"Swelling occurs due to absorption of water by clay minerals in rock upon excavation-induced unloading. Swelling manifests as movement of the ground into the tunnel opening or additional tunnel support loading. Swelling of rock at the excavation boundary contributes to softening."
Definition from the "Caldecott Improvement Project: Geotechnical Baseline Report," prepared by Jacobs Associates, June 2009. 100 pp.
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