## 4 Discussion  
### Compressive Strength of Layered Cemented Filler  
Figure 3 shows the uniaxial compressive strength curves for different interlayer inclination angles based on the data in Table 2. Observing the change magnitude of each curve, the distribution pattern of the filling strength with the change of inclination angle is not apparent when the maintenance ages are 3 d and 7 d. The influence of the interlayer inclination angle on the filling strength increases gradually with the maintenance age. Comparing Figure 3a,b, it is found that the uniaxial compressive strength of sandblasted delaminated specimens and natural delaminated specimens, influenced by different interlayer inclinations, are the same trend; both show the characteristics of the inflection point, the uniaxial compressive strength decreases with the increase of inclination between the interlayer inclination \(0^{\circ}\) and \(20^{\circ}\) and increases with the increase of inclination between the inclination \(20^{\circ}\) and \(30^{\circ}\). However, the increase of sandblasted delaminated specimens compared with the natural delamination specimen is not apparent: the maximum increase is only 0.08 MPa. It is not difficult to find that the strength of the filling body at each maintenance age is the lowest at an inclination angle of about 20\({}^{\circ}\). According to the research on the mechanical properties of laminated rocks by domestic and foreign scholars: the minimum value of compressive strength of laminated rocks is generally in the range of 20\({}^{\circ}\) to 60\({}^{\circ}\) of the angle between the maximum principal stress and the direction of the level [27; 28; 29], which varies according to the lithology. Therefore, it is judged that the minimum compressive strength of the tailing sand colluvial fill used in this test occurs near the interlayer dip angle of 20\({}^{\circ}\) under the influence of natural delamination. To quantify the effect of sandblasting treatment delamination on uniaxial compressive strength, define the strength gain rate. \[k_{1}=\frac{\sigma_{d}-\sigma_{n}}{\sigma_{n}} \tag{7}\]  
where \(k_{1}\) is the strength gain rate; \(\sigma_{n}\) is the compressive strength of naturally delaminated specimens (Mpa); \(\sigma_{d}\) is the compressive strength of sandblasted delaminated specimens (Mpa). Figure 4 shows the variation pattern of strength gain rate with interlayer dip angle and arranged in the order of maintenance age. Overall, the four curves from left to right gradually tend to be concentrated from dispersion. It indicates that the hydration reaction is incomplete at the early maintenance stage [30], and the link between the sand and gravel on the layered surface and the filler is not tight. In contrast, the mechanical properties of the filler are weak, and the benefits brought by sandblasting treatment are magnified. In the uniaxial compressive test, when the interlayer dip angle is 0\({}^{\circ}\), the sublevel is in the same direction as the primary stress, the more voids caused by the early age are continuously compacted, and the sand and gravel are combined with the filler to improve its uniaxial compressive strength. With the growth of interlayer dip angle, the angle between the principal stress direction and the sublevel keeps increasing, coupled with the shorter maintenance age leading to more voids existing around the sand and gravel, and thus subject to the increased partition force along the sublevel direction; hence, the voids that can be considered as initial damage will continue to expand along the sublevel direction while compacting. In addition, the non-tightly connected gravel slides in the direction of the sublevels causing damage; thus, reducing the mechanical properties of the filler to the extent that the early gain rate is negative at interlayer dips of 20\({}^{\circ}\) and 30\({}^{\circ}\). With increased curing age, the strength gain rate tends to be concentrated and stabilized.