The strata within the fault fracture zone contain numerous macro-, micro-, and nano-pores. The sealing of deep pores is influenced by gravity, capillary force, van der Waals force, and interatomic forces, with different intermolecular forces illustrated in Figure 3. Therefore, it is necessary to select slurry with different particle size gradients for compositional matching to ensure the full filling of various pores and water-conducting channels, displace free water and air, form a dense overall rock-slurry consolidated body, and reduce the permeability of the strata.

Due to the relatively developed macroscopic and microscopic fractures in the surrounding rock of the shallow plastic zone, the grout-stopping rock plate is insufficient to support the overhanging load. Therefore, large-particle grout should be selected for extrusion and sealing to prevent damage to the surrounding rock. The deep surrounding rock is rich in microscopic and nano-pores, so nano-scale grout is selected for fracturing, extrusion, sealing, and leakage prevention, reflecting overall segmental differences.

Grouting pressure alters the stress distribution state of the injected rock mass, causing deformation of joints and fractures, which in turn affects the flow pattern of the grout, indicating a coupling effect among stress, fractures, and seepage fields. Therefore, the control of grouting pressure gradient requires simultaneous consideration of particle size gradient and spatial joint and fracture structure adaptation conditions. For tunnels passing through fault zones with fragmented surrounding rock, low-pressure coarse-grained pre-grouting should be adopted, followed by high-pressure fine-grained fracturing grouting.

Under deteriorated geological conditions, grout diffusion becomes nonlinear seepage. If the selected grouting pressure is too low, it may result in insufficient grout diffusion range, unable to fully displace the liquid and gas in the pores. When the selected grouting pressure is too high, it may cause excessive grout diffusion, disturbing other stable areas and increasing engineering risks. Therefore, at the initial stage of grouting, lower filling, compaction, or infiltration grouting pressures (1–4 MPa) are adopted for larger fracture openings. After the macroscopic and microscopic pores are basically closed, fine-grained grout is used for high-pressure fracturing grouting (4–10 MPa). When the fracturing grouting pressure exceeds the shear strength of the rock and soil mass, the structural surface will be fractured and damaged, and the grout will move along the fracture surface, continuously compressing the rock and soil mass and filling nanoscale fractures and water conduits, ultimately forming a relatively hard cross-shaped grout skeleton that jointly bears the shrinkage deformation of the overlying rock and soil mass. The schematic diagram of the effect of dual-gradient grouting is shown in Figure 4.

Due to the concealment of the grouting process in engineering, phenomena such as grout leakage, grout reversal, and water infiltration are prone to occur after the grout splits the rock mass (Li et al., 2014). Adding appropriate admixtures can significantly shorten the gelation time of the grout, increase its viscosity, effectively control the diffusion radius of the grout, and enhance the sealing effect of connected pores and water-conducting fractures.

Due to the influence of early grouting holes, the grout injected into later grouting holes will deviate with grouting pressure or other factors, and the injection volume will decrease. By using materials such as ordinary cement-water glass double-fluid grout and fast-hardening sulphoaluminate cement single-fluid grout, and injecting from top to bottom and in a sequential and skip-hole manner, it is possible to effectively achieve constrained grouting step by step, ensuring that the grout is injected to achieve compaction. Furthermore, by gradually increasing the grouting pressure in sequence, it is beneficial for the diffusion of the grout and the improvement of the compactness of the grout stone. At the same time, the later grouting holes also serve as a check and evaluation of the grouting effect of the earlier holes. The grouting process for mixed liquid configuration is shown in Figure 5.

