# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
## 3 Results and Discussion
### Unconfined Compression Strength  
The unconfined compression strength of a given soil specimen is the peak axial stress resisted by an unconfined cylindrical soil specimen. The absence of confining pressure in the specimen eases the determination of the undrained shear strength; this value is simply half of the unconfined compressive strength of the sample. A uniform strain rate of 0.25 mm/s was applied to each treated soil.  
Table 11 presents untreated samples' unconfined compressive strength values with varying curing periods. Results show an increase in unconfined compressive strength as each specimen was subjected to a more extended curing period, a similar trend observed by [PERSON] et al. [56]. This can be attributed to the prolonged pozzolanic reactions within the specimens [57] and thixotropic hardening, which is characterized by the return of soil to a harder state under constant water content or volume conditions from a softening state [58]. This occurs due to the presence of energy imbalances by the rearrangement of soil particles by compacting or remolding soil samples. The disturbed soil from compaction gradually recovers its original strength or stiffness by attractive forces, creating a structure until the equilibrium state of the soil strength is reached.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Hyperbolic Parameters**} & \multicolumn{2}{c}{**Hyperbolic Parameters**} \\
**Specimen** & \multicolumn{2}{c}{**Defining \(\alpha\)**} & \multicolumn{2}{c}{**Defining \(\beta\)**} \\ \cline{2-5}  & \(K_{a}\) & \(m\) & \(K_{b}\) & \(r\) \\ \hline Untreated soil & 0.0038 & \(-2.308\) & 0.0041 & \(-1.032\) \\ Soil + 5\% gypsum & 0.0195 & \(-0.356\) & 0.0046 & \(-0.653\) \\ Soil + 10\% gypsum & 0.0315 & \(-0.124\) & 0.0110 & \(-0.405\) \\ Soil + 15\% gypsum & 0.0326 & \(-0.019\) & 0.0076 & \(-0.965\) \\ Soil + 5\% gypsum + 10\% RHA & 0.0506 & \(-0.002\) & 0.0073 & \(-1.083\) \\ Soil + 10\% gypsum + 10\% RHA & 0.0092 & \(-0.378\) & 0.0074 & \(-0.881\) \\ Soil + 15\% gypsum + 10\% RHA & 0.0163 & \(-0.483\) & 0.0067 & \(-0.921\) \\ \hline \hline \end{tabular}
\end{table}
Table 10: Hyperbolic parameters for volumetric change of fine-grained soil and treated variants in a saturated condition.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Curing Period (Days)** & **UCS (kPa)** \\ \hline
7 & 131.84 \\
14 & 163.18 \\
21 & 241.52 \\
28 & 259.29 \\
35 & 383.39 \\ \hline \hline \end{tabular}
\end{table}
Table 11: Unconfined compressive strength of samples not subjected to capillary soaking with varying curing days.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Shear Modulus Hyperbolic Parameters**} \\ \cline{2-5}
**Specimen** & \(K\) & \(n\) & \(\phi_{o}\) (deg) & \(\Delta\phi\) (deg) & \(R_{f}\) \\ \hline Untreated soil & 46.478 & 0.938 & 35.701 & \(-4.394\) & 0.951 \\ Soil + 5\% gypsum & 22.669 & 0.215 & 45.930 & _7.777_ & 0.943 \\ Soil + 10\% gypsum & 56.505 & 1.126 & 41.278 & \(-0.735\) & 0.940 \\ Soil + 15\% gypsum & 50.507 & 0.626 & 42.060 & \(-1.523\) & 0.943 \\ Soil + 5\% gypsum + 10\% RHA & 107.153 & 1.565 & 42.932 & 0.445 & 0.949 \\ Soil + 10\% gypsum + 10\% RHA & 67.541 & 0.772 & 38.085 & \(-3.277\) & 0.966 \\ Soil + 15\% gypsum + 10\% RHA & 90.821 & 1.001 & 37.775 & \(-2.802\) & 0.970 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Hyperbolic parameters for stress–strain behavior of fine-grained soil and treated variants in a saturated condition.