## 4 Collected Database and Statistical Analysis  
A comprehensive and detailed dataset consisting of 181 data points was created for the development of predictive models. The independent variables were selected according to two key criteria: (1) they represent standard geotechnical index properties routinely measured in site investigations, and (2) they have been established as significant factors influencing swelling pressure, as discussed in the Section 1 based on the existing literature. Prior to modelling, a statistical analysis was conducted to understand the behaviour and relationships of the input variables in the dataset. The goal was to evaluate their distribution, variability, and possible interdependencies and to identify any multicollinearity or redundancies that might affect model interpretation. Table 1 provides descriptive statistics for all the input parameters analysed in this study. It includes measures of central tendency, dispersion, and distribution characteristics. In Figure 3, the distribution histograms of the parameters considered are depicted. Table 1 and Figure 3 provide a detailed understanding of the characteristics of the clayey soils included in the dataset, making it easier to interpret their properties. The soil fraction below #0.075 mm has an average of 81.2% with a standard deviation of 20.2%, indicating a notable level of variability. Additionally, its mode is 100%, indicating that a considerable number of samples were composed almost entirely of fine-grained material. Regarding the LL, a nearly symmetric distribution with a mean of 51.3% is observed, accompanied by a moderate level of variability (standard deviation of 16.3%). PI shows a slight right skew with a mean of 27.3% and moderate variability. Natural water content (w) exhibits relatively low variability with a mean of 23.9%. The variable \(\rm\gamma_{d}\) demonstrates a symmetric distribution and relatively low variability. Lastly, the \(\rm p_{s}\) shows a right-skewed distribution with a mean of 163.0 kPa and a median of 149.2 kPa. The high variability in the data is indicated by a standard deviation of 105.7 kPa. Next, all soils were classified according to the USCS. The results of this classification are presented in Table 2. In addition, Figure 4 presents the distribution of the 181 undisturbed soil samples on the Casagrande plasticity chart, with colour intensity representing sample density. Most samples are concentrated above the A-line, corresponding predominantly to clays of medium to high plasticity (CL and CH, according to the USCS). A dense cluster can be observed between LL values of 30 and 60% and PI values of 12-32%, indicating a dominance of plastic clayey soils. Only a few samples (approximately 20%) fall below the A-line, within the silt categories (ML or MH). Specifically, the results of this classification indicate that 143 samples correspond to clays (C), 48 to silts (M), 93 to high-plasticity \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Property** & **Mean** & **Median** & **Mode** & \begin{tabular}{c} **Standard** \\ **Deviation** \\ \end{tabular} & **Minimum** & **Maximum** & **Kurtosis** & **Skewness** & \
\begin{tabular}{c} **1 st** \\ **Quartile** \\ \end{tabular} &
\begin{tabular}{c} **3 rd** \\ **Quartile** \\ \end{tabular} \\ \hline Pass & & & & & & & & & & & \\ \#0.075 & 81.2 & 88.3 & 100.0 & 20.2 & 28.2 & 100.0 & 1.29 & \(-\)1.55 & 77.8 & 94.1 \\ mm (\%) & & & & & & & & & & \\ \(\mathrm{LL}\) (\%) & 51.3 & 50.3 & 37.2 & 16.3 & 27.7 & 85.1 & \(-\)0.99 & 0.11 & 37.8 & 65.4 \\ PI (\%) & 27.3 & 26.0 & 18.5 & 11.1 & 7.9 & 57.8 & \(-\)0.32 & 0.53 & 18.7 & 34.9 \\ w (\%) & 23.9 & 25.8 & 26.3 & 7.5 & 8.9 & 49.1 & 0.30 & \(-\)0.20 & 19.7 & 29.0 \\ Y4 & 16.0 & 16.0 & 15.1 & 1.3 & 13.0 & 19.2 & \(-\)0.58 & 0.06 & 15.1 & 16.8 \\ ps (MPa) & 163.0 & 149.2 & 40.2 & 105.7 & 17.2 & 514.2 & 0.58 & 0.88 & 79.6 & 221.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistical characteristics of main parameters considered in this study. Figure 3: Histograms for the dataset.