## 6 Conclusions  
1. Carbonaceous slate is severely influenced by tectonic stress, and the UCS of rocks with different structures is significantly different. The average \(I_{s(50)}\) of lump-structure carbonaceous slate is 0.16 MPa, and the UCS ranges from 1.15 MPa to 5.18 MPa. The average \(I_{s(50)}\) of layer-structure carbonaceous slate in which the loading direction is perpendicular to the lamina plane is 0.97 MPa, and the UCS ranges from 15.34 MPa to 23.12 MPa. The average \(I_{s(50)}\) of layer-structure carbonaceous slate in which the loading direction is parallel to the lamina plane is 0.58 MPa, and the UCS ranges from 6.13 MPa to 17.63 MPa. This means that lump-structure rock is extremely soft, and the anisotropy for layer structure is significant.
2. It was found that the values of the correction power index \(m\) are from 0.42 to 0.51 for carbonaceous slate in the Luang Prabang Suture Zone, Laos. These \(m\) values are close to the 0.45 suggested by the standard of the ASTM. The \(I_{s(50)}\) = \((D_{e}/50)^{0.44}\)\(I_{s}\) can be used for calculating the \(I_{s(50)}\) of lump-structure carbonaceous slate, the \(I_{s(50)}\) = \((D_{e}/50)^{0.46}\)\(I_{s}\) can be used for calculating the \(I_{s(50)}\) of layer-structure carbonaceous slate in which the loading direction is perpendicular to the lamina plane, and the equation \(I_{s(50)}\) = \((D_{e}/50)^{0.49}\)\(I_{s}\) can be used for calculating the \(I_{s(50)}\) of layer-structure carbonaceous slate in which the loading direction is parallel to the lamina plane.
3. The PLS and UCS of carbonaceous slate in the Luang Prabang Suture Zone are well correlated. The carbonaceous slate UCS prediction results calculated by the ISRM's proposed relationship are substantially larger than the test results. Comparing four  
Figure 7: Fitting curves of the relationship between UCS and PLS: (**a**) Linear equation; (**b**) nonlinear equation.  
different fitting relationships of the zero-intercept linear equation, non-zero-intercept linear equation, power function equation, and logarithmic equation, the relationship between the UCS and PLS satisfies the zero-intercept linear equation better. It can be expressed as UCS = 18.45I\({}_{s(50)}\) and can be used for geotechnical engineering rock UCS estimation.
4. The relationship between the UCS and PLS of rocks is related to the rock type and geological environment. If the revealed surrounding rock of tunnel engineering is extremely soft, it will be extremely difficult to prepare the standard samples for UCS. For this situation, the PLS can estimate rock UCS effectively.  
Conceptualization, J.W. and Y.Y.; methodology, Y.Y. and Z.T.; formal analysis and investigation, Y.Y. and D.L.; writing--original draft preparation, Y.Y.; writing--review and editing, Z.T. and Y.Y.; funding acquisition, Z.T.; resources, Q.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Grant No. 51978041).  
Not applicable.  
The authors acknowledge the financial support of the National Natural Science Foundation of China and relevant organizations. The authors are grateful for the comments provided by the anonymous reviewers.  
The authors declare no competing interest.