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This paper describes the design of the new tunnel invert lining of the 9-foot tunnel at Mud Mountain Dam, Washington, USA. The tunnel diverts all bed load sediments into the tailwater. Major invert abrasion has been observed in the existing steel lining. The new invert design consists of 0.59 m2 and 0.79 m2 granite blocks that are 0.25 m thick and placed tightly together along the tunnel. Stability analysis showed factors of safety ranging from 1.2 to 2.6 against uplift. This will be achieved with strip drains placed in the bedding material along the tunnel. A service-design-life analysis was performed using abrasion prediction modelling.
This model was based on abrasion measurement data acquired from granite field tests at Pfaffensprung sediment bypass tunnel, Switzerland. The estimated annual abrasion depths for the granite were approximately 0.50 mm/year for average sediment transport conditions.
A major drawback of sediment bypass tunnels is the potential for severe invert abrasion due to intense bedload sediment transport. This paper briefly describes the abrasion phenomena as well as the available models used to predict invert material loss. The application and calibration is demonstrated on the basis of the Mud Mountain sediment bypass tunnel, Washington, USA.
