|dc.description.abstract||Numerous reports on expansive shales have received wide attention in recent decades. Damages and greater financial loss associated with expansive shale rocks have been documented in different countries. In Norway, challenges and problems in tunnels and underground excavations associated with the Upper Cambrian-Middle Ordovician black shale, Alum Shale, have been experienced since 1900s.
When evaluating various research reports on causes of concrete degradation through Alum Shale swelling, it is obvious that there is still the need to analyze further which minerals and mineral-related swelling processes are responsible in order to prevent harmful concrete compositions and to ensure safe and long-lasting structural integrity after construction work.
In this pretext, an extensive laboratory work was undertaken to address the extent to which the mineralogy and swelling potential can be correlated, based on the hypothesis that gypsum influences the swelling property of the Alum Shale. In addition, possible effects of clay minerals and other unidentified mineral reactions on the swelling phenomenon were investigated.
Fourteen samples, two of which are from a basement of an old building at Oslo city area (Møllergata) and twelve from the Gran Tunnel project site were studied. For each sample petrography and mineralogy were determined by optical microscopy, secondary electron microscope and X-ray diffraction and the swelling potential was obtained from free swelling and swelling pressure indices.
Swelling potentials were obtained on seven samples with conventional and labour-intensive schemes determined by the free swelling index of rock powder submerged in water and swell measurements in response to consolidation, saturation, and rebound in an oedometer.
The high pH value (low acidification), the low sulphur reactivity from pyrites and the absence of catalytic effect of pyrrhotite in pyrite oxidation showed how low the formation of gypsum could be in the shale. Gypsum was not detected in the 12 samples from Gran. The low amount of gypsum (about 1,5% and 2,8%) in samples from the Oslo city area showed the insignificant effect of swell in the Alum Shale. Rather, a fairly good positive correlation between measured illite/muscovite contents and swelling pressure was observed under these conditions.||nb_NO