Design and Construction of the Venaus Powerhouse Cavern in Calcschists

This paper examines the design and observed performance during construction, associated with staged excavation and support requirements of the Venaus powerhouse cavern, a component of the Pont Ventoux-Susa hydroelectric scheme (in Northern Italy, near the city of Torino), which has been recently put into operation. This cavern, with a maximum height of 49 m, a length of 50 m, and a span of some 23 m, has been excavated in calcschists at a depth of 250 m below surface. Rock reinforcement consisted essentially of 9 to 15 m long primary rock anchors, for long term support, and 6 m long secondary rock dowels, together with a 25 cm thick reinforced shotcrete lining. Following comprehensive geological and geomechanical investigations with laboratory and in situ testing, design analyses have been performed by numerical modelling with both continuum and discontinuum methods. On the basis of the observed performance during early excavation of the upper cross section of the cavern, a discontinuum model, calibrated by back analysis procedures, was used for predicting the overall excavation stability. It is noteworthy that the design concepts adopted allowed this large cavern to be excavated and stabilized without the use of a cast-in-place concrete arch for roof support.