# Reducing Soil Permeability Using Bacteria-Produced Biopolymer
## 1 Introduction  
The growing development and consequent increase in construction in the main urban centres around the world results in a need for infrastructures over and/or inside geotechnical formations lacking the most favourable characteristics. In the past, these formations were frequently considered inappropriate for civil engineering. The main limitations of these types of soils are usually related to insufficient mechanical properties and permeability characteristics that are inadequate for most building practices. Some ground improvement techniques are frequently used to enhance the properties of these soils to overcome these limitations and to allow the safe construction of infrastructures. Currently, chemical stabilisation using cement-based binders is one of the techniques most used [1]. However, the application of cement-based materials raises some concerns from an environmental point of view, in particular: (i) the activity associated with the production of cement creates significant CO\({}_{2}\) emissions; in fact, in 2014, this activity contributed to about 8-10% of the global CO\({}_{2}\) emissions [2; 3; 4; 5]; (ii) the use of cement-based materials for soil stabilisation creates an irreversible composite material [4]; (iii) the use of cement-based binders for soil stabilisation induces an increase in the pH of the soil, with detrimental effects on the vegetation and microbial communities [4]. In order to decrease the ecological footprint caused by the cement-based materials, some alternative bio-based materials that promote the enhancement of the soil's properties have been investigated during the last few years. These biomaterials can be obtained from the activity of microorganisms and/or the use of substances derived from them. Bacteria-producing materials [6; 7; 8] and enzymes [9; 10; 11; 12; 13] have been used to catalyse thehydrolysis of the urea in porous media to promote bioccmentation and biopolymers have been used to modify the properties of soils [5; 14]. The effect of the bioccmentation process on such properties is usually due to the association of two factors, the creation of some bonds in the solid skeleton and the filling of the empty spaces between the soil particles with calcium carbonate crystals. Results published with this methodology using sandy soils have showed that the bioccmentation process promotes an improvement in the mechanical behaviour [7; 9; 10; 11; 15; 16; 17] and a decrease in the coefficient of permeability of the bioccmented soil [16; 18]. During the last few years, several studies have investigated different biopolymers for soil improvement, specifically in terms of the: (i) improvement of the mechanical behaviour with the use of Guar gum [19], \(\delta\)-glucan [20; 21], and xanthan gum [22; 23; 24; 25; 26]; (ii) change in the compressibility characteristics with the use of Guar gum [27], cationic e-polylysine [23], xanthan gum [19; 23; 26], gellan, and agar gums [4; 21; 28]; (iii) change in the plasticity properties with the use of Guar gum [29], cationic e-polylysine [23], and xanthan gum [22; 23]; (iv) reduced permeability coefficient with the use of Guar gum [30], chitosan gum [31], and xanthan gum [20; 25]; (v) use of Chitosan gum to remediate contaminated soils [4; 32]; (vi) reduce the erosion of the soil with the use of xanthan gum [21] and casein and sodium caseinate salt [31]. Recent research concerning the effect of xanthan gum (XG) on the hydraulic conductivity of XG-soil mixtures can be summarised as follows: (i) The biostabilisation of sandy and silty soils with XG induces the filling of the pore spaces of the soil with hydrogels, which obstruct the water flow (i.e., pore-clogging) and consequently reduce the permeability coefficient of the XG-soil mixture. This effect tends to be more significant with an increase in the XG content of the mixture [19; 20; 25; 33; 34]. In fact, the experimental results of [PERSON] et al.