The tunnel is located on the southwestern margin of the transition between the Sichuan Basin and the Tibetan Plateau. The Minjiang River, Dadu River and other river systems with severe cutting result in large topographic height differences and complex landforms. The surface elevation of the region is 2,715–4,225 m, and the exogenous forces are mainly glacial water erosion and freezing and thawing, accompanied by biological weathering. The tunnel site topography is a typical plateau landform as shown in Figure 1.

As shown in Figure 2, the strata exposed in the tunnel site mainly consist of Quaternary Holocene sediments: the colluvial layer is coarse breccia soil, gravel soil and block rock soil, the alluvial-diluvial layer is silty clay (breccia), fine pebble soil, pebble soil and drift rock soil, and the slope residual layer is silty clay (breccia), coarse (fine) breccia soil and gravel soil (breccia). The underlying bedrock is metamorphic quartz sandstone with slate of the Upper Triassic Zagunao Formation. The strata in this area also include granodiorite, fine-grained biotite monzogranite and mylonitized granite.

The tunnel is located west of the junction between the Yangtze block and the Qiangtang-Sanjiang orogenic system, the Kang-Yunnan basement complex belt of the Yangtze block in the east, and the Yajiang remnant basin of the Qiangtang-Sanjiang orogenic system in the west. The tunnel site is located in the southeastern margin of the Songpan-Ganzi orogenic belt, where folds and faults are well developed and predominantly trend NNE, followed by NS and NNW. In terms of first-order structure, the tunnel site is located at the junction of the Sichuan-Yunnan syncline, Kang-Yunnan syncline and Longmenshan fold belt of the Songpan-Ganzi fold belt on the western margin of the Yangtze platform, adjacent to the Kang-Yunnan axis to the west, the Longmenshan structural belt to the north, the Western Sichuan platform depression of the Sichuan platform depression to the east, and the Liangshan depression fold bundle to the south. In terms of secondary structure, it is located in the Xianshuihe structural belt. The orientations of the structures near the tunnel site are mainly N20°∼30°W, which is consistent with the strike of the Xianshuihe fault zone, as shown in Figure 3. The faults developed around the tunnel and were greatly influenced by the Yalahe fault and Selaha-Kangding fault. The Yalahe fault is near the entrance end of the tunnel, and the rocks at the entrance end of the tunnel have been seriously crushed. The rocks are obviously cataclastic, and joints have developed. The tunnel site is dominated by NW-trending faults, followed by NE-trending faults (interpreted faults).

The hydraulic fracturing method, one of the recommended methods for determining rock stress issued by the ISRM Commission on Testing Methods in 1987, was used to measure the in-situ stress of deep holes near the tunnel. The test section of each borehole was determined according to the test requirements and the actual situation of the corresponding test hole, and the straddle packer system was used for the test. Water was injected into the test section through the drill pipe, and the water injection pressure was recorded through the wireless pressure sensor placed on the surface. The measured record curve was analyzed to obtain the characteristic pressure parameters and the minimum principal stress of the formation. Then, according to the corresponding theoretical formula, the magnitude of the maximum horizontal principal stress at the measuring point and the rock mechanical parameters such as the hydraulic fracturing tensile strength of the rock were calculated. The baseline orientation and fracturing crack orientation can be obtained through the impression tool and orientation tool, and the orientation of the maximum horizontal principal stress can be further calculated. Thirteen boreholes were tested for in-situ stress by the hydraulic fracturing method. Their positions are shown in Figure 4, and the measurements are shown in Table 1. Based on the measured data, the in-situ stress magnitude, maximum principal stress direction and lateral pressure coefficient of the tunnel section were analyzed.

Table 1 shows that the maximum horizontal principal stress (S _H) measured in the Xianshuihe fault zone ranges from 6.18 to 41.69 MPa, the minimum horizontal principal stress (S _h) ranges from 4.37 to 29.84 MPa, and the vertical stress (S _V) ranges from 7.41 to 35.85 MPa within the hole depth range of 279.5–1,353 m. According to the measured borehole stress data, a scatter diagram of the stress value and depth is shown in Figure 5. In general, the in-situ stress increases gradually with increasing depth. Through linear fitting, the formula of the principal stress gradient can be obtained: S H = 0.0533 H − 14.6247 (1) S h = 0.0382 H − 10.2708 (2) where H represents the depth, m. The maximum and minimum horizontal principal stress gradients are approximately 5.33 MPa/100 m and 3.82 MPa/100 m, respectively, and the S _h gradient is less than 2.66 MPa/100 m of the vertical stress gradient.

