# Enhancing the Strength of Mine Residue Soil by Bioremediation Combined with Biopolymers
## 3 Results
### Effect of the Biostimulation and Bioaugmentation Techniques  
Figures 2 and 3 and Table 3 summarize the results from the UCS tests performed with the mine residue soil samples that were biostimulated (BS, Figure 2a) or bioaugmented (BA, Figure 2b), whether or not they were combined with the addition of the two biopolymers (XG or CMC). In both cases, the results are compared with the control (soil without biopolymer but BS or BA) and with the addition of Portland cement combined with BS and BA. Figure 2 also includes the results of the UCS tests of the individually applied stabilizers to assist in the interpretation and discussion of the results. Figure 3 illustrates the variation between the unconfined compressive strength (\(\mathrm{q_{u\,max}}\)) and undrained Young's modulus defined for 50% of \(\mathrm{q_{u\,max}}\) (\(\mathrm{E_{u\,50}}\)) for all the tested samples, representing the average values, the variability regarding the average value and the increase or decrease in the respective control value. Figure 1: UCS results—effect of the stabilizers applied individually (CMC = carboxymethyl cellulose; XG = xanthan gum). In general, it may be concluded that the biostimulation of the native bacteria produces worse results than the bioaugmentation technique. Indeed, independently of the combination of biostimulation and any of the studied stabilizers, the results are always below the control test (Control), revealing that biostimulation after two percolation cycles is not effective. Moreover, no substantial difference was observed between the biostimulated samples, even when combined with any of the stabilizers, which supports the conclusion that this technique is not effective for this particular mine residue soil. However, in the case of the bioaugmentation technique, the stress-strain compressive behavior is characterized by a higher strain at failure, inducing a more ductile behavior, and by an unconfined compressive strength that is higher than the control values (control and BA-control) for the stabilizers XG (BA-XG) and Portland cement (BA-cement). The poor performance of the biopolymer CMC (BA-CMC) may be due to the combination of percolation cycles and the extraction and homogenization steps, where some of the bonds promoted by the CMC seemed to be washed and/or destroyed, suggesting that CMC shows a poor mechanical behavior over time. Another possible explanation is the degradation of the biopolymer CMC by the bacteria, which needs to be confirmed. The best unconfined compressive strength result obtained with the biopolymer XG (BA-XG) is supported by the studies of [PERSON] [9], [PERSON] et al. [11] and [PERSON] et al. [13]. Figure 2: UCS results—effect of two different bio-techniques: (**a**) biostimulation (BS); (**b**) bioaugmentation (BA). (CMC = carboxymethyl cellulose; XG = xanthan gum). Moreover, [PERSON] et al. [11] observed that the addition of XG to soil promoted a significant increase in the unconfined compressive strength over time (up to three years), i.e., the XG exhibits a very good performance and stability over time. Thus, it may be concluded that despite the disturbance associated with the percolation, extraction and homogenization steps, the biopolymer XG continues to be active, and is able to stabilize the mine residue soil. Comparing the samples containing Portland cement, it may be seen that the percolation, extraction and homogenization steps induced the destruction of a great majority of the cementitious bonds responsible for the enhancement of the mechanical properties of the cement-stabilized soil. This justifies the lower unconfined compressive strength of the BA-cement sample compared to the cement sample. However, it is known that the Portland cement develops cementitious bonds (due to hydrated reactions) over long periods, during months or even years [32], justifying the increase in the unconfined compressive strength of the sample BA-cement when compared to the control sample (BA-control).