Under what circumstances should the external protective casing be set for pipe jacking, and the source should be indicated?

Pipe jacking is only a process, and it is not mandatory to use pipe jacking. I don't quite understand what you mean by casing. My understanding is that the so-called protective sleeves, such as steam pipes, must cross the railway and must not be excavated. If the pipe jacking is carried out directly, how to deal with the insulation layer? Pipe jacking technology will be adopted here. First, jack up a pipe, and then insert the pipe with insulation layer. If it is a construction unit, according to the construction plan, if it is a design unit, you should consider the cost and existing construction technology. Otherwise, the construction unit will smile with pictures.