International Geologiical Congress - Oslo 2008


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GTE-01 General contributions to engineering geology and geotechnics


Distribution and impact of swelling clay in basalt


Bartal H°jgaard, Jar­feingi (The Faroese Earth and Energy Directorate) (Faroe Islands)


The proposed road tunnels on the island of Boroy from the Faroe Islands in the North Atlantic are projected to run through the Malinstindur Formation, which is predominantly composed of basaltic compound lava flows. The stratigraphic interval for the tunnels has previously been drilled for the Noroyar sub-sea tunnel between the islands of Eysturoy and Boroy, which encountered problematic swelling clay bearing basalt that required extensive pre-injection concrete and rock bolt support. To foresee such complications a simple modified version of the free swelling test has been developed for minute clay samples obtained from geotechnical cores 4 cm in diameter. Furthermore, the ethylene glycol soak test has been used to investigate the impact swelling clay has on basalt. Tests showed that samples from the cores and bases of altered olivine cumulate basalt flows were prone to disintegrate in ethylene glycol. Aphyric and plagioclase porphyritic flows did not suffer substantial deterioration even though swelling clay was present within vesicles. The destructive nature of the olivine porphyric basalt is probably due to permeable texture caused by micro fracturing and the high content of swelling clay, most likely, formed from the alterantion of the olivine. Fracture description methods, such as Rock Quality Designation (RQD) and Rock Mass Quality (Q), should be used with care on swelling clay bearing rocks, as they can continue to disintegrate with time after exposure, thus altering the state of fracturing and modifying the initial RQD and Q values derived from newly drilled core.


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