Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Clair Peak DamLocation: MarylandSummary: Grouting from the embankment dam surface to fill voids along theoutside of a spillway conduitIn April 2003, a police officer traveling on State Highway 235 near Lexington Park,Maryland reported a large “pothole” in the roadway. A highway repair crewdispatched to the site promptly filled the 12-foot long, 1.5-foot deep hole withasphalt patching material, and the road remained open to traffic (figure B-29).However, one lane of the roadway was closed a few hours later when it wasobserved that the patched area had again subsided and a sinkhole was locateddirectly above the spillway conduit.The lake at Clair Peake Dam had been in existence for many years before thehighway was widened in 1983. As part of the highway modification, the originallow-level concrete spillway pipe was abandoned by backfilling with concrete (TheState Dam Safety Division was unable to determine why this was done). After the26-foot high embankment dam was widened by placement of new fill on thedownstream side, a new 24-inch diameter CMP spillway was installed under theB-30Figure B-29.—A sinkhole, which appeared in a heavily traveled roadwayabove a 20-year old CMP spillway, was filled with asphalt. Part of theroadway was closed when the asphalt patch subsided a few hours later.