Karst and tunnelling: a reverse impact case history

Abstract

In this paper we present a case history in which tunneling heavily impacted onto a karst system. This paper deal with a hydraulic tunnel under pressure crossing a mountain ridge constituted by calcareous and gypsum units, karstfied in recent geological time. The tunnel was driven right above the water table, and above the base level of karstification, in the transfer zone. During time, the concrete lining suffered deterioration mainly due to cavitation processes that led to pierce the lin-ing and the water flowing from the tunnel into the surrounding rock-mass. The water pressure into the tunnel changes daily, according to the needs, from 400 to 600 kPa. That led to inflowing water into the karstfied rock-mass creating a bub-ble of water infilling the karst system tubes and conduits above the tunnel for a highness of 40 m to 60 m and asymmetric shaped to the lower end of the moun-tain ridge and its sides. Therefore, many springs at higher level of the natural wa-ter table were activated and the base water flow in the karst emerging springs largely improved. When for the execution of the remedial works the tunnel was empty a surplus of water flow of about 6 Mm3 was recorded, all this water was drained from the water bubble created into the karst system. Once the tunnel had been cladded by a steel tube and the water flow reopened no more water losses occurred.