## 2 Materials and Methods  
### Materials and Classification Tests  
To achieve the research objective, expansive clay soil was excavated from Demsarkho-Lattakia at a depth of 3 m from the ground level, surpassing the backfill soils to access the targeted soil layer. Fine-grained marine sand was used for mixing with the expansive clay, which was washed to eliminate fine particles and obtain clean sand suitable for experiments. Mixtures were prepared by adding different percentages of sand (10%, 20%, 30%, 40%, and 50%) to the expansive soils based on the dry weight. To determine the grain size distribution, experiments were conducted using the dry sieving method for fine sand and hydrometer sedimentation for clay soil according to ASTM standards [59,60]. Figure 1 illustrates the granular gradient curves of the mixed soil, based on the percentage of sand added to it. Specific weight tests were carried out in accordance with ASTM D854-98 [61], revealing a specific gravity of 2.7 for the expansive clay and 2.65 for the sand. The chemical composition of clay has been listed in Table 1.  
The Afterberg Limits experiments were performed in accordance with ASTM international standard D854-14 [62] on mixtures with varying percentages of sand, ranging from 0 to 50%. The results of the experiments, depicted in Figure 2, reveal the variation in the Liquid Limit (LL), Plastic Limit (PL), and Plastic Index (PI) with respect to the quantity of sand added. These findings are in line with previous studies investigating the effect of adding sand to the expansive soil for the purpose of soil improvement [14,22,63].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Chemical Composition** & \% \\ \hline Alumina (Al2O3) & 11.5 \\ Ferric (Fe2O3) & 5.5 \\ Calcium (CaO) & 12 \\ Magnesium (MgO) & 2.4 \\ Silica (SiO2) & 48.8 \\ Sodium (Na2O) & 1.2 \\ Potassium (K2O) & 0.36 \\ Loss of ignition (LoI) & 17.24 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical properties of clay.  
Figure 1: The granular gradient curves of the tested mixtures.  
The clear clayey soil used in this study, without sand, was classified as A-7-5 according to the AASHTO classification system [64] and as CH according to the Unified Soil Classification System [65]. The degree of volume change was determined to be high to very high according to [66; 67; 68]. The swelling pressure of the utilized expansive soil, prepared under standard proctor parameters of \(\gamma_{d}\) = 13.95 kN/m\({}^{3}\) and \(SR\) = 91.2%, revealed a substantial swelling pressure of almost 200 kPa while the swelling pressure of the utilized expansive soil, prepared at \(\gamma_{d}\) = 15.3 kN/m\({}^{3}\) and \(SR\) = 75%, was observed to be almost 520 kPa. The percentage of clay particles (with a diameter smaller than 0.02 mm) was found to be 73.9%. The free swell [69] for clear clayey soil was determined to be 127%.  
Standard proctor experiments were carried out to determine the maximum dry unit weight and optimal moisture for each mixing percentage, as illustrated in Figures 3 and 4. The results indicate a continuous increase in the maximum dry unit weight, which is consistent with the findings of previous studies [63; 70]. This can be attributed to the reduction in soil pore volume and suction stress resulting from the replacement of a soft component, which can hold a large amount of water, with a coarse component that has a low water-holding capacity. This is further supported by the optimal moisture values of the mixtures, which decrease as the percentage of added sand increases, as shown in Figure 4. For more details on the Proctor, Alterberg, and consolidation tests, see [PERSON] and [PERSON] [71].  
Figure 4: Relationship between optimal moisture and percentage of added sand.  
Figure 3: Relationship between maximum dry unit weight and percentage of added sand.  
Figure 2: The relationship curves between the liquid limit, plastic limit, and plastic index with the percentage of added sand.