This paper describes the measures taken to stabilize the slope above a tunnel portal during construction. In Takasaki Tunnel, Chiba Prefecture, geology of the portal section is very poor, which consists of highly weathered mudstone. Vartical earth stitching bolts were installed prior to the tunnel excavation to minimize the influence of excavation on slope stability. The behavior of the ground and support works including earth stitching bolts were monitored during construction. The observed data indicate that vertical earth stitchng boltseffectively worked to reduce the subsidence of the ground and then to maintain slope stability.
