This Recommendation: ? makes a classification of different methods for inspecting and repairing underground ducts; ? describes both trenchless methods and conventional methods; ? is limited to the methods for underground ducts in which no cables are installed; ? focuses on methods for underground ducts that have single-way duct unit systems and are made of plastic materials such as polyvinyl chloride (PVC) and polyethylene (PE); ? deals with pipes having diameters ranging from 90 to 110 mm.