## 6 Conclusions  
This research proposes a new similar material for foundation pit excavation model tests. On the basis of the orthogonal tests, the iron ore powder and barite powder content, the cement content, the molar concentration of the cementing agent, and the mesh number of the quartz sand for four controlling factors, set up at four levels, were used to obtain the corresponding mechanical properties of the similar materials. Taking the Shenzhen metro station expansion project as engineering background, a similar model test was carried out in order to investigate the mechanical characteristics of limited-space foundation pit excavation. Based on the experimental results, the following conclusions were obtained:  
1. A new similar material composed of iron ore powder, barite powder, quartz sand, liquid paraffin, rosin, gypsum powder, and water was developed. It can adjust the range of mechanical properties with 0.37-5.37 MPa of uniaxial compressive strength, 42.0-279.0 MPa of elastic modulus, and 2.23-2.65 g/cm\({}^{3}\) of gravity. The values of the mechanical parameters of the new similar materials are widely distributed, and thus they can effectively reflect the variation of the physical properties of various soils. Rosin and liquid paraffin can be selected as cementing agents in order to easily adjust the mechanical properties of materials.  
Figure 11: The earth pressure monitored from model test during the excavation process: (**a**) process 1, (**b**) process 2, (**c**) process 3, (**d**) process 4.  
2. Range analysis was used to investigate the sensitivity of influence of the different factors on the mechanical parameters of the similar material. The study demonstrated that the cement content has a significant impact on similar materials. In the range of 3-9% cement content, the elastic modulus decreases as the cement content increases, while the compressive strength first increases and then decreases. The mesh number of the quartz sand plays a critical role in controlling the density and cohesion of the specimens.
3. Based on the developed new type of similar material, a 3D geomechanical model test on the excavation process of a metro station foundation pit was carried out. The changing mechanical processes of a limited-space soil mass excavation was stimulated truthfully. It was verified that the similar material can meet the test conditions. The variation and distribution of earth pressure in the test results can provide a practical reference for excavation engineering. The comprehensive effects of adjacent structures should be considered in the construction of a station foundation pit. It is necessary to strengthen the support for parts with large earth pressure changes.  
Conceptualization, Z.Z. and Y.G.; investigation, J.C. and Y.C.; writing--original draft preparation, Z.Z.; project administration, X.Z.; funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China (Grant No. 2021 YFB2600800), the National Natural Science Foundation of China (Grant No. 51978425), the Natural Science Foundation of Hebei Province (Grant No. E2021210032), and the Hebei Provincial Central Guidance Local Science and Technology Development Project (Grant No. 226Z5405G).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.