Decommissioning of Outlet Works
“When the existing conduit deteriorates to a point where it can no longer serve its intended design purpose, a decision must be made to renovate, remove/replace, or to abandon it. In some cases, the designer may find it technically and economically more feasible to abandon the conduit by grouting it closed and leaving it in place. For instance, abandonment has some advantages over removing the conduit because a large trench is not required to be excavated transverse to the embankment dam. Backfilled excavations in an existing embankment are a source of differential settlement and potential hydraulic fracture. If abandonment is selected, a filter diaphragm or collar should be part of a design to intercept any flow that could potentially occur through defects in the grouted conduit or along the interface between the existing conduit and earthfill”.[1]
“Another possible reason for abandoning an existing conduit would be when a proposed embankment dam raise would result in much higher embankment loads over portions of the conduit. Structural analysis may determine that the higher embankment loads would fail the existing conduit and measures to strength it are not feasible. To prevent failure and to reduce the potential for internal erosion and backward piping erosion of embankment materials, the existing conduit would need to be abandoned and a new conduit constructed. The abandonment of the existing conduit may best be postponed until after the replacement conduit has been constructed and is fully operational. The existing conduit is used for diversion while the new conduit is constructed”.[1]
“Any abandonment activities should also evaluate the need for partial or full demolition of the entrance and terminal structures, gate houses, plugging of gate chambers and shafts, and removal of certain mechanical equipment. Blasting for demolition should not be permitted. Shaft structures can be backfilled with compacted sand instead of concrete or grout”.[1]
Best Practices Resources
Technical Manual: Conduits through Embankment Dams (Federal Emergency Management Agency)
Citations:
Revision ID: 4504
Revision Date: 11/18/2022