## 2 Engineering Geological Conditions
### Characterization of Goaves  
The Tianzimiao tunnel site is a goaf site, and there are four layers of exploited and recoverable coal seams in the lower part: mainly including seam no. 3, 8, 12, and 15, among which the goaves of seam no. 3, 12, and 15 are located within the boundary of the Yangquan Coal Mining Group. The seam no. 8 coal goaf belongs to the mining area boundary of the Yangquan Coal Mining Group and the Hanhegou coal mining area. In order to ensure the safety of the Tianzimiao tunnel, the goaves of seam no. 3 and 8 have been grouted in the tunnel construction process. The distribution of the main recoverable coal seams in the study area: seam no. 3 is 2.0 m thick, and its depth from the tunnel floor is 21.5 to 26.4 m; seam no. 8 is 0 to 1.5 m thick (unsteadily distributed), and its distance from the tunnel floor is 77.9 to 84.5 m; seam no. 12 is approximately 1.5 ms thick, and its depth from tunnel floor is 110.8 to 116.1 m; seam no. 15 is approximately 6.0 m thick, and its depth from the tunnel floor is around 165 m.  
According to the available dates of the Yangquan Coal Mining Group, seam no. 3 in the study area was mined in 1959 to 1960, seam no. 12 was mined in 1969 to 1981, seam no. 15 was mined in 1982 to 1996, and the individual panels of seam no. 15 were mined in 2001. The drilling preliminary ascertained the depth, thickness, and compactness of the goaves in the study area, and it verified the mining conditions of seam no. 8 that were not reflected by the mining data; thus, important results were achieved. During the drilling process, it was found that there was water leakage in the goaf of seam no. 3, and there was a small amount of water leakage and borehole collapse in the goaf of seam no. 12. In addition, there were many instances of drilling bit burying in the drilling process. The above-mentioned phenomenon suggests that the integrity of the rock mass was poor, and the rock mass in the collapse zone of the deep goaf was relatively broken due to the impact of rock mass collapse above the goaves. In order to further test the integrity of the rock strata and the collapse and compaction of the overlying rocks in the goaf, water injection tests were carried out in Z1, Z2, Z3, Z6, 1-4, and 4-4 drillings boreholes. The water injection volume, the change in the groundwater level, and the test time were recorded, and the water injection curves of each borehole were obtained. The results of the water injection test show that: (1) the goaf of seam no. 3 has been grouted, the water flow velocity is very low, and there are slight differences in different sections; (2) the goaves of seam no. 12 and 15 have been preliminary compacted, and there are still some gaps in compaction, but the water flow rate of the water injection test is also very low with a maximum value of 0.065 m\({}^{3}\)/min, indicating that the porosity is not large [33]. The scope of the goaf exposed by borehole drilling is relatively limited, and a combination of various exploration methods was used to facilitate a more comprehensive understanding of the distribution of shallow goaves (for seam no. 3 and 8) under the tunnel site and the treatment effect of goaves. Additionally, this also provides a scientific and rational basis for whether or not follow-up treatment will be needed. Due to the fact the shallow goaves were mined at a very early stage and there are small coal mining pits, the distribution of shallow goaves in the study area could not be accurately ascertained by the method of investigation and interview. Therefore, a seismic exploration was carried out in the section of K12 + 853 to K13 + 400 of the left line and RK12 + 902 to RK13 + 400 of the right line of the tunnel, so as to determine the mining situation and distribution of seam no. 3 and 8. Based on the comprehensive analysis of the seismic time-distance profile of each survey line, the distribution maps of the goaves of seam no. 3 and 8 within the range of the left and right lines in the tunnel were obtained (Figure 4).