A laboratory study of the expansion of an Irish pyritic mudstone/siltstone fill material

Extensive damage to dwellings in the east of Ireland, arising from the expansive pyritic fill material used beneath the floor slabs, prompted the laboratory study presented in this paper to quantify the expansion and study its influencing factors. A novel experimental arrangement, in which sample height and water supply were varied, was used to test a large quantity of active pyritic infill taken from an affected house in the greater Dublin area.The expansion of ten separate fill specimens was measured over a 175 day period from December 2010 to May 2011. Remarkably consistent results were obtained when the expansions were normalised by the sample height, with expansion rates of approximately 2–3 mm/year/m height of fill for specimens standing in water, where the rates were largely insensitive to the amount of water supplied. More rapid expansion was noted for one specimen to which no additional water was supplied during the test. A steady reduction in pH of the water, although relatively small, was consistent with pyrite oxidation reactions occurring in the samples during the test, and also implies the occurrence of leaching of ions from the pores in the mudstone and also that oxidation reactions may have been occurring in the saturated mudstone. Changes in expansion were found also to correlate strongly with changes in the ambient temperature in all specimens.

► This paper presents measurements of the expansion of Irish pyritic fill material.
► Expansion normalised by the specimen height was consistent among nine specimens.
► The amount of water supplied had little impact on the expansion.
► Changes in temperature were influential, albeit with a lag involved in some cases.
► The results were used with success to predict floor slab heave in affected homes.