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.