Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damscommon for repair of leaking joints and cracks in conduits. Polyurethanegrouts are generally viscous liquids which react with water to form a solid orsemisolid material. The properties of common chemical grouts are described inthe USACE’s Chemical Grouting (1995a).Chemical grouts are classified as hydrophobic or hydrophilic depending on howreactive they are when mixed with water. Hydrophilic grouts can incorporate alarge amount of water into the cured products, and may shrink substantially ifallowed to dry out completely. Therefore, hydrophilic grouts should be used inlocations which are not kept wet at all times. Hydrophilic grouts cure withlower strengths than epoxy types of grout and are not considered to produce astructural bond in the area being grouted. Hydrophilic grouts form aclosed-cell flexible foam barrier to stop the seepage of water through the joint.These grouts are usually injected when the ambient air temperatures in theexisting conduit is above 32 °F. Figure 196 shows an example of resin injectionequipment used for small repair projects.For examples of grout cracks in an existing conduit see the case histories forPablo and Ridgway Dams in appendix B.14.3 Conduit abandonmentWhen the existing conduit deteriorates to a point where it can no longer serve itsintended design purpose, a decision must be made to renovate, remove/replace, orto abandon it. In some cases, the designer may find it technically and economicallymore feasible to abandon the conduit by grouting it closed and leaving it in place.For instance, abandonment has some advantages over removing the conduit becausea large trench is not required to be excavated transverse to the embankment dam.Backfilled excavations in an existing embankment are a source of differentialsettlement and potential hydraulic fracture. If abandonment is selected, a filterdiaphragm or collar should be part of a design to intercept any flow that couldpotentially occur through defects in the grouted conduit or along the interfacebetween the existing conduit and earthfill. For guidance on the design of filterdiaphragms and collars, see sections 6.4 and 6.5, respectively. At embankment damswith small reservoirs, abandonment of a conduit may be done in conjunction withthe installation of a siphon. See section 11.4.1 for guidance on the design andconstruction of siphons. For an example of a conduit abandonment, see the casehistory for St. Louis Recreation Lake Dam in appendix B.354