## 1 Introduction  
Studying the shear behavior at the interface between the exterior surface of solid structures and the surrounding earth material is essential for understanding the soil-structure interaction, which plays an important role in the geotechnical engineering design of various structures including deep foundations, retaining wall structures (e.g., anchored, reinforced, soil nailing), landfills, and slope stability [1]. Thus, interfacial shearing of soil has been investigated by several researchers using various laboratory testing methods such as the interface direct shear, ring shear, simple shear, and resonant-column torsional shear tests [2, 3, 4, 5, 6]. The mechanical behavior of soil-solid structure interfaces depends on the physical properties of the soil as well as the structural surface characteristics. Among them, the particle shape, which includes angularity and size distribution, surface roughness, cementation, and normal stress, mainly influences the interfacial shearing behavior of soil [7]. Moreover, these factors induce variations in the apparent adhesion and interface friction angle (\(\delta\)), which are considered as the main parameters for the design and analysis of the actual soil-structure interaction. In the general design of geotechnical structures having shear resistances or frictional behaviors between soil and solid materials, \(\delta\) is considered to be in the range of 1/2-2/3 times the internal friction angle (\(\phi\)) of soil, and the magnitude of interface adhesion (\(c_{a}\)) is typically considered to be a fraction of the soil cohesion (\(c\)) [8; 9].  
Biopolymers have been recently suggested as environment-friendly soil-stabilizing materials owing to their origin and low greenhouse gas footprint [10; 11; 12; 13]. Accordingly, recent research has investigated the feasibility of biopolymer-based soil treatment (BPST) for geotechnical ground improvement practices in terms of soil strengthening and permeability reduction [14; 15; 16; 17; 18; 19; 20]. Several studies have assessed the shear strength parameters (i.e., internal friction angle and cohesion) and strengthening efficiency (e.g., unconfined compressive strength) of biopolymer-treated sand through triaxial shear, direct shear, and uniaxial compression test approaches [14; 15; 18; 21; 22; 23; 24]. Additionally, the shear parameters of BPST exhibit moisture content-dependent characteristics, which are expected to be associated with the rheological properties of biopolymer hydrogels inside the soil. As most biopolymer hydrogels (e.g., xanthan gum, guar gum) are thixotropic, they exhibit a shear thinning characteristic with decreasing viscosity and yield stress with a large strain rate [25]. The rheological characteristics of biopolymer hydrogels could affect the interface shearing behavior between biopolymer-treated soil and structures, when structural members (e.g., piles, geosynthetics) are implemented into biopolymer-treated grounds. Thus, understanding the interaction between biopolymer-treated soil matrix and adjacent structure surfaces becomes crucial for stability analysis and BPST application. Previous studies on BPST have mostly focused on the biopolymer-induced soil strengthening, while there is no research considering the interfacial interaction behavior between biopolymer-treated soil and adjoining structural (i.e., solid, non-geomaterial) members reported yet. Hence, in this study, the effects of the moisture state and biopolymer concentration on the internal and interfacial shearing behavior of xanthan gum-treated soil are reported. For this purpose, the mobilization of shear strength and volume change response of both internal and interfacial behaviors of xanthan gum-treated sand were studied via direct (laboratory) and interfacial shear tests according to a wide range of normal stress, moisture state, biopolymer concentration, and interface type conditions.