# Enhancing the Strength of Mine Residue Soil by Bioremediation Combined with Biopolymers
## 2 Materials and Methods
### Testing Program and Sample Preparation  
Table 2 summarizes the experimental testing program.  
The laboratory testing program started with the homogenization of the mine residue soil. After air-drying the soil, a Proctor test [27] was performed to evaluate the optimum water content (19.5%) to be added to the soil to achieve the \"best\" compaction conditions. The wet mine residue soil sample was introduced in a PVC mold (76 mm height, 37 mm diameter) in three layers. For each layer, the sample was statically compacted with a steel bar 20 times; the surface of the first and second layers was scarified before the introduction of the next layer; the top surface was smoothed with a spatula. The samples were cured for seven days in a room with temperature (20 \(\pm\) 2 \({}^{\circ}\)C) and humidity (95 \(\pm\) 5%), which served as the control.  
For the samples stabilized with biopolymer (XG or CMC) or Portland cement, the dry mass of each stabilizer was mixed with the wet mine residue soil and then introduced into the PVC mold following the previously described procedure.  
For the bioaugmented samples, a bacterial B2A2W2 suspension of approximately 5.0 \(\times\) 10\({}^{7}\) cells per gram of the dry mine residue soil was added to the top layer, following the remaining part of the procedure described above.  
For the BS and BA samples, the testing procedure can be summarized in the following steps: (i) mix all the materials and introduce the samples to the PVC mold, as described above; (ii) percolate the culture media R2 Ab through the samples for 7 days; (iii) extract the sample from the PVC mold to undergo homogenization, followed by the re-introduction of the samples to the PVC mold, as described previously, and curing for 7 days; (iv) repeat steps (ii) and (iii). The extraction and homogenization steps, followed by the re-introduction  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test ID** & **Biopolymer** & **Bio-Technique** & **Number of UCS Tests** \\ \hline control & No & & 2 \\ cement & No & no & 2 \\ CMC & Yes & & 2 \\ XG & Yes & & 2 \\ \hline BS—control & No & & 2 \\ BS—cement & No & & 2 \\ BS—CMC & Yes & & 2 \\ BS—XG & Yes & & 2 \\ \hline BA—control & No & & 2 \\ BA—cement & No & & 2 \\ BA—CMC & Yes & & 2 \\ BA—XG & Yes & & 2 \\ \hline \hline \end{tabular} CMC = carboxymethyl cellulose; XG = xanthan gum; BS = biostimulation; BA = bioaugmentation.  
\end{table}
Table 2: Experimental testing program.  
of the sample to the PVC mold, aim to evaluate if the tested bio-based techniques remain active in a real adverse scenario where the sample has been disturbed.  
The percolation system is a constant head permeability test with the modifications proposed in [28]. The tests were performed under a constant head of 5 kPa.  
Finally, the samples were extracted from the PVC mold and weighted, and UCS tests were performed to assess the stress-strain compressive behavior. The UCS test was carried out in a universal testing machine (Wykeham Farrance Tristar 5000 kg) under a constant axial strain rate of 1%/min, as stated in the standard [29]. The maximum uniaxial compressive strength (q\({}_{\text{u max}}\)) of the samples is a fundamental design parameter obtained from the stress-strain curve [30], as well as the strain at failure (\(\varepsilon_{\ell}\)) and the undrained Young's modulus, defined for 50% of q\({}_{\text{u max}}\) (E\({}_{\text{u 50}}\)).