# Viscoelastic Strains of Palaeozoic Shales under the Burger's Model Description
## 4 Theoretical Basics of Rock Rheology in Triaxial Test Conditions for the Burger's Model
### Parameters of Burger's Model for the Paleozoic Shales from the Baltic Basin  
The estimated parameters of the Burger's model are presented in Table 1, Figures 6 and 7. As commonly known, rock creep is sensitive to many factors, including stress state, temperature, or humidity [35; 39]. The influence of the level of sample loading (\(\sigma_{1}-\sigma_{3}\))/(\(\sigma_{1}-\sigma_{3}\))\({}_{max}\), i.e., the stress state on creep in the case of the analyzed shales is clearly visible (Table 1, Figure 6). Figure 6: Parameters of the deviator part of the equation showing the viscoelastic physical model for the analyzed shales with 0.95 confidence bounds: (**a**) Maxwell shear modulus \(G^{M}\), (**b**) Kelvin shear modulus \(G^{K}\), (**c**,**d**) Maxwell \(\eta^{M}\) and Kelvin \(\eta^{K}\) viscosity, respectively. The behavior of the analyzed shales is variable depending on the deviator or axiator changes. In the case of deviator loading, its increase causes a decrease of all parameters in [PERSON]'s model, which means that the samples display lower stiffness and larger susceptibility to creep. Similar conclusions can be drawn from the analysis of viscosity coefficients. At higher values of deviator stress, creep velocity described as \(\frac{\tau_{00}}{\eta^{M}}\) is higher than at lower levels of loading. A different situation can be observed in the case of volumetric change (Figure 7). Sample stiffness increases with increased loading \((\sigma_{1}-\sigma_{3})/(\sigma_{1}-\sigma_{3})_{max}\), and the analyzed rock has a smaller tendency to creep. Creep velocity decreases with loading increase according to the relationship \(\frac{\sigma_{00}}{\eta^{M}}\). Unfortunately, large dispersal of the results in the case of volumetric changes hampers their quantitative analysis. The samples from particular lithostriagraphic units (Pelplin, Paslek, Jantar, and Sasino) are marked with different colors in Figures 6 and 7. It is evident that creep in shales is different among the particular units. Results obtained for the Jantar Formation differ significantly from the samples from the Pelplin and Paslek Formations, which in turn behave similarly. Results for the Sasino Member are strongly dispersed and difficult to assign to any of the mentioned clusters. The results are influenced by the mineral composition of the analyzed samples. As mentioned in Section 2, samples from the Pelplin and Paslek Formations have a similar mineral composition, while samples from the Jantar Formation and Sasino Member are characterized by a variable mineral composition, strongly influencing the observed results. The determined shear \(G^{M}\) and bulk \(K^{M}\) moduli allow [PERSON]'s longitudinal elasticity modulus and [PERSON]'s coefficient to be defined. Results obtained by other methods are  
Figure 7: Parameters of the axiator part of the equation showing the viscoelastic physical model for the analyzed shales with 0.95 confidence bounds: (**a**) Maxwell bulk modulus \(K^{M}\), (**b**) Kelvin bulk modulus \(K^{K}\), (**c**,**d**) Maxwell \(\eta^{M}\) and Kelvin \(\eta^{K}\) viscosity, respectively. conformable with the results presented in [PERSON] et al. [34] in earlier analyses of the Paleozoic shales from the Baltic Basin. In the drawings Figures 6 and 7 one can notice a large scatter of the presented results. Due to the digital measurement of displacement and force (MTS system), the errors associated with the measurements themselves in experiments are relatively small. However, the greatest variability of the results is caused by the natural heterogeneity of the research material, because the shale samples came from the drill core. No preliminary selection of research material was carried out. This is clearly visible in Figures 6 and 7, where a large spread of the obtained model parameters for individual formations is observed.