## 1 Introduction  
Creep is one of the main causes of slope deformation and damage, and creep characteristics describe how the deformation and strength of rock and soil change over time. Slope deformation has a complex nonlinear relationship with time. Significant time-dependent creep properties have been reported in several landslides, including the Zentoku landslide in Japan [1], the creep landslide near Saga City, Kyushu Island, Japan [2], the creep landslide in the Gundogdu district of Babadag town in Denizli, Turkey [3], and a large number of dam slopes in the Three Gorges reservoir area in China [4, 5]. Field investigations and monitoring analyses of these landslides have revealed that slope excavation, rainfall, and the rise and fall of the water level in a reservoir are all important factors that affect creep characteristics [6]. In recent years, a series of technologies have emerged that enable people to monitor landslide motion patterns with temporal and spatial resolution [7; 8]. However, the active mechanism of landslides can only be understood if the mechanical properties of the constituent materials of landslides are fully investigated [9]. Numerous studies have performed laboratory creep tests as the primary method to analyze the creep characteristics of rock and soil. Such tests generally include uniaxial compression creep tests [10; 11; 12; 13], triaxial creep tests [14; 15; 16; 17; 18], and direct shear creep tests [19; 20; 21]. Rainfall alters the water content of rock and soil in slopes, which modifies their physical and mechanical parameters, creep characteristics, and affects. Triaxial creep tests are often used to study the effect of water content on the creep properties of rock and soil owing to their long testing times and better sealing ability than other tests. [PERSON] et al. proposed a nonlinear creep damage model that considered the pore water pressure and used triaxial creep tests on saturated and dry sandstones to verify their model's validity [22]. [PERSON] et al. investigated the effect of dry and wet cycles on the creep behavior of sandstone using triaxial creep tests [23]. [PERSON] et al. conducted triaxial creep tests and analyzed the effects of freeze-thaw cycles, chemical erosion, and other factors on the creep parameters and long-term strength of rock [24; 25]. [PERSON] et al. and [PERSON] et al. investigated the effect of water content and the drainage method on the creep properties of clay using triaxial creep tests [16; 26]. Long-term strength and creep characteristics have also been a research focus of slope creep stability analyses [5; 14; 19]. The long-term strength of rock and soil refers to gradual (generally decreasing) changes in the material strength with increasing loading time. Numerous in-depth studies have been conducted on the long-term strength of landslides [27; 28; 29] and a consensus has been reached that a necessary condition for creep damage is when the shear stress of the rock and soil exceeds its long-term strength. Numerous tests have confirmed that the long-term strength of rock and soil directly undergoing creep damage is less than the peak strength. Many studies have addressed the creep properties of landslide sliding zones [18; 19; 30], with most laboratory studies generally conducted on gravel-free soils [31; 32; 33] (with particle diameters finer than 2 mm, e.g., clay, silty clay). However, natural sliding zones often contain large particles with diameters greater than 2 mm, which cannot be ignored. [PERSON] et al. and [PERSON] et al. conducted creep tests on sliding zone specimens containing gravel and showed that the proportion of large particles significantly affected the creep behavior of the sliding zone [34; 35]. Nevertheless, the use of large particles in creep behavior studies on sliding zones is scarcely reported. Existing research on creep in sliding zones has focused on homogeneous samples or samples with small grains owing to size limitations of the test instrument.