In hot desert regions, attention is turning to local sources of aggregate that were previously regarded as ‘too difficult' to exploit. This paper describes an investigation of such a deposit in Qatar. The material, known as Wadi gravel, is a gravelly sand of Tertiary age cemented by gypsum. A field survey found the gravel content to be in the range of 10 to 20%. As-dug samples had excessively high sulfate content, which could not be removed with conventional aggregate operations. However, intensive treatment in a local sand-processing plant using multi-stage crushing, screening and washing reduced the sulfate to acceptable levels. Petrographic analysis was carried out to identify rock types potentially susceptible to Alkali Silica Reaction (ASR). Trial concrete mixes using the processed Wadi gravel were found to produce satisfactory C40 and C60 concrete. Full-scale building trials were constructed to demonstrate the practicality of using the gravel and assess in-service performance. The smooth surface and rounded particles of Wadi gravel improved the concrete workability and strength, and durability-related properties were similar to concrete made with imported gabbro. Strata similar to Wadi gravel occur widely in hot desert regions and could be useful sources of coarse and fine aggregate for concrete.

Deposits of Wadi gravel are available in many parts of the Gulf region, but not widely utilized as aggregate for concrete, mainly due to the possibility of internal sulfate attack, plus the perceived risk of alkali aggregate reactivity (AAR). This paper describes the investigations for AAR of the Wadi gravel in this case, as part of the wider study described in Part 1 of this paper.Wadi gravel from the Mekaines site in Qatar was subjected to petrographic analysis, plus the gel-pat and accelerated mortar-bar test methods. The AAR potential was found to be low to normal. The accelerated mortar-bar test exhibited ‘innocuous’ behaviour after 14 days of immersion in alkali solution. When separately testing the constituent rock types of the Wadi gravel, limestone and quartz returned innocuous results, while rhyolite, granite and quartzite returned potentially alkali silica reactive (ASR) results and some reaction was confirmed using post-expansion petrographic examination.Wadi gravel was classified as potentially reactive in the RILEM AAR-4.1 accelerated concrete prism test, but of ‘low reactivity’ in the BS 812-123 test over the longer period of 12 months. Overcoming the potential problems of gypsum content and AAR successfully provides a valuable local resource of Wadi gravel aggregate for concrete.

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The failure in 1984 of Carsington Dam near the end of its construction led to major advances in several areas of geotechnical engineering. It also led to major advances in understanding the nature and extent of geochemical and mineralogical reactions that can occur in earthworks and how these can be allowed for in design and construction. These aspects had not been considered in the original investigations and design. This resulted in much greater degradation of the fill materials than expected during construction, pollution of surface waters and the tragic deaths of four site personnel. The lessons learned in understanding the impact of geochemistry at Carsington were fundamental in further work by the authors in developing improved test methods for sulphur compounds and protocols for assessing their impact. The relevance of these lessons for civil engineering works remains as great as ever. Seven general principles for dealing with geochemical issues are presented.