###### Abstract  
In recent decades, loess landslide events have attracted increasing attention in the South Jingyang tableland. To elucidate the mechanical mechanism of landslide initiation in the region, this work collected undisturbed loess and paleosol samples taking from the Q\({}_{2}\) strata in the South Jingyang tableland. A range of direct shear tests were carried out to explore the strength evolution law of shear zone soil subjected to a varying initial moisture content. In addition, soil water characteristic curves (SWCCs) were also charted and used for predicting the unsaturated shear strength. The findings show that the basic physical properties of the paleosol are different from those of loess due to their different pedogenic environments. The normal stress level and initial moisture content jointly determine whether the shear behavior is strain hardening or strain softening. The shear strength and strength parameters evidently diminish with an increasing initial moisture content, and cohesion contributes to the vast majority of strength attenuation. Paleosol samples possess higher values in shear strength and strength parameters than loess samples due to their stronger inter-particle cementation. The predictive formulas of unsaturated shear strength for undisturbed loess and paleosol are proposed, respectively, based on the Vanapalli model, and the calculated values of the strength prediction model are in perfect agreement with the experimental values. loess; paleosol; shear behavior; soil water characteristic curve (SWCC); prediction of unsaturated shear strength +
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destruction of inter-particle cementation [17; 18; 19; 20; 21; 22]. Similar research results have also been reported in other loess regions abroad. [PERSON] et al. [23] studied the strength of loess deposited in Belgrade, and showed a decrease in shear strength owing to greater initial moisture. [PERSON] and [PERSON] [24] conducted triaxial tests to investigate the underlying causes of frequent rainfall-triggered slope failures in the Akaroa harbor area in Banks Peninsula, New Zealand, and the stress-strain curves showed an increase in maximum deviatoric stress as moisture decreased. These plentiful research results offer in-depth insights into the shear behavior of loess, but there remains little investigation of paleosol shear strength. Moreover, the limited research on the paleosol mainly focuses on environmental or paleoclimatic changes [25; 26; 27]. However, the existing research has proven that loess and paleosol are different with respect to physical and mechanical properties [16; 28]. Consequently, it is essential to conduct a comparative analysis of the strength evolution laws for undisturbed loess and paleosol under different moisture content conditions. Establishing an effective strength prediction model can provide a good approximation of shear strength for unsaturated soil in the absence of experimental data. Past studies have stated that the SWCC is a critical parameter for estimating the unsaturated shear strengths of soils [29; 30]. The prediction of loess shear strength has also made gratifying progress. [PERSON] et al. [30] proposed a new model for estimating unsaturated shear strength on the basis of the SWCC. [PERSON] et al. [31] established a shear strength formula with matrix suction as a variable. An equation using the SWCC to predict the unsaturated shear strength of loess over a wide suction range was also proposed by [PERSON] et al. [32]. The results obtained from these strength prediction formulas have good consistency with the measured strength, and they are convenient to apply in engineering practice. However, in our review of the existing literature, we found that there are abundant shear strength prediction models for unsaturated loess, but the research on prediction models for the paleosol is insufficient. Further research is, therefore, needed to establish an effective prediction formula for paleosol shear strength.