The excavation, cooling, and ventilation of the underground openings results in the gradual desaturation and depressurisation of the surrounding rock, which eventually resaturates when ventilation ceases. Transient processes associated with ventilation include:
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drying out of tunnel support materials,
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creation of desiccation cracks in the surrounding rock,
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precipitation of salts in these cracks due to evaporation of porewater,
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an increase in salinity of the porewater, and
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carbonation of cementitious materials, due to reaction with CO2 in the air.
An Excavation Damaged Zone (EDZ) also forms around the underground openings. The EDZ, however, reseals in the post-closure period as it resaturates (see Chapters 3 and 4 in NAB 24-20 Rev. 1, Nagra 2024m).