A study on the mineralogical ageing of iron ore rock was performed. During mining, oxidizing conditions were imposed in the mines for a century and chemo-mineralogical transformations occurred in iron ore pillars: chemical alteration of reduced minerals (siderite and berthierine) in the inter-oolithic cement, recrystallization of berthierine and neo-crystallization of a hematite crust. This corrosion occurred at a large scale in old pillars exposed to the mine atmosphere for 70–100 years. Because of the alteration of the interoolithic cement, which is not compensated by the neo-mineralizations, the cohesion decreased, thus resulting in a decrease in micro- and macro-mechanical properties (from the millimetric scale of micro-indentation tests to the decimetric scale of triaxial tests). This chemical weathering of anthropic-origin plays a major role in the long-term behaviour of ironstone. It is probably one of the main causes explaining collapses of underground iron mines in Lorraine (France).
This paper focuses on the problem of underground iron mines collapses in Lorraine (North-Eastern France).A partial extraction method (i.e., room and pillar system) was used under sensitive zones with houses and surface infrastructures until 1980. Some of these safety pillars were left in a state of abandon for a hundred years. In these areas of partial extraction, many collapse occurred often because of the failure of iron ore pillars. Underground iron mines are in shallow (~200 m) environments with hydrous conditions which evolve in time: saturation under a water-table before the mining, partial saturation during mining because of the ventilation of rooms (hr =85–100%), resaturation at the end of mining due to the water-table rising and thus the mine flooding. Some of the mine collapses occurred during this flooding stage.
Knowledge of the long-term strength of rocks is of prime importance for estimating the stability of such man-made structures. In chemically reactive environments (e.g., iron ore exposed to O2 and water in the mine) and in the brittle regime relevant to in situ mine conditions (low temperature and confining pressure), time-dependent behaviour of rocks depends on the physiochemical mechanism of subcritical stress corrosion cracking (e.g., Atkinson 1984, Heap et al. 2009, Nara et al. 2007) and on chemical reactions responsible for mineralogical transformations. Ageing of rocks results from the chemical and/or physical weathering of mineralogical, geochemical, geomechanical and petrophysical properties. Previous studies on ageing/weathering were carried out to characterize building materials such as concrete, granite, calcareous shale and basalt (e.g., Nandi & Whitelaw 2009, Moon & Jayawardane 2004), to analyse landslides (e.g., Zhao et al. 2011) and (reservoir and cap) rocks of underground constructions such as storage sites (e.g., Tu et al. 2005, Oztoprak & Pisirici 2011) and to study the mechanisms of rock-soil transition (e.g., Begonha & Braga 2002, Arel and Onalp 2004). In the case of ironstone, ageing is an important parameter to take into account when performing mine stability analysis.