In addition, some scholars have discussed the construction of tunnels in the upper soft and lower hard strata and the stress and construction deformation of surrounding rock by model test. However, it is difficult to obtain the computational parameters of the composite formation when conducting numerical calculation and analysis, which need to be obtained through rock mechanics tests.

Due to the influence of mechanical properties of composite formation, the construction of large-span stations in the upper soft and lower hard strata is often the focus of research.

There are many research studies on tunnel mechanics and tunnel excavation deformation control in the composite stratum. The research on composite strata mainly focuses on the construction of tunnels and coal mining. When the underground engineering is built in this kind of composite stratum, it will cause engineering disasters such as uneven stress of surrounding rock collapse due to improper construction measures or other potential risks. The upper part is the weak stratum with higher weathering degree, and the lower part is the hard stratum with lower weathering degree. This kind of stratum is often caused by geological movement and different degrees of weathering. With the development of underground engineering, the upper soft and lower hard composite strata will inevitably be encountered. The research results can provide reference for the test and analysis of mechanical properties in similar complex strata with difficult sampling. Moreover, the numerical simulation and the experimental stress-strain curves both show a similar trend, indicating that the deformation of the composite strata mainly occurs in the soft part. The change of volume proportion of high-strength materials has a significant impact on the overall strength of composite specimens. Without considering the structural interface effect, the greater the strength ratio of similar materials, the greater the impact on the overall strength of composite specimens. The results of uniaxial compression test show that artificial similar materials could be used to simulate the composite rock samples which are difficult to sample. Through the uniaxial compression test of artificial composite rock samples, the effects of the strength ratio of similar materials and the thickness ratio of higher strength materials on the mechanical properties of composite specimens were analyzed. For this purpose, the artificial composite samples composed of two kinds of materials with different strengths were prepared by using cement and kaolin as similar materials. Therefore, the feasibility of artificial sample preparation and the test of mechanical properties of rock samples in the composite strata become the key to solve the study of mechanical properties of the upper soft strata and the lower hard composite strata. The study of the mechanical properties of similar material samples by artificial manufacture may enable an alternative method to solve this problem.

In the upper soft and lower hard composite strata, it is very difficult to sample the rock and test the mechanical properties of the samples.

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