## 3 Results and Discussion
### Non-Stabilised vs. Stabilised Soils  
Figure 5 is a comparison of the behaviour of unstabilised and stabilised soils along with the pore water pressure changes occurring at the critical yielding points of the stress-strain curves. The failure stresses experienced by the stabilised soils were generally greater than those of the non-stabilised soils. This was mostly due to the hardening of the soils, ultimately resulting in pozzolanic activities from treatment with the calcium-based agents. For soils containing 10% gypsum, the approximate increment in failure stress when the soils were stabilised was much higher (450%) under a constant effective pressure of 100 kPa as compared to the stabilised soils (140%) when the constant effective pressure was 200 kPa (Figure 5a,b). For soils containing 20% gypsum, the increase in stress at failure when stabilised with the additives (lime and GGBS) was about 200% and 70% under constant effective pressures of 100 kPa and 200 kPa, respectively (Figure 5c,d). When the amount of gypsum was raised to 30% in the soil, treatment with the additives caused a rise in the failure by about 120% and 70% under the constant effective pressure of 100 kPa and 200 kPa, respectively (Figure 5e,f). Thus, it is clear that the increase in cell pressure did not seem to necessarily cause a remarkable increment in the failure stress of the soils when stabilised with the binders.  
Figure 5: _Cont._  
Figure 5: Stress–strain curves with pore pressure changes: (**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.  
It is important to note that, for normally consolidated (or lightly over-consolidated) clays, axial stress would be expected to reach an ultimate value at a relatively large strain and is usually accompanied by a rise in pore water pressure to a steady value. Meanwhile, for heavily over-consolidated clays, the axial stress would tend to increase to a peak value and then decrease with subsequent increase in strain. In this case, the pore water pressure would increase initially and then decrease [29].  
An examination of the pore water pressure curves indicates that the non-stabilised soil containing 10% gypsum and subjected to constant effective pressures of 100 kPa and 200 kPa was lightly over-consolidated. However, when stabilised with the additives, heavy over-consolidation tended to occur. Moreover, as could be observed, the stabilised soils were seemingly more over-consolidated under the lower cell pressure. Moreover, shearing carried out under the lower cell pressure showed that the stabilised soil possessing 10% gypsum was more over-consolidated when compared to those containing higher amounts of gypsum which perhaps explains why the percentage increase in the failure stress of the former was the highest (i.e., about four times that of the non-stabilised soil). In the same vein, as indicated by the stress-strain and the corresponding pore pressure curves, the stabilised soils containing 20% and 30% gypsum tended to be mostly normally consolidated, especially when shearing was carried out under the higher cell pressure compared to the those having 10% of gypsum, leading to the lower volumetric strain and higher failure stress observed overall.