Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams9.1 Types of inspectionsInspection intervals may vary, depending on the overall conditions determined fromprevious inspections and the existence of any dam safety concerns. Periodicinspections can vary in scope and purpose and by the organization or personnel(damtender, agency/district level, etc.) performing the inspection.Dam safety organizations and embankment dam owners may employ a variety ofinspections during the life of a conduit (figure 118). These inspections may includethe following types (Reclamation, 1988, p. I-2):194• Initial or formal.—Initial or formal inspections include an in-depth review of allpertinent data available for the conduit to be inspected. Design andconstruction data are evaluated relative to the current state-of-the-art to identifypotential dam safety problems or areas requiring particular attention. Athorough onsite inspection of all features is conducted, and an attempt is madeto operate all mechanical equipment through their full operating range, ifpossible. Many State and federal agencies require formal inspections on a setfrequency (e.g., every 6 years).The first time the reservoir behind an embankment dam is filled is critical to itsintegrity. The embankment dam will experience the hydraulic loading for thefirst time and will begin to adjust to this loading. During first filling, thewetting front begins to penetrate the embankment dam. History has shownthat a much higher frequency of incidents occur at this time. Also, the conduitthrough the embankment dam will be tested for the first time.Good practice dictates that the embankment dam be monitored by frequentinspections during this crucial period. Round-the-clock surveillance is notuncommon for high hazard facilities. Special lighting provisions may beinstalled to permit adequate nighttime visibility.There may be several “hold” periods during initial fill to allow stresses in theembankment dam to partially stabilize and instrumentation to level off prior tothe continuation of filling. The rate of reservoir rise may be limited to allow forthe wetting front to slowly penetrate the embankment dam. A rate of reservoirrise in the range of 0.5 to 2 feet per day is a common. Limiting the rate of risefor small reservoirs that do not usually have large outlet systems may not befeasible. If the outlet conduit has a small capacity and large inflows follow ahigh precipitation event, no method for controlling the rate of rise exists.The first fill monitoring may be staggered to accommodate the amount of wateravailable to fill the reservoir. For some embankment dams, many years may berequired to reach their fully operational reservoir level. Often, an embankment