Permanent and temporary combined roundabout tunnel structure for tunnel to pass through karst cave and construction method

The invention provides a permanent and temporary combined roundabout tunnel structure for a tunnel to pass through a karst cave and a construction method, and belongs to the technical field of tunnel engineering. One end of the roundabout tunnel is communicated with a starting section, the other end of the roundabout tunnel penetrates through the upstream of the karst cave and then extends to a terminal section, a tunnel bottom box culvert communicated with the roundabout tunnel is arranged in the terminal section, the other end of the tunnel bottom box culvert is communicated with the downstream of the karst cave, and a water passing hole is formed in the side, facing the upstream of the karst cave, of the roundabout tunnel in the karst cave. And a plug is arranged in the part, between the upstream and the starting section of the karst cave, of the roundabout tunnel. A roundabout tunnel is arranged on the outer side of the karst cave to serve as a temporary construction channel, a front tunnel construction w