## 4 Creep Strain Fields And Discussions
### Non-Uniform Strain Fields  
As an important advantage with respect to classical measuring techniques, the application of DVC to X-ray tomographic images allows the computation and analysis of non-uniform strain fields inside samples. In Figure 7, we show the full axial strain fields in the three samples under two similar stress levels and time periods, respectively at 60-67% and 27-30 days, and 90-94% and 13 days. As a common feature of three tested samples, the axial strain fields were strongly non-uniform. More precisely, the axial strain fields showed a clear layered distribution. However, the concentration degree and orientation of strain layers differed between the three samples. As mentioned above, this layered distribution of axial strain was inherently related to the motion of bedding planes. For the perpendicular sample (\(\perp\)), as the bedding planes were strongly compacted by the applied stress which was normal to them, one got regular and clearly identifiable horizontal strain concentration layers. For the parallel sample (\(\parallel\)), even the applied stress is parallel to the bedding planes, one still observed horizontal strain concentration layers, probably due to the compaction of initial cracks. However the concentration degree of strain layers was less marked than the perpendicular sample (\(\perp\)). For the inclined sample (\(\parallel\)) with an angle of 45\({}^{\circ}\), the applied axial stress generated both normal and tangential stresses on the bedding planes, which could exhibit a frictional sliding. As an interesting result, the strain concentration layers in this sample were also inclined and almost parallel to the bedding planes. On the other hand, the strain concentration layers were more and more marked and the difference between the three samples was larger when the applied stress was higher. In order to investigate the evolution of strain field under a prescribed constant stress, the axial strains fields in three samples at two different instances under a fixed axial stress are presented Figure 8. It is clear that the strain fields evolve with time due to the creep of COx claystone. As the mineral inclusions had an elastic behavior, the creep deformation occurred mainly inside the clayey matrix. Further, the creep deformation enhanced the concentration degree of strain layers, which became more and more marked with time. Figure 6: Evolutions of average axial and radial strains during In situ creep tests on three samples with different loading orientations. Figure 8: _Cont_. Figure 7: Distributions of accumulated axial strain under different stress levels and at different instances: (**a,d**): parallel sample (\(|\)) with \(q_{c}/q_{peak}\) = 60% and \(q_{c}/q_{peak}\) = 94%; (**b,e**): inclined sample (\(|\)/) with \(q_{c}/q_{peak}\) = 67% and \(q_{c}/q_{peak}\) = 90%; (**c,f**): perpendicular sample (\(\perp\)) with \(q_{c}/q_{peak}\) = 60% and \(q_{c}/q_{peak}\) = 90%. In order to deeply investigate the evolutions of radial strain, the local distribution of strain on some cross sections are now presented. It is noticed that the 3D analysis used in the present study allowed providing the local strain field on any cross section of tested sample. As a representative example, a horizontal cross section also called slice was cut at the mid-height of each tested sample, as shown in Figure 9. The distributions of accumulated radial strain on the selected slices are presented in Figure 10 for the three tested samples under two different stress levels, the same as those used in Figure 7 for the axial strain fields. It is found that at the low axial stress, the concentration degree (or heterogeneity) of radial strain in the parallel (\(\parallel\)) and perpendicular (\(\perp\)) samples was significantly less marked than that of the inclined sample (\(//\)). This is consistent with the average radial strains discussed above. The frictional sliding along bedding planes in the inclined sample (\(//\)) enhanced the non-uniform distribution of radial strain.