On 1 September 2022, a giant loess landslide occurred in Huzhu Tu Autonomous County, Qinghai Province, China. This catastrophic event brought to light a unique loess fluidisation phenomenon. In specific parts of the landslide, the loess completely transformed into a viscous, fluid-like state, whereas other parts showed a deepseated slide that retained their structural integrity. In this case, loess with different sliding patterns exhibited varying levels of mobility and destructive potential. Based on the field investigation, electrical resistivity tomography was employed to investigate the groundwater condition of the slope. Subsequently, ring-shear tests were carried out to examine the mechanical properties of the sliding zone loess under different saturation degrees and its response to rainfall as a triggering factor. The results indicate that the natural water content in the original slope was unevenly distributed, influenced by local terrain and groundwater runoff. Following the initial slide caused by cumulative rainfall, the overlying sliding material with high degree of saturation was likely to fluidise due to the increase in excess porewater pressure caused by continued shearing, ultimately resulting in flow-like movement features. In contrast, in areas with a deeper groundwater table, the initial shear could only be sustained over a short distance. This study reveals a mechanism of multiple movement patterns that may coexist in giant loess landslides.

Fluidisation in saturated subgrades of transport infrastructure is a huge problem in many countries around the world caused by dynamic cyclic loads due to heavy haul trains on railways and heavy trucks on highways. The mechanism of subgrade fluidisation has been experimentally studied to a significant extent. However, numerical studies that have been carried out for studying fluidisation are limited. The first part of the paper includes a critical review of previous studies on the mechanism and the effect of cyclic loading factors on fluidisation. It is vital to conduct a comprehensive study with numerical modelling to simulate the actual field conditions of transport infrastructure to find reliable and cost-effective solutions to mitigate subgrade fluidisation. This goal can be achieved only by choosing a soil constitutive model that can capture the changes to the soil stiffness and strength due to excess pore pressure generation and dissipation, along with accumulated deformations in clay soil subjected to cyclic loading. Therefore, in this study, the SANICLAY constitutive model is selected as the suitable candidate to fulfil those requirements. It is implemented in the ABAQUS/Standard finite element program using the user-developed material subroutine UMAT. In the second part of the paper, the validation of the SANICLAY model that accounts for the anisotropy and structure of natural clay was presented using triaxial test data found in the literature for undisturbed clay. Application of the model to simulate cyclic loading shows that the version of SANICLAY used in the simulations needs modifications to capture the stiffness and strength degradation during cyclic loading.

The demand for more efficient heavy-haul rail networks over soft subgrades poses significant geotechnical challenges and requires a comprehensive understanding of stress conditions as well as the failure potential of subgrade soil under moving wheel loads and increasing rail speeds. Unfavourable stress conditions in the subgrade can result in various types of failures, three of which are identified in this article: (i) excessive plastic settlement, (ii) progressive shear failure, and (iii) subgrade fluidisation (mud pumping). Through a series of advanced testing schemes using cyclic triaxial, hollow cylinder, and an in-house dynamic filtration apparatus, critical stress conditions and soil characteristics prone to subgrade instability are discussed. The results demonstrate that under adverse combinations of loading frequency (f) and cyclic stress ratio (CSR) the continuous application of cyclic loads can lead to an unstable state of soil where excess pore pressure and axial strain increase rapidly. This study also reveals that low to medium plasticity soils (PI < 22) are more vulnerable to subgrade fluidisation, where the rapid internal migration of pore water transforms the upper soil to a fluid-like state with substantial loss in soil stiffness. The layered response of soil through dynamic filtration tests showed larger hydrodynamic forces induced by differential hydraulic gradients in the top layer during cyclic loading causes moisture to move upwards. Various factors that can influence soil instability such as the degree of compaction degree, clay content, soil fabric and stress rotation are also addressed in this paper. Finally, novel solutions for stabilising subgrade such as a vertical drain-composite system and the use of eco-friendly biopolymers are presented.