### Project Overview  
The Gaoligongshan tunnel is a key control project of the Dali-Ruili Railway. The terrain and geological conditions are extremely complex. Along the way, it passes through many unfavorable geological bodies, such as fault fracture and fissure water development zones.  
The tunnel is set as an auxiliary tunnel plan of \"1 through parallel heading + 1 inclined shaft (main and auxiliary shaft) + 2 vertical shafts (main and auxiliary shaft)\". Among them, the main shaft of the No. 1 shaft has a depth of 762.59 m and inner diameter of 6 m; the auxiliary shaft has a depth of 764.74 m and inner diameter of 5.0 m. The main function of the main shaft is to discharge the harmful gas generated and transport the waste residue during the construction process, and the main function of the auxiliary shaft is to transport fresh air, deliver construction materials, equipment, construction personnel, and serve as a safe escape. The stratum of No. 1 shaft is shown in Figure 2.  
At about 19:00 p.m. on 15 January 2018, during the mucking and leveling process at a depth of 630.3 m of the auxiliary shaft, the shaft wall on the left side of it suddenly collapsed, forming a \"bowl shaped\" water inrush point, with a water inflow of about 300 m\({}^{3}\)/h, resulting in flooding (as shown in Figure 3). On 19 January 2018, YunGui company organized four parties to hold an expert meeting. The analysis showed that, due to the complex hydrogeological conditions around the shaft and lack of understanding of the distribution characteristics of external water pressure of the lining, the pre reinforcement measures in the construction project were unreasonable, thus resulting in the accumulation of groundwater around the shaft wall exceeding its bearing capacity; the shaft wall collapsed and gushed water.  
Figure 3: No. 1 vertical auxiliary shaft flooded. (**a**) Water inrush; (**b**) Gushing water; (**c**) Flooded shaft.  
Figure 2: Strata section of No. 1 vertical shaft.  
According to the core sample of the drilled rock mass, the revealed depth is 764.74 m. The specific conditions are as follows: 0-9.3 m is strongly weathered stratum, mainly saturated soil, which contains sub prismatic gravel sand, filled with brown-yellow silly clay between particles; breccia is seen in some parts, followed by pebble soil, mainly strongly weathered quartz sandstone, filled with fine round gravel and sandy soil. At 30-70 m is the strongly weathered mixed granite stratum, and the surrounding rock is mainly brownish-yellow mixed granite and gneiss-like structure, containing medium-coarse sand grains; maroon-red iron infestation is occasionally seen along the joint surface. At 9.3-30 and 70-764.74 m are weakly weathered mixed granite strata, with hard lithology, block and fragment structure, general stability, general cementation, and a relatively complete/broken rock mass. The integrity of local surrounding rock of shaft wall is relatively poor, and a small amount of vertical weak interlayer is developed, which easily causes wall caving and dropped pieces (as shown in Figure 4).  
Affected by atmospheric precipitation and surface water, the strata of vertical auxiliary well No. 1 have good water richness, no obvious water barrier, irregular water outlet direction and horizon, hydraulic connection with the outside of the wellbore, and a relatively stable supply. The water pressure increases proportionally with the wellbore, and it is predicted that the maximum water inflow volume of the wellbore is 6200 m\({}^{3}\)/d, and the maximum hydrostatic pressure is 7.6 MPa. According to the measurement data of the exploration holes, the logging contents include conventional logging (apparent resistivity, spontaneous potential, natural gamma, and gamma), inclinometer, and temperature logging. The logging curves determine four aquifers, which are 154.50-156.25, 340.00-343.70, 495.45-498.90, and 693.50-694.75 m (as shown in Figure 5).