Loess soil is a homogeneous, porous, light yellow or brown wind deposit with good sorting. The size of loess particles is often in the range of silt, with some clay and, sometimes, sand (Pye, 1995; Ding et al., 2019). This type of soil is widespread in most parts of the world, including Asia, North America, South America, and Europe (Banak et al., 2013; Li et al., 2020). Similarly, loess sediments have a significant distribution in Iran, and are mainly concentrated in the northern regions and Golestan Province, but can also be seen in the Central Plateau and even Southern Iran (Karimi et al., 2009). Loess is prone to geological disasters due to its collapsible property, honeycomb structure with large pores, and great water content sensitivity (Zhou et al., 2002). Landslides destroy roads and buildings, block rivers, buries villages, and cause enormous loss and financial damage every year. In general, earthquakes and rainfall are the two main causes of landslides in loess soils, and this theory is widely accepted among experts (Havenith and Bourdeau, 2010; Sorbino and Nicotera, 2013; Peng et al., 2015; Wu et al., 2022). To investigate the causes of landslides, numerous research, field and laboratory experiments have been conducted to analyze the landslide mechanism. In Europe, for example, Szokoli et al., (2018) used geophysical methods to investigate the slide mechanism. They used the Earth electrical resistivity tomography plotting (ERT) and pressure measurement using a pressure probe (PreP) to identify the propagation of cracks in a loess slope with a slow movement in the banks of the Danube in the south of Hungary.
Also, Hong et al., (2021) based on their studies suggested that liquefaction could be the cause of some rapid landslides and great displacement in uniform loess slopes. Based on the field survey and examination of a landslide that occurred in a village in the Xiangning Province of China in 2019, Shi et al., (2020) showed that the penetration of surface water into weak layers in the depth of the slope reduced the shear strength of loess and led to a failure. The strong ground movement caused by the earthquake affects the soil structure, facilitates water flows, and rapidly increases the pore water pressure, which ultimately leads to a decrease in the shear strength of the slope loess materials (Zhang and Wang, 2007). Meanwhile, the penetration of rain and snow into the loess slopes is mainly controlled by fractures and surface fissures such as tensile cracks and large pores (Chen et al., 2018). Due to the importance of the role of water in landslides and the mentioned points, Bai et al. (2014) developed a system called the ASWS to study the effect of daily precipitation and previous precipitation on the triggering of landslides.
In general, it should be stated that each landslide has a major cause, while the causes of landslides can also be quite complex. As a result, analyzing landslides in complex geological conditions and the induced forces only through field surveys and laboratory studies can be very sophisticated. The use of modern and complex numerical methods to analyze the stability of compound landslides has become widespread among researchers and engineers (Xie et al., 2021; Azarafza et al., 2021; Fawaz et al., 2014). To analyze slope stability in practical and scientific works, the limit equilibrium method is widely used (Avci et al., 1999; Azarafza et al., 2021). In combination with modern computer programs, these methods can analyze slope stability more quickly. Currently, several software programs for numerical analysis of slope stability are used by researchers and engineers, including the finite-element PLAXIS (Fawaz et al., 2014; Afiri and Gabi, 2018; Zhao and You, 2020), the discrete-element FLAC (Danneels et al., 2008; Sarkar et al., 2012), and the PFC (Zhang et al., 2018; Yang et al., 2021).
Some researchers (Kainthola et al., 2013) use more accurate limit equilibrium methods including the Morgenstern-Price and Spencer method to determine the critical slip surface and safety factor. Other studies have shown (Chen et al., 2021) that in these methods, cohesion and internal friction angle of the soil are key factors in the calculation of the safety factor. However, these methods have not paid attention to the damage to the soil structure of the slopes following earthquakes and precipitation, which requires data obtained from field surveys and experiments. Similarly, modern numerical methods rarely address local damage caused by the leakage of water into the slope.
This paper aimed to analyze the landslide in AghEmam Village, Golestan Province, in northeastern Iran, which occurred in April 2019, and to determine the mechanism of the landslide. Field surveys, laboratory experiments, and numerical simulations are used to develop models of slope failure mechanisms, in which the effects of earthquakes and precipitation are considered the main causes of landslides. Based on drone mapping, the exact geometry of the slope and the accompanying phenomena of the landslide can be observed accurately and directly. Moreover, changes in the resistive properties of loess slope affected by the presence of water and the dynamic loads caused by earthquakes will be studied by triaxial tests. Numerical simulations can reconstruct the initial geometry of landslides and show when and where landslides begin in the studied landslide. In the following, the effects of rainfall and earthquakes in the studied slope can also be studied. The mechanism of landslides in the loess materials caused by earthquakes and rainfall can be effectively understood using these approaches. At the end of this section, it should be noted that the displacement caused by the earthquake and the change in the water content of slope materials due to the penetration of precipitation are considered in the analysis of the landslide mechanism in this paper.