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Title: Centrifugal model tests of flood embankments
Author: Hird, C. C.
ISNI:       0000 0001 3579 0493
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 1974
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Three new large British centrifuges have been constructed in recent years for the purpose of testing soil models. A centrifugal model, in which the stresses are brought into similarity with those in a prototype by increasing the self-weight of the soil, can be used either as a means of providing data against which analyses may be checked or as an analogue to solve difficult boundary value problems for which no satisfactory analysis exists. Both uses contribute to the progress of soil mechanics as a science and to its application in engineering design. This thesis describes the use of one of the new centrifuges to investigate a design problem: the stability of floodbanks in the Thames estuary when they are subjected to abnormally high tidal conditions and associated uplift pressures in a permeable layer beneath the marsh on which they are built. The objective of the investigation was defined in general terms so as to allow the use of soils prepared in the laboratory in the first series of model tests, described here; a second series of model tests utilising prototype soil samples was envisaged but has not yet been carried out. Conventional laboratory tests were performed on the soils chosen fox the investigation in order to provide information which could be put to use in building the models and in analysing the results of the model tests. The interpretation of the laboratory tests proved more difficult than anticipated because of the need to define undrained strength relationships for two different clay soils taking into account, for one of them, experimental evidence of anisotropy. Whilst neither the soil conditions nor the loading conditions of the prototype could be closely simulated, it was deemed necessary to first bring the soils in the model into a state of equilibrium, by consolidating them in the centrifuge, and then to subject them to a rapid change of loading consisting of an increase of self-weight or an increase of uplift pressure or a combination of the two. Before the tests could be carried out, a means of controlling water movement in the model had to be developed. Substantial data of pore pressure and settlement during consolidation are presented which show that a state of equilibrium was indeed reached by the model soils. The rapid changes of loading generated two types of failure: failure of the embankment by slipping and failure of the marsh on the landward side of the embankment by bursting; these events occurred independently in the model so that the presence of even-very nigh uplift pressures did not influence the stability of the embankment. The results are expressed in the form of an interaction diagram. The tests have been interpreted by extending the model behaviour qualitatively to the prototype, which must be done with caution, and by comparing the embankment failures with the predictions of stability analyses previously used in design. Despite uncertainties, which are discussed, reasonable predictions. of model behaviour are obtained in the absence of high uplift pressures; in the presence of high uplift pressures previous design assumptions appear over-conservative but may not be should bursting of the marsh occur near the toe of the prototype embankment. In conclusion some statements relating to the stability of the prototype embankments are made, some limitations of centrifugal modelling are pointed out and some views concerning the role of the centrifuge in British design practice are expressed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available