Figure 6 shows the relationship between the principal stress directions and depth of the measured holes near this tunnel, and the dominant orientation of the maximum horizontal principal stress is NW. The direction of the maximum horizontal principal stress is mainly N40°W ∼ N75°W, and the azimuth angle of the maximum horizontal principal stress becomes more concentrated in distribution with increasing burial depth. Faults in the tunnel site mainly strike NW, and the direction of the maximum horizontal principal stress is consistent with the strike of fault.

The lateral pressure coefficient k is used to study the distribution law of horizontal stress. The closer this value is to 1, the closer the stress state is to hydrostatic. The larger the value is, the more dominant the horizontal tectonic force. The lateral pressure coefficient k is also expressed by the ratio of the average value of the two horizontal stresses to the vertical stress, and the calculation formula is as follows: k = ( S H + S h ) ( 2 S V ) (3)

According to Formula (3), the lateral pressure coefficients of each measured hole are shown in Table 2. The lateral pressure coefficients range from 0.560 to 1.744. The lateral pressure coefficients of holes DZ-S-02, BDZ-S-02, DZ-S-03-1, DZ-S-05, DZ-S-04 and DZ-DS-S-04 are greater than 1, and the in-situ stress is S _H＞S _h＞S _v, indicating that horizontal tectonic activity is dominant in the vicinity of these holes. The lateral pressure coefficients of holes DZ-DS-01, BDZ-S-03, DZ-S-06, DZ-S-07 and DZ-X-01-2 are mainly less than 1, and the in-situ stress mainly shows S _H＞S _v＞S _h, indicating that the self-weight of the rock plays a dominant role in determining the stress state near these holes.

To understand the influence of the Xianshuihe fault zone on the in-situ stress as a whole, the boreholes near a tunnel are divided into four categories according to the distance perpendicular to the nearest fault strike: 0–500 m, 500–1,000 m, 1,000–1,500 m and 1,500–2000 m. The relationships between the measured maximum horizontal principal stress, minimum horizontal principal stress, maximum principal stress direction and the burial depth and distance to the fault are shown in Figures 7A,B,C, respectively. As shown in Figures 7A,B, the maximum horizontal principal stress and the minimum horizontal principal stress linearly increase with increasing burial depth. Regardless of the distance from the fault, the rate of increase in the maximum horizontal principal stress with burial depth is greater than that in the minimum horizontal principal stress. In addition, when the burial depth is less than 600 m, the farther the borehole is from the fault, the greater the measured maximum horizontal principal stress and minimum horizontal principal stress. When the burial depth is greater than 600 m, the variations in the maximum horizontal principal stress and minimum horizontal principal stress with increasing distance to the fault are not consistent. Therefore, the fault has a significant differential impact on the in-situ stress of the shallow stratum. The fastest rates of increase in the maximum horizontal principal stress and minimum horizontal principal stress, which are 0.0910 MPa/m and 0.0534 MPa/m, respectively, are observed at the boreholes less than 500 m from the fault. When the distance from a borehole to the fault is greater than 500 m, the trend of the rate of increase in maximum horizontal principal stress with the distance from the borehole to the fault is not obvious, and the rate of increase in the minimum horizontal principal stress increases slightly with increasing distance from the fault. As shown in Figure 7C, when a borehole is less than 1,000 m from the fault, the direction of the maximum principal stress of the borehole is different from the dominant NW direction of the maximum principal stress in the area, instead showing a NE direction. The in-situ stress direction near the fault is complex and may be deflected under the influence of the fault.

