Residual Shear Strength Properties of the Soil from Landslide Area, Using Ring Shear Test

Landslide research, specially in Japan, is in good progress in recent years. Because of the environmental consciousness and disaster mitigation strategy, many geo-technical researchers of the world are involved in the research related to landslide. Although there are many slope stability equations developed by veteran geo-technical engineers in the past, they were very difficult to apply directly in the landslide analysis. Chandler et. al. developed the back analysis method and the stability analysis for landslide area become popular in the world. However, the residual shear strength has been adopted arbitrarily in this method since the beginning. Along with the development of soil mechanics, the concept of residual shear strength has also been developed and many geo-technical professionals have developed the machines to conduct the residual shear test. Ring shear test machine was one of the best out-come of that development. Bishop put one more step by designing new ring shear testing machine, suitable for landslide clays, in 1971. After that there have been series of developments to design the ring shear machines fitting the specific objectives. Portable machine by Bromhead and high speed machine by Sassa are few examples. Although new machines have been developed, special care has not been taken to test the real landslide sliding surface soil in un-disturbed condition. It is extremely difficult to get the undisturbed sliding surface soil, recently, as most of the drainage wells do not penetrate the sliding surface. Almost all of the geo-technical professionals agree on the dependency of residual shear strength on the clay minerals. If the dominant clay mineral in the main scarp and sliding zone soil is identical, then the residual shear strength might be similar. This concept can solve the problem of soil testing on less complicated landslides as the test can be done on main scarp soil. This will add one factor for back analysis too. The construction of drainage well is extremely difficult in these days, even in developed countries, because of its high cost. In this situation, the residual shear strength of main scarp soil can give good idea to the planners. This research is mainly focused on the relationship of residual shear strength of main scarp and sliding surface soil, the importance of clay fraction during the shearing process and dependency of residual shear strength with other strength parameters. Five landslides have been selected in Niigata Prefecture, Japan. Undisturbed soil samples have been collected from those landslides. The ring shear test result of those samples has been compared with that of main scarp soil of the same landslide block. The result has been found to be very close. Likewise, shearing has been found mainly on the clay fraction during multi-stage ring shear test. This shows the importance of clay minerals in residual shear strength of the soil. Very good relationship has been found between the residual shear strength and liquid limit. The residual shear strength properties in various scarps of the same landslide area have been found to be identical. This paper describes the test result and its analysis in detail