Abstract: The Suoertou landslide, a giant old landslide with a volume of 72.85×106 m3 in Zhouqu county, Gansu province of China, has been reactivated with a small rate since 1970’s. The slow-moving character indicates the typical rheological behavior of the landslide. It is well known that mechanical characteristic of slip zone plays an important role in landslide activities. In order to clarify the creep behavior of slip zone and the relationship between long-term strength and residual strength, a series of directed creep shear tests under residual state is employed. It is found that the creep behavior of residual slip zone is strongly dependent on normal and shear stress, with two critical shear stresses and two critical initial strain rates. Moreover, it can be found that the critical shear stress and strain rate, when the creep develops into accelerated stage, have a linear relationship with normal stress separately. Based on the linear relationship between critical shear stress and normal stress, it can be speculated that the shear stress condition of the Suoertou landslide has been in accelerated creep state already. Meanwhile, the relationship between long-term strength and residual strength is analyzed and compared. It is found that long-term strength of the landslide’s slip zone is slightly greater than its residual strength, and the magnitude of their difference is negatively correlated with normal stress.

Key words: soil at slip surface, residual creep test, long term strength, residual strength