###### Abstract  
The shear strength of unsaturated soils exhibits significant nonlinearity, while previous studies often simplified it with linear strength models. The objective of this paper is to investigate the distinctions in the stability of three-dimensional (3D) tunnel faces when using linear and nonlinear strength models. A new 3D rotational failure mechanism and an extended form of the Mohr-Coulomb (M-C) failure criterion were integrated into the kinematically limited analysis (KLA) framework to describe the failure characteristics of tunnel faces. Subsequently, the factor of safety (\(F\)S) of the 3D tunnel faces was calculated using the strength reduction method (SRM). In the discussion section, the impacts of nonlinear shear strength, matric suction in the unsaturated soils, and the 3D geometric parameters of the tunnel on the stability of the tunnel face were analyzed. The outcomes indicate that, in unsaturated soil conditions, diverse nonlinear strength calculation models and soil types exert disparate influences on the \(F\)S of 3D tunnel faces. The main novelty of this study lies in establishing an effective method for assessing the stability of tunnel faces in unsaturated soils. m pacs: 03.67.  
geotechnical engineering scenarios [22; 23; 24]. During the course of optimizing the kinematic mechanism, [PERSON] and [PERSON] [25] were the pioneers involved in proposing a 3D kinematic mechanism involving double blocks, which paved the way for more advanced analyses in tunnel stability assessments. On this basis, [PERSON] et al. [26] proposed a modified multi-block mechanism to obtain more accurate solutions. The horned failure mechanism has been utilized in the examination of tunnel face safety, and its results are in good agreement with both numerical simulations and experimental observations [27]. In the context of the New Austrian Tunnel Method (NAT), the tunnel face is not subjected to supporting pressure [28; 29]. Consequently, evaluating the stability of the tunnel face using the \(FS\) is a more appropriate approach than considering supporting pressures. There are two approaches for delineating the \(FS\): the strength reduction method (SRM) and the gravity augmentation method (GAM). In the GAM, the \(FS\) is expounded as the quotient of the internal energy dissipation rate to the external energy dissipation rate. Nonetheless, within the SRM paradigm, the \(FS\) is articulated as the quotient of the real soil strength to the critical strength in the state of limit equilibrium. This approach yields a more cautious resolution when contrasted with the GAM, consequently rendering SRM a prevalent choice for assessing the stability of Mohr-Coulomb (M-C) materials. Numerous scholars have delved into the influence of matric suction on shear strength within the domain of geotechnical engineering, particularly concerning the unsaturated constituents of soil strength [30; 31; 32; 33]. To account for unsaturated properties in soils, [PERSON] et al. [34] modified the M-C criterion and proposed the concept of matric suction angle. Following this, various estimation formulas were advanced to assess the strengths of unsaturated soils across diverse soil classifications [35; 36; 37; 38; 39]. Extensive research has been conducted in the academic realm concerning the analysis of unsaturated soil strength, with a focus on topics such as slope stability and retaining walls. However, there is a noticeable lack of studies addressing the safety of tunnel faces. As an instance, [PERSON] et al. [40] determined the critical groundwater level depth for retaining walls under varying distributions of matric suction in unsaturated soil, utilizing the temporal equilibrium method. Additionally, [PERSON] and [PERSON] [41] explored the consequences of rainfall-induced failures on engineering slopes under unsaturated conditions.