To investigate the distribution characteristics of in-situ stress in the control area of a single fault in the Xianshuihe fault zone, the measured in-situ stress values of boreholes perpendicular to the strike of the Selaha-Kangding fault are selected for analysis. Because the factors affecting the in-situ stress are mainly the burial depth and fault, the horizontal principal stress of the same burial depth is selected for comparison between different boreholes to eliminate the influence of burial depth. Considering that there are some differences in the range of the measured sections of the boreholes, boreholes with the same depths are selected for analysis. According to the above analysis, the in-situ stress obeys different laws in deep and shallow strata. Therefore, for the borehole measured sections selected, burial depth ranges of less than 600 m and more than 600 m are selected for comparison (namely, burial depths of 450 and 750 m). Among them, boreholes DZ-X-01-2, BDZ-S-03, DZ-S-03, DZ-S-04 and DZ-S-05 are selected for the 450 m burial depth, and boreholes BDZ-S-02, DZ-S-02, DZ-DS-01, DZ-S-03-1 and DZ-S-06 are selected for the 750 m burial depth. The stress values of each burial depth are obtained based on the approximate linear relationship between the measured stress value and the burial depth. The relationships between the maximum horizontal principal stress, the minimum horizontal principal stress, the lateral pressure coefficient and the distance from the borehole to the fault are shown in Figure 8. As shown in Figure 8A, for the boreholes near the Selaha-Kangding fault, when the burial depth is 450 m, the overall maximum horizontal principal stress and minimum horizontal principal stress increase with increasing distance from the fault. The maximum horizontal principal stress and the minimum horizontal principal stress of the borehole farthest from the fault increased by nearly 47 and 66%, respectively, compared with the maximum horizontal principal stress and the minimum horizontal principal stress of the borehole nearest to the fault. Figure 8B shows that when the borehole is close to the fault, k is less than 1, which means that the geostatic stress is dominant; however, when the fault distance is greater than 600 m, k is greater than 1, which means that the tectonic stress is dominant. This may be because in the shallow area, the tectonic influence is weakened due to the deterioration of rock mass properties near the fault, so the horizontal stress is reduced more. As shown in Figures 8C,D, when the burial depth is 750 m, the variation pattern of the maximum horizontal principal stress, minimum horizontal principal stress and lateral pressure coefficient with the distance from the borehole to the fault is not significant, which may be due to the greater influence of gravity of the rock mass in the deep area and the weakening of tectonic influence, so the change in horizontal stress is not obvious.

To ensure the safety of tunnel construction, it is necessary to determine the in-situ stress distribution along the tunnel. Therefore, based on the inversion of the in-situ stress field at the tunnel site, the contour map of the maximum principal stress of the section where a tunnel is located and the line map of the maximum principal stress at the tunnel depth are extracted, as shown in Figure 9. It can be seen from this figure that the maximum horizontal principal stress of the tunnel is 3∼44 MPa, and the in-situ stress of the entrance and exit of the tunnel is low, while the in-situ stress in the surrounding rock of the tunnel barrel is high. The maximum horizontal principal stress of the tunnel barrel is affected by the depth and fault. The maximum horizontal principal stress increases with increasing depth, while the in-situ stress release at the fault leads to a decrease in the in-situ stress, which corresponds to an area of lower in-situ stress at the fault on the contour map of the maximum horizontal principal stress. However, the in-situ stress increases away from the fault, and there is usually a maximum in-situ stress in the tunnel segment between two faults due to the dual influence of burial depth and fault. For example, the highest maximum horizontal principal stress between the Yalahe fault and the Sandaoqiao fault is 34 MPa, which is approximately three times the maximum horizontal principal stress at the Yalahe fault and the Sandaoqiao fault. The highest maximum horizontal principal stress between the Sandaoqiao fault and the Selaha-Kangding fault is approximately 28 MPa, which is approximately three times the maximum horizontal principal stress value at the Sandaoqiao fault and approximately twice the maximum horizontal principal stress value at the Selaha-Kangding fault. From the variation in the maximum horizontal principal stress between the Sandaoqiao fault and the Selaha-Kangding fault, it is obvious that the variation in the maximum horizontal principal stress of the rock mass between the two faults is roughly the same as that of the burial depth of the tunnel barrel. This phenomenon indicates that the maximum horizontal principal stress of the middle segment is mainly controlled by depth. Moreover, there is stress concentration near the fault due to tectonic influence.

The relationship between the maximum horizontal principal stress in the intersection area of the tunnel barrel and fault and depth is shown in Figure 10. The maximum horizontal principal stress in the intersection area of the tunnel barrel and fault increases roughly with increasing depth, except at the Sandaoqiao fault, Yala River fault and the interpreted fault of mile D6K276 + 996. That is, the depth of the tunnel barrel section of one fault is less than that of the tunnel barrel section of another fault, but the in-situ stress value of the former may be greater than that of the latter. For example, the depth of the Sandaoqiao fault (1,467 m) is greater than that of the D6K270 + 565 fault (1,396 m), while the maximum horizontal principal stress value of the former (10 MPa) is less than that of the latter (15 MPa). This indicates that different faults have different effects on the in-situ stress.

