# Reducing Soil Permeability Using Bacteria-Produced Biopolymer
## 1 Introduction
[34] show that treatment of sand and silty soil with a content of 2% of XG leads to a reduction in the coefficient permeability to 10% of the value shown for the untreated soils. (ii) The results of the mixture of XG (0.5%-3%) with sand-bentonite are not in line with the behaviour described previously for biostabilised silty and sandy soils since a slight increase in the permeability coefficient is observed with the increase in the XG content [35]. These results highlight the effect of the finer particles (silt or clay) in the soil type in terms of the effectiveness of the treatment with biopolymers. In fact, the increase in the XG content strengthens and makes more numerous the bonds between the clay particles (ionic/electrostatic, hydrogen, and Van der Waals). This promotes the creation of more and bigger-sized aggregates of clay particles. This leads to the widening of the water flow channels and a consequent slight increase in the coefficient of permeability, within one order of magnitude [35]. (iii) The experimental findings concerning the effect of the curing time on the permeability coefficient of treated soils show some contradictory results. Thus, while [PERSON] et al. [33] show a decrease in the permeability coefficient of treated Ottawa sand (0.43-0.6 mm size sand) over 11 days of curing, the results obtained by [PERSON] et al. [25] with treated Narli sand (0.075-1.0 mm size sand) do not show a clear tendency over the same range of curing time. On the other hand, for a longer curing time, most of the experimental results show an increase in the permeability coefficient with the increase in the curing time for mixtures of XG with sand and silt, which is justified by the dehydration of the XG expected over time [25; 34]. Indeed, SEM images confirm the occurrence of shrinkage over time, which increases the size of the pore spaces, and consequently an increase in the soil's permeability [34]. (iv) The increase in the permeability coefficient observed over the curing time (in the long term) is more significant in treated sands than in treated silts since the finer particles of silt delay the evaporation of the water in the voids and the dehydration of the hydrogels deposited in the pore spaces [34]. (v) The gas permeability also decreases after the biostabilisation with XG; indeed, a mixture of 3% xanthan gum with kaolin clay decreases the gas permeability of the mixture in relation to the untreated clay by up to two orders of magnitude [36]. This effect is more significant for a higher water content since there is more water available for the hydration of the biopolymers, which promotes a more effective pore-clogging [36]. As the XG is a polysaccharide, there is an important concern, related to its long-term durability when it is used in geotechnical engineering since this biopolymer (like the generality of biopolymers) is biodegradable over time [5]. In spite of this issue, some works have revealed a satisfactory durability, for example: (i) a mixture of sand, gravel, and kaolin treated with XG shows a reasonable durability against water-induced deterioration [37]; (ii) a sand biostabilised with XG submitted to slake durability tests showed a better resistance to disintegration than with the use of Portland cement [38]; (iii) the long-term strength (until 730 days) of a sand treated with XG does not reveal a decrease in the strength, that is, traces of the biodegradability [39]. It should be mentioned that sometimes, the biodegradability of the XG can become an advantage, for instance, in temporary works performed in sensitive areas, from an environmental point of view and where, after the work, it is necessary to remove all traces of the intervention performed.