Analysing geotechnical aspects of concrete pipe culverts

Porter, Simon (2015) Analysing geotechnical aspects of concrete pipe culverts. [USQ Project]

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Abstract

Concrete pipe culverts are important structures used throughout the world to convey water and provide access beneath roadways. Culvert design requires an understanding of the structural, hydraulic, construction and geotechnical aspects that influence the functioning of the structure. Geotechnical aspects of culvert structures significantly affect the design and include factors like bedding and backfill type, installation conditions and loading analysis. A disparity exists between current design methodology and in field observations. Such design methods are affected by certain assumptions which may not be met in field during construction due to site restrictions, cost, material availability or other factors.

The impact of analysis technique for assessing the distribution of live and dead loads of soil on the pipe, as well as the soil structure interaction, can significantly affect the design. Some of these techniques do not adequately provide for the assessment of alternative construction methods or backfill materials. Therefore, the impact of these changes may not match the impact assessed in the design. There is the potential for modern Finite Element Method (FEM) programs to more accurately evaluate culvert loadings and describe alternative construction conditions. This research has been prepared in order to compare the various analysis methods within FEM packages, as well as the current design standard, while also assessing the contribution that changed construction methods have upon culvert loading. This involved outlining design scenarios for the trench condition with varying trench widths, backfill heights (ranging from 0.3 m to 1.2 m) and bedding and backfill materials (including conforming granular backfill, non-conforming granular backfill, aggregate, stabilised sand and controlled low strength materials (CLSM)). These scenarios have been analysed utilising the Culvert Analysis and Design (CANDE) FEM program, with a linear elastic, Mohr Coulomb and Duncan soil model, as well as by the Australian/New Zealand Standard Design for installation of buried concrete pipes (AS/NZS 3725). These techniques were then used to assess alternative construction methods in industry for two case studies. One was assessing aggregate backfill while the other was assessing the use of stabilised sand for low cover applications.

The findings identified that the Australian Standard design method resulted in significantly reduced factors of safety for alternative bedding and backfill materials, whereas in some cases the CANDE models observed the opposite result. However, for conforming granular materials meeting HS3 support conditions the AS/NZS 3725 method resulted in higher safety factors than the FEM program. The AS/NZS 3725 design method also significantly underestimated the capacity of pipes for low covers (below 400 mm) in comparison to a FEM analysis method.

Drawbacks of the FEM models were identified with the Mohr Coulomb model, which resulted in non-convergence for non-plastic materials with low shear strength, due to the high surface loading. The Duncan model, while being identified as the most realistic model (of the models compared), has limitations for assessing alternative materials as the model requires specific parameters based upon the material properties.

A comparison of the scenarios identified that the stabilised sand and CLSM materials could result in potentially greater safety factors against failure, as well as reduced displacements, in comparison to the other materials. Aggregate backfill also had the potential to reduce the loading upon the pipe and displacements of the backfill, however, this is significantly affected by construction processes. The non-conforming materials performed similarly to the conforming granular materials, however, those with higher clay content generally exhibited higher displacements. There are also issues associated with the response of such material to moisture. For the cover and trench widths assessed, it was determined that narrower trenches were favourable for the material types, however, an ability to adequately compact the material needs to be maintained. Finally, for the highway loadings utilised it was identified that the deeper cover depths were preferable to the shallow cover depths due to the increased load distribution outweighing the disadvantage of increased dead loading.

Analysis of the case studies revealed that the alternative materials performed better than the conforming material when using FEM modelling. However, this was inconsistent with the standard method which identified that particular alternative materials may be inadequate. The standards results were also inconsistent with the field performance of these culverts.

The improved understanding of the performance of various construction methods can allow for better decision making in the field. From the results of the research, superior (or in some cases similar) performance can be achieved by alternative or non-conforming backfill in comparison to conforming materials. Conservative design processes are apparent in some aspects of the current standard design method which can be improved through the use of more accurate FEM models. Future research analysing the field performance of these alternative construction techniques is required in order to assess the FEM programs ability to analyse the material. An expansion on the range of material properties available to the Duncan
model could also improve the models ability to analyse alternative backfill materials.


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Item Type: USQ Project
Item Status: Live Archive
Additional Information: Bachelor of Engineering (Civil) project
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Civil Engineering and Surveying
Supervisors: Ghabraie, Kazem
Date Deposited: 07 Jun 2016 02:00
Last Modified: 07 Jun 2016 02:00
Uncontrolled Keywords: concrete pipe culverts, geotechnical, water convey, finite element method, culvert analysis and design, Australian standard design method
Fields of Research : 09 Engineering > 0905 Civil Engineering > 090599 Civil Engineering not elsewhere classified
URI: http://eprints.usq.edu.au/id/eprint/29277

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