Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 8—Potential Defects Associated with Conduitsand have differing products of corrosion. Soil-related bacterial corrosion producesoxidation scale, which is active in organic, poorly drained soils of nearly neutral pH.This scale is usually black, but upon being exposed to aerated conditions in conduits,becomes rust colored. Water-related bacterial corrosion produces nodular oxidation,which exists on pipe surfaces associated with a water source of nutrients. Localperforations on the pipe invert characterize nodular oxidation. Nodular oxidationresults from sulfate-reducing bacteria activity.Polyethlene plastic pipe is not subject to galvanic action and will not corrode.Naturally occurring water and soil conditions will not affect the pipe.8.2 Poor design and constructionGood design and construction practice can extend the service life of a conduit.However, poor design and construction practice can greatly shorten it. Much of thefollowing discussion was adapted from USACE’s Evaluation and Repair of ConcreteStructures (1995b, pp. 3-1 to 3-14) for reinforced cast-in-place concrete. Some of themost common areas where poor design and construction practice can affect conduitsare:• Poor design practice.—Design errors may be divided into two general types: thoseresulting from inadequate structural design and those resulting from lack ofattention to relatively minor design details. Common design errors include:1. Inadequate structural design.—Inadequate structural design exposes theconcrete to greater stress than it is capable of carrying, or greater strainthan its strain capacity. This may result in excessively high compressivestresses and appear as spalling. Similarly, high torsion or shear stressesmay also result in spalling or cracking. Also, high tensile stresses will resultin cracking. To prevent this from occurring, the designer must complete athorough and careful review of all design calculations. Any renovationthat makes use of existing conduit must be carefully reviewed.2. Poor design details.—While a conduit may be adequately designed to meetloadings and other overall requirements, poor detailing may result inlocalized concentrations of high stresses in otherwise satisfactory concrete.These high stresses may result in cracking that allows water to access theinterior of the concrete. In general, poor detailing does not lead directlyto concrete failure; rather, it contributes to the action of one of the othercauses of concrete deterioration described in this chapter. A frequentcause of cracking in conduits is improperly spaced joints. Thermalcracking can also result in conduits where joint spacings are too long orare not provided in the conduit to accommodate for changes of length. Ingeneral, all of these problems can be prevented by a thorough and careful187