# A Review on the Importance of Microbial Biopolymers Such as Xanthan Gum to Improve Soil Properties
## 4 Xanthan Gum Biopolymer
### Effects of Xanthan Gum on Soil Properties for Geotechnical Engineering
#### 4.3.1 Mechanical Properties  
A study from [PERSON] et al. [30] concerned the strength behaviour of a sand-xanthan gum mix in three different phases of the gel: initial, dry and re-submerged. For the initial phase, the test was performed as soon as the samples were prepared; in the dried phase, the samples were in a drying room for 28 days; and the re-submerged samples were submerged in distilled water for 24 h before performing the tests. The study concluded that even a very small addition of xanthan gum to the sand can change the strength behaviour of this type of soil. The tests proved that increasing the xanthan gum content in the initial gel increases cohesion, but the friction angle remains constant. In the dry gel phase, both the cohesion  
Figure 3: Prices of five common biopolymers (based on Jang [28]). angle and friction increased with the addition of more xanthan gum content due to the formation of the biofilm that occurs on the surface of particles, as well as the formation of viscous hydrogels that induces the bioclogging effect. Otherwise, the friction angles of the sand-xanthan gum mixture decreases as the xanthan gum content increases (above 1%) in the submerged condition. This is because the interaction on the surface friction between inter-particles is reduced due to the increased pressure generated [11; 13; 30]. One of the reasons that explain the improvement in the shear strength in soil-xanthan gum mixtures is due to the presence of different chemical functional groups, such as hydroxyls and esters. Moreover, its long chain also allows additional locations in which the characteristic chemical reaction of a specific functional group can occur. As described for other polymers, the chemical bonds between the biopolymer and the soil particles are formed due to the adhesive forces that hold the soil particles on their surfaces. At microscopic scales, this effectiveness is observed through connective forces existing at the gel-particle interface, which are forces that contain and allow ionic/electrostatic or covalent bonds (i.e., chemisorption), hydrogen bonds (i.e., strong polar attraction) and Van der Waals bonds (i.e., physical absorption) [19; 22; 24; 28]. Tests performed to determine the effect of xanthan gum content (from 0.25% to 2%) on the modulus of elasticity of biostabilized sandy and silty soils by [PERSON] et al. [29] show, for both soils, that the modulus of elasticity increases with the biopolymer content. Several studies have shown that xanthan gum can be used successfully to improve the unconfined compressive strength and stiffness of sands [17; 31; 32; 33], silty sands [34], clays [17; 35], residual soils [36], bentonite, kaolinite [17; 25; 37], and soft marine clays [24], as well the undrained shear strength of mine tailing material [38]. Additionally, experimental results revealed that the most favourable xanthan gum content for soil stabilization is in the range of 1-1.5% for ambient curing and oven curing [39] and 2.5% for curing in submerged conditions [35]. [PERSON] et al. [40] concluded that the optimum xanthan gum content depends on the soil type and water content. Experimental results also showed that the mechanical properties (unconfined compressive strength and stiffness) tend to improve with curing time (up to 28 days) for bentonite and kaolinite [39; 40], clays [17; 35] and sandy soils [17; 31; 32; 40]. A similar time evolution was obtained for the shear strength and its strength parameters (cohesion and angle of shear strength) of clayed soils obtained from direct shear tests [35; 39]. Although this tendency was confirmed for a xanthan gum-treated bentonite and kaolinite for a curing time up to 90 days [39], the results obtained from a biostabilized sand with silt indicate that the soil strength does not change considerably after a curing time of 5 days [40].