## 4 Analysis of the Causes of Landslides
### Analysis of Subjective Factors of Roadbed Landslides  
The influence of various subjective unfavorable factors leading to the occurrence of composite roadbed slip and collapse accidents was analyzed in the following. * Adverse effects of construction. Unfavorable factors during construction can have a negative impact on the bearing capacity and stability of PHC pipe pile composite roadbeds. Unconventional construction practices, such as biased filling (0.3 m subgrade and 0.6 m pre-compression load) and a fast loading rate (2d to complete the filling, equivalent to 0.8 m of embankment), can lead to problems. The verticality deviation of PHC piles often exceeds the specified requirement of 1%, reaching a maximum of 6.78%. Excessive deviation and pile leakage can result in increased pile spacings and insufficient bedding thicknesses. When the right width is used as a construction right-of-way and heavy machinery such as vehicles and rollers pass through, the mechanical vibration forms a bias load, which is likely to produce a horizontal extrusion effect on the outer piles. * Inadequate cost control issues. The cost of using piles should not be reduced, as it is scientifically necessary to ensure that the pile lengths, strengths, and stiffnesses, as well as thicknesses and materials of bedding layers, are sufficient. For example, in the road section design, if the pile length is shortened and the pile spacing is increased compared to the original design, this can lead to problems. If the project cost is blindly reduced, it often results in necessary treatment and protective measures not being implemented properly, indirectly affecting and limiting the intentions of the designers. This increases the risk of accidents in the project and may lead to inherent quality hazards. * Inappropriate site management issues. If special circumstances, such as the half-width construction of the roadbed, are not properly handled, this can lead to embankment cracking and seepage issues. These cracks can cause continuous damage, reducing the strength of the roadbed and impacting its load-bearing capacity. Moreover, misleading test results and imperfections in the testing  
Figure 8: Schematic diagram of load transfer and soil arch effect of the PHC pipe pile composite roadbed. (**a**) Load transfer of PHC pipe pile composite roadbed; (**b**) soil arch effect. system can affect the judgment of construction and engineering management personnel, leading to hidden quality problems. * On-site monitoring issues. The cumulative deep horizontal displacement and section settlement data for section K31+290 before the slide are presented in Figure 9.  
As can be seen in Figure 9, lateral displacement of the soil body mainly occurs within the depth range of 0-17 m, and it follows a parabolic pattern with increasing and then decreasing cumulative lateral displacement along the depth direction. The largest cumulative lateral displacement of the soil body is observed at a depth of approximately 5 m, reaching 110 mm. The displacement rate curve exhibits a similar parabolic shape, with the maximum displacement occurring at a depth of 5-10 m. The latest field observation recorded a displacement rate of 25 mm/d, which is significantly higher than the warning value of 5 mm/d for deeper layers. Starting from 2 months before the slide collapse, the cumulative settlement remained unchanged on the right side of the section, while the left side of the road experienced significantly larger cumulative settlement. This difference in cumulative settlement within the same section is attributed to the construction activities on the left half of the section. The settlement rate during the first two months was stable at less than 4 mm/d, but it increased to 11.6 mm/d prior to the slide, surpassing the warning value of 10 mm/d. Based on the observation data from section K31+290, both the deep horizontal displacement and surface settlement values exceed the warning threshold.