## 3 Results
### Study Countries  
The distribution of TBM performance research among countries is illustrated in Figure 3A. Presently, there are 49 countries that have contributed to the TBM performance literature. China takes the lead with 442 articles, constituting 49.11% of the total. Iran secures the second position with 90 articles, representing 10% of the total. The United States  
Figure 2: Descriptive statistical distribution of the TBM performance literature. follows closely in third place with 67 articles, making up 7.44% of the total. Subsequently, we have Turkey, Australia, Italy, Germany, the UK, Switzerland, South Korea, and Malaysia as contributors in the field. The prominence of a country, its physical and geographical context, and the specific tunneling challenges it faces all play pivotal roles in shaping the development of TBMs and, by extension, TBM performance research. Consequently, TBM performance research has been predominantly concentrated in regions such as Europe, North America, East Asia, and South Asia, where tunneling activities and infrastructure development are extensive. Close collaboration and cooperation among countries have played a pivotal role in driving the progress of TBMs. During the period spanning from 2000 to 2021, numerous international conferences have been convened, facilitating effective communication and collaboration within the TBM research community. Prominent conferences such as the Conference on Tunnel Boring Machines in Difficult Grounds, TBM Digs, World Tunnel Congress (WTC), the 39 th General Assembly of the International Tunnelling and Underground Space Association (ITA), and the 2009 Rapid Excavations and Tunnelling Conference have served as valuable platforms for scholars from diverse countries to exchange ideas and insights. These conferences have had a profound impact on the progression of TBM performance research by promoting cross-border knowledge sharing and fostering international research cooperation. China has emerged as the leading contributor of articles in the field of TBM performance research from 2000 to 2021, making it a subject of significant analytical interest. During this period, China contributed a total of 442 relevant articles, which garnered a combined citation frequency of 3995, with an average of 14.53 citations per article and an h-index of 34. The number of articles published in China has exhibited an overall upward trajectory, with a notable surge after 2011, accounting for 95.02% of the total publications  
Figure 3: TBM performance literature published by major national and research institutions. (**A**) Leading countries in the world that publish TBM performance literature. (**B**) The research institution that published the most TBM performance literature. (Dalian University of Technology: DUT; Colorado School of Mines: CSM; Wuhan University: WU; Central South University: CSU; Chinese Academy of Sciences: CAS; Zhejiang University: ZU; Islamic Azad University: IAU; University of Tehran: UT; Pennsylvania State University: PSU; Tarbiat Modaris University: TMU). from China. In contrast, as depicted in Figure 2, the number of publications from countries other than China has gradually plateaued over the past decade. This suggests that there is still substantial potential for further development in the field of TBM performance research, particularly in China. It is worth noting that China's h-index in this domain is relatively low when compared to the global average. This discrepancy can be attributed to the relatively lower citation rate of articles published in China, which is inversely related to the number of articles published. Consequently, there is ample room for China to enhance its influence and impact within the realm of TBM performance research by improving the citation rates and the quality of its publications. As per the assessments made by funding organizations, China has made significant strides in the realm of TBM performance research, even though it entered this field relatively late.