## 4 Conclusions  
Specific ground behaviors, such as low rock strength, high geological stress, large deformations, and high expansibility, can result in unsuitable circumstances that typical support systems (for example, rock/cable bolts and steel sets) cannot sufficiently maintain, causing safety issues.  
Figure 14: Deformation contour of a rectangular tunnel with and without grouting.  
Figure 13: Deformation contour of a circular tunnel with and without grouting.  
Moreover, to address the increasing demand for coal in energy production and the worsening of geological and geotechnical conditions, the application of pipe-jacking technology (one of the alternatives for efficient civil engineering in the main gate of underground coal mining in the weak rock strata) was studied under several conditions using a series of numerical analysis and the finite element method.  
According to the results of the numerical analysis, the following can be concluded:  
1. We recommend using pipe-jacking technology, combined with or without the grouting method, as a reasonable and safe method that allows the main gate of an underground coal mine with weak geological structures to remain serviceable in the long run.
2. Although the tunnel length is limited by high failure proximity and large crown deformation, concrete-pipe supports can be installed as a primary support system to control the ground problems without the use of auxiliary supports.
3. Based on the failure proximity coefficient, the tunnel length can reach up to 50 m, obtained with a critical coefficient of one. This shows that, if the tunnel length exceeds 50 m, some serious damage will occur in the concrete-lining support.
4. High tunnel inclination angles and highwall slopes result in an increasing, overburdened depth and high geological stress, which are the main factors causing significant crown deformation.
5. A tunnel that is circular is much more stable than one that is rectangular. However, selecting which shape to use and what cross-sectional area is optimal depend a lot on the goal of the mining project.
6. According to the worst-case scenario in which [PERSON]'s modulus is reduced, it is clearly shown that improvements to the surrounding rock have to be considered. In addition, it suggests that the grouting method can be a highly efficient approach used to solve these types of problems in an actual scenario and can extend the tunnel's length in order to be supported by concrete lining.  
Conceptualization, U.B., T.S. and H.S.; methodology, U.B. and H.H.; software, U.B. and H.H.; validation, U.B., T.S. and A.H.; formal analysis, U.B., H.H. and A.H.; investigation, U.B., H.H., T.S. and A.H.; resources, U.B., T.S., H.S. and A.H.; data curation, U.B. and T.S.; writing--original draft preparation, U.B.; writing--review and editing, U.B., T.S., H.S. and A.H.; visualization, U.B.; supervision, T.S., H.S. and A.H.; project administration, U.B. and A.H.; funding acquisition, T.S., H.S. and A.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The data used to support the findings of this study are available from the first author upon request.  
The authors express special gratitude to [PERSON], a very kind colleague and researcher who gave precious advice. The authors also express sincere thanks to the mining company in Indonesia for permission to conduct the fieldwork, providing information, data, and samples for the current research. Moreover, the authors express great appreciation for the JDS program, Japan International Cooperation Center (JICE).  
The authors declare no conflict of interest.