Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsdifficult to obtain good compaction of the fill soils under and alongthe CMP and around the antiseep collars .........................B-10B-11 Seepage along the sides of the CMP resulted in loss of soil support, causing conduit and joint deformation ...........................B-10B-12 Arial view of Arkabutla Dam, Mississippi.........................B-12B-13 Cross section of the outlet works conduit.........................B-14B-14 Metal plate bolted across the joint...............................B-15B-15 Damaged metal plate .........................................B-16B-16 Cracking at the embankment dam crest above the sinkhole on the upstream slope of the embankment dam. Note the simple staking ofthe area to monitor movement of the crack .......................B-19B-17 Portable pump used to initiate draining of the reservoir ............. B-20B-18 Initialization of the breach in the embankment dam near the left abutment ................................................... B-21B-19 Discharge of water through partially breached section and down thedownstream slope of the embankment dam ....................... B-21B-20 Large spall at the construction joint located at station 12+13 ......... B-23B-21 Exposed aggregate located at station 11+28.......................B-23B-22 Popout located at station 10+79 . . . . . . . . . . . . . . . .................B-24B-23 Spall at the construction joint located at station 4+93 ............... B-24B-24 Because of soft foundation soils, numerous 60-foot long pipe piles were installed in winter 2001 to support a new reinforced cast-in-placeconcrete spillway structure . . . . . . . . . . . . . . . . . . . . ................. B-27B-25 This the downstream end of the spillway at the end of construction in 2002....................................................... B-27B-26 Less than 2 years later, an engineer inspecting the bridge over the spillway reported that seepage flow was visible from under thedownstream end of the spillway. The seepage flow was clear, and nomigration of soils was evident. A few months later, the roadway onthe dam crest collapsed, and large quantities of sediment wereobserved in the pool below the dam . . . . . . . . . . . . .................B-28B-27 The road was closed immediately, and lake level was lowered. However, the sinkhole rapidly enlarged ..........................B-28B-28 Seepage flow at the downstream end of the spillway appeared to be boiling. Attempts to create a sandbag weir around the boil to reduceleakage under the spillway were unsuccessful . . . . . .................B-29B-29 A sinkhole, which appeared in a heavily traveled roadway above a 20-year old CMP spillway, was filled with asphalt. Part of the roadwaywas closed when the asphalt patch subsided a few hours later ........ B-30B-30 Ground penetrating radar investigations were performed from theembankment dam crest .......................................B-32B-31 Ground penetrating radar identified the location of voids along the CMP ......................................................B-32B-32 The failed CMP and voids were filled with a stiff compaction grout . . . B-33xxvi