## 5 Conclusions  
Through uniaxial compression experiments on different types of rock masses, the compressive strength, elastic modulus and failure characteristics of the samples were obtained. The influence of component content, number of layers and rock position on the combined rock mass was analyzed. The conclusions are as follows:  
(1) The mechanical properties of the superimposed rock mass, such as strength and elastic modulus, are between the sandstone and shale. The strength and elastic modulus of superimposed rock mass decrease greatly compared with the shale, but increase little compared with sandstone. Sandwiched rock mass can be divided into soft sandwiched rock mass and hard sandwiched rock mass. The overall strength of the soft sandwiched rock mass decreases with the increase in sandwich thickness, while the overall strength of hard sandwiched rock mass increases with the increase in rock thickness. Compared with the former, the mechanical properties of the interbedded rock mass tend to be more stable; the strength and elastic modulus of the interbedded rock mass only change a little after changing the composition content and formation location. Compared with the sandwiched rock mass, the strength of the interbedded rock mass is similar to that of the sandwiched rock mass, but the elastic modulus of the interbedded rock mass decreases with the increase in the number of layers.  
(2) The stress-strain curve of the combination can be divided into the compaction stage, elastic stage, plastic stage and post-peak bearing stage. The component content of rock mass plays a decisive role in the compaction stage. When the component content is unchanged, the interbedded rock mass will produce a larger strain value.  
Figure 14: Failure characteristics of the hard interlayer.  
Figure 15: Failure characteristics of interbedded rock mass.  
(3) The failure modes of the combination are mainly shear failure and splitting failure. The failure mode of the rock mass with the soft sandwich is related to the content of the rock layer. With the increase in the sandwich content, the failure degree of sandstone is gradually weakened, and the failure mode shows the trend of splitting failure to shear failure. For hard sandwich rock mass, when the sandwich content is high or low enough, the failure of the rock mass mainly occurs in the surrounding rock. When the content of the sandwich is similar to that of the surrounding rock, both the sandwich and surrounding rock will appear. The properties of soft and hard interbedded rock mass are more stable and the damage degree is less.  
(4) Layered rock mass is the most common type of rock mass in practical engineering. The research results of this paper can provide a reference for the complex layered rock mass problems encountered in oil and gas exploitation and underground excavation. The cementation between the layered rock samples prepared in this experiment is the same. In fact, the cementation between the rock layers also affects the strength of the rock mass. Therefore, the cementation between rock layers can be further studied as a variable.  
Conceptualization, Y.Z.; methodology, Y.Z. and H.W.; software, T.Z.; validation, Y.Z.; formal analysis, Q.N.; investigation, Y.Z.; resources, Y.Z.; data curation, T.Z. and H.Y.; writing--original draft preparation, T.Z.; writing--review and editing, Y.Z.; visualization, Y.Z.; supervision, Y.Z. and W.W.; project administration, Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the Natural Science Foundation of Hebei Province (E2021210036 and E2021210128).  
Not applicable.  
Due to the need for follow-up research, participants of this study did not agree for their data to be shared publicly, so supporting data is not available.  
All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.