Partial excavation method for steel frame and dike integrated support for in-situ extension of large-section tunnel

The invention provides a partial excavation method for a steel frame and dike integrated support for in-situ extension of a large-section tunnel. The partial excavation method involves an existing tunnel, an expansion section and backfill which is arranged on the portion from the bottom face of the existing tunnel to the arch foot, wherein the expansion section comprises an upper excavation part and a lower excavation part, the upper excavation part is arranged above the horizontal plane of the backfill, and the lower excavation part is arranged below the horizontal plane of the backfill. Whenthe existing tunnel is extended, by backfilling the exiting tunnel, the surrounding rock stress and the surrounding rock disturbance range are effectively controlled; according to different sizes oftunnel excavation section areas, the excavation areas can be reasonably utilized to reduce the rock pressure of the excavated tunnel; a temporary steel frame vertical support of an upper excavation part and a lower dike form a