## 2 Materials and Methods
### Specim Preparation  
The soil was first oven-dried followed by crushing and sieving to 2.36 mm to remove the gravel. With respect to the dry mass of soil, three concentrations of biopolymer agent, including (0.5, 1, and 2%) of XG by dry soil weight, were used. In the current study, the dry mixing technique followed by [PERSON] et al. [8] was adopted. The XG powder was first uniformly mixed with dry soil for about 15 min. Then, the desired mass of water, corresponding to the optimum water content obtained from the compaction test, was sprayed into the soil mixture and thoroughly mixed. To avoid the formation of aggregations and ensure a homogenous mixture of biopolymer with soil particles, the resulting mixtures were sealed with double-layer plastic wrap and placed at a controlled room temperature for 24 h. To make a soil sample, the prepared soil mixtures were placed into a cylindrical metal mold and evenly compacted in 5 layers. The mold had a diameter of 50 mm and a length of 150 mm, and the specimens with the same diameter and length of around 110 mm were extruded from the mold after each compaction set. The dry density of each specimen was ensured to be above 95% of the maximum dry density. UCS tests. To define the optimum curing time, the prepared specimens were placed in a controlled temperature room at various times, including (0, 1, 4, 7, 10, 14, 28, and 35 days). The specimens without curing time were taken as wet samples, and the rest were considered dry specimens. The UCS tests in accordance with ASTM D2166-06 [22] were performed for the all-prepared specimens. Three replicated specimens were prepared and examined to minimize experiment errors. With regard to the results of the UCS test, the optimum curing time for the stabilized samples was selected for the following experiments. Triaxial tests. The effect of saturation conditions on the strength of biopolymer-treated soil was investigated by conducting triaxial tests.The consolidated undrained (CU) triaxial tests for the saturated samples and unconsolidated undrained (UU) triaxial tests for the dried specimens were performed at three different confining pressures (50, 100, and 200 kPa). The sample preparation was consistent with the UCS tests under optimum curing times. Once the samples were cured accordingly, the CU and UU triaxial tests were conducted. For the UU tests, no saturation was carried out on the given specimen, and the dry specimen was first subjected to the considered confining pressure, and then the shearing was directly applied. Wetting and drying cycles tests. To determine the effect of wetting and drying cycles on the soil strength, six wetting and drying cycles with respect to the results of curing time were considered. To conduct the part of experiments, PVC molds (inner diameter of 51 mm and length of 130 mm) with bottom caps were first constructed. This extra space allowed the specimen to expand once it got saturated. Each cycle was initiated with one wetting cycle in which the cured dry sample was placed into a PVC mold, and then the mold was mounted in water for 24 h. The drying cycle was then started by expelling the specimen out of the mold and letting it dry under room temperature conditions for the given optimum curing time. Following the completion of each cycle, the UCS test was performed to achieve the compressive strength of each treated and untreated soil sample. Soaking test. The soaking test for the given concentrations of XG-treated soil was conducted to examine the effect of such biopolymer treatment on the moisture stability of soil mixtures. All prepared treated and untreated specimens were submerged in water for over five days, and the degree of disintegration was visually assessed. SEM analysis. An untreated sample and a specimen treated with 2% XG were prepared and cured accordingly, and SEM imaging was conducted for the samples. Imaging was done on the TESCAN MIRA 3 scanning electron microscope, using 5 kV voltage with magnifications of 2500 and 15,000 times for low and high magnification images, respectively.