Investigation of destructive ground heave attributed to pyritic fill affecting new-build properties in the Dublin area of Ireland

As structural damage caused by expansion of pyrite bearing bedrock and fills had been documented in the UK and other countries since at least the 1960s, it was surprising that some thousands of cases involving extensive damage to residential and other properties near Dublin, Ireland came to light during the first decade of 2000. Although the stability of properties was not immediately endangered, their appearance and functionality were impaired with serious impact on their value, all of which was obviously very distressing to the owners. Various possible causes relating to the ground conditions, construction methods, structural design and materials used in construction were investigated. However, from the characteristics of the damage and the type of fill used beneath concrete floor slabs it was suspected that the presence of pyrite in the fill was resulting in it being subject to damaging expansion. The paper documents investigations carried out in 2008 into the damage caused to houses in one development in north Dublin. Detailed mineralogical and chemical characterisation of the materials concerned confirmed that both cubic and framboidal forms of pyrite were present in the fill material and there was evidence for in-situ expansive mineral formation. Early signs of isolated chemical attack to sub-surface footings and foundation structures was also evident and although no physical degradation of mortar and concrete was apparent, there was potential for deterioration of these buried structures in the longer-term. Various solutions to the problem were suggested, but it was concluded that replacement of the fill with effectively pyrite-free material was the best strategy for remediation. A lack of awareness in the construction industry, particularly the domestic housing sector, of the potential for problems due to pyrite was apparent. This resulted in protracted legal proceedings and new standards and specifications for fill materials were issued. This paper aims to contribute to the understanding of the behaviour of pyrite and its potential for causing problems in construction situations as even with compliance with standards, problems cannot be otherwise ruled out.