Striding-type quick excavation method for down steps of surrounding rocks of grade IV-V of dual-track tunnel

The invention provides a striding-type quick excavation method for down steps of surrounding rocks of a grade IV-V of dual-track tunnel and relates to the field of engineering construction. The methodsuccessively comprises the following steps: a down step blast area is divided into a left area and a right area which are symmetrical with an excavation center line of a working area face of a blasttunnel as the interface, shot holes are disposed on the left side of the excavation center line, and one-time drilling and in-place charging are carried out. One-time explosion initiation is carried out, and ventilation is carried out quickly after blasting; loading equipment and matched self-unloading equipment are carried out for quick tapping; danger elimination is carried out; an arc bridge isjointed in length, a roof truss frame is erected for each interval of a roof truss, a locking pin anchor tube is disposed, U-shaped rebar is used for welding reinforcement, and a reinforcement mesh is hung; and arc frames, loc