Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damslocated within a larger access conduit. This type of arrangement is discussed insection 3.1.1.1.• The valleys of the pipe corrugations may be too tight to get a good bondbetween the transducer and the steel. Transducers vary in size, but 1 /8 inch isabout the smallest diameter transducer.In performing an ultrasonic thickness survey, measurements should be taken atintervals sufficient to gather an adequate number of thickness measurement datapoints to help ascertain the true wall thickness of the pipe. Measurements should betaken circumferentially about the pipe. Particular emphasis should be placed ontaking measurements near the invert of the pipe, as this area is more susceptible towall thinning (metal loss) due to abrasion and corrosion. Factors to consider indetermining the amount of measured wall thinning are:• Pipe manufacturers tend to make the pipe wall slightly larger than the thicknessspecified to ensure they meet minimum wall thickness requirements.• Testing accuracies depend upon the test instrument and the transducers. Mostultrasonic thickness equipment has an accuracy of at least 2 percent. Accuracyis also dependent upon proper calibration of the instrument.If wall thinning is encountered, additional ultrasonic thickness surveys should beconsidered periodically (i.e., every 5 years, or more frequently, depending upon theseverity of the wall thinning, corrosion damage, etc.). A history of these wallthickness surveys may indicate the expected yearly decrease in wall thickness, ifuniform corrosion damage is occurring.A stress analysis of the pipe to determine structural adequacy is recommended, if theultrasonic thickness survey indicates a wall loss greater than 10 percent of its originalspecified value (see ASTM A 796 and the USACE’s Culverts, Conduits, and Pipes[1998a, pp. 4-4 and 4-5] for guidance on performing stress analysis for CMP). Adetailed wall thickness survey allows an accurate structural assessment of the pipe tobe performed. The results of the thickness surveys can be compared to theminimum acceptable plate thickness specified by design criteria to determine if thepipe has a sufficient safety factor and if corrective action may be required.Inspection of welds can also be performed by ultrasonic techniques. This processrequires the use of an angle beam transducer to detect flaws in the weld metal.Ultrasonic sound waves are transmitted through a transducer, which reflects theminto the weld area at an angle of 30 or 45 degrees. The angled reflection of thesound waves allows the flaw area to be detected and accurately sized. Theinterpretation of the results requires a great deal of experience, and should beperformed by someone with a level 2 certification in ultrasonic testing.258