# Strength and Compressibility of Ammonia-Soda Residue from the Solvay Sodium Plant
## 3 Results and Discussion
### Constant Rate of Strain Consolidation Tests  
During the CRS test, the vertical force (\(P\)), axial strain, and pore pressure (\(u_{b}\)) were measured continuously at the specified time intervals. The main problem with the interpretation of the CRS tests was a slight increase in the pore pressure at the sample bottom. The maximum increase in \(u_{b}\) was only 2-3 kPa, hampering the determination of the coefficient of consolidation \(c_{v}\) and the coefficient of permeability k. The lack of increase in \(u_{b}\) was also independent of the assumed strain rate in the test. Figure 9 shows the obtained axial deformations as a function of the effective stresses \(\varepsilon_{a}=f(\sigma^{\prime})\). The analysis of the results revealed the relationship between the stress-strain behaviour during compression and the strain rate. As can be seen from Figure 9, the overall compressibility of the samples decreased with the increasing strain rate.  
The influence of the strain rate led to the proportional shift of the curve towards higher effective stresses (to the right). This effect was most pronounced at pressures around the yield stress. Using the graphic Casagrande method, the yield stress value of 155 kPa was determined based on the oedometer test. Figure 9 also shows superimposed data from the reference oedometer test. The shape of the primary compression curves for the different strain rates was similar. It was observed that the curves from the CRS study shifted to the right, showing higher yield stress than in the oedometer study.  
Moreover, it can be observed that for a given strain, the higher strain rate leads to higher effective stress. The strain rate values selected for the CRS test were generally higher than the observed strain rate in the oedometer test, which caused the compressibility curves derived from the CRS test to lie above the compression curve from the oedometer test. The compressibility curve for the CRS test, obtained with strain rate of 0.04 mm/min, was the closest to the oedometer curve. Therefore, an attempt was made to run the test at a value lower than the strain rate of 0.04 mm/min to fit the results better. A new compression curve was obtained after taking the value of the strain rate of 0.005 mm/min and performing the test. However, the expected curve shift to the left did not take place. On the contrary, the curve shifted to the right and was higher than the curve obtained with the 0.04 mm/min strain rate. It should be remembered that the selection of very slow strain rates leads to the significantly extended test time, which may also cause the ageing effects and the gradual development of intermolecular bonds, which leads to an increase in preconsolidation pressure and a shift of the CRS compression curve to the right [59].  
Additionally, in the course of the analysis, the compression index (\(C_{c}\)) was calculated, which was 0.16. This value indicates that ASR can be comparable with soils of a slight to low compressibility.  
Figure 9: Compression curves for the different strain rates in CRS consolidation tests with superimposed data from the reference oedometer test.