Abstract: An underground interchange tunnel is proposed to be built at the intersection of Lu′ao Road and Haicang Shugang channel in Xiamen. The tunnel bifurcation directly transitions from a singlehole fivelane largespan section to a small cleardistance section of the main tunnel with three lanes and a ramp with two lanes. The directtransition method is adopted without a multiarch section. The maximum excavation span reaches 30.46 m, and the maximum excavation area reaches 450.41 m2. The design of the construction and supporting structure of the bifurcated tunnel suffers from many difficulties. First, the direct transition of the bifurcated tunnel from the longspan section to the small cleardistance section results in complicated process transformation during the construction process as well as in difficult control of the construction mechanics. Second, the surroundingrock stress of the bifurcated tunnel is affected by the clear distance of the tunnel and its crosssectional form. Thus, triaxial test and numerical analysis are conducted to investigate the construction mechanics of the junction between the largespan and small cleardistance sections and the small cleardistance tunnel. The following conclusions are drawn. (1) The granite samples collected on site are elastic and brittle rocks. The strength and stability of the rock in a triaxialstress state are much greater than those in a biaxialstress state. The elastic modulus of the granite sample is positively correlated to the confining pressure. The HoekBrown and MohrCoulomb strength criteria are used to modify the triaxial test results to obtain the physical and mechanical parameters of the rock mass. (2) The surrounding rock at the junction between the largespan and small cleardistance sections will ultimately be sealed. It is recommended to use highperformance shotcrete and an Ibeam support to ensure that the surrounding rock will enter the triaxialstress state as soon as possible. (3) Tunnel excavation in the small cleardistance section mainly affects the crosssectional displacement at the end of the largespan section along the vertical direction, and it causes different degrees of internalforce changes at the end of the largespan tunnel. The internal force changes at the left and right arch feet are the most dramatic. The influence range of the maintunnel construction on the internal force and displacement of the largespan tunnel is 16 m behind the tunnel face, and that of the ramp is 12 m behind the tunnel face. (4) During the construction of the small cleardistance section, the optimal longitudinal excavation distance between the main and ramp tunnels is 16 m. When the clear distance between the main and ramp tunnels is more than 6 m, they can be designed and constructed as independent single tunnels.

Key words: highway tunnel, small cleardistance tunnel, large span, construction mechanics, underground interchange, triaxial test, numerical simulation