The occurrence of rockburst is affected by many factors, so these factors should be comprehensively considered to establish their prediction criteria. In this paper, the strength-stress ratio method is used for prediction and combined with the results of the geological analysis method. The geological analysis method mainly aims at macroscale prediction, taking geological exploration data as the basis and similar engineering data as the analogy to make a simple macroscale prediction of the research object from the aspects of in-situ stress, lithology, hydrology, structural geology and so on. The strength-stress ratio rule is used to establish the quantitative index of rockburst prediction according to the in-situ stress and rock mass properties, which is calculated according to the following formula (National Railway Administration of the People Republic of China, 2016): p = R c σ max (4) where p is the rockburst classification index, and the relationship between its value and rockburst classification is shown in Table 3. R _c is the saturated uniaxial compressive strength of the rock mass; σ _max is the maximum in-situ stress. Although the occurrence of rockburst is caused by stress redistribution due to excavation, the stress redistribution is based on the initial stress of the rock mass. Therefore, the initial maximum horizontal principal stress of the surrounding rock is used in this paper to reflect the relative magnitude of stress redistribution after excavation.

The strength-stress ratio method and geological analysis method were used to analyze and predict a tunnel rockburst. Due to the complex strata, broken rock mass and severe weathering, the surrounding rock is mostly grade Ⅳ∼Ⅴ, and only some parts of ηγ³N₂ are grade Ⅲ. First, it is preliminarily found that the surrounding rocks of grade Ⅳ and below do not have the conditions of rockburst, so rockburst is not considered, and only the possibility of rockburst occurring in the surrounding rocks of grade Ⅲ and above is considered, so the rockburst discrimination and prediction analysis is carried out only for the surrounding rocks of grade Ⅲ. According to the rock burst prediction results of a tunnel obtained by the strength stress ratio method and geological analysis method (as shown in Figure 11A), rockburst occurs mainly in grade III surrounding rock of ηγ³N₂, where the rock mass strength is higher, and the section is mainly in the middle of a rock mass between two adjacent faults, so the maximum principal stress value is relatively high (greater than 30 MPa). The tunnel is mainly subject to moderate rockburst, and the length of the tunnel with moderate rockburst is 980 m.

The classification index for large deformation prediction of soft rock segments is calculated according to the strength-stress ratio formula (5)(National Railway Administration of the People Republic of China, 2016): q = R b σ max (5) where q is the large deformation classification index, and its value and corresponding large deformation classification are shown in Table 4. R _b is the strength of the surrounding rock mass; σ _max is the maximum in-situ stress.

According to the numerical results of the tunnel axis stress corresponding to the mileage section of the in-situ stress simulation and geological prospecting section division, the identification and division of the large deformation of the surrounding rock were carried out, and the prediction results of large deformation were obtained (as shown in Figure 11B). The results of large tunnel deformation include slight large deformation, moderate large deformation, and severe large deformation, and most of them occur in fault zones or affected areas of fault zones. Although the in-situ stress at the fault is relatively low, the rock strength here is also low, so large deformation easily occurs. In the statistical range, the length of the section with severe large deformation is 1900 m, accounting for 11% of the total length of the tunnel, and almost all of them occur at the fault. In these sections, the in-situ stress is higher than 12 MPa, mostly at the moderate stress level. Due to the low strength of the surrounding rock, the final strength-stress ratio is low. The moderately large deformation section is 3,170 m, accounting for 18.35% of the total length of the tunnel. Most of these sections are faults or fault-affected areas. There are two situations: one is that the in-situ stress value is equal to or even slightly lower than the in-situ stress value of the section with severe large deformation, and the strength of the surrounding rock is slightly higher than that of the section with severe large deformation; the other is that the in-situ stress value of the ηγ³N₂ stratum is very high (all greater than 40 MPa), and the strength of the surrounding rock is relatively high. In both cases, moderate deformation is more likely to occur. The section with slightly large deformation is 2,940 m, accounting for 17.02% of the total length of the tunnel. Most of these sections are affected areas of fault zones or T₃z and myl strata. The in-situ stress level of this section is relatively low, mostly below 20 MPa, and the strength of the surrounding rock is relatively high.