Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsDepending on the model, camera-crawlers used in conduits with very smalldiameters (about 4 to 14 inches) have cameras with some pan, tilt, and zoomcapabilities, a wide range of tether pulling capacity (200 to 1,000 feet), and somesteering capabilities. Camera-crawlers used in conduits with diameters of 15 inchesor larger are steerable, have a greater cable tether-pulling capacity (500 to 1,500 feet),and have cameras that can provide a wider array of optical capabilities, including pan,tilt, and zoom. As the technology of CCTV inspection equipment advances, greatertether lengths and optical capabilities will become available. Actual tether limitsobtainable in the field, vary greatly, depending upon a number of factors, such asconduit diameter, bends, invert slopes, and existing invert conditions, such assediments, mineral encrustations, and bacterial growths.In large diameter conduits, the video camera can be attached to a scissor mechanismmounted to the transport vehicle. The scissor mechanism, controlled by theoperator, can raise or lower the video camera as needed for inspection. In addition,the video camera usually has a high powered zoom, which can be used to providecloseup views of areas that might be difficult for the transport vehicle to get near.These features allow examination of very large conduits with diameters as large as40 or 50 feet.If required, some models of camera-crawlers allow for the attachment of retrievaltools, such as alligator clamps, grippers, and magnets. These tools can be used toremove light debris or damage. The attachment of any type of retrieval tool willrequire additional clearance within the conduit to operate the retrieval tool. Somemodels of crawlers have robotic cutters attached to them. These cutters can be usedto remove debris or protrusions in concrete, steel, or reinforcement. Most cameracrawlersystems are portable and can be carried to conduit access locations(figure 130). The use of an all-terrain vehicle (ATV) may be beneficial for transportof equipment in difficult access areas.Sometimes the conduit is too small and a transport vehicle cannot be used, orobstructions/invert conditions exist that prevent the transport vehicle fromtraversing the conduit. For these types of situations, a small color video camera(1.5 to 3 inches in diameter) with maximum pressure depth ratings up to 1,000 feetof water can be used. Figure 131 shows an example of this type of video camera.This video camera can be attached to metal or PVC poles (commonly referred to aspush poles) and manually pushed up the conduit. Push poles are normally used forstraight sections of conduit. The use of push poles for advancement is generallylimited to about 400 feet of conduit length. If bends exist in the conduit, a flexiblesnake device (spring steel wire, coiled wire, or flexible polypropylene-jacketedfiberglass push rod) can be used instead of the push poles. A coaxial cable connectsthe video camera to a video cassette recorder and television monitor. Snake devicesare generally limited to about 75 to 200 feet of conduit length.224