## 4 Discussion
### Mechanical Performance Under WD Cycles  
At \(-1500\) kPa matric suction, all soils displayed strain-softening behaviour, typical of unsaturated fine-grained soils with apparent cohesion [39]. Cement-treated soils exhibited the highest undrained shear strength (\(S_{\text{u}}\)), owing to the formation of calcium silicate hydrates (CSHs) and calcium aluminate hydrates (CAHs), which enhance interparticle \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Material**} & **Target** & **Dosage** & **Quantity** & **Cost per** & **Total Cost** & **CO\({}_{2}\)** & **Total CO\({}_{2}\)** \\  & **Density of** & **(\%)** & **Required for** & **Ton (GBP)** & **for 1 m\({}^{3}\)** & **Emissions** & **Emissions for** \\  & **Soil (kg/m\({}^{3}\))** & **(\%)** & **1 m\({}^{3}\) of Soil (kg)** & **Soil (GBP)** & **per Ton (ton)** & **1 m\({}^{3}\) Soil (kg)** \\ \hline Cement & & 6 & 96 & 150 & 15 & 0.9 & 86.4 \\ Guar gum & & 1 & 16 & 1500 & 24 & 0.1 & 1.6 \\ \hline \end{tabular}
\end{table} 
Table 1: Cost and CO\({}_{2}\) emission comparison for cement and guar gum to stabilise 1 m\({}^{3}\) of soil. bonding [40]. However, the progressive \(S_{\text{u}}\) reduction (579 to 288 kPa from 1 to 15 WD cycles) underscores the vulnerability of cementitious bonds to cyclic moisture intrusion and microstructural fatigue [40; 41; 42]. Interestingly, untreated soils maintained higher \(E_{50}\) values than treated soils at \(-1500\) kPa matric suction, possibly due to their naturally compacted and stiff state with high matric suction. This is congruent with studies showing suction-induced stiffness dominates compacted unsaturated clays [43]. In contrast, the lower modulus values in treated soils may reflect the presence of hydrated gels and entrapped voids introduced during treatment, contributing to increased deformability. Despite exhibiting lower \(S_{\text{u}}\) than untreated soils, guard gum-treated soils maintained a comparatively higher modulus of toughness (\(U_{\text{t}}\)). This implies enhanced energy dissipation capacity under monotonic loading--a critical factor in resisting progressive cracking and deformation in cyclic environments [44]. At \(-33\) kPa matric suction, the untreated soil showed the most pronounced reduction in \(S_{\text{u}}\) (39%) with WD cycles from 1 to 15, corroborating [PERSON] et al. 's [45] observations of exponential strength loss with WD cycles. Guard gum treatment yielded superior toughness and relatively stable \(S_{\text{u}}\) (124 to 92 kPa, \(-26\)% drop), indicating biopolymer-mediated enhanced ductility and pore water buffering. These results agree with [PERSON] et al. [44], who reported that moisture entrapment within biopolymer films reduces pore pressure gradients and crack propagation. The saturated state provided a worst-case scenario for long-term treatment efficacy. As expected, all soils showed reduced undrained shear strength (\(S_{\text{u}}\)) due to the complete loss of matric suction. In untreated soils, the undrained shear strength decreased from 28 kPa to 17 kPa, marking a substantial 38% reduction over 15 WD cycles. In contrast, samples stabilised with cement experienced even greater degradation, with strength plummeting from 72 kPa to 27 kPa, reflecting an extraordinary 62% decline under the same conditions, indicative of extensive chemical bond degradation with WD cycles. Microcracking and leaching of calcium-based products likely facilitated a reduction in solid-phase continuity [40; 41; 42]. Despite this, cement-treated soil maintained the highest absolute strength among all treatments, affirming its structural advantage even under extreme saturation. Guard gum-treated soils demonstrated moderate strength loss (49 to 29 kPa from 1 to 15 WD cycles, \(\sim\)40% drop) but preserved the highest toughness across all WD cycles. This performance reinforces the notion of guard gum as a ductility enhancer, promoting distributed deformation rather than brittle failure.