Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiesroadway. The new pipe was installed on a steep slope completely through the newand old embankment zones, with the upstream end of the pipe set at the normalpool elevation and the downstream end at the toe of the new earthfill.An inspection of the embankment dam in 1996 noted some “sinkholes” in thedownstream slope of the embankment near the location of the pipe, but no repairswere made. Another inspection of the embankment dam and pipe exterior thefollowing year noted substantial deposits of sediment in the stream channel justbelow the downstream end of the pipe. An interior evaluation of the pipe could notbe made, because the sediment deposits at the downstream end obscured the pipe,and debris placed at the upstream end by a local beaver precluded inspection at theupstream end.After the roadway collapsed in 2003, an inspection revealed extensive deteriorationof the CMP and that substantial loss of embankment material had occurred. Groundpenetrating radar (GPR) detected large voids in the embankment along the pipe, anda decision was made to construct a new spillway at a different location (figures B-30and B-31). The failed CMP spillway and adjacent voids were filled with a stiffcement and flyash-based “compaction grout” (figure B-32). A specialty contractorperformed the grouting and pumped the grout into the 24-inch diameter CMP usinga trailer mounted, diesel powered, piston type concrete pump, specially mounted forgrout injection work. The pipe fill mixture was a flowable, nonshrink, moderatestrength (500-700 lb/in 2 ) grout with the following specifications:• 600 pounds type I Portland cement• 500 pounds flyash• 500 pounds pea gravel• 1800 pounds concrete sand• 40 to 45 gallons of water• 40 ounces of superplastizerThe compaction grouting to fill the voids in the embankment dam was performedfrom the roadway and median along the abandoned pipe alignment, with each groutpipe extending to approximately the pipe invert elevation. The grout-hole layoutplan consisted of 128 compaction grout locations based on a 5-foot offset squaregrid throughout the anticipated treatment zone. The compaction grouting wasperformed utilizing a diesel powered track drill to install the grout casing through thepavement and fill soils to the invert elevation of the pipe. The grout was a blend ofB-31