First Filling and Monitoring

Earth embankment dam.

The initial filling of a reservoir is the first test that the dam will perform the function for which it was designed. A carefully managed first filling is crucial to the future success of a dam. According to a study completed by the Bureau of Reclamation on internal erosion failure modes, “approximately two-thirds of all failures and one-half of all dam incidents occur on first filling or in the first 5 years of reservoir operation.”^[1]

"An initial reservoir filling and surveillance plan should be prepared by the design staff. Initial filling should be well documented, including a record of reservoir elevations and controlled water releases during the filling. The record should include complete written justification and design approval of any deviations from the planned filling. The surveillance record should include all information obtained from inspection of the dam, appurtenant structures, abutments, and reservoir rim during the initial filling."^[2]

"If an embankment dam were to fail under conditions that could be appropriately defined by a limit-equilibrium analysis, it would do so at one of three critical periods... during construction... first filling... [and] achievement of maximum pore pressure under a full reservoir."^[3]

"The second critical period is the first filling of the reservoir. If the dam survives the initial filling and if there is no blowup at the toe, the dam can be considered safe (in effect proof-tested) against failure by piping due to heave... Under many circumstances, including the presence of relatively thin cores or ample well-drained downstream shells, pore-pressure maxima follow so rapidly after the first filling that the survival of the first filling can be considered to be a demonstration of the ultimate safety of the dam under full-reservoir conditions. However, if the impervious section of the dam is thick and impermeable enough to create a time lag between the rise of the reservoir and the rise of piezometric levels in the core or supporting downstream zones, pore-pressure equilibrium may not occur for several years after the reservoir is first filled and the critical period may be delayed."^[3]

"The factor of safety of a dam that survives its first filling and the associated increases in pore pressure will increase with time, unless this factor of safety is so close to unity that cyclic loading produced by fluctuations in the level of the pool causes strain softening and a critical loss of strength."^[3]

"Requirements for initial filling should be available and should be agreed upon. During this time extra precautions and procedures for operation should be established because unpredictable situations may occur. During the first filling the facility should be attended continuously."^[4]