## 2 Materials and Instruments  
### Artificially Prepared Collapsible Loess  
Loess is mainly formed by wind deposition under arid and semi-arid climates. During the deposition, soil particles tend to accumulate vertically, forming a denser bedding structure, while the horizontal arrangement of particles is relatively loose. This kind of bedding structure leads to variation in the physical and mechanical properties in vertical and horizontal directions. Furthermore, seepage of water, temperature difference, and plant root systems also significantly influence this phenomenon. Therefore, the sedimentary history and physical, chemical, and biological effects after deposition cause anisotropy of loess. Due to its anisotropy, even in the same site, the structure and physical and mechanical properties of the loess with different locations and depths will change greatly [40; 41; 42]. If the remodled natural loess was used in the model test, there will be two main problems. One is the variation in soil properties caused by anisotropy, and the other is that the remolding processes will destroy the structure of the loess. Soil conditioners (e.g., cement, quicklime, organic matter, etc.) and some physical reinforcement methods (e.g., geotechnical grids, etc.) are usually used to improve the soil structure and decrease the anisotropy in engineering practice. In order to ensure the repeatability of the test, six materials, namely barite powder, river sand, kaolin, cement, industrial salt, and calcium oxide, were prepared to make artificial collapsible loess to simulate the loess structure to the greatest extent (Figure 1). It should be noted that artificial loess cannot reproduce the material composition and structure of natural loess, which may lead to incorrect estimation of base bearing capacity. Figure 1: The materials of artificially prepared collapsible loess. In this study, four kinds of artificially prepared collapsible loess (APCL) with different mass ratios (the ratio of mass of each component in an APCL) were made by considering the components of intact loess. On the basis of keeping the mass fraction of cement, industrial salt, and calcium oxide unchanged at 3%, 5%, and 2%, respectively, the mass fractions (percentage a component occupies in total mass) of barite powder, river sand, and kaolin were 25%, 35%, 30% (type 1); 30%, 30%, 30% (type 2); 30%, 35%, 25% (type 3); and 35%, 30%, 25% (type 4), respectively. The material ratios referring to the chemical compositions of natural loess (SiO\({}_{2}\) accounts for 50%-60%, Al\({}_{2}\)O\({}_{3}\) accounts for 9%-12%, CaO accounts for 7%-10%, MgO accounts for 1%-39%, and K\({}_{2}\)O accounts for about 2%). Barite powder (barium sulfate) and river sand (silica) are bulk density-sensitive materials; river sand can adjust the internal friction and deformation modulus of APCL; cement (mainly consists of calcium carbonate, silica, alumina and ferric oxide, etc.) is an auxiliary bonding material; kaolin (mainly consists of alumina, silica, etc.) and industrial salt (mainly consists of various soluble sodium salts, potassium salts, etc.) can act as cementation between soil particles and the reaction of calcium oxide with carbon dioxide and water will produce calcium carbonate cementation, and will produce calcium carbonate cementation, which can form the special structure of collapsible loess. The grading curves of 4 APCLs are shown in Figure 2.  
The free-drop method was used in preparing the samples, which can form a skeleton structure similar to that of natural loess [43; 44; 45]. Firstly, the ground materials were proportionally weighed and mixed evenly, and a soil sieve with an aperture of 1 mm was placed at a height of 20-40 cm above the ring cutter, and the mixed material was placed on it. Then, the soil sieve was shaken so that the mixed material on the sieve continued to fall until the sample was higher than the ring cutter. At this point, the sample surface was smoothed and pressed to the specified degree of compaction.