Expansion of structural backfill used as hardcore aggregate under floor slabs in the Dublin area of Ireland has resulted in significant structural damage and distress to buildings recorded since 2006. The hardcore or backfill aggregate typically comprises mudrocks of Carboniferous age that contain pyrite. The most recent estimates suggest that pyrite-induced reactions have resulted in structural damage affecting in excess of 10000 buildings in the Dublin area. This has a severe impact on the value of property and the remediation costs are set to run to hundreds of millions of euros. A court case held in the High Court in Dublin in 2010 (upheld in 2014 on appeal to the Supreme Court) found that structural damage to Ballymun Youth Centre, Dublin had been caused by pyrite-induced reactions leading to swelling of the backfill as a result of forceful crystallization of gypsum. This paper describes the logic, chemical, X-ray diffraction and petrological techniques and procedures used to evaluate pyrite-induced swelling for this court case and in many hundreds of evaluations since. The paper also suggests procedures to reduce the costs of testing.

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