The Deformation and Instability Characteristics of Fractured Landslides during Typhoon-Triggered Rainstorms: Observations from an In Situ Field Experiment

Fractures are the dominant conditions for rainfall infiltration into slopes, which can aggravate the instability of landslides. However, few studies have been conducted to analyze in detail the instability and deformation characteristics of creeping landslides with fractures. In view of this, this study investigated a landslide in Fu'ao Village, Wencheng County, Zhejiang Province in China to determine how fractures affect landslide deformation and instability during heavy rainfall through an in situ simulation experiment. In the experiment, three consecutive days of rainfall were set up based on Typhoon Megi in 2016, and two conditions were simulated, namely, rainfall + water filling fractures and rainfall + filled fractures (also referred to as the first and second conditions, respectively). The changes in the volumetric water content (VWC), pore water pressure (PWP), and deep displacement of the landslide at a depth of less than 5 m under the two conditions were observed using field monitoring instruments. The simulation results were as follows: (1) The volumetric water content of the shallow soil showed a more sensitive time-varying response to rainfall, while that of the soil at a depth of more than 200 cm showed a lagged response to rainfall, with a lag of about 10 h, which decreased significantly in the case of the unfilled fractures; (2) Under the first condition, the pore water pressure at different depths showed almost the same changing amplitude of 5 kPa or less. Under the second condition, the pore water pressure increased significantly with depth during the rainfall, with the changing amplitude reaching 30 kPa; (3) The displacement of the borehole equipped with the inclinometers near the front of the slope was higher than that at the borehole equipped with the inclinometers near the rear of the slope. The displacement under the first condition was up to 6 mm, which was significantly greater than that under the second condition. Therefore, fractures have significant effects on the instability of landslides induced by typhoon-triggered rainstorms, and one important measure to prevent and control this type of landslides is to fill fractures in the landslides in time.