Abstract: A model test is carried out to study the distribution of surrounding rock pressure and its mechanical characteristics based on open excavation section of Xinzuofang unequal-span double-arch tunnel of Lanzhou-Chongqing railway. Test results show that lateral pressure acting on the tunnel is smaller than the vertical stress of surrounding rock, and lateral pressure at the crown is smaller than that at the bottom of side wall, moreover, lateral pressure coefficient of small hole side is about 0.55 on average, while the big hole side is about 0.65. The stress of surrounding rock on the top of middle wall is bigger than that on the crown; the surrounding rock stress of big hole side is about 1.2 times that of the small hole. The tunnel is in a small eccentric bending state, and thrust stress in big hole is about 20% to 30% greater than that in small hole side; the destroy of tunnel structure firstly occurs at hance and invert near the middle wall in succession, finally the whole structure collapse; thus, the hance and invert near the middle wall are the place that should be focused on in the design and construction. Based on the above, the mode of surrounding rock pressure distribution is obtained finally. The results can directly provide guidance for the design and construction of Xinzuofang tunnel; what’s more, it’s also helpful to improving the design and construction concepts related to the unequal-span double-arch tunnel.

Key words: unequal-span double-arch tunnel, surrounding rock pressure, mechanical characteristics, model test