## 3 Results Analysis and Discussion
### The Effect of Different Fiber Additives on Soil Behavior
In the case of using random fiber reinforcement to improve the soil properties, the differences in the results obtained according to the type of fiber selected as reinforcement are significant [47]. Three different fiber types were used in experimental studies. These fibers are glass fiber, basalt fiber, and macromesh fiber, respectively. Glass fiber and macromesh fiber were used in two different lengths of 24 mm and 48 mm, while basalt fiber was used only in 24 mm length. Therefore, in the comparison of these fibers, experimental groups made with a length of 24 mm were evaluated. As can be seen in Figure 19a, using basalt fibers as reinforcement with Dr = 30%, FI = 24 mm, and Fc = 1% yielded higher ultimate bearing capacity values compared to the use of glass fiber and macromesh fiber at all reinforcement depths. In other groups, the ultimate bearing capacity values of macromesh fiber-added soils are higher than basalt and glass fiber-added soils. Strength increases are related to the shear stresses that occur in the soil as a result of the incorporation of fibers into the soil, revealing the tensile strength of the fibers. [PERSON] et al. [48] investigated the effect of fiber additives of different types (polyppropylene, basalt, and glass) on the mechanical performance of sand, similar to our study. When the results were examined, polypropylene fiber with the same content gave more strength to the treated floor due to its good flexible structure and strength properties compared to other fibers. [PERSON] et al. [49] reported that the better interconnection properties of polypropylene fiber also contribute to the ductility gain. In addition, the superiority of macromesh fiber-reinforced samples compared to glass and basalt fiber-reinforced samples is due to the higher surface roughness. [PERSON] et al. [50] and [PERSON] et al. [51] studies, they reported that the fiber reinforcement mechanism is based on friction and the roughness of the surface of the fibers strongly affects the sand fiber interaction. A fiber's original surface roughness has an impact on how well its strength is mobilized by nearby soil particles. Hard sand particles that touch the fiber surface during the cutting process cut abrade it, causing plastic deformation and even the loss of a portion of the surface layer. The development of holes and grooves on the surface of the fiber increases the interactions between that surface and the sand matrix by forming an interlock. Compared to basalt fiber and glass fiber macromesh fiber, their positive effects on strength increase were more limited as they have a smoother surface. When the experimental results with hybrid fibers are examined, as observed in the separate use of the fibers, the increase in the relative density, the increase in the depth of reinforcement, the increase in the fiber additive ratio from 1% to 2%, and the increase in the fiber length have positively affected the increase in the ultimate bearing capacity value. In [PERSON] et al. [36], [PERSON] et al. [52] and [PERSON] et al. [53]'s studies, it has been stated that the simultaneous use of fibers with different lengths and aspect ratios (hybrid) improves soil properties more effectively than a single fiber. However, in the experiments, the ultimate bearing capacity values obtained from the soils with macromesh fiber additives are mostly higher than the ultimate bearing capacity values obtained from the soils with hybrid fiber additives. Among the hybrid experimental groups, the highest ultimate bearing capacity was obtained from the group reinforced at 3B depth, Dr = 30%, Fc = 2%, and in which basalt and macromesh fiber were used together. In addition, the ultimate bearing capacities obtained from the glass fiber-reinforced tests were lower than all the other fibers used. However, much higher qu values were obtained when basalt and macromesh were mixed with fiber.