Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 10—Evaluation by Geophysical and Nondestructive Testing10.7 Mechanical and sonic caliperCalipers can provide information about the internal diameter of the conduit(USACE, 2001c, p. 32). Calipers are used for detecting any changes or defects thatcause changes in interior dimensions, such as pits, holes, cracks, deformations,damage, or corrosion. Caliper measurements are made using mechanical, sonic, orultrasonic methods. For the mechanical method, metal “feeler” instruments contactthe inside wall of the conduit. The positions of the feelers are sensed electronicallyand recorded on a printout. The sonic or ultra sonic calipers use transducers thatbeam a pulse to the conduit wall. The pulse is reflected by the wall back to thetransducer and interpreted based on its time of transit. Typically, calipers are used inconduits with diameters 18 inches or larger. Calipers are deployed using a wire cableor smart pig. Some caliper tools can be used underwater.10.8 RadiographyThe radiography method encompasses any type of penetrating radiation, such asX-rays, gamma rays, beta particles, neutron beams, or proton beams. Radiography isuseful for detecting cracks, voids, and defects, or for viewing the internalcomposition of a conduit. Differences in thickness and density are easily measuredand can be seen on a screen or recorded onto film. Although radiography isgenerally quick, efficient, and accurate, the complex nature of the equipment, costsinvolved, training, and certification requirements often limit its use.10.9 Surface hardnessThe rebound hammer and penetration resistance methods are quick, simple to use,and inexpensive to perform. These methods can be performed by field personnelwith a limited amount of training and instruction. These methods are useful inassessing the general quality of concrete and locating areas of poor quality concretein a conduit. However, these methods do have a number of limitations, includingimprecise measurements of the in-situ strength of concrete. The rebound hammerand penetration resistance methods require man-entry access into the conduit.10.10 Conduit evaluation by destructive testingAlthough not utilized often in conduits, destructive testing can be used to gathermore data. Concrete cores can be cut from selected locations to obtainrepresentative samples. Samples are often taken from deteriorated areas and fromgood quality concrete for comparative purposes. A petrographer can examine the259