# Reducing Soil Permeability Using Bacteria-Produced Biopolymer
## 2 Materials and Methods
### Testing Procedure  
The samples used in the permeability tests were produced in six different types: natural soil, a mixture of soil with the xanthan-like biopolymer produced by bacteria (Lab XG) (0.33%), a mixture of soil with the commercial biopolymer xanthan gum for the contents of 0.33% and 0.16%, and also a mixture of soil with cement for the contents of 0.33% and 1.0%. Each type of mixture was tested for different hydraulic gradients (6.9, 8.2, 9.4, and 10.6) and after four different curing times (3, 7, 14, and 28 days). The details of the testing programme are shown in Table 2. Although all the specimens were prepared equally, the permeability tests were repeated three times for each condition tested, to assure the quality and the reproducibility of the results obtained. The stabilised soil specimens used in the permeability tests were prepared according to the following steps: (i) larger particles of the soil were removed with a sieve with a 2.0-mm mesh; (ii) the soil was dried in an oven at 105 \({}^{\circ}\)C for 24 h; (iii) the dry soil and powdered grout agent content (presented in Table 2) were mixed; (iv) the soil, the grout agent (LAB XG, commercial XG, and cement), and the water content of 14.3% (optimum water content for untreated soil using the standard Proctor test) were mixed until a homogeneous paste was obtained; (v) the paste was put inside PVC moulds (38 mm in diameter, 76 mm height) in 3 layers; (vi) each layer was compacted in order to obtain the dry unit weight of 16.2 kN/m\({}^{3}\) (optimum conditions for untreated soil using the standard Proctor test); (vii) the contact surfaces between two successive layers was scarified before the introduction of a new layer; (viii) the specimens were stored during the curing time (Table 2) in a humid chamber with a temperature of (20 \(\pm\) 2 \({}^{\circ}\)C) and humidity (95 \(\pm\) 5%) control; (viii) after the curing time, the specimens were assembled in the equipment used to carry out the permeability tests (Figure 3); (ix) a container (to measure the volume of the water that flowed through the specimen) was placed under each one; (x) the water level was adjusted in order to assure the hydraulic gradients stated in Table 2; (xi) the permeability tests were performed. Although the optimum compaction conditions obtained from the standard Proctor test changed when Portland cement and the xanthan gum were added to the soil [26; 45], all the specimens used in the work were prepared for the optimum water content for untreated soil, making it possible to compare all the results without including additional effects. Figure 2: Diagram of the production of the _Stentrophomonas maltophilia_ Faro439 strain biopolymer. The permeability tests were performed according to the scheme illustrated in Figure 3, which consists of a constant head permeability test. This scheme allows us to test eight specimens simultaneously. An overflow drain in the upper container assures a constant level of water during the tests. As is usual in this type of test, the total volume of water that flows through the specimen is measured (the accumulated water inside the containers placed under each specimen was weighed on a precision balance). The system has a set of taps that make it possible to control the water flow for each specimen. The permeability coefficient (k) was calculated according to [PERSON]'s law:  
\[\text{k}=\frac{\text{Q}}{\text{A}\times\text{i}}=\frac{(\Delta\text{V}/ \Delta\text{t})}{\text{A}\times(\Delta\text{h}/\text{L})} \tag{1}\]  
where Q is the water flow rate, \(\Delta\text{V}\) is the average of the accumulated volume of water that passed through each sample during three measurements, \(\Delta\text{t}\) is the period of time, A is the cross-sectional area of the specimen, i is the hydraulic gradient, L is the height of the specimen, and \(\Delta\text{h}\) is the sum of the L/2 and the water column height.