# Strength and Compressibility of Ammonia-Soda Residue from the Solvay Sodium Plant
## 3 Results and Discussion
### Drained Triaxial Tests  
#### 3.3.1 Stress-Strain Behaviour  
The substantial differences between samples were the initial bulk density, initial level of compaction, apparent cementation, initial void ratio, and natural water content. Higher initial physical parameters are typical for samples collected from the first layer of ASR. The full stress-strain characterisation was conducted in the range from 0.2% to 35% (for samples collected from a depth of 8 m) and up to 30% (for samples from a depth of 16.5 m). Cauchy stress was used according to Equations (1) and (2) to calculate the mean effective stress (\(p^{\prime}\)) and deviator stress (\(q\)):  
\[p^{\prime}=\frac{\sigma_{1}^{\prime}+2\sigma_{3}^{\prime}}{3} \tag{1}\]  
\[q=\sigma_{1}^{\prime}-\sigma_{3}^{\prime} \tag{2}\]  
where \(\sigma_{1}^{\prime}\) is the effective principal stress, and \(\sigma_{3}^{\prime}\) is the effective confining stress. The deviator stress versus axial strain curves are shown in Figure 4. As one can see in Figure 4a, the samples from the smaller depth (ASR I) led to strain-hardening behaviour at high confining pressures. In this case, the deviator stress was gradually increased with the simultaneous increase of axial strain. No distinct failure or any tendency to rupture even up to 30% axial strain was observed. Such behaviour is typical for loose sands and is represented by a monotonically increasing curve (Figure 4a) with no clear drops and, therefore, no pronounced peaks (the  
Figure 4: Drained triaxial test results: (**a**) axial strain \(\varepsilon_{\text{a}}\) vs. deviator stress q (ASR I), (**b**) axial strain \(\varepsilon_{\text{a}}\) vs. principal stress ratio \(\sigma_{1}^{\prime}/\sigma_{3}^{\prime}\) (ASR I), (**c**) axial strain \(\varepsilon_{\text{a}}\) vs. deviator stress q (ASR II), and (**d**) axial strain \(\varepsilon_{\text{a}}\) vs. principal stress ratio \(\sigma_{1}^{\prime}/\sigma_{3}^{\prime}\) (ASR II). failure points). The increasing strength of ASR I with the higher confining pressure probably comes from the cementation bonds among soil particles caused by the inclusions consisting of fine calcite. Moreover, the ASR may have greater stiffness due to the interlocking of particles and densification. When the confining pressure is high, the voids in the ASR closed quickly, and dislocations become blocked. It is also favoured by the considerable amount of water that drained from the sample during the consolidation stage, bringing the grains closer together and reducing porosity. At low confining pressure (i.e., \(\sigma_{3}^{\prime}\) = 75 kPa), the stress-strain curve showed a specific weak strain-softening. From the practical point of view, it is appropriate to consider the peak value of the deviator stress as the failure criterion. None of the specimens from layer I could be taken as failure as defined by the Mohr-Coulomb criterion. The maximum values of the deviator stress were identified after 25% of the axial strain was exceeded. Hence, as the criterion of the failure, 20% of the axial strain was accepted. Moreover, the high values of the ultimate axial deformation for the discussed case may be associated with the low consolidation of the specimen and the weak microstructure (no bonds) of the material [44]. Figure 4c shows deviator stress versus axial strain curves for the samples from the greater depth (ASR II). It can be noticed that the shear resistance of an ASR decreases with further shearing after the shear stress has reached its peak value. Hereafter, the slope of the deviator stress versus axial strain curve decreased to its residual value in an axial strain of about 30%. This phenomenon is referred to as strain-softening. Such behaviour is commonly associated with dense sands. Notwithstanding, some studies have established that the observed strain-softening is not a material response but merely a consequence of nonhomogeneous deformations caused by the frictions between the specimen and the top and bottom loading platens or by the initial non-uniformity of the specimens [45; 46; 47].