It is generally believed that loess is not prone to liquefaction. However, on December 18, 2023, a magnitude 6.2 earthquake occurred in Gansu Province, China (35.70 degrees N, 102.79 degrees E), triggering a large-scale loess liquefactioninduced flow slide spanning 2.5 km, approximately 10 km from the epicenter. To understand the disastercausing mechanism, this study obtained the physical and mechanical properties of loess in the source area through field surveys and laboratory tests, and characterized the liquefaction behavior of saturated loess layers. The findings indicate that the strong ground motion, saturated loess, and gentle slope collectively contribute to the prevailing dynamic, geological, and topographic conditions. The saturated loess layer primarily comprises silt particles with particle sizes less than 0.075 mm accounting for approximately 92.2 % of its composition. The saturated loess layer at a depth of 11m was liquefied under the action of seismic waves with a peak ground acceleration of 0.40 g, however, due to the unique pore structure of loess, it is observed that pore pressure development rate lags behind strain rise rate during liquefaction process. The majority of strain accumulation occurred during a distinct post-peak stabilization phase following peak seismic activity while pore pressure continues to escalate even after vibration ceases. The results provide scientific insights into understanding the cause contributing to loess liquefaction induced-flow slide disasters due to earthquake.

On December 18, 2023, the M S 6.2 Jishishan earthquake triggered a large-scale liquefaction disaster of loess sites in Jintian and Caotan villages, Zhongchuan town, Minhe County, Haidong City, Qinghai Province. To clarify the micro-mechanism of the liquefaction disaster, the Q3 Malan loess layer of the disaster site and its overlying red silty clay layer samples were selected and quantitatively analyzed for the differences in physical properties, structure, microstructural parameters, and mineral compositions. Based on the discrepancy results, the micro-mechanisms between loess microstructure and macro-mechanical properties of soil and liquefaction disaster were investigated. The research shows that compared with red silty clay, the dynamic index of loess corresponding to the physical indices of Zhongchuan loess obviously exceeds the critical threshold of liquefaction under actual seismic intensity. Additionally, its pore structure is dominated by point contact and weakly cemented overhead macropore structure, and its quantitative pore microstructure parameters and mineral composition show significant liquefaction potential. The comprehensive analysis of the liquefaction mechanism shows that the rapid deformation of the soil skeleton and the destruction of the cementation and contact system of the water-sensitive minerals under seismic loading and hydraulic force lead to the collapse of the overhead macropores, the damage of structural strength, the increase of the complex pore channels, the rapid accumulation of pore water pressure, and the reduction of the effective stress, which leads to the liquefaction of the loess.

Loess has a unique structure that makes it susceptible to liquefaction during intense seismic activity. Liquefaction is closely linked to microstructural changes due to hydraulic coupling. This study examined the threedimensional microstructure evolution of loess in various liquefaction states using dynamic triaxial tests and high-precision micrometer CT scanning. As the ratio of pore water pressure (Rwp) increases, the size of loess particles tends to decrease while the roundness is inclined to increase. Moreover, the morphology and orientation of particles remain relatively stable under such circumstances. In addition, increasing Rwp will decrease the number of macropores, increase the number of mesopores and fine-pores, and decrease the size of throats and channel length, with which petite throats and pores become more prominent. Consequently, liquefaction gradually opens closed pores, enhances soil connectivity, and divides large pores to increase small to mediumsized pores, improving pore distribution uniformity. Liquefaction induces the pore shape coefficient to decrease, the number of slim pores to increase, and irregular and circular pores to decrease. These findings provide a scientific foundation for preventing and evaluating loess liquefaction disasters and shed light on the microscopic mechanisms of loess liquefaction.

Loess is a typical structural soil with properties such as water sensitivity, collapsibility, and seismic vulnerability. The dynamic response of a water transmission pipe crossing a fault zone is highly complex in a loess site. The Hanjiang River to the Weihe River Diversion Project (Phase II) crossing the Weihe fault was selected as the prototype for a shaking table test, through which the responses of acceleration, dynamic stresses, strains, and pore-water pressure were systematically investigated. The acceleration response of a geologic body similar to the experimental model was greatly affected by factors such as fault location, degree of soil saturation, distribution of internal structure, and so on. The acceleration along elevation experienced its highest amplification factor of approximately 2.0, mainly due to seismic waves with frequencies above 2.0 Hz being amplified. To ensure the seismic fortification of the pipeline near the fault belt, it is recommended to utilize an acceleration of 1.2 times within the severe impact zone. This zone involved 36.0 m of the hanging wall and 30.0 m of the footwall, which are approximately 6.0 times and 5.0 times of the fault belt width. It is recommended to use an acceleration of 1.1 times for areas within 200 m from the fault of the hanging wall; refer to the fault-avoiding distance of the seismic design code. The deformation mode of the water pipeline was expansion/shrinkage in the transverse and slight bending in the longitudinal section. The pore-water pressure response demonstrated coupling features of hysteresis, accumulation, and dissipation. The seismic collapsibility modes of the loess at the studied site were generalized into four stages: energy accumulation, state mutation, failure extension, and successive failure. Seismic subsidence could be expressed by adopting a piecewise function with a maximum value of approximately 18.0 cm. Based on the similarity calculation, the maximum seismic subsidence of the prototype can be recommended as 3.6 m. Liquefaction occurred when the input acceleration amplitude reached 0.5g. These shaking table test results provided reasonable parameters for the seismic design and construction of the Hanjiang to Weihe River Diversion Project across this fault.