Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historieswaterstops in the joints of the conduit be designed and installed properly.Unfortunately, this did not happen. There is no written record why this error wasnot discovered sooner. A cross section of the outlet works conduit is shown infigure B-13. The designers intended that the lower one-third of the conduit beconstructed as one continuous monolith. Above the field joint shown in figure B-13,the conduit was designed to be cast in 25-foot long monoliths.At each monolith joint, a rubber waterstop was placed in the top two-thirds of theconduit. The waterstop was placed in the center of the conduit walls and extendedfrom 2 feet below the field joint on one side of the conduit to 2 feet below the fieldjoint on the other side. No waterstop was installed along the lower one-third of theconduit, but the contractor also constructed the lower one-third of the conduitmonolithically. This was not the design intent. With no waterstop along the bottomof the conduit, the very fine sand in the foundation was eroded through each jointand was continually being flushed downstream during operation of the outlet works.Problems with the joints were discovered soon after the project went into operation.Lead wool was used for several years to control the erosion of fine sand into theconduit, but problems were experienced in keeping the lead wool in the joints. Thedesigners estimated that the maximum settlement of the conduit would eventually be0.25 feet. However, by 1950, the conduit had settled as much as 0.75 feet. Much ofthis unexpected settlement was attributed to the loss of sand through the joints ofthe conduit. The first attempt to grout the joints was undertaken in 1950. Grout“takes” were not significant in 1950, except at joints 5-6 and 6-7, where 47 and98 cubic feet of grout, respectively, were pumped. There are 13 monoliths, withmonolith 1-2 being at the upstream transition and monolith 12-13 being at thedownstream end of the conduit. Monolith 6-7 is about 60 feet downstream of thecenterline of the embankment dam. Monolith 4-5 is about 10 feet downstream ofcenterline of dam. Additional settlements since 1950 have been less than 1 inch;however, in 1970 an attempt to grout the joints again was made due to sand beingeroded into the conduit through joints. Water and trace amounts of sand were againnoted coming from some joints in 1977, but attempts to grout the joints resulted inonly insignificant amounts of grout “take.” Since 1977, at least three attempts havebeen made to stop water coming from the joints using chemical grout. Grout takeswere significant only at joints 3-4, 5-6, and 6-7, where 410, 33, and 68 cubic feet ofgrout, respectively, were placed. Joints 3-4, 5-6, and 6-7 have caused the mosttrouble, but essentially all joints have had to be grouted at least once with eitherchemical grout or neat cement grout.The above background provides an introduction to the problem experienced in thefall of 2003. While in the conduit to replace the filler in the outer dove-tail portionof the joints, joint 6-7 broke lose and started to make sand at a significant rate (2 /3cubic yard in 1 hour). Even though it has been known since construction that therewere no waterstops in the lower portion of the conduit, it was thought that the jointsB-13