*3.4. SEM Images Results*

Figure 9 shows SEM images results for some selected samples. Figure 10a shows the M2 soil sample taken at 500X, showing a fiber coming out from the soil matrix. Moreover, Figure 9b,c correspond to samples M2 and M4, with a fiber still attached to the soil and presenting a good impregnation of the soil to the fiber.

Figure <sup>10</sup> presents SEM-EDS images of kaolin soil. Figure 10a shows a <sup>3</sup> <sup>×</sup> <sup>10</sup> <sup>−</sup><sup>3</sup> mm<sup>2</sup> spectrum SEM-EDS analysis of the structure and chemical composition of the soil used. Oxygen is the main element present in the material with a concentration of about 48.78%; aluminum and silicon are also significant components with concentrations of about 23.46% and 24.66% respectively. Finally, the soil has low concentrations of thallium and iron, being 1.98% and 1.12% of the respective concentrations. Concentrations are given as percentage of weight. Figure 10b presents a 49 µm<sup>2</sup> spectrum SEM-EDS analysis of the soil showing bacterial growth in the soil under natural conditions. These are rod-like microorganisms with an average 0.5 µm width and 15 µm length and a concentration about 10<sup>4</sup> microorganism/mm<sup>2</sup> .

**Figure 8.** Soil samples after failure in compression tests: (**a**) samples before the test, (**b**) M0, (**c**) M2, (**d**) M4), (**e**) M6, (**f**) M8, (**g**) M10, (**h**) M12.

**Figure 9.** SEM images for (**a**) the end of a fiber; (**b**) the M2 soil sample, and (**c**) the M4 sample.

**Figure 10.** SEM-EDS for kaolin soil images, (**a**) soil used, (**b**) soil with bacterial growth.

Figure 11 shows the biological activity developed in sample M12. Figure 11a displays a microorganism that grew in the soil pores and extended around the micro-channels on the soil matrix with an average width of 2.6 µm, thickness of 0.4 µm, and thin film shape. Microorganism appeared after 4 days in some samples. In Figure 11b, another view of the microorganism described can be seen, as it rises from the bottom and has some particles adhering to it. Figure 11c shows the macro-scale image of microorganism growth in sample M12 at the external layer.

**Figure 11.** (**a**) Pseudo colored SEM images of M12 internal biological activity; (**b**) another area of the same sample showing a different configuration; (**c**) M12 external biological activity.
