators, and governmental bodies. This paper presents a state-of-the-art review of current developments in the assessment of seismic vulnerability of tunnels and underground structures. Methods for the development of fragility functions for the assessment of bored tunnels in rock or alluvial, and cut and cover tunnels and subways in alluvial, against ground seismic shaking and earthquake-induced ground failures are presented. Emphasis is placed on the estimation of the capacity of the examined structures, the selection of appropriate intensity measures to express seismic intensity, the development of rational probabilistic seismic demand models and the estimation of epistemic and aleatoric uncertainties, related to the seismic fragility of underground structures. Through the discussion, acknowledged gaps in the relevant literature are highlighted

Effects of SSI and lining corrosion on the seismic vulnerability of shallow circular tunnels

The paper presents a numerical approach for the construction of seismic fragility curves for shallow metro tunnels considering the soil-structure-interaction (SSI) and the aging effects due to corrosion of the lining reinforcement. The tunnel response under ground shaking is evaluated through 2D non-linear dynamic analyses, for increasing levels of seismic intensity. An elasto-plastic model is used to simulate the soil non-linear behavior under ground shaking, while the effects of lining mechanical properties, soil conditions and ground motion characteristics are also accounted for. The effect of corrosion on the lining behavior is encountered through proper modification of the lining strength properties. Damage state thresholds are defined based on the exceedance of the lining capacity. The fragility curves are estimated in terms of peak ground acceleration at the ground free field conditions for different time periods considering the associated uncertainties. The proposed approach is applied for the fragility assessment of selected soil-tunnel configurations. The derived fragility functions are compared with existing empirical and analytical fragility models, highlighting the important role of soil conditions and corrosion effects in the vulnerability of tunnel structures. The proposed fragility models contribute towards an advanced vulnerability and risk assessment of transportation systems and infrastructures