A new type of sliding zone soil and its severe effect on the formation of giant landslides in the Jinsha River tectonic suture zone, China

Most of the major rivers in the eastern margin of the Tibetan Plateau are distributed along the tectonic suture zone, where many giant landslides are present. The catastrophic Baige landslide that occurred in 2018 developed in the Jinsha River tectonic suture zone. What is the geo-structure prone to sliding in the Jinsha River tectonic suture zone? How does it control landslide evolution? A special soft rock/soil, altered clay, is found in the Jinsha River tectonic suture zone, and this altered clay may play an important role in giant landslide initiation. To understand the mechanism of the strength weakening of the altered clay and its effect on the formation of giant landslides, taking the Baige landslide as a typical case study, detailed field surveys and laboratory experiments were performed, and the process and mechanism of the transformation of altered clay into sliding zone soil were analyzed. The results indicate that (1) the altered clay is rich in platy layer clay minerals, so it has strong water sensitivity and expansibility. Furthermore, it has significant strain-softening behavior and extremely low residual strength due to the directional arrangement of clay minerals. (2) The evolution process of the Baige landslide was controlled by altered clay and rock mass discontinuities. Altered clay developed along ophiolite discontinuities and formed the weak interlayer of the slope. Due to long-term gravity creep, intermittent rainfall, and earthquakes, the strength of the altered clay was continuously weakened, and the sliding zone gradually penetrated, which finally led to the failure of the Baige landslide. The results of this manuscript are helpful for understanding the evolution mechanism of giant landslides in tectonic suture zones around the world.