SMALL DAMS PETITS BARRAGES

Fig. 7.15 – Example of the historical timeline used by US Bureau of Reclamation to
assist in the structural evaluation of existing outlet conduits (from FEMA, 2005)
Removal and replacement of defective outlet conduits is the traditional method for
rehabilitation. However a variety of “trenchless” technologies are now available for renovating
deteriorated outlet pipes. Table 7.2 summarizes some of the various trenchless technologies,
the conditions under which each method is appropriate, and the associated pros and cons.
Sliplining is one of the most common methods used (Figure 7.16). Understanding the
existing seepage conditions around a defective outlet is a critically important consideration
when evaluating whether or not sliplining should be attempted.
If the control valve is located upstream of the conduit, the conduit is not always
pressurized and, if not watertight, can play an important drainage role for the surrounding
embankment. In such a case, rehabilitation of the pipe by sliplining may alter the seepage
flow regime and in some cases actually exacerbate adverse seepage conditions around the
outside of the pipe (see Figure 7.5). Figure 7.5 shows seepage conditions prior to sliplining,
when seepage through the dam embankment is entering the defective (or leaking) conduit.
After sliplining, drainage into the pipe is prevented, and pore pressures within the embankment
may increase.
It is important to provide adequate filter protection around the outside of the pipe near its
downstream end to intercept seepage around the outside of the pipe. Further details for
evaluation, design and construction of sliplined conduits are provided in FEMA (2005).
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