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The collapsibility of loess has constantly been a major problem in engineering geology. The determination of the collapsibility process and characteristics of loess are crucial to foundation construction and residents’ life in the Loess Plateau. Thus, a large-scale in situ irrigation collapse experiment was conducted on a 10 m2 test pit. A total of 79 benchmarks are used in this experiment. These benchmarks are divided into three categories, namely, deep-buried steel pipe benchmarks, ground benchmarks inside the pit, and ground benchmarks outside the pit. The irrigation and observation time spanned 40 days. In the entire irrigation experiment, a three-day water suspension period resulted in two peaks on the collapsibility velocity curve, thereby showing a remarkable difference in the collapsibility curve from the standard one. In terms of the microstructural deformation types of loess and infiltration process of irrigation water, we found that the collapsibility of loess is a dynamic process apart from being particularly sensitive to water. That is, even after a full collapse of the irrigation process, new pores will eventually form once the water content in the loess begins to decline, thereby providing conditions for another loess collapse. Therefore, multilevel collapsibility can occur during continuous irrigation, which is a problem that has been unexplored in previous applications.

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Malan loess is an eolian sediment in arid and semi-arid areas. It is of great significance to study the pore structure of Malan loess for its evolution, strength, and mechanical properties. In order to quantitatively characterize the absolute permeability tensor of Malan loess and to simulate the seepage process of Malan loess, this study calculated the specific yield of intact Malan loess with a homemade seepage experimental device and recorded the water flow process on the surface of Malan loess during the seepage process. Modern computed tomography was used to scan the intact Malan loess samples from Jiuzhoutai, Lanzhou (western part of the Loess Plateau, China); the specific yield of the intact loess was used as the parameter value for the threshold segmentation of the scanned image for the 3D reconstruction of the connected pore space, the solver program in AVIZO software was used to solve the absolute permeability tensor of Malan loess using the volume averaging method combined with the CT scan to reconstruct the 3D pore space, and the simulation of the seepage process was carried out. The simulation results showed that Malan loess is a highly anisotropic loess; the absolute permeability in the vertical direction is 9.02 times and 3.86 times higher than the permeability in the horizontal direction. The pore spaces are well connected in the vertical direction (forming a near-vertical arrangement of pipes) and weakly connected in the horizontal direction. In the seepage simulation, it was found that the water flows first along the vertically oriented channels and then fills the horizontally oriented pores; the absolute permeability coefficient was calculated to be 0.3482 μm2. The indoor seepage experiment was consistent with the simulation experiment, which verifies the reliability of the calculated model.