Abstract: The stability of slopes at portals is often critical to a successful tunnel construction. This paper presents a slope stability analysis on the portals of a highway tunnel. The strength parameters are estimated throughout back analysis as well as engineering analog. The impacts of active and passive strengthening methods on the slope stability and its sliding influence zone are evaluated. The comparative study results indicate that, if the active strengthening methods are used, strength reduction and deformation zone in the sliding body are limited. Larger deformation and strength reduction can be found in the potential sliding body if the passive strengthening methods are utilized. The cost and effectiveness ratio is also low for the passive strengthening methods. The final design used active strengthening methods at both entrance and exit portals. The sheet pile wall was installed at the tunnel entrance and the prestressed ground anchors were installed at the exit portal. The tunnel operation period demonstrates that the slopes are stable and satisfactory.

Key words: highway tunnels, slopes at tunnel portals, stability mitigations, cost effectiveness ratio