## 2 Materials and Methods  
### New Definition of Structural Parameter  
When natural loess is not disturbed by external influences, it exhibits inherent structural properties. The strength of the soil structure is mainly manifested in the arrangement and connection of the soil particles. Loading, disturbance, and immersion in water can alter or destroy the original structural form of the soil. Loading can change the arrangement and association characteristics of soil particles. Disturbance can weaken and break the cementation between soil particles. Immersion does not only dissolve and weaken the chemical composition of the soil, so that the suction in the soil disappears and the water film thickens, but also releases the expansion and contraction potential of the soil [23; 24; 25; 26; 27; 28]. [PERSON] [16; 17] first proposed to seek a quantitative index of soil structural properties by releasing the structural potential, i.e., the structural properties of undisturbed soil are completely released by loading, disturbing, and immersion methods. Based on the idea of integrated structural potential, many scholars have conducted substantial research [29; 30; 31]. [PERSON] [19] obtained the difference of principal stresses of three given strains from the stress-strain curves of undisturbed loess, saturated loess, and remodeled loess under the condition of consolidated drainage triaxial shear. The ratios of principal stress difference between undisturbed and remodeled loess and between primary and saturated loess define the structural parameter of the soil. It comprehensively reflects the structural characteristics of soil particle arrangement and particle bonding. [PERSON] [32] defined the strain-structure parameters of soils by the shear strengths of undisturbed, remodeled (wet compacted state is kept the same as undisturbed), and saturated soils under triaxial stresses for a given strain. The ratio of strength between undisturbed soil and saturated soil, as well as the ratio of strength between undisturbed soil and remolded soil, comprehensively reflects the arrangement and connection characteristics of soil particles. The structural parameter defined above shows good sensitivity, stability, and rationality. However, in the testing process, it is necessary to test the as-is, remodeled, and saturated samples, and the workload is relatively large. Therefore, this paper combines the triaxial shear test and takes Shaanxi undisturbed loess as the research object to give a structural parameter that is applicable to complex stress states and does not require too much workload, is achievable through a relatively simple test, and is easy and convenient to obtain. The defined structural parameter \(m_{\tau}\) is:  
\[m_{\tau}=\frac{\tau_{o}}{\tau_{\tau_{\tau}}} \tag{1}\]  
In Formula (1), \(\tau_{o}\) represents the shear strength of intact loess and \(\tau_{\tau_{\tau}}\) represents the shear strength of remolded saturated loess. From the definition of \(m_{\tau}\), it can be seen that the greater the strength of the association between soil particles, the more unstable the arrangement, and the lower the strength of the soil body under external load after disturbance and saturation by water immersion, i.e., the greater the structural parameter. The definition of the structural parameter is expressed in terms of shear strength, which proves that the parameter is a material constant that is a composite response to the strength of the earth material, and that the definition is more standardized and widely applicable. Furthermore, the new definition utilizes the strength parameters of remolded saturated soil rather than those of both saturated and remolded soil, which simultaneously addresses the structural stability and variability of the soil. This approach simplifies the testing conditions and reduces the required sample size.