In local sections of tunnels passing through fault zones, the surrounding rock is fragmented. Without grouting, there is no effective adhesion point at the anchoring end of NPR anchor rods/cables, leading to detachment of the anchoring end from the deep surrounding rock and failure (Tao et al., 2024c). The ultimate anchoring force is less than 150 kN, preventing the anchor cables from exhibiting their characteristic of constant resistance to large deformation. The surrounding rock cracks severely. The anchoring conditions of NPR anchor cables under different grouting conditions are shown in Figure 6. After conventional fracturing grouting, the deep expansion of fractures in the surrounding rock is restricted to some extent. However, in water-rich fault zones, some anchor cables lose stable anchoring points (i.e., partial failure of the anchoring end), resulting in constraint failure and subsequent stress damage to the protective structure. After implementing dual-gradient grouting, coarse-grained grout infiltrates and compacts the macroscopic fractures in the surrounding rock, followed by the injection of fine-grained grout to form a complete and stable grout network skeleton. This fills and seals the microscopic fractures in the surrounding rock, enhancing its strength and integrity. It provides a stable anchoring environment for NPR anchor rods/cables, facilitating the full utilization of their extraordinary mechanical properties of constant resistance to large deformation and energy absorption. The average anchoring force can be increased to over 350 kN.

Cloud model characteristics: To determine the crack evolution law and the diffusion range index of grouting particle reinforcement, PFC particle flow is used to construct a fragmented surrounding rock mass in the tunnel passing through the fault zone area, simulating the deformation and plastic zone expansion law of the surrounding rock. In the shallow area of the surrounding rock, after low-pressure grouting with coarse-grained slurry, high-pressure grouting with fine-grained slurry is carried out in the deep area of the surrounding rock. Based on the actual situation on site and considering the computing power and accuracy of the computer, an 80 × 4 × 80 m (length × width × height) stratum model is first constructed, and a seepage field grid is generated. The grouting pressure is set at 4/8MPa, and the model characteristics are shown in Figure 7.

Based on field-measured in-situ stress data, the horizontal structural stress in the Cretaceous ground is 8.1 MPa, with a vertical stress of 4.1 MPa. Using the Fish language, in-situ stresses in the x, y, and z directions were applied to the wall elements within the model. The top position of the seepage boundary was set at the groundwater level (y = 0), with hydraulic pressure distributed linearly with depth. The hydraulic pressure at the tunnel face was set to 1 standard atmosphere, and the tunnel portal was defined as an open boundary.

During the calculation process, 10,000 steps are computed initially, with particle displacement and velocity corrected every 100 steps. Following the generation of the in-situ stress field, flexible boundaries were applied to mitigate model boundary effects. The “Wall” element is removed, and particle elements at the boundary are rigidly fixed. Furthermore, the damping of boundary particles was enhanced to approximate free-field boundary conditions. Subsequently, a stability calculation was performed again with rock mass parameters assigned. These converted in-situ parameters were specifically assigned to the particles representing the rock mass. Upon parameter assignment, grouting reinforcement was implemented, after which a third equilibrium calculation was carried out.

It is infeasible to directly adopt experimental results as the values for engineering rock mechanics parameters. The assignment of numerical calculation parameters is one of the key issues in geotechnical engineering research, greatly affecting the calculation results. The selection of parameters not only needs to consider the inherent properties of rocks, but also needs to be combined with the occurrence environment and structural characteristics of rock masses. Based on the method established by Hoek et al. (2002), Hoek and Diederichs (2006), and Hoek and Brown (2019) based on the GSI geological classification index, the numerical calculation parameters are ultimately obtained as shown in Table 1.

The evolution law of crack morphology in low-pressure coarse particle grouting is shown in Figure 8. Under the grouting pressure of 2–4 MPa, the grouting cracks first expand horizontally on both sides in the coupled seepage field grid, and then spread diagonally upward at a 30° angle on both sides. The low-pressure coarse particle grout fills the cracks and penetrates into the grout, without disturbing the internal structure of the rock mass. The evolution law of particle morphology in low-pressure coarse particle grouting is shown in Figure 9. The grout particles are extruded outward, and there is no obvious displacement partition between the grouted reinforcement area and the non-grouted reinforcement area. Taking the horizontal line of the grouting hole as the dividing line, the particles at the top of the shallow layer on the inner side displace vertically upward, the particles at the bottom of the shallow layer on the inner side displace vertically downward, and the particles at both ends of the shallow layer on the inner side are pressed and migrated towards the horizontal tension cracks. From Figures 9a–d, the particles at the top of the shallow layer on the inner side move diagonally at a 30° angle towards both sides. This is due to the compaction of particles in the surrounding area of the grouting hole by the preceding grout, which exerts a compressive force causing the rock particles to be squeezed and compressed. The unbalanced force between particles and particles, as well as between particles and solid elements, reaches 1e⁻⁵N. The software default calculation model meets the equilibrium solution conditions, and the model terminates the calculation.

