A duct is a length of underground piping (usually made of PVC), which is used to house electrical and communication cables. Ducts can be laid underground using trenchless techniques, just as piping is laid, however they are not used to carry product, but cables. The use of ducting enables construction crews to install cables using trenchless technology, which would not be possible otherwise.
Trenchlesspedia Explains Duct
To install underground cables individually would require digging open trenches and the potential for cable damage. However, with the use of ducts, numerous cables can be pulled through a duct, which has already been laid in the ground. Ducts can be laid alongside each other to create capacity for more cables, and they can also be protected with the use of concrete sheaths so that cables will not be damaged through other construction activities. If a cable is faulty, a new cable can be pulled through the duct in its place. Spare capacity within the duct also creates the opportunity for new cables to be added later without any open trenches.