## 3 Results and Discussion  
This study presents an analysis of results related to the residual shear strength obtained from highly plastic clayey and silty samples, which are part of the overburden of the third coal layer (Figure 2). The analysis of the grain size distribution shows that the siltstone consists of 13-15% clay fraction, 81% silt fraction, and 4-6% sand fraction, while the gray clay layer consists of 30-34% clay fraction, 65-69% silt fraction, and 0-2% sand fraction. The results of these tests are presented in Figure 6.  
Figure 5: Appearance of the gray clay samples after testing in the ring shear apparatus (**left**) and direct shear apparatus (**right**). The liquid limit is in the range of LL = 46.3-47.5% for siltstone and LL = 63.0-68.0% for gray clay samples. The plasticity index values are in the range of Ip = 19.7-21.6% for siltstone and Ip = 22.5-23.2% for gray clay. According to the plasticity chart in EN ISO 14688-2 (2018) [42], based on the liquid limit and plasticity index values, the siltstone samples belong to class CIM, while the gray clay samples belong to class CIM. The consistency index values for siltstone are in the range of Ic = 0.84-0.94 and for gray clay from Ic = 0.91-0.97 (Figure 7). Based on the consistency index values, the samples are in a semi-hard consistency state. The shear progression in the ring shear and direct shear apparatuses for the siltstone and clay samples is shown in Figures 8 and 9. Since these are remolded samples, they are dominated by unstable, hydrocolloidal bonds, making the activation of resistance smoother and more complex as it is accompanied by visible sample deformation, which is a result of the movement, even rotation, and shearing of elementary particles. Based on the low clay content and medium plasticity of the soil, combined with the specific shape of the stress-horizontal deformation curve, we can conclude that the movement mechanism is classified as transitional according to [PERSON]'s, 1981 [43]. Transitional soils are characterized by a  
Figure 6: Particle size distribution graph. Figure 7: Identification and classification indicators of the tested samples. unique mixture of cohesive and non-cohesive properties. The low clay content suggests that, while the soil retains some cohesive characteristics, it may not exhibit the same level of plasticity as pure clay soils. The medium plasticity indicates that the soil has enough plastic behavior to deform under stress, but not excessively, which aligns with transitional behavior. Figure 8b illustrates a drastic drop in shear strength, which can be attributed to the higher content of clay fractions and the high plasticity of the soil. The platey clay minerals present in the soil exhibit a strong tendency to orient themselves parallel to the shear plane during deformation. This orientation contributes to a significant reduction in soil strength. According to [PERSON]'s, this behavior is classified as a sliding mechanism. [PERSON], 1970-1985 [2] concluded that, due to an increase in moisture, which results from the deformation and reorientation of clay particles parallel to the shear direction, there is a distinct drop in strength in overconsolidated clay after reaching the peak drained strength. Figure 8: Shear stress versus rotation angle for (**a**) siltstone (**b**) gray clay. In direct shear tests, after the first and second cycles and returning the shear box to its original position, due to the orientation of particles in the shear plane, samples U-5 (curve at normal stresses of 100 and 400 kPa), U-7 (curve at 100 kPa), and U-9 (curve at 100 and 200 kPa) exhibited accumulation and the \"rabbit's paw\" effect, leading to a sudden increase in shear stress (Figure 9). The mechanism of material displacement in the form of accumulation and stretching (spreading) resembles a rabbit's paw, and this phenomenon is colloquially referred to in Serbian as the \"rabbit's paw\" effect.