The Tizicao landslide is located in Maoxian County, Sichuan Province, Southwest China (Fig. 1), with geographical coordinates of 31°53′14.89″N and 103°40′51.12″E. It lies on the right bank of the valley of Minjiang River and faces the Shidaguan Town on the left bank of the river, with National Highway G213 passing by its foot.
The Tizicao landslide is located in the lower part of the right bank slope of the Minjiang River. Its rear and toe are at an elevation of 2415 m and 1950 m (Fig. 2a), respectively, and thus the relative elevation difference reaches 465 m. The relative elevation difference between the toe of the landslide and the lower riverside of the Minjiang River is 220 m. The Tizicao landslide is therefore determined as a high-locality landslide.
The Tizicao landslide is approximately 680 m long and 570 m wide (Fig. 2b), with an average thickness of approximately 45.1 m and a volume of 1388.2 × 104 m3. On July 3, 2017, a local collapse occurred in the front of the landslide body in the north, with a volume of approximately 6.0 × 104 m3, and it blocked the Minjiang River for several hours. Fortunately, there were no casualties. In case of large-scale sliding, this landslide will directly threaten the lives of 30 people on the slope body, even seriously threaten the lives of 113 people in the Shidaguan Town below the landslide, and will directly strike more than 30 buildings and the National Highway G213 at a distance of 2 kilometers. Meanwhile, the Minjiang River may be blocked to form barrier lakes and induce secondary floods, which will endanger the towns, villages, and infrastructures along the Minjiang River and directly threaten the properties of about RMB 86 million. Nonetheless, the landslide gradually became stable by the end of 2017 after multiple deformations and partial collapses in 2013 − 2017, owing to the anti-sliding of local locking segments in its front.
The site area is in the shape of a round-backed armchair (Fig. 2), high in the west and low in the east. The aspect and mean gradient of the failed slope are 44°−78° and 30°−40°, respectively, with a shape of steps vertically. The lower riverside of the Minjiang River to the upper section at an elevation of 1900 − 2000 m is a straight steep slope with a slope of about 70°, where bedrocks are mostly exposed. The failed slope is mainly distributed in the section at an elevation of 2000 m to the rear of the landslide, with a mean slope of about 25°. The alternate steep and gentle steps, and the development of free-face conditions provide favorable topographic conditions for the development of the Tizicao landslide.
The average monthly rainfall is about 134.7 mm (2012 − 2018), with a peak value of 400 mm. The rainfall mainly concentrates from June to October, accounting for 85% of the annual rainfall. As shown in Fig. 3a, the historical monthly rainfall and deformation data of the landslide show that the deformation mainly occurred from June to September, especially the deformation quickly developed in September. The daily rainfall statistics and the curve of typical surface displacement monitored from June 1 to October 30 in 2017 are shown in Fig. 3b. The violent landslides occurred after 20 days of heavy rain and 10 days of short-term rain, with the maximum displacement reaching 1210 mm. It can be concluded that the landslides lag behind rain.
The Tizicao landslide mainly consists of thin silty clay on the surface and broken phyllite (Q4del) below, while the sliding bed mainly comprises the carbonaceous phyllite of the Devonian Weiguan Group (Dwg2). The phyllite is greyish-black and greyish-yellow and is interbedded with a small number of quartz veins locally. The phyllite layers show an attitude of 197°∠71°. The carbonaceous phyllite is weak and broken, indicating that it has poor physical and mechanical properties, with a potential risk of collapse, slide, and deformation during the rainy season.
The massive amount of developed broken carbonaceous phyllite in the slope body is closely related to active structures. Shidaguan Town lies in the middle part of the NS-trending seismic belt, where intensive earthquakes in history developed. Since the beginning of the 20th century, it successively experienced the Diexi earthquake in 1933 (Ms 7.5), the Songpan-Pingwu earthquake in 1976 (Ms 7.2), the Wenchuan earthquake in 2008 (Mw 7.9), and the Jiuzhaigou earthquake on August 8, 2017 (Ms 7.0; Fig. 4). Among them, the Diexi earthquake, with a hypocenter depth of 6.1 km and an epicenter of merely 1.3 km away from the landslide center, caused a variety of disasters, such as massive rockfalls, landslides and debris flows, and devastated the former Diexi Town (Wang et al. 2019a). The Diexi earthquake shattered numerous mountains due to its epicentral intensity up to 10 degrees, thus resulting in broken rock masses on the slope surface. In addition, a great number of secondary folds (Fig. 5a) develop in the bedrock of the site area and clear corrugations are visible on the steep wall below the front landslide toe, and fractured rock masses, secondary folds, hollow structures, and argillaceous fillings (Fig. 5b) can also be observed at the slope foot. Many holes or empty spaces reserved in the fractured rock masses after earthquakes, formed the hollow structures, some of which were filled with argillaceous materials. Therefore, earthquakes and tectonic movements are the critical factors causing broken phyllite masses in the landslide body.