Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 10—Evaluation by Geophysical and Nondestructive TestingFigure 146.—Ultrasonic thicknessgauge for measuring metal thickness.• Thickness measurements can be taken without removing coating (with theexception of asphalt and concrete linings) on the interior of the pipe.• Measurements can be taken on the exterior surfaces for exposed pipe while thepipe is in operation (i.e., full of water).• Thickness measurements taken on the interior of the pipe help determinewhether corrosion is occurring on the outside of the pipe shell for buried pipeor pipe encased in concrete.• Measurements of paint thickness and oxidation/rust thickness can also be takensimultaneously during the plate thickness survey.However, there are some limitations in performing a survey of this type. Factorsthat may prevent obtaining readings for specific sections of the metal or steel are:• Water on the invert.• Measurements cannot be taken for certain interior coatings, such as asphalt andcement-mortar lining.• Rough, uneven, corroded surfaces can limit the bond between the transducerand the steel pipe, thus preventing thickness readings.• Normally, the ultrasonic thickness survey is performed by man-entry into thepipe. However, for inaccessible pipes the thickness survey can be completedusing a specially equipped pig that moves through the pipe. In some cases, ifthe exterior surface of a pipe is accessible, the survey can be completed withoutrequiring man-entry into the pipe. This situation is applicable when the pipe is257