Prediction of the failure of a slope comprising weathered granite soil under multistep excavation based on multidimensional displacement measurements

Measurements of slope displacements can be effective tools for the early warning of the collapse of slopes under excavation in construction projects in mountainous areas, while evaluating of instability based on measured displacement has not yet been accomplished for slopes under excavation. Measurements of displacements on sandy soil slopes under multistep excavation were made, and the measured data were analyzed to determine the characteristics of creep deformation of the slope during and after the excavation and to establish a procedure for evaluating instability of the slope under excavation in this paper. The following facts were derived from the examination of the measured data. Displacement was generated significantly in the latter stage of excavation, and it was generated not only during excavation but also after excavation. The ratio of the displacement after excavation to that from the start of the excavation to the start of the next excavation became larger as the displacement developed immediately before failure. It indicates that creep displacement after the excavation was more significant at latter excavation just prior to failure. The normal displacement converged to constant as the shear displacement increased under a steady state immediately before the failure in direct shear conditions, and the normal displacement can be an indicator of the instability of the slope. The normal displacement cannot remain constant when the inclination of the slope surface is different from that of the slip surface in the excavated slope in this paper. Different indicators are necessary in this case. The angle α between the slope surface and synthetic displacement (RD) on the slope derived from measured data was introduced instead of normal displacement. The synthetic displacement (RD) on the slope indicated the scalar of the synthesis of the displacements normal and downward to the surface of the slope and surface displacement. It converged to constant after excavation immediately before failure, and shear displacement showed an accelerative increase for constant α. This indicated that the angle α could be an indicator of the instability of the slope. This result showed that the angle α being constant indicated that the stress condition was almost in failure and the displacement increased acceleratively. Further examinations based on the measured data on other slope under excavation should be necessary for confirming the results in this paper based on only a case of experiment.