Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiesof several very hard rains, there were no stability problems. About 160 feet of theconduit, entirely within the rockfill zone, was exposed.The drain, consisting of a coarse inner zone and a fine outer filter zone, was wrappedaround the outer surface of the exposed conduit. At the upstream end of theexcavation, an expanded filter diaphragm was placed from the eastern bedrocksurface to the western rock fill excavation surface. The diaphragm extended about5 feet above the crown of the conduit.The drain consisted of a coarse drainage fill (AASHTO No. 7 stone) to act as acarrying medium for collected seepage, and a finer sand zone (Vermont concretesand) to act as a filter to prevent the infiltration of fines from the surroundingrockfill materials. The 18-inch thick course zone was placed around the drainpipeand filled the irregular space between the overhanging eastern bedrock wall and theconduit surface. The fine filter zone was also a minimum of 18 inches thick, but wasbroadened to fill the excavation wedge along the western side of the conduit. Seefigure B-78 for cross section.The filter and inner coarse drainage fill zone were designed to meet filter criteria withrespect to each other, and the filter was designed to handle the silts within the finematrix material within the adjacent rock fill zone. The low-plasticity glacial silts,whether foundation silts, core material, or rock fill matrix fines, have always been theprimary concern with respect to internal erosion. A relatively fine filter material isrequired to meet filter criteria.The exact limits of the rockfill zone were not known, although some of the originaldrawing information suggested that the proposed excavation would completelypenetrate the rock fill to the contact with the shell zone. As the excavationprogressed, it became apparent that the rock fill zone would not be completelypenetrated as hoped. Therefore, it was decided to install a horizontal drain throughthe remaining wedge of rockfill to intercept seepage before it dispersed into therockfill zone. The drain was drilled within the confined space between the west sideof the conduit and the bedrock surface. Drilling was very difficult due to thepresence of an unknown concrete plug adjacent to the conduit. However, the drainwas completed, and seepage flows were intercepted near the upstream limits of therockfill zone. The discharge pipe from the horizontal drain was extended to thedownstream toe of the dam, along with the drainage pipe placed at the bottom of theconduit drainage zone along both the east and west sides of the conduit.Because of the confined spaces and irregular bedrock surfaces, the drainage and filterzones were placed in thin lifts and compacted with hand-operated vibratorycompaction equipment (figure B-79). Although the work was time consuming, theuniformly graded materials were very easy to place and compact. To accommodateB-109