## 4 Analysis of the Causes of Landslides
### Analysis of Objective Factors of Roadbed Landslides  
The influence of various objective unfavorable factors leading to the occurrence of composite roadbed slip and collapse accidents was analyzed in the following. * Geological conditions. The spatial combination of topography, stratigraphy, lithology, and geological structure constitutes the slope structure of a landslide, and the slope structure determines the boundary and spatial form of the landslide. In the K31+100-K31+388 section of the highway phase II project, the soft foundation is particularly complex due to factors such as topography, geomorphology, and construction environment. The soft soil layer in this section has a low shear strength, a high compression coefficient, a low bearing capacity, and a high compression in the underlayment, which makes the design and construction control of soft foundation treatment challenging. Accurate geological data are essential for the design and construction of the roadbed. However, it is difficult to obtain a clear understanding of the geological conditions in each section, especially in an area where plains and hills meet. Therefore, continuous adjustments and corrections are necessary during the actual construction to minimize potential hazards. Insufficient analysis of geological data and field test results can indirectly impact the anti-disturbance ability of composite roadbed construction. The complexity of the geological conditions poses risks to the project quality and adds to the difficulty of construction control. * Limited construction environment. The construction environment of the K31+100\(\sim\)K31+388 section is challenging due to various uncontrollable factors. Land acquisition difficulties have resulted in the construction of this section being carried out only on the left width, with the right width serving as a construction right-of-way. This half-width construction method has posed challenges in maintaining the integrity of the bedding form and achieving evenness in the composite roadbed treatment. These issues have impacted the overall settlement problem of the section, leading to cracks in the embankment and increased difficulty in implementing waterproofing measures during construction. Additionally, the high rainfall in the area prior to the slide collapse has further compounded the situation, as rainfall infiltration alters the gravity and physical properties of the soil body. Rainwater infiltration also transforms into groundwater, exerting dynamic and hydrostatic pressure on the slip surface [30]. The difficulty in land acquisition also results in a shorter construction period and varying filling thicknesses and loading speeds. These factors not only affect the formation of the composite roadbed bearing system but also influence the amount and speed of pile piercing. Excessive loading rates can lead to increased lateral extrusion displacement of the composite roadbed and a decline in roadbed stability. Fishponds are widely distributed around the road section, and the poor nature of the stratum is accompanied by differences in geological conditions, which leads to discontinuous changes in the topography and weak horizontal binding force on the top of the PHC pipe piles. Such a localized phenomenon of low safety often occurs in road sections with significant geological variations and is often the origin of landslides. However, it is difficult to avoid such localized variance situations in the design stage. * Limitations of roadbed bearing. From a site perspective, the PHC piles at the center of the road after the slide collapse, such as the fracture occurring at about 2.5-7.5 m, and the settlement of the roadbed against the center of the road were particularly large. The localized sudden shear damage of the soil between the piles at its weakest point resulted in the nearby piles experiencing a load similar to a dynamic load. This load mainly consisted of an approximate horizontal driving force, which caused excessive horizontal thrust.