## 1 Introduction  
Chemical treatment is considered a soil improvement technique by adding agent stabilizers to the soil. Portland cement and lime as preferred binder materials are traditionally used to enhance the engineering properties of soil, such as soil strength, hydraulic conductivity, and erosion resistance of the weak ground. Although cement has been widely employed for different construction purposes and ground improvements, overdependence and overuse of cement have had a major impact on the environment and causing many environmental concerns recently. After the coal and oil and gas industry, the cement industry is the third major sources of carbon dioxide (a potent greenhouse gas) emitter with annually up to 8% of global CO\({}_{2}\) emissions [1]. This highlights the need to use sustainable techniques in civil and geotechnical engineering practices. Recently, an attempt has been made to use biopolymers as environmentally friendly additives in geotechnical engineering applications. Biopolymers are organic polymers produced from natural resources. Based on their source of origin, biopolymers can be classified into three main groups: plant-based, animal-based, and biopolymers produced by microorganisms [2]. Gunar [3], lignin [4, 5], agar [6], beta-glucan [7], and alginate [8] are some examples of plant-based biopolymers whose applications in geotechnical engineering were previously studied. Regarding animal-based biopolymers, chitosan which is produced from the waste crustacean shells, and caisin, a protein-based biopolymer extracted from milk and dairy products, are two animal-based biopolymers whose applications in geotechnicalengineering were investigated [9; 10]. Microorganism-based biopolymers such as xanthan gum, gellan gum, and dextran are produced from the fermentation process. Their potential for soil properties improvement has been reported in previous studies [11; 12; 13; 14]. The enhancement of soil strength through biopolymer additive agents has been investigated previously. [PERSON] et al. [8] examined the effect of different biopolymers, including xanthan gum, beta-glucans, guar gum, chitosan, and alginate on the improvement of soil strength by conducting various laboratory experiments. They reported substantial increases in biopolymer-treated soil strengths over a longer period. Lignin, a by-product of paper industries and sugarcane mills, has shown its effectiveness in improving soil strength [15; 16]. Through conducting UCS tests, the effect of wetting and drying cycles on the guar gum and xanthan gum-treated silty sand was assessed by [PERSON] et al. [3]. They confirmed the effectiveness of a 0.5% xanthan gum additive in lowering the loss of soil strength reduction after each wetting and drying cycle. Among all applicable biopolymers in geotechnical engineering, xanthan gum has been shown to be one of the best cost-effective materials and was selected for use in the current study. To improve the efficacy of biopolymer treatment, in-situ influencing elements must be taken into account. Even though xanthan gum proved to increase the compressive and shear strength of soil [8; 13], previous research mainly concentrated on investigating fundamental strengthening behavior and validating feasibility. In particular, inadequate consideration of in situ three-dimensional stress situations has been made. Besides, in previous studies, the shearing behavior of xanthan gum-treated soil has been mostly investigated through conducting shear box tests for dry samples which may not properly represent the underground conditions. Additionally, a few studies investigated the effect of wetting and drying cycles on xanthan gum-treated soil. To address the mentioned gaps, this study aims to investigate the shear strength properties of xanthan gum (XG) under different confining pressures and biopolymer concentrations.