## 2 Methodology
### Database  
For models training and testing, two independent databases were compiled, respectively, for UCS and ITS studies, containing 121 records in the first case and 94 in the second. All samples were prepared under a controlled environment in the framework of a laboratory testing program developed at the University of Coimbra. This program aimed to characterize the compression and tensile behavior of soil-binder-water mixtures reinforced with fibers through unconfined compressive strength tests and indirect tensile strength tests (the later ones also called split tensile strength tests). Soils characteristics (grain size composition, organic matter content, water content, Atterberg limits), binder content, curing time and fibers characteristics (changing origin, length, fiber content and mechanical properties) were parameters considered in the study [4; 6; 10; 11; 12; 47; 48]. The soils used in the preparation of the laboratory samples comprise natural soils (collected in the Mondego river lower valley area and in a gravel-silty pit) and laboratory-made soils (starting from natural soils a specific property was varied, e.g., organic matter content and sand content), ranging from cohesive to cohesionless soils, organic to nonorganic soils, presenting different geotechnical properties. In all cases, soils were chemically stabilized with Portland cement, the most widely used binder in soils stabilization [49], applied in different amounts ranging from 75 to 500 kg/m\({}^{3}\). Concerning the fibers, four distinct types have been used trying to encompass all the types of fibers usually applied in soils stabilization. Thus, it was selected a natural fiber (Sisal) and three artificial fibers, a synthetic one (polypropylene), and two metallic fibers (Dramix and Wiremix, varying the fibers anchorage conditions), characterized by different mechanical properties, namely stiffness and tensile strength. The fibers length changed from 12 to 30 mm, and they were applied in different amounts ranging from 2 to 150 kg/m\({}^{3}\). A detailed description of all materials may be found in [4; 6; 10; 11; 12; 47; 48]. As models input, a set of 16 variables were selected. Among all variables available in the framework of the study, these 16 features are identified in the literature as influents on mechanical properties behavior [30; 50; 51; 52; 53]. Moreover, from a statistical point of view, they were also identified as relevant, as shown in the correlation matrix depicted in Figure 2, which relates to the UCS study. Considering that the formulations prepared for both studies (UCS and ITS) are similar, the equivalent representation for ITS is also similar. For that reason, it was not included in the paper. In addition, the selection of the variables was also supported on a try and error procedure using the evaluation metrics described above. Below, all 16 input variables considered in this study are listed on both mechanical properties' prediction of reinforced soil-binder-water mixtures with fibers:  
* Soil sand content (%)--%_Sand_
* Soil silt content (%)--%_Silt_
* Soil clay content (%)--%_Clay_
* Soil organic matter content (%)--%_OM_
* Liquid limit--W_LL_
* Plastic limit--W_PL_
* Water content (%)--_o_
* Cement content (%)--_a_\({}_{w}\)
* Cement dosage (kg/m\({}^{3}\))--_D_Kg/m\({}^{3}\)
* Ratio between water and cement contents--\(\omega_{0}/a_{w}\)
* Age of the mixture (days)--_t_
* Length of the fiber (mm)--_L_filter
* Fiber content (%)--_T_filter_
* Fiber dosage (kg/m\({}^{3}\))--_F_Kg/m\({}^{3}\)
* Tensile strength of the fiber (MPa)--_fct_filter_
* Deformability modulus of the fiber (GPa)--_E_filter_  
Table 1 summarizes the main statistics of all 16 inputs variables, as well as of the output variables (UCS and ITS), showing the wide range of binder and fiber contents. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Variable**} & \multicolumn{2}{c}{**Minimum**} & \multicolumn{2}{c}{**Maximum**} & \multicolumn{2}{c}{**Mean**} & \multicolumn{2}{c}{**Std.