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18 Abandonment of Low-Level Outlet Conduits…Think it Through Before you Grout it Through By: Richard F. Walker Jr, PE Introduction Deterioration of low-level outlet conduits is a common problem, especially for older embankment dams. When the existing conduit deteriorates to a point where it can no longer serve its intended design purpose, a decision must be made to rehabilitate, remove and replace, or abandon it. Removal and replacement is the most reliable approach, but will most likely require draining of the reservoir and result in the highest cost to the owner. Rehabilitation can be a reasonable alternative and is typically accomplished by one of two methods, sliplining or cured-in-place pipe (CIPP) liners; however, it may not be applicable for severely deteriorated conduits (See Figure 1) or changes in loading conditions. In some cases, the owner and/or designer may find it technically and economically more feasible to install a new alternate means of water conveyance and abandon the conduit by grouting it closed and leaving it in place. The advantages of abandoning an existing conduit through an embankment dam include: • Excavation—A large excavation is not required through the embankment dam. • Reservoir operation—Abandonment can in some cases be done while the reservoir is full. • Costs—Costs are generally less than other rehabilitation and replacement methods. The disadvantages of abandoning an existing conduit through an embankment dam include: • Grouting—Difficulties may be encountered while trying to fill the existing conduit with grout. • Loss of use—A replacement means of providing downstream flow and flood discharge capacity will be required. The most common way to abandon an existing conduit is by backfilling with grout or concrete. Owners of dams with release requirements may choose to abandon the existing conduit after installing a new one in a new location. In embankment dams with small reservoirs and no regulation or requirements on regular water flow or release, an abandoned conduit’s function can be accomplished by an alternative means of water conveyance, such as a siphon or high-level outlet conduit. This article will focus on abandoning conduits without replacement in-kind for small-sized dams. Information is presented on alternatives for water conveyance, the implications of abandoning a conduit in-place, and the means and method of abandoning the conduit. Figure 1. Severely deteriorated CMP outlet conduit. [4] Figure 2. Typical drop-inlet structure for outlet conduit.