Abstract: When shallowburied largecrosssection tunnel crossing underneath operational highway in soft rock stratum, problems such as over settlement, collapse, and hidden danger to highway easily occur. Hence, a case study is conducted on the #4 inclined portal of the Tongzi tunnel crossing underneath an operational highway in soft and waterrich strata in Guizhou, China. Then, comprehensive reinforcement inside and outside tunnel is proposed, which include: (1) The temporary steel bridge guarantees the smooth flow of the highway and scatters concentrated load. (2) In the shallow section of the tunnel portal, surface grouting is carried out and the surrounding rock stability is improved by blocking the water paths in the cracks. (3) The pipeshed support, combined with pregrouting with a doublelayer micropipe, and the application of bench reservation core soil are used to ensure the surrounding rock stablility during the excavation with a milling machine. (4) A new type of locking anchor is proposed to enhance the stability of the anchoring rock. Finally, the effectiveness of the scheme is confirmed by monitoring and measurement. Compared with the conventional mountain tunnel portal construction plan, the methods recommended can effectively control the complex terrain tunnel settlement and overbreak, ensure the smooth flow of surface traffic, reduce the consumption of shotcrete, and avoid collapse, roof collapse, landslides, and other geological disasters.

Key words:  , shallowburied largecrosssection tunnel, highway undercrossing, tunnel portal construction, soft and waterrich strata, settlement control, comprehensive reinforcement inside and outside tunnel