Research on Construction Methods for Ultralarge Y-Shaped Tunnel Sections

Table 2

Comparison of advantages and disadvantages of forward and reverse excavation of bifurcated tunnel.

Construction method	Advantages	Disadvantages
Forward excavation	① The construction proceeds in a single direction and the construction process is relatively simple.	① The construction speed is slow for the large-span section of the main line. If construction proceeds from a large mileage to a small mileage, the construction process will be delayed.
	② The construction sequence proceeds from the large to small section at the junction, and the construction risk and difficulty are lower.	② It is not possible to realize two-way construction or eliminate delays in construction.
Reverse excavation	① Construction proceeds from small mileage to large mileage, followed by reverse excavation; therefore, the tunneling speed is not limited by the speed of constructing the large-span section of the main line.	① The construction direction changes many times, the construction process is complicated, and influence on the surrounding rock is unknown.
	② Two-way construction can be realized, which speeds up the construction process, to a certain extent.	② Mutation stress concentrations in the cross section of the pilot tunnel at the junction makes construction more difficult and higher risk
		③ The extension section of the pilot tunnel has a tunnel span of more than 20 m and there are certain construction difficulties and risks