Prefabricated underground structures were first researched in China to address the serious social and environmental issues associated with underground construction. Five metro stations have been built on Line 2 of the Changchun Metro in China using this new prefabrication technology. The joints connecting prefabricated elements are the most critical components in such prefabricated structures. In this study, experiments and numerical simulations investigating the influences of different grouted mortise and tenon joint geometrical parameters were conducted to determine the optimal parameters and ensure proper performance. To do so, a finite element model was built with the appropriate characteristics using the experimental results, and an analysis of the influence of different geometrical parameters was conducted. The results indicate that increasing the dip angle of the tenon could improve the flexural rigidity of the joint, but its effect was relatively small compared to that of the other parameters. Increasing the width of the tenon only had a positive effect on the flexural rigidity of the joint when the width was relatively small and under small axial loads. Increasing the length of the tenon helped to enhance the flexural performance of the joint; however, this advantage was not obvious when the tenon length was relatively long. Proper indentation of the joint improved the flexural capacity under a small axial load, but was not beneficial under a high axial load. The findings of this study are of value to help researchers and engineers more effectively design prefabricated underground structures.
With the rapid development of rail transit construction in China, social awareness of impacts to environmental quality during metro construction has continuously increased. Furthermore, long and tight construction periods, large resource consumption, and a decrease in young laborers in civil engineering (and the accompanying shortage of skilled labor and decreased guarantee of structural quality) present significant challenges for traditional metro station construction methods. These situations are particularly prominent in cities in the northeast of China, such as Changchun city, where it gets so cold that a four to fivemonth winter break is required during metro construction as it is difficult to guarantee construction quality at such low temperatures. This results in significant deadline pressure. To address these problems in cold regions, Yuanjiadian station on Line 2 of the Changchun Metro (shown in Figure
Metro station constructed of prefabricated components: (a) rendering and (b) picture.
For prefabricated aboveground buildings, the relevant technology and management systems have already been established as there has been a considerable increase in engineering applications of such structures in recent years [
The joints connecting prefabricated elements have been recognized as the potentially weakest parts of a prefabricated structure, and the different geometric structures of different joints can affect their mechanical behavior. Therefore, in order to address the limitations of prefabricated technology in large underground applications, it is necessary to study the effects of the multiple geometrical parameters that define the joints between prefabricated components. In this study, experiments were conducted and used to optimize numerical simulations evaluating different geometrical joint parameters. Using the results of this comprehensive analysis of the influence of joint geometry on joint performance, we then provide guidance on selecting a reasonable and effective joint geometry.
The five metro stations built in Changchun using prefabricated structures are all cutandcover stations supported by anchorpile systems. Each of these horseshoeshaped twostory stations is 20.5 m wide and 17.45 m high. The station structures were built by assembling seven 2 m wide prefabricated components into the section geometry shown in Figure
Geometry of a Changchun Line 2 station prefabricated element.
Grouted mortise and tenon joint.
Four main geometrical parameters, the dip angle
Geometric design of a mortise and tenon joint.
Threedimensional sketch of the indentation of a mortise and tenon joint.
Prototype experiments on a 1 : 1 scale loaded in the key working direction (the crosssectional direction in Figure
Plan and facade view of experimental specimens: (a) long joint and (b) short joint (all dimensions are in mm).
Figure
Loading configuration and cases.
Loading apparatus used for specimen tests.
Numerical models the same size as the experimental specimens were built using finite element modeling (FEM). Because the geometrical parameters of the mortise and tenon joint were the key factors under investigation, the rebar and grouting segments were neglected in the model. As can be seen in Figure
Numerical model of long tenon without grout (same as experimental specimen).
The density, modulus of elasticity, and Poisson’s ratio of C50 concrete were used in the model, with values of 25 kN/m^{3}, 34.5 × 10^{6} kN/m^{2}, and 0.2, respectively. The nonlinear Coulomb friction law was used to model the concreteconcrete contact with a normal stiffness modulus, shear stiffness modulus, cohesion, internal friction angle, and tensile strength of 34.5 × 10^{6} kN/m^{3}, 8 × 10^{6} kN/m^{3}, 0 kN/m^{2}, 50°, and 0 kN/m^{2}, respectively.
Figure
Loading and boundary conditions of the numerical model.
Figures
Comparison of loaddeflection curves of experimental and numerical results.
Comparison of momentrotation curves of experimental and numerical results.
Based on the verified numerical model, an additional numerical model of larger size, shown in Figure
Calculation model for influence analysis of different geometrical parameters.
Table
Numerical simulation cases.
Axial load (kN)  Joint parameters  Variable parameters  Fixed parameters 

500 , 
Dip angle 


Width 

 
Length 



Indentation (ratio) 


