Abstract
From the site investigation data, difficult geotechnical conditions were anticipated for the 9.6-m-diameter tunnel-boring-machine (TBM) drive between the Olympion and Syntagma stations of the Athens Metro. In addition, due to the difficult geological conditions, including zones of highly sheared phyllites and mylonites, it was anticipated that ancient wells and canals would exist at or above tunnel level. To reduce the uncertainties and hence the risk of collapse beneath a busy main road, a decision was made to drive a 208-m-long pilot tunnel, approximately 2.5 m in height and 3 m in width. Extensive face mapping was undertaken as the drive progressed, and, in the light of the information obtained, an RMR* value was produced, from which the temporary lining of the walls was determined. Where appropriate, ground improvement was undertaken. The TBM was successfully driven through the area.
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