Understanding the reactivation causes of ancient landslides is imperative for the prevention of landslides. However, the reasons for the reactivation of thick loess-mudstone ancient landslides and evolutionary mechanisms are unclear. This paper investigates the Gaojiawan thick loess-mudstone ancient landslide as an example using field investigation, InSAR time series analysis, and laboratory testing methods to analyze the reactivation deformation characteristics and reactivation causes of the thick loess mudstone ancient landslides, which were and verified by numerical simulation. The results show that fault fracture zones and groundwater primarily control the reactivation of Gaojiawan's thick loess-mudstone ancient landslide. Due to the fragmentation of rock mass and the development of structural planes in the fault fracture zones, as well as the excavation and unloading zone formed by the surrounding rock of the tunnel, it is beneficial to the enrichment of groundwater. It intensifies the interaction of groundwater-rock-fault fracture zones, especially for the red mudstone with more clay mineral content. The strength degradation is significant after encountering water, resulting in an imbalance in the stress state in deep strata and the reactivation of the landslide.

BackgroundAt approximately 4:00 PM on 18 July 2023, a heavy rainstorm lasting one hour triggered a significant mudstone landslide in Dongping, Weiyuan County, Gansu Province, Northwest China. The landslide resulted in the burial of houses, the fracturing and destruction of roads, and posed a serious threat to 16 households. The estimated economical loss from this disaster reached 3.2 million yuan. This study presents a detailed field investigation of the Dongping landslide, focusing on the deformation and failure characteristics through a multi-layered analysis of sliding strata, rock mass structure, slope configuration, and failure mechanism. Moreover, the study explores the key triggering factors of the Dongping landslide, with particular attention to the roles of seismic activity, rainfall, and preferential flow in the development of large-scale mudstone landslides.ResultsThe stratigraphic profile of the Dongping landslide reveals a two-layer structure, consisting of overlying loess and underlying mudstone, with the sliding surface primarily located within the underlying Neogene red mudstone. The initiation location of the Dongping landslide is situated at the rear of the slope, while the main slip-resistant is located in the middle of the landslide, exhibiting a predominantly thrust-sliding. After encountering resistance in the middle section, the front part of the sliding mass continued to move, leading to the formation of secondary landslides. The overall movement of the Dongping landslide is characterized by rotational sliding, with the sliding mass remaining relatively intact.ConclusionsThe initiation of the large-scale mudstone landslide in Dongping was driven by multiple factors. The heavy rainfall served as the direct triggering factor for the landslide occurrence. However, some historical factors, including seismic activity and previous sliding surface, had already weakened the slope structure by degrading the mechanical properties of the landslide mass and creating preferential flow channels, thereby setting the stage for the Dongping landslide. Structural fractures in the landslide area, along with sinkholes formed by a combination of tectonic joints, soil properties, and human activities, constituted preferential seepage pathways for water within the slope. These pathways provided the hydraulic conditions necessary for rainfall-induced landslides, making them the primary controlling factors in the occurrence of the Dongping landslide.

The geomorphologic and environmental evolution of the Loess Plateau is greatly affected by the strong fault activity, leading to the frequent occurrence of geological hazards, particularly the loess-mudstone landslide (LML). Thus, it is crucial to investigate the formation mechanism of such landslides in active fault zones. In this study, a field survey was conducted in the Weibei tableland of Baoji where the active fault zone is developed. To study the creep behavior of LML sliding zone soil, the triaxial creep tests of multi-stage loading under different water content, confining pressure, dry density, loess-mudstone binary structure (LMBS) contact surface angle, and thickness were carried out using the sliding zone soil samples (loess, mudstone, and LMBS samples) obtained from Wolongsi landslide in the study area as an example. Experimental results revealed that: (1) Water content has a significant weakening effect on the strength of LML sliding zone soil. The strength of the LMBS sample is extremely water-sensitive. The weakening effect of water on the long-term strength of LML sliding zone soil mainly manifested in promoting the elastoplastic deformation of loess and the viscoplastic deformation of mudstone. (2) The long-term strength of the LML slip zone soil increases linearly with the increase in confining pressure and increases exponentially with the dry density. (3) The degrading effect of the stratigraphic interface dip angle on the long-term slope strength is mainly reflected in the change in the weakening degree of water on the strength of the LML sliding zone. In addition, according to the test results, the traditional Nishihara model is improved by introducing nonlinear parameters, and a new constitutive equation describing LML sliding zone soil is established. The new constitutive model can accurately describe the creep curve's whole process especially sensitively identifying the accelerated creep stage. Finally, after conducting a field investigation and laboratory tests, the main hazard factors affecting the occurrence of the LML were analyzed in the presence of fault activity on the Weibei Plateau of Baoji, China, and its formation mechanism was revealed as well.

The complex structure of Neogene mudstone plays an important role in geological disasters. A close relationship exists between the mechanisms of mudstone landslides and the disintegration characteristics of rocks. Therefore, understanding the disintegration characteristics of Neogene mudstone at different depths is crucial for enhancing engineering safety and assessing landslide stability. This study employed Neogene mudstone from different depths to perform disintegration and plastic limit experiments and revealed the sliding mechanisms of landslides involving Neogene mudstone, providing theoretical support for mitigating mudstone geological disasters. Our results demonstrate that Neogene mudstone from different depths experiences varied stress conditions and pore water pressure due to geological actions, significantly affecting the disintegration characteristics. By ignoring the factors of the slip surface, the slake durability index of mudstone decreases with increasing burial depth, while the plasticity limit index tends to rise. The influence of groundwater, geo-stress, and pore structure on Neogene mudstones at different depths results in overall weak stability and disintegration. Landslide occurrences are likely connected to the mechanical properties of mudstones at the slip surface, where a low slake durability index and higher plasticity index make the mudstones prone to fracturing, breaking, and disintegrating once in contact with water.