Transparent soil model test of a landslide with umbrella-shaped anchors and different slope angles in response to rapid drawdown

Umbrella-shaped anchors, as new stabilizing structures, have been increasingly adopted in engineering practice due to their simple structure, ease of installation, and high load capacity. Reservoir operation, especially during rapid drawdown, affects the long-term stability of reservoir bank slopes as this process affects the stress, seepage and deformation characteristics of the water-level fluctuation (WLF) zone. Thus, the characteristics of the deformation and failure mode of WLF zones reinforced by umbrella-shaped anchors under rapid drawdown conditions are important for the design and construction of anchoring measures. Five slope-anchor scale models based on the Yingpan landslide were established as transparent soil models considering different slope angles and drawdown rates. Displacement, velocity and macroscopic deformation data were obtained using particle image velocimetry (PIV) technology. The results indicated that the deformation process of both reinforced and unreinforced slopes under rapid drawdown conditions could be divided into four phases: initial, shallow sliding, multisliding and stable phases. In the anchor-reinforced models, loss of soil particles occurred when the water level dropped to reach the anchor heads, causing the anchor plates to loosen and slope deformation to accelerate, especially near the lower row of cables. The anchor heads failed first, and progressive slope sliding ensued. Under rapid drawdown conditions, soil liquefaction and subsequent loosening and sliding occurred, generating a far larger zone of accumulation than the zone of collapse. This indicates that soil liquefaction is another important factor of the link between rapid water level drawdown and slope instability causing local collapses. Combining the experimental results and 47 sets of field monitoring data, it could be concluded that the slope stability decreases with increasing slope angle of the WLF zone. This occurs because a higher water level is needed for deformation initiation in the WLF zone in the case of a higher slope angle.