Construction method for converting double-arch tunnel and small-clear-distance tunnel in underground transverse passage

The invention discloses a construction method for converting a double-arch tunnel and a small clear distance tunnel in an underground transverse passage, which comprises the following steps of: 1) a first hole is excavated in the outer side wall of the right line of a double-arch tunnel section, additionally opening the transverse passage when the first hole of the double-arch tunnel section is excavated to the junction of the double-arch tunnel section and the small clear distance tunnel section, and performing small clear distance tunnel section underground excavation in the transverse passage, meanwhile, the double-multi-arch tunnel is reversely excavated, and the double-multi-arch tunnel is caught to go out of the tunnel; (2) the double-arch 1 # hole is firstly sealed into a ring, anda transverse channel is excavated in the vertical side wall of the double-arch 1 # hole; 3) a transverse passage side wall arch frame is broken in the pilot tunnel range; (4) in the bottom falling process of the transverse ch