The evolution law of crack morphology in high-pressure fine-particle grouting is shown in Figure 10. Under grouting pressures ranging from 4 to 8 MPa, the grouting cracks expand and develop horizontally on both sides and at a 45° angle within the coupled seepage field grid. The high-pressure fine-grained slurry undergoes fracturing grouting, diffuses along the fractures, forms a gel, consolidates the surrounding rock, and compresses it densely. The evolution law of particle morphology in high-pressure fine-particle grouting is shown in Figure 11. The slurry particles are extruded outward, and there is a clear displacement zoning between the grouted reinforcement area and the non-grouted reinforcement area. The boundary is defined by the horizontal line of the grouting hole and the 45° oblique lines on both sides. The particles at the top of the shallow layer on the inner side displace vertically upwards, while the particles at the bottom of the shallow layer on the inner side displace vertically downwards. The particles at both ends of the shallow layer on the inner side undergo horizontal tension and small-scale extrusion movement towards the low-value areas of the cracks. From Figures 11a–e, the particles at the top of the shallow layer on the inner side move and extrude towards the areas above the 45° angles on both sides, due to the application of high grouting pressure force squeezing and compressing along the weak planes of the rock mass. The particles in the surrounding rock between the horizontal line of the grouting hole and the 45° oblique lines on both sides are basically in a stable state. No local deformation occurs under stress concentration conditions.

The Tabaiyi tunnel of the Jian (Ge) Yuan expressway in Yunnan province is designed as a dual-line separated tunnel, with a burial depth of approximately 150∼298 m and a length of about 2,600 m (Zhou et al., 2024). The tunnel site area features water-rich fault zones F7 and F15, which intersect the tunnel at an angle of nearly 51°, as depicted in Figure 12. The fault rocks, weathered slate, and mudstone contain a high content of montmorillonite. Upon contact with water, they undergo strong argillitization, absorbing water and disintegrating into fault gouge. Furthermore, they are prone to weathering when exposed to air, resulting in extremely poor self-stability.

After the tunnel entered the fault zone, the surrounding rock underwent significant deformation, with a maximum shrinkage deformation of 3.1 m, resulting in a quasi-circular collapse pit with a diameter of approximately 1.7 m on the crown and spandrel. Among them, the primary support of the right track from K63 + 035 to K63 + 050 underwent significant deformation, with twisting occurring at the connection position of the steel frame on the arch, resulting in abnormal noise. Subsequently, the I-beam fractured, and approximately 4,500 m³ of water and mud inrush occurred, with a mud length of 70 m. This pushed the hanging trolley and the secondary lining trolley backwards by more than 20 m, as shown in Figures 13a,b, seriously hindering the normal progress of tunnel construction.

According to the “Highway Tunnel Design Specifications JTG-2018“ and the classification standards for tunnel surrounding rock grades in the geological survey report, the surrounding rock of the Tabaiyi Tunnel on the Jian (Ge) Yuan Expressway is classified into Grade IV and Grade V surrounding rocks. At the same time, based on the third-party settlement measurements and the degree of surrounding rock deformation during the excavation of the pilot tunnel, the tunnel is divided into four zones: ordinary deformation zone, fault fracture zone influence zone, fault fracture zone, and water-rich fault fracture zone. The surrounding water resources around the tunnel are abundant, and the surrounding rock is easily affected by surface precipitation and groundwater. Therefore, engineering problems such as uncontrollable surrounding rock deformation often occur during tunnel excavation, with the water-rich fault fracture zone being the most severe, followed by the fault fracture zone and the fault fracture zone influence zone, and the ordinary surrounding rock deformation zone being relatively mild.

