Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsBecause of water demands and minimum flow needs downstream of theembankment dam, and other concerns, the original investigation program did notinclude dewatering and inspection of the outlet works conduits. Based on the resultsof the observed grout loss, however, it was determined that the outlet worksconduits must be dewatered and inspected to further evaluate seepage and safetyconcerns, including the possible cause of the grout loss. Following dewatering, thecondition of the conduits was thoroughly assessed in a two-phase program. Duringthe first phase, a nonintrusive investigation was performed using geophysical impulseresponse to evaluate support conditions, impact echo to evaluate the condition ofthe concrete, and structural condition surveys (mapping of cracks and locations ofseepage discharging from the cracks, inspecting the general condition of theconcrete, and surveying the conduit invert along four upstream-to-downstreamprofiles). During the second phase, information from the nonintrusive investigationwas used to design a program of intrusive investigations, including (1) drillingthrough the concrete to inspect the condition of the subgrade and backfill aroundthe conduits including the presence of voids around the conduits, and (2) installationof vibrating wire piezometers to measure water pressures and estimate seepagegradients along the bottom and side walls of the conduits at different locations alongthe conduit profile.Small voids were found beneath the conduit at a number of locations near observedcracks with seepage discharge. The lost grout was not found. Based on theadditional information, final design and construction of corrective actions wereinitiated. The USACE was going to remove and replace the outlet works within 5 to10 years as part of a comprehensive flood control project. The design of correctiveactions took into consideration the limited planned life expectancy of the outlet.Corrective actions included in the rehabilitation design included both primary andsecondary “lines of defense.” Primary lines of defense included (1) installation of sixrelief wells into the foundation soils to collect and safely discharge foundationseepage—these wells were installed about 15 feet downstream of the roller gates anddischarged up through the floor of the conduit through a flap valve; and (2) sealingof all cracks in the conduit floors and walls with an elastic filler that would adhere tothe concrete and expand and contract during seasonal changes in the width of thecrack. The secondary lines of defense included (3) grouting around the base andexterior of the outlet conduits to seal existing voids and increase the effectiveness ofthe relief well system (see figure B-45); and (4) construction of a new floor with afiltered underdrain system.While the investigation program found voids around the conduits and unfilteredseepage water discharging through cracks in the conduit floor, instrumentation alsoshowed locations within the embankment and foundation with relatively high waterpressures and seepage gradients, particularly near the discharge end. Correctiveactions would not be performed on the basin downstream of the discharge. Toaddress concerns related to potential high discharge gradients in the downstream toeB-52