233 research outputs found

    São Paulo cavern-shaft collapse viewed as a trap-door problem.

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    Στο παρόν άρθρο εξετάζονται με τη βοήθεια αριθμητικού μοντέλου οι συνθήκες που οδήγησαν στην κατάρρευση του συνδεδεμένου συστήματος φρέατος-θαλάμου στην μεγαλούπολη Sao Paulo της Βραζιλίας το 2007. Οι δύο θάλαμοι αντιδιαμετρικά του φρέατος εξορύχθηκαν με τη Νέα Αυστριακή ή Τμηματική Μέθοδο. Ο σταθμός Pinheiros όπου εκδηλώθηκε το περιστατικό αυτό βρίσκεται εντός γεωλογικής ζώνης γνωστής ως Caucaia Shear Zone. Εντός της ζώνης αυτής ο βιοτιτικός και γρανιτικός γνεύσιος είναι έντονα κατακερματισμένος που διασχίζεται από τέσσερεις οικογένειες ασυνεχειών (οι δύο εξ αυτών είναι παρακατακόρυφες και οι άλλες δύο με κλίσεις προς τις αντίθετες παρυφές των θαλάμων). Σύμφωνα με το σύστημα ταξινόμησης RMR του Bieniawski oι θάλαμοι εξορύχθηκαν σε βραχόμαζα κλάσης II,III,IV (η τελευταία αναφέρεται σε σαπρόλιθο), με υπερκείμενα σαπρόλιθο και εδάφη αποσάρθρωσης κλάσης V. Η προσομοίωση της κατασκευής του συστήματος φρέατος-θαλάμου έγινε με τον τρι-διάστατο κώδικα πεπερασμένων διαφορών FLAC3D™. Δόθηκε ιδιαίτερη προσοχή στην ποσοτική περιγραφή των επί τόπου γεωλογικών συνθηκών. Λόγω του έντονου κατακερματισμού της βραχομάζας αυτή προσομοιώθηκε με ένα κινηματικό μοντέλο χαλάρωσης της συνοχής και της γωνίας τριβής. Από τις προσομοιώσεις βρέθηκε ότι σε κάοια στιγμή της εκσκαφής του θαλάμου σχηματίζεται συζυγές σύστημα ζωνών διατμήσεως που εκκινούν από τις γωνίες της προπορευόμενης σήραγγας το οποίο ενώνεται με μια κυλινδρική ζώνη διάτμησης που σχηματίζεται γύρω από το φρέαρ. Καταυτόν τον τρόπο σχηματίζεται σφήνα βραχομάζας η οποία και καταρρέει ως μηχανισμός θυροπαγίδας.This paper refers to the numerical simulation of the conditions that have lead to the collapse of the shaft-cavern collapse in São Paulo, Brazil (2007) constructed with the Conventional Tunnelling Method (CTM) or the New Austrian Tunneling Technique (NATM). The Pinheiros station where the incident has occurred, is located in an area known as the Caucaia Shear Zone, resulting in a highly fractured medium (four main families of discontinuities, i.e. two subvertical and two dipping towards the tunnel walls).The main observed lithologies were biotite gneiss and granite gneiss. According to the Bieniawski classification, the following rock mass classes were observed: II,III,IV (partially corresponding to saprolite), and V (partially corresponding to residual soils). The shaft-tunnel construction is simulated by virtue of the 3D finite differences code FLAC3D™. Special emphasis is given on the appropriate quantitative description of the geological conditions. A kinematic cohesion-friction softening model of the discontinuous rock mass is used. It was found that failure is manifested with the evolution of shear bands starting from the corners of the cavern periphery as this approaches the shaft while retreating. Before this, another cylindrical shear band initiates from the bottom of the shaft and also propagates upwards to connect with the former and hence producing the final collapse

    Doctor-patient differences in risk and time preferences: a field experiment

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    We conduct a framed field experiment among patients and doctors to test whether the two groups have similar risk and time preferences. We elicit risk and time preferences using multiple price list tests and their adaptations to the healthcare context. Risk and time preferences are compared in terms of switching points in the tests and the structurally estimated behavioural parameters. We find that doctors and patients significantly differ in their time preferences: doctors discount future outcomes less heavily than patients. We find no evidence that doctors and patients systematically differ in their risk preferences in the healthcare domain

    Doctor-patient differences in risk preferences, and their links to decision-making: a field experiment

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    We conduct a field experiment among patients and doctors, with the aim to test i) whether the two groups have similar risk preferences; and ii) whether differences in risk preferences between doctors and patients are associated with the likelihood of patients seeking a second medical opinion. We measure risk preferences using an adaptation of the Holt and Laury (2002) test to the healthcare context (Galizzi et al., 2013). We find no evidence that the two groups of doctors and patients systematically differ in their preferences towards risk. We find, however, robust evidence that differences between doctors’ and patients’ risk preferences associate with patients’ decision to look for a second opinion: the more diverse preferences are, the more likely patients are to seek a second opinion

    How Has the COVID-19 Pandemic Affected Students’ Online Social Presence?

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    The coronavirus disease 2019 pandemic has affected higher education institutions worldwide as they had to switch from face-to-face to online teaching almost overnight. This abrupt change made a huge impact on teaching, learning, and particularly, student engagement. This paper focuses on online social presence as an element of student engagement, which represents how students feel under synchronous online teaching. A survey was conducted among 244 first-year students to evaluate the impact of online social interaction, online collaboration, online contact with staff, online engagement, and online active learning on online social presence. Structural equation modeling was used to test and evaluate these multivariate relationships. Our study illustrates that all variables have a significant positive relationship with online social presence. In particular, online social interaction and online collaboration show a more powerful relationship with student online social presence. Thus, digital technologies should be adopted in a way that encourages students to actively interact with their peers
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