Road Foundation Layer Research Articles

A philosophy for a performance specification for road foundations

The road foundation layers perform several functions both during construction and when the road is in service, for example load-spreading, temporary haul routes, and a base for the overlying construction layers. The critical loading condition is usually directing trafficking where the applied stresses are greatest. The capping and sub-base layers during construction require adequate stiffness and strength to resist these stresses. The current UK specification for road foundations is based on a recipe approach, and, unless permission is granted to use an analytical design, the pavement foundation designs are based entirely on the California Bearing Ratio (CBR) to characterise the subgrade, capping and sub-base materials. Here CBR is used as an index of both material strength and stiffness, although it measures neither directly. Such an approach is potentially inefficient and does not readily facilitate the use of new and marginal materials or alternative design procedures. Recent technical advances in laboratory and in situ testing of pavement foundation materials now allow the performance parameters of stiffness, strength and resistance to permanent deformation to be measured both for design and during construction. This in turn enables a performance-based specification for road foundation layers to be introduced to provide some assurance of the as-constructed quality, and by permitting the use of secondary or recycled aggregates, to contribute to the parallel goal of sustainable construction. This paper sets out an idealised philosophy for a performance-based specification for road foundations, examines the individual elements of the specification in relation to current knowledge and makes recommendations for a phased introduction alongside CBR-based methods.

Pre-Treatment of MSWI fly ash for useful application

At TNO a feasibility study has been carried out into the possibilities of removing a number of easily leachable elements from MSWI fly ash. The aim of this study was to find a way to useful application of MSWI fly ash (after treatment) as a road base construction material. A combination of a slight washing step and a stabilization/solidification step with cement and other additives appeared to be convenient to meet the severe standards of the Dutch Building Materials Decree. The slight washing step removes more than 90% of the cadmium and chloride that were originally present. Over 50% of zinc and sulphate are removed. The remaining solid material (approximately 70% of the original quantity) could be easily processed into a bound road foundation layer, adding some 20% cement and other additives. The leaching characteristics of the stabilized material meet the most severe standards of the future Building Materials Decree in The Netherlands. This means that the material may be applied without any provisions. In contrast, stabilized but unwashed MSWI fly ash could not even meet the less severe standards. A global cost estimate shows that, at least for the Dutch situation, the processing costs of the combination (washing, processing of the washing liquor and stabilization) equal the costs for disposal of MSWI fly ash.

Self-stabilization of road bases - fact or fiction

Self-stabilization is defined as a natural improvement in the wet strength or plasticity of a road foundation layer, not caused by traffic compaction or the addition of conventional stabilizing agents, such as lime or cement. The evidence for improvement in strength due to self-stabilization is reviewed, and it is concluded that, although many are of the opinion that it does take place, and although considerable increases in strength have been attained during laboratory experiments, documented evidence of its importance in engineering practice is lacking. (A). /TRRL/