Abstract: This paper presents a case history of a local deformation and failure of a daylighted slope in Shenzhen during the construction period. The major potential factors triggering the local instability of the slope are analyzed through on-site geological survey, combined with investigation and design data, on-site construction procedures and monitoring data. On this basis, the Phase2 numerical model that can reflect the characteristics of the slope, such as rock stratum, the outfall ditch and the supporting structure, is established. The slope construction process under four different conditions is analyzed, and the potential triggering mechanism of the local instability of the slope is studied. The results show that the outfall ditch is the main inducing factor of the local deformation and instability of the slope. The potential triggering mechanism is that the excavation of the outfall ditch leads to the weak sliding surface at the rock stratum, and the water in the catchment leads to the softening of the rock mass at the foot of the slope, resulting in the failure of the daylighted bedding plane in the fourth-grade slope. When there is no water the catchment after the excavation of the outfall ditch, the safety factor of the slope without support structure is 1.03, which is in an unstable state. After the support is applied, the safety factor increases to 1.09, which basically meets the stability requirements. When the water in the catchment is presented, the safety factor is 0.85 when there is no support, which is in an unstable state. After the support is applied, the safety factor is 0.99, which is in a critical slip state. The ground anchor with supporting beam can effectively prevent the local slope instability induced by the drop ditch when there is no water in the catchment, but the supporting effect is not good when the water in the catchment slope softens.

Key words: Outfall Ditch, Daylighted Slope, Stability, Rock Surface, Numerical Simulation