Based on advanced drilling and geological prediction data, the surrounding rock in the section prone to mud and water inrush is primarily composed of strongly weathered carbonaceous slate and fault gouge, interspersed with strongly to moderately weathered carbonaceous sandstone. Geological data from the excavation process of this section reveals the presence of water-bearing weak structural planes in the vicinity. The primary grouting methods employed in this trial section are low-pressure compaction grouting and high-pressure fracturing grouting.

Based on the aforementioned analysis, the double-gradient advanced grouting method is adopted for advanced reinforcement and treatment of the sudden mud and water inrush section. The on-site test is based on the analysis of the excavation exposure and the three-bench construction method, combined with engineering experience. The thickness of the radial reinforcement ring is 8∼12 m outside the outer contour line of the tunnel. To restrict the grout diffusion radius beyond the reinforcement range, the grouting materials are mainly composed of a gradient combination of ordinary Portland cement and ultra-fine cement, with single-fluid supplementary grouting used in the final reinforcement stage. The grouting parameter design is as follows:

Based on the characteristics of the strata, a construction process combining drill rod retreat-style segmented grouting with horizontal sleeve valve tube bundle segmented grouting is adopted. The segment length for drill rod retreat-style grouting is 2∼4 m, while the grouting process for the horizontal sleeve valve tube bundle involves drilling to the final hole, then inserting 2∼3 Φ25 horizontal sleeve valve tubes into the hole, and using segmented grouting. Before grouting, 3∼5 grouting holes are drilled to assess the strata ahead of the tunnel face and determine the grouting process, as shown in Figure 15.

For the completion criteria of single-hole grouting: ① The grouting pressure reaches the design final grouting pressure and is maintained for 5∼10 min, indicating the completion of grouting for that hole. ② During the grouting process, if the actual grouting volume reaches 1.5 times the design grouting volume but the pressure fails to reach the design final pressure, the grout setting time can be adjusted to make the pressure reach the design final pressure and complete the grouting. For the completion criteria of the entire section: ① All grouting holes meet the completion criteria of a single hole, with no omissions in grouting; ② Based on the distribution of grouting volume, design inspection holes are taken at weak links, with 5% of the total grouting holes designated as inspection holes.

Figure 16 depicts the excavation face of the double-gradient grouting area. The grout fills large pores and connected channels through compaction, and then spreads in a fracturing manner within the weak structural planes of the fault zone, forming grout veins with a width of 6∼15 cm. Under dynamic control of grout particle ratio and grouting pressure parameters, the diffusion range of the grout is effectively controlled, as evidenced by the thin ends of the grout veins and the rapid cessation of grouting. The fracturing grout veins in other surrounding rock reinforcement areas vary in width, mainly ranging from 0.3 to 2 cm, and tend to converge towards the main structural plane. Double-gradient grouting forms a spatial grid with the main grout vein as the backbone and secondary grout veins as branches, which, together with the consolidated interlayer of the fault zone, constitutes a grouted reinforcement body, providing a good anchoring environment for subsequent active support with high prestressed NPR anchor cables. Moreover, during the excavation process of the double-gradient grouting area, the surrounding rock remains relatively intact with good stability, and no rockfall or collapse occurs. This effectively controls the asymmetric deformation of the fault zone rock mass under the influence of artificial excavation disturbance, achieving the goal of managing large deformations caused by water-rich fault zones with sudden mud and water inrush. Figure 17 is a statistical diagram of the convergence deformation of the surrounding rock in sections with constant resistance large deformation anchor cable compensatory support based on double-gradient grouting reinforcement and conventional support.