Abstract
A surface failure occurred after completion of the handy dynamic cone penetration test at 169 points in a 0.46 ha area, which was designed to clarify the ground characteristics related to the preceding collapse. The slip layer of these collapses coincided with the layer with a Nc value of less than 1 accompanied by a slight increase to the deeper layer. We named the layer located deeper than 50 cm, a "fragile layer". Out of 169 points surveyed 54 points had a fragile layer. All of them were located downward of the knick line, and 50% of their bottom boundary was 7080 cm deep. Meanwhile, 15 old scars 77 ± 6 cm in depth had no fragile layer, and were also traced only downward of the knick line. Comparison between the depth of the old scars and that of the bottom boundary of the fragile layer remaining on the slope suggested that the fragile layer plays an important role in the generation of these collapses. Shear strength of the fragile layer was 0.5 kN/m2, but it abruptly changed to 10.0 kN/m2 in deeper layers. These fragile layers may be missed if we use the conventional penetration test criteria, i.e., Nc-value of about 510 accompanied by a significant rise in the adjacent lower layer, but should be a key to detecting a surface failure in the future.
