Headrace tunnel geo-hazard identification, beyond the rock mass classification methods Vanni A., Vietti D. Lombardi Engineering Limited, Giubiasco, Switzerland Online published on 14 April, 2023. Abstract Headrace tunnels and infrastructure tunnels are different. Beyond the evident equivalence in what pertains-their nature (a long underground cavity from A to B), the intended use makes the relevant geo-hazard and-related risk assessment different. The construction of any tunnel induces disturbances in the rock mass. For an infrastructure tunnel, in most cases the fundamental disturbance is limited to an alteration of the stress state around the excavation (in all its possible expressions), and the worst-case operating condition is not different from the worst-case construction condition. The geo-hazards are identified by relating each other geological features, rock mass properties, stress state and infiltration conditions at tunnel depth. A headrace tunnel, if pressurized, beyond altering the stress state around the excavation, brings pressurized water into itself and in rock mass extensive portions. This causes the rock mass to find a new equilibrium, not only in terms of stress state, but also in terms of physical and chemical equilibrium with water. The new equilibrium will be reached during operation in days, or months, up to years, and often leads to the worst-case operating condition. Dynamic conditions such as transients can bring further conditions requiring specific approach and solutions. If not properly anticipated, the consequences can be catastrophic. Several occurrences are known and directly experienced by the relators. For the latter, the design solutions - including alternatives to the tunnel alignment and different solutions for the final lining - can’t be based on rock mass classifications only but have also to compulsory include the expected new long-term equilibrium between rock mass and operating tunnel. The paper focusses on the geo-hazards that must be completely identified to properly assess the operating risks, which represent the main causes of major damages investigated in the recent past. Top |