Construction method for large cross-section tunnel passing through longitudinal hard-soft-hard strata

The invention discloses a construction method for a large cross-section tunnel passing through longitudinal hard-soft-hard strata. The construction method comprises the following steps that boundariesof the hard-soft-hard strata are determined; upper steps of the two hard rock strata are excavated by blasting, rock pillars are reserved separately, excavation areas are reinforced with feet-lock bolts, and primary supports of the upper steps are constructed separately; the soft stratum in the area where the upper steps are excavated is reinforced; the two rock pillars are removed, and temporarysupports are constructed; an upper step of the soft stratum is excavated, and primary supports are constructed and a bottom reinforcing layer is reserved; lower steps of the two hard rock strata areexcavated by blasting, rock pillars are reserved separately, excavation areas are reinforced with feet-lock bolts, and primary supports of the lower steps are constructed separately; the soft stratumin the area where the lower steps are excavated is reinforced; the two rock pillars and the temporary supports are removed, a lower step of the soft stratum is excavated, bottom primary supports, an inverted arch and a second lining are constructed, and construction joints are reserved; and the construction steps are repeated until the tunnel construction is completed. According to the construction method for the large cross-section tunnel passing through the longitudinal hard-soft-hard strata, the construction risk is reduced greatly, and social and economic benefits are remarkable.