# Instability of High Liquid Limit Soil Slope for the Expressway Induced by Rainfall
## 1 Introduction  
High liquid limit soil [1] refers to fine-grained soil with a liquid limit greater than \(50\%\) and a plasticity index greater than \(26\), which is widely distributed in areas with abundant rainfall and lakes. Because of high water content and low strength, the high liquid limit soil cannot be directly used in roadbed filling [2] and may lead to serious problems such as roadbed cracking, uneven settlement, and slope instability [3]. [PERSON] et al. [4] suggested that slope instability under rainfall may be attributed to reduced shear strength in the soil and the interface of reinforcement steel. Rainfall will reduce the shear strength of soil, raise the groundwater level, and increase the pore water pressure. In addition, the transient saturation zones induced by prolonged and intense rainfall will appear in areas above the stable groundwater level. Meanwhile, the pore water pressure in the corresponding area will increase. Rainfall infiltration promotes fracture propagation, leads to fracture intersection and formation of permeability zone, accelerates the process of rock and soil softening, and further reduces the safety factor of the slope [5]. The effect of soil density on the stability ofslopes under rainfall was studied by [PERSON] et al. [6]. The effect of rainfall on the stability of multi-layer unsaturated soil slope had been studied [7]. Based on the rainfall data of the Three Gorges Reservoir area, [PERSON] et al. [8] incorporated the initial conditions of pore water pressure distribution into the slope stability analysis. The dynamic characteristics of the loess slope under artificial rainfall were studied by the model test, considering the intensity and duration of rainfall on the slope stability [9]. By analyzing the characteristics of the saturated zone of the soil slope and the influence of water characteristic parameters on the slope stability, a landslide prediction model was proposed based on the critical curve of the shallow saturated zone and a one-dimensional model [10]. The effects of rainfall intensity and hydraulic characteristics on slope stability were studied through the strength reduction method and finite element seepage method [11; 12; 13]. But the impact of the existence of anti-slide piles was ignored. Continuous rainfall was an important factor resulting in landslides because the rainfall infiltration increased the water pressure in the steep tension cracks and promoted the expansion of the sliding surface [14; 15]. The slope instability influenced by rainfall was caused by the complex interaction of the topographic, hydrological, and geological conditions of the slope [16]. It was very important to evaluate the controlling factors, such as slope angle, initial water content, soil properties, rainfall intensity, and intermittent rainfall, to determine the slope stability [17]. A large number of landslide cases show that the instability of the slope is highly consistent with rainfall time and rainfall frequency [18]. Due to the structural surface complexity specified by the steepness of the slope and its direction, [PERSON] et al. [19] defined that the geometric slope index requires additional correction of the potential evaporation determined by the slope of different directions and dips. The flooding of the remaining gaps could cause loss of slope stability during and after the flooding process [20]. Although the rainfall intensity and time were two important factors affecting slope stability of shallow landslides, shallow landslides occur in the transient saturation zone of sandstone slope. The depth of the sliding surface was independent of the type and duration of rainfall [21]. The stability of soil slope with low permeability coefficient was mainly affected by rainfall within 5 days, while the instability of soil slope with high permeability coefficient was mainly affected by the maximum rainfall of the day [22].