Figure
Bending momentrotation curves for different tenon dip angles under axial loads of (a) 500 kN, (b) 1000 kN, (c) 1500 kN, (d) 2000 kN, (e) 2500 kN, (f) 3000 kN, and (g) 4000 kN.
Overall, increasing the dip angle within a reasonable limit can improve the flexural rigidity of the joint in the nonlinear stage, but the increase in flexural rigidity slows as the axial load increases. When the axial load is sufficiently high, there is little difference in flexural rigidity under any dip angle. It was also observed that, for the joint with the 90° dip angle, the stress component in the axial load direction increased significantly as there was greater stress concentration at the top of the tenon when the applied axial load was very large; this reduces the carrying capacity of the tenon in the later periods of loading. Therefore, all factors should be comprehensively considered when selecting the dip angle of the tenon for prefabricated structures used to construct metro stations.
Figure
Bending momentrotation curves for different tenon widths under axial loads of (a) 500 kN, (b) 1000 kN, (c) 1500 kN, (d) 2000 kN, (e) 2500 kN, (f) 3000 kN, and (g) 4000 kN.
Overall, increasing the width of the tenon only has a positive effect on flexural rigidity when the width is relatively small and under small axial load.
Figure
Crack development with loading for different joint lengths. (a) 195 mm, (b) 95 mm.
Key bending moments of joints with different tenon lengths and (a) modified epoxy resin grout at 500 kN axial load, (b) modified epoxy resin grout at 1600 kN axial load, and (c) modified cementbased material at 1600 kN axial load.
As can be observed in Figure
Comparison of experimentally observed joint deformations for different tenon lengths.
Figure
Bending momentrotation curves for different tenon lengths under axial loads of (a) 500 kN, (b) 1000 kN, (c) 1500 kN, (d) 2000 kN, (e) 2500 kN, (f) 3000 kN, and (g) 4000 kN.
In summary, an appropriate increase in the length of the tenon can be beneficial to its flexural rigidity. A tenon length of 195 mm, as used in the joints of the prefabricated structures for building the metro stations evaluated in this study, provides a flexural capacity better than that at tenon lengths of 145 mm and 95 mm. When the length of the tenon is sufficiently long (195 mm, 245 mm, 295 mm, and 345 mm), the positive effects of using a longer tenon are not obvious. Therefore, actual loading conditions should be taken into consideration when choosing the tenon length as an excessively long tenon may lead to bending failure.
Figure
Bending momentrotation curves of different tenon indentations under axial loads of (a) 500 kN, (b) 1000 kN, (c) 1500 kN, (d) 2000 kN, (e) 2500 kN, (f) 3000 kN, and (g) 4000 kN.
Based on the above findings, reasonable indentation can be stated to ensure effective mortising of the tenon, which is beneficial for the flexural capacity of the joint, especially under a small axial load. However, a larger indentation does not guarantee better performance of the joint. For the 1 m wide prefabricated element evaluated in the numerical model, an indentation rate of 30% provides the best performance.
This study analyzed the influence of geometric parameters on the behavior of mortise and tenon joints used in prefabricated structures installed underground. To evaluate the effect of a wide range of parameters on the performance of the joints, a finite element model simulating suitable contact was constructed based on the results of experimental joint testing. The following conclusions can be drawn from the findings:
Increasing the dip angle of the tenon can improve the flexural rigidity of the joint, but the effect is relatively small compared to that of other parameters. When the axial load is sufficiently high (>4000 kN), the stress concentration at the top of a tenon with a 90° dip angle increases significantly. Therefore, the flexural capacity of the joint is reduced.
When the axial load is small, increasing the width of the tenon is beneficial to the flexural capacity of the joint for relatively small widths (200–350 mm); increasing the tenon width does not significantly improve the flexural capacity of the joint at relatively high widths (400–450 mm).
Increasing the length of the tenon helps to enhance the flexural performance of the joint. However, this advantage is not obvious when the length of the tenon is relatively long (195 mm, 245 mm, 295 mm, and 345 mm).
Proper indentation of the mortise and tenon joint improves its flexural capacity under a small axial load. However, indentation changes are not beneficial when the axial load is high (>3000 kN). For the 1 m wide prefabricated element evaluated in the numerical model, an indentation rate of 30% is recommended for small axial loads.
In summary, the different geometrical parameters (dip angles, widths, lengths, and indentations) of the mortise and tenon joints used in prefabricated structures should be selected after considering the various axial loads, loading conditions, and relationships between parameters in order to ensure environmentally friendly, readily assembled prefabricated structures for constructing underground structures such as the stations installed in Line 2 of the Changchun Metro.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflicts of interest.
Xiuren Yang supervised the project and developed the concept and methodologies with Zhongheng Shi. Fang Lin performed the experimental and numerical studies.
This research was funded by the National Key S&T Special Projects (grant number 2017YFB1201104).