This thesis has investigated the static liquefaction failure, during construction, of the Nerlerk underwater berm. This hydraulically-placed sandfill structure was designed to form part of an artificial caisson island, intended for use as a year-round exploration drilling platform in the Canadian Beaufort Sea. The aim of the investigation has been to examine how liquefaction was possible in a fill which, according to CPT data, was predominantly dilative; in particular, to study the influence, on stability, of 'pockets' of liquefiable material in the sandfill. Instead of a conventional deterministic approach, based on mean strengths, a stochastic type of approach has been necessary, in order to accurately model material heterogeneity. For this research, the double-hardening constitutive model Monot has been used, this being capable of modelling a wide range of material behaviour, from liquefiable to strongly dilative. The material parameters have been calibrated from an extensive laboratory database and expressed in terms of density via the state parameter for sands. The state parameter statistics have been derived, based on a detailed statistical evaluation of an extensive CPT database from the Tarsiut and Nerlerk island sites. These statistics were used for generating uni-variate random fields of state parameter, from which the Monot material parameters could then be backfigured, in order to model the spatial variability in the fill (i.e. heterogeneity). A number of sophisticated stochastic investigations have been performed, using an advanced two-dimensional finite element algorithm encompassing Monot. The results have shown that liquefaction of the Nerlerk berm is consistent with CPT data: i.e. it may be possible for a predominantly dilative fill to liquefy, under static loading conditions, due to the presence of semi-continuous loose zones arising from deposition-induced anisotropy. For such problems, in which material anisotropy is present, a deterministic approach is not appropriate and can be misleading, as this was shown to represent an upper bound response to the stochastic simulations.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:488082 |
| Date | January 2000 |
| Creators | Onisiphorou, Christakis |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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