Triggering mechanism of seismic landslides from the perspective of co-seismic accumulation of dynamic pore water pressures

Significance 

Earthquakes, with their devastating power, not only cause direct structural damage but also trigger secondary geological hazards, such as co-seismic landslides. The assessment of the stability of slopes in the wake of an earthquake has been a challenging task for the engineering geological research community. Three main approaches have been traditionally used for seismic slope stability analysis: the pseudo-static approach, the stress-deformation approach, and the Newmark displacement approach. The pseudo-static approach, while simple, has limitations, such as difficulty in determining the pseudo-static coefficient and ignoring time-history characteristics of ground motion. The stress-deformation approach provides accuracy but is computationally intensive and complex. The Newmark displacement approach, on the other hand, offers a simplified yet effective method for analyzing shallow landslides. However, it has its own set of assumptions that may not always hold true in real-world conditions. One significant limitation of the traditional Newmark displacement approach is its neglect of the effect of dynamic pore water pressure (DPWP), an important parameter for assessing the seismic stability of pore pressure-prone soil slopes. Previous research has considered DPWP induced by horizontal ground motion but often overlooks the contribution of vertical ground motion. This study addresses this gap by proposing an improved Newmark displacement model that accounts for DPWP accumulation due to both vertical and horizontal ground motions.

In a recent study published in the peer-reviewed Earthquake Engineering & Structural Dynamics Journal by Professor Jian Ji, Mr. Wenliang Zhang, Dr. Tong Zhang, and Dr. Jian Song from Hohai University, introduced an improved Newmark displacement model for seismic slope stability analysis, taking into consideration the  DPWP accumulation due to both vertical and horizontal ground motions. The research team presented a detailed and comprehensive analysis of the effects of DPWP on seismic slope stability through various case studies. It uses a combination of horizontal and vertical ground motion data to demonstrate how DPWP can significantly influence the yield acceleration and seismic displacement of slopes, especially when soils are near-saturated. The findings indicate that the traditional Newmark model, which ignores DPWP or only considers DPWP induced by horizontal motion, can lead to underestimation of seismic slope displacement, particularly in near-saturated conditions.

The authors also highlighted the importance of considering both vertical and horizontal ground motions simultaneously in seismic slope stability analyses. It emphasizes that the direction and magnitude of vertical ground motion can have a substantial impact on slope stability, and these factors should not be overlooked. They provided valuable knowledge into the behavior of slopes under earthquake-induced conditions, shedding light on the complex interplay of factors that affect their stability.

Furthermore, the authors performed frequency content analysis, demonstrating that the accumulation rate of seismic slope displacement is influenced by the frequency distribution of vertical ground motion. This data is crucial for understanding the dynamics of slope behavior during earthquakes and underscores the need for a more comprehensive approach to seismic slope stability analysis.

The researchers quantitatively described the dynamic changes of the seismic yield acceleration of near-saturated soil slopes by developing DPWP due to both vertical and horizontal ground motions. This quantitative analysis involved several steps and considerations: Firstly, the researchers selected specific parameters that characterize the near-saturated soil slopes under consideration. These parameters include the slope geometry (e.g., slope angle and height), soil properties (e.g., cohesion, friction angle, and effective cohesion), and initial saturation level (Sr0). These parameters are essential for defining the behavior of the slope. Secondly, the researchers utilized real ground motion data obtained from different earthquake events. They considered various types of ground motion with different frequency content and amplitude to capture the range of seismic excitations that slopes might experience during an earthquake. Thirdly, the researchers developed a model to calculate the DPWP accumulation in the soil during the earthquake. This model accounts for both vertical and horizontal ground motions, recognizing that these motions can induce changes in pore water pressure in different ways. Moreover, the seismic yield acceleration represents the maximum acceleration that a slope can withstand before it fails or starts to move. The researchers computed the yield acceleration for the near-saturated soil slope under the influence of DPWP accumulation caused by both vertical and horizontal ground motions. Furthermore, to quantitatively describe the dynamic changes in yield acceleration, the researchers conducted a time history analysis. They tracked how the yield acceleration evolved over time during the earthquake event. This analysis considered the combined effect of horizontal and vertical ground motions and the influence of DPWP. They compared the results of their analysis with cases where DPWP was not considered or only horizontal DPWP was taken into account. This comparison allowed them to quantify the impact of DPWP accumulation due to both vertical and horizontal ground motions on the yield acceleration of the slope. By conducting this quantitative analysis, the researchers were able to provide a comprehensive understanding of how DPWP accumulation affects the seismic yield acceleration of near-saturated soil slopes. This information is crucial for assessing the stability of slopes during earthquakes and for designing effective mitigation strategies to reduce landslide risks in areas prone to seismic activity.

In conclusion, the research by Professor Jian Ji and colleagues presented an important advancement in earthquake-induced landslide research. Their proposed improved Newmark displacement model, which considers DPWP accumulation due to both vertical and horizontal ground motions, provides a more accurate and comprehensive approach to assessing the seismic stability of slopes. The study’s findings have important implications for earthquake engineering and geotechnical practices, as they highlight the need to consider a wider range of factors in seismic slope stability analyses, particularly in near-saturated soil conditions. This research contributes valuable knowledge to enhance our understanding of landslide risks associated with earthquakes and aids in improving mitigation and resilience strategies for vulnerable areas.

Reference

Jian Ji, Wenliang Zhang,Tong Zhang, Jian Song. Seismic displacement of earth slopes incorporating co-seismic accumulation of dynamic pore water pressure.  Earthquake Engng Struct Dyn. 2023;52:1884–1907.

Go to Earthquake Engng Struct Dyn.

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