## 3 Results and Discussion
### Undrained Shearing  
The behaviour of the non-stabilised and stabilised soils in the shearing stage is shown in Figure 4. For the non-stabilised soils, samples with higher quantity of the sulphates tended to fail at higher stresses compared to those having lower proportions of sulphates. Moreover, shearing carried out under greater constant effective pressure (i.e., cell pressure and back pressure) tended to fail at higher deviator stresses. The soils containing lower amounts of sulphates were, therefore, softer than those with higher proportions of the same. This behaviour of gypsum acting as a stabilising agent was apparent under the consolidation stage as previously observed. This would normally consist of a complex set of processes occurring concurrently but systematically and involving ionic exchanges in the initial stages in which case the addition of gypsum would increase the ion concentration in the soil as a result of hydrolysis, further leading to a reduction in the diffuse double layer thickness of the clay and a decrease in pore spaces. Subsequently, hydration would occur, contributing to increased cementation of the stabilised soil mix. In the process, crystallisation of gypsum would also ensue, enabling bonding, wrapping, and filling of the pore spaces resulting in a denser microstructure and, hence, increased cohesion. Figure 3: Volumetric and pore pressure responses: (**a**) 10% gypsum content and constant effective pressure of 100 kPa; (**b**) 10% gypsum content and constant effective pressure of 200 kPa; (**c**) 20% gypsum content and constant effective pressure of 100 kPa; (**d**) 20% gypsum content and constant effective pressure of 200 kPa; (**e**) 30% gypsum content and constant effective pressure of 100 kPa; (**f**) 30% gypsum content and constant effective pressure of 200 kPa; (**g**) consolidation of unstabilised soil under constant effective pressure of 100 kPa; (**h**) consolidation of unstabilised soil under constant effective pressure of 200 kPa; (**i**) consolidation of stabilised soil under constant effective pressure of 100 kPa; (**j**) consolidation of stabilised soil under constant effective pressure of 200 kPa. Generally, the unstabilised soils are characterised by ductile behaviour with contraction and then yielding observed at approximately 40 kPa (constant effective pressure of 100 kPa) and 75 kPa (constant effective pressure of 200 kPa), followed by strain hardening (Figure 4a,b). Furthermore, it is clear that the behaviour of the unstabilised soils with 20% and 30% gypsum were almost indistinguishable under the two constant effective pressures, thus indicating the lower effect of sulphate content beyond 20%. Beyond the yielding point, the percentage increase in stress between samples having 10% and 20-30% of gypsum was approximately 20% under the two constant pressures. On the other hand, the stabilised soils tended to behave differently with characteristics almost opposite to those of the non-stabilised ones. As Figure 4c,d indicate, the stabilised soils were initially ductile with the yielding followed by an almost perfectly plastic behaviour up to about 6-8% of the strain before finally undergoing a small amount of strain-softening. However, it is interesting to note that the stabilised soils with a lower percentage of the sulphate tended to produce a higher level of plastic failure compared to those possessing higher amounts of the sulphates. Hence, beyond the yielding point and at the constant effective pressure of 100 kPa, failure tended to occur at stresses of about 350 kPa, 290 kPa, and 210 kPa for soils having 10%, 20%, and 30% of the gypsum content, respectively. On the other hand, the failure rates were higher under a higher constant effective pressure although the behaviour of stabilised soils containing 20% and 30% of the gypsum were almost the same. In this regard, plastic failure occurred at approximately 385 kPa and 300 kPa for the stabilised soil containing 10% gypsum and 20% (or 30% gypsum, respectively.