# Effect of Local Cyclic Loading on Direct Shear Strength Characteristics of Shear-Zone Soil
## 1 Introduction  
Despite extensive investigations having been carried out in the recent decades to investigate the static shear strength of the sliding zone in landslides, only a few studies have been performed on the effects of local cyclic loading, i.e., surge waves, groundwater level fluctuation, traffic loading, precipitation, or seismic loading, which are often encountered periodically in reservoir landslides but do not last long. One of the reasons for this discrepancy is that in general, local cyclic loading cannot trigger large-scale rapid landslides. However, the phenomenon of cyclic loading-induced landslides has been noticed in much of the recent research. For example, [PERSON] et al. [1] noted that under every stress loading process, the pore pressure raised until it increased to the vertical stress. [PERSON] et al. [2] found that cyclic loading could trigger a liquefaction effect near the slip zone. [PERSON] et al. [3] observed that shear strength can be reduced under cyclic loading and may eventually make the materials behave like high mobility materials. Moreover, effective cell pressure and deviatoric stress had a great effect on pore pressure in triaxial tests [4,5]. In saturated sliding-zone soils, pore pressure may increase to cell pressure under cyclic loading, leading to soil liquefaction [6]. Reservoir bank landslides and the surge waves they generate are a calamitous disaster that can cause massive property damage and fatalities [7]. For example, the impulse water waves generated by the Vajont landslide tsunami in Italy (1963) swept away several villages and caused nearly 2000 fatalities [8]. During the 2004 Mid-Niigata earthquake, cyclic loading triggered numerous landslides and a barrier dam in front of the landslide [2]. Since the impounding of the Three Gorges Reservoir (TGR), reservoir landslides havebeen arousing wide public concern as they can generate dynamic loading [9; 10]. The current situation is that many reservoir landslides have not been reinforced and remain in a state of continuous deformation under local cyclic loading in the TRGA, for example, the Huangtupo landslide [11] and the Outang landslide [12]. Hence, it is of great significance to study the stress-strain characteristics of reservoir landslides under local cyclic loading. Understanding the deformation evolution of shear-zone soil under the condition of monotonic loading and cyclic loading is essential for calculating the stability and designing the prevention structure of reservoir landslides [13; 14]. Monotonic loading and cyclic loading will cause permanent and fatigue deformation in specimens under different stress levels [15; 16]. Under these deformations, as a slope is damaged, it generates tensile cracks at the trailing edge [9; 17]. The main study methods involve computer simulation, model testing, indoor testing, and in situ surveys [18; 19; 20; 21]. In addition, some linear and nonlinear methods were used to predict the shear strength parameters from soils' physical properties [22; 23; 24]. Monitoring data from reservoir landslides has shown that reservoir level variation is amongst the most significant elements governing the stability and deformation features since the impoundment of the TGR [25]. The cyclic loading effect on the displacement behaviors has become more and more obvious [26]. Detailed deformation and strength studies are necessary to explain how reservoir level changes influence the stability of a landslide. In this paper, the shear-zone soil of the Huangtupo landslide was taken as the research object, and a cyclic direct shearing test system was used to perform a special dynamic test. Then, the shear strength characteristics of the shear-zone soil under various stress states and the stress-deformation behavior of the shear-zone soil with local cyclic direct shear testing were obtained to simulate the stress-strain evolution law of reservoir landslides under the fluctuation of the water level.