###### Abstract  
Climate-induced wetting and drying (WD) cycles significantly affect the long-term performance of geotechnical structures. This study explores expansive Oxford clay's mechanical and volumetric responses stabilised with ordinary Portland cement (OPC) and guar gum (GG) under repeated WD cycles. We prepared 108 samples in total--36 untreated, 36 treated with OPC, and 36 treated with GG. These samples were compacted to 90% of their maximum dry density and subjected to 1, 5, 10, and 15 WD cycles, with nine samples for each treatment at each cycle. During the WD cycles, we monitored volumetric strain and moisture content. Mechanical performance was assessed through unconsolidated undrained triaxial tests conducted at matric sutions of \(-1500\) kPa, \(-33\) kPa, and under saturated conditions. We evaluated the undrained shear strength (\(S_{\text{u}}\)), secant modulus of elasticity (\(E_{50}\)), and modulus of toughness (\(U_{\text{t}}\)). The results showed that OPC-treated samples consistently exhibited the highest \(S_{\text{u}}\) at \(-1500\) kPa across all WD cycles, followed by untreated and GG-treated samples. At \(-33\) kPa, OPC-treated samples again outperformed the others in \(S_{\text{u}}\), while GG-treated samples performed better than the untreated ones. Under saturated conditions, GG-treated samples displayed a similar \(S_{\text{u}}\) to OPC-treated samples, significantly higher than untreated samples. Energy absorption capacity, measured through \(U_{\text{t}}\), peaked for OPC-treated samples at \(-1500\) kPa but favoured GG treatment at \(-33\) kPa and under saturation. X-ray computed tomography (CT) revealed severe degradation in untreated samples, characterised by extensive cracking, minor cracking in OPC-treated samples, and minimal damage in GG-treated samples. This highlights the superior resilience of guar gum to wetting-drying cycles. These findings underscore the potential of guar gum as a sustainable alternative to cement for enhancing the WD resilience of expansive soils, particularly under low-suction or saturated conditions. Oxford clay; wetting and drying cycles; cement and guar gum stabilisation; undrained shear strength; modulus of elasticity and modulus of toughness +
Footnote †: journal: Applied Sciences  
0000-0002-4000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-0000-00  
moisture regimes. Fluctuations in moisture content due to extreme precipitation or extended dry periods can substantially impact the mechanical behaviour of soils, reducing shear strength, stiffness, and structural integrity [2; 3; 4]. Clayey soils, common in many regions, are particularly vulnerable to environmental processes that change their hydro-mechanical properties. Among these processes, cyclic wetting and drying (WD) has become a major mechanism of soil degradation, especially in semi-arid and tropical climates, where seasonal moisture changes frequently occur [5; 6]. These wetting and drying cycles cause repetitive swelling and shrinkage, resulting in cumulative damage such as fissuring, disaggregation, microstructural changes, and declines in strength and stiffness [7; 8; 9]. At the microstructural level, WD cycles induce alterations in pore structure, particle orientation, and fabric connectivity, which ultimately affect permeability and collapse potential [7; 10; 11]. Furthermore, physico-chemical interactions, such as cation exchange, mineral dissolution, and re-precipitation, further exacerbate degradation [12]. Despite increased awareness of these processes, our understanding of the long-term mechanical evolution of soils subjected to repeated WD cycles is still limited, with most studies focusing on a limited number of WD cycles [13; 14]. Cement stabilisation is a widely accepted technique that decreases plasticity, limits swelling potential, and significantly enhances shear strength properties [15; 16].