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2005 Higher Degree Theses

The following postgraduate students were awarded higher degrees for their theses in 2005.

Doctor of Philosophy

Don Cameron
Analysis of Buried Flexible Pipes in Granular Backfill Subjected to Construction Traffic
Supervisor: Prof John Carter

This thesis explores the design of flexible pipes, buried in shallow trenches with dry sand backfill. The thesis reports the comprehensive analysis of twenty-two full-scale load tests conducted between 1989 and 1991 on pipe installations, mainly within a laboratory facility, at the University of South Australia. The pipes were highly flexible, spirally-wound, uPVC pipes, ranging in diameter from 300 to 450 mm. Guidelines were required by industry for safe cover heights for these pipes when subjected to construction traffic. The tests were designed by, and conducted under the supervision of, the author, prior to undertaking this thesis.

As current design approaches for pipes could not anticipate the large loading settlements and hence, soil plasticity, experienced in these tests, finite element analyses were attempted. Extensive investigations of the materials in the installations were undertaken to permit finite element modelling of the buried pipe installations. In particular, a series of large strain triaxial tests were conducted on the sand, which was used as fill in the installations, to provide understanding of the sand behaviour in terms of critical state theory. Subsequently a constitutive model for the soil was developed.

The soil model was validated before implementation in an element of finite element program, AFENA (Carter and Balaam, 1995). Single element modelling of the triaxial tests proved invaluable in obtaining material constants for the soil model. The new element was applied successfully to the analysis of a side-constrained, plate loading test on the sand. The simulation of the buried pipe tests was shown to require three-dimensional finite element analysis to approach the observed pipe-soil behaviour. Non-compliant side boundary conditions were ultimately adjudged chiefly responsible for the difficulty in matching the experimental data. The value of numerical analyses performed in tandem with physical testing was apparent, albeit in hindsight.

The research has identified the prediction of vertical soil pressure above the pipe due to external loading as being the major difficulty for designers. Based on the finite element analyses of the field tests, a preliminary simple expression was developed for estimation of these pressures, which could be used with currently available design approaches to reasonably predict pipe deflections.

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Andrew de Ambrosis
Investigation of the Facing Response of Soil Nailed Excavations
Supervisor: Prof John Small

This thesis details work conducted to investigate the behaviour of facing elements in soil nailed excavations.

The body of research current at the time of writing is reviewed, with particular emphasis placed on the historical ‘evolution’ of the perceived role of the facing. A knowledge gap pertaining to the size of the loads experienced by the facing is identified and a program of work proposed to address the shortfall in understanding.

The development and validation of a three-dimensional finite element program, capable of directly calculating the response of facing elements to the construction of soil nailed excavations, is detailed. A full-scale experimental soil nailed wall, constructed as part of The French National Research Project Clouterre, is modeled using the program and comparisons between observed and calculated behaviour are presented and discussed in the context of other simulations of the same project.

A parametric study is conducted targeting factors that were, either shown by existing research to be of significance or revealed during the programming process to influence the calculations. In general the results confirm the current understanding regarding facing behaviour. Notwithstanding, it is shown that there is a strong theoretical basis for the argument that construction related issues can have a significant impact on the behaviour of the facing. In light of this, a number of possible indicators as to the impact of construction processes are discussed, and in conclusion a program of further research exploring these issues proposed.