Abstract: Serious potential risks, such as high geostress, weak surrounding rock, fault and fracture zone, secondary geological disasters, etc., are existing in the tunnel supporting structure in mountainous areas in southwest China during construction phase. As a result, cases analysis are carried out on Ya′anKangding Highspeed Tunnel and WenchuanMa′erkang Highspeed Tunnel so as to analyze the mechanical behaviour characteristics of tunnel supporting structure under different potential risks based on site monitoring data. The analytical results show that: (1) The contact pressure and steel arch stress of tunnel in weak surrounding rock with low strength and selfbearing capacity are much higher than those of tunnel in common surrounding rock, and the loadsharing ratio of secondary lining increases significantly. (2) When the tunnel passes through fault and fracture zone, the stabilizing time for supporting structure stress is quite long, and the mechanical behavior of the supporting structure presents threestage evolution law, i.e. falling quickly in the early stage, slowly decline in the middle stage, and becoming stable in the final stage. (3) As for portal section of tunnel passes through loose deposits, the stability of slope is easily disturbed, the mechanical behavior of the supporting 〖HJ1.5mm〗structure has significant asymmetric characteristics, and the surrounding rock pressure mainly concentrates on the deepburied side. (4) The geostress and rock properties jointly control the stability of surrounding rock and the mechanical behavior of supporting structure; the high geo stress hard rock tunnel also has certain rheological time effect, but due to the greater strength and stability of the hard surrounding rock, the supporting structure is relatively less stressed and the safety reserve is higher. (5) The surrounding rock pressure and structural force of soft rock tunnel with high geostress increase significantly, and the mechanical behavior of the supporting structure shows remarkable rheological characteristics during the construction period, even after about 200 days of excavation, it still growing slowly.

Key words: southwest mountainous area, tunnel engineering; construction period, surrounding rock deformation, supporting structure; mechanical behavior