C.steel sleeve: in case based on process request, if the length is more than 30m, the steel base support could be used through chief tube forepart &removed as repeated usage purpose.

• C.；钢套管 套管为钢制套管时，根据施工工艺要求，若套管长度大于30米，主管穿越前端可使用钢制托管支撑，当穿越主管出套管后，托管支撑拆掉后可供重复使用。