Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams7.1.1 Design measures to prevent failurePreventing this type of failure requires conduits to be properly designed andconstructed with durable materials that are unlikely to develop defects. Chapter 2discusses important design considerations for conduit materials. CMP’s areparticularly susceptible to this type of failure. Joints in articulated conduits must bedesigned to accommodate movement to tolerable limits to avoid separation of thejoints.Once soil around the conduit begins to move into a defect in a conduit, either frombackward erosion piping or internal erosion, a serious problem exists. Quick actionis usually advisable. Sinkholes can develop, and if the defect is large enough, perhapsan embankment dam breach could even develop. The only reliable long termsolutions to preventing a failure or accident associated with this failure mode are torepair the defect in the conduit or renovate or replace the damaged section(s) ofconduit. Short term remedial measures like grouting seldom are adequate tocompletely stop the seepage from moving the soil particles. Several options foraddressing the defect in the conduit are available, including:• Sliplining the conduit• Removal and replacement of the conduit• Repair of the conduitChapters 12, 13, and 14 have more extensive discussion on methods for renovation,removal and replacement, and repair of conduits.Once a defect develops in a conduit, quick action is needed to prevent seriouserosion of the surrounding soils. At Tin Cup Dam, an emergency repair involvingsliplining a deteriorated 2- by 3-foot outlet works conduit (masonry pipe) with a16-inch diameter HDPE pipe was implemented to address sinkholes that hadformed above it (Luehring, Bezanson, and Grant 1999, p. 3). The annulus spacebetween the HDPE slipliner and the masonry pipe was grouted, but the grout waslater found to have floated the HDPE conduit, and sufficient grout was not injectedto fill the annulus space completely. Later inspections showed that cavities werepresent next to joints in the masonry pipe that were not filled during the groutoperations. Additional seepage problems became apparent soon after the repair.Consequently, extensive additional repairs were required the next year. This exampleillustrates how emergency repairs may avert an immediate threat, but may not be asuitable long term solution. This also illustrates that problems perceived to beassociated with a conduit may have additional causes. In the final repair of theembankment dam, evidence was found of construction problems, including use ofmaterials containing roots and other debris. Other poor construction practices and164