MONITORING THE PROCESS OF GROUND FAILURE IN REPEATED LANDSLIDES AND ASSOCIATED STABILITY ASSESSMENTS

Field monitoring of two repeated landslide areas, namely the Sodechi a nd the Yonaihata landslide areas on the island of Honshu, Japan, has r evealed that the spread of ground failure occurred as in normal landsl ide, even though these landslide events were repeated slides in clayey material. A rapid increase in surface displacement (the beginning of third-stage creep) occurred simultaneously with an increase in strain in the ground (the cause of local failure along the sliding surface), resulting in large, continuous landslide movements. Thus, an entire sl ope began to collapse when the full length of the failure surface beca me connected. Ring-shear tests of landslide clay were conducted to det ermine three kinds of shear strength parameters: (1) fully softened st rength parameter phi' (parameter for peak strength of remolded clay); (2) ''specimen-separated strength'' parameter phi'(ss) (newly proposed in this study); and (3) residual strength parameter phi'(r). Stabilit y analysis revealed that phi'(ss) was the most appropriate shear-stren gth parameter to express slope stability in the ground failure transmi ssion stage. Thus, this result provides a first step in applying the m echanism of ground failure transmission to the problem of slope stabil ity by defining the specimen-separated strength parameter phi'(ss). (C ) 1998 Elsevier Science B.V. All rights reserved.