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Improved use of pulverised fuel ash as general fill

Lookup NU author(s): Professor Barry Clarke


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In the UK, the introduction of a tax on landfill and the tightening of planning consents for disposal provided powerful incentives for the use of industrial by-products such as pulverised fuel ash (PFA) in construction. PFA (known as fly ash in some parts of the world) has a long history of successful use as general fill, including applications in environmentally sensitive locations. However, the current commonly used Specification for Highway Works treats PFA as a manufactured material, and applies an end-product specification for compaction. This form of specification is restrictive as it requires that the maximum dry density and optimum moisture content be determined for each consignment of PFA delivered to site, a process that can take up to two days. In addition, the Specification requires testing to determine effective shear strength parameters under fully saturated conditions for embankment design. No guidance is given on whether peak or post-peak parameters are to be selected for design purposes or, indeed, on appropriate factors of safety. As a result, most engineers test PFA in the fully saturated condition, select post-peak effective shear strength parameters for design, and apply a conservative factor of safety. As the testing protocol represents a worst-case scenario the application of such a conservative approach can be questioned. A method specification is more commonly applied to natural fill materials used as general fill. Historical evidence shows that PFA has been placed using a method specification, and that those fills still function as designed. Thus the introduction of a method-based specification for the use of PFA as general fill would make possible its greater use and thus reduce the UK-wide need for landfill.

Publication metadata

Author(s): Clarke BG; Winter MG

Publication type: Article

Publication status: Published

Journal: Proceedings of the Institution of Civil Engineers: Geotechnical Engineering

Year: 2002

Volume: 155

Issue: 2

Pages: 133-141

ISSN (print): 1353-2618

ISSN (electronic): 1751-8563

Publisher: ICE Publishing