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When the deep tunnel is excavated, the pressure of the confined water is relatively high, causing the water inrush to have a hydraulic fracturing effect. The method of theoretical analysis was adopted to study this effect. A mechanical model for fracturing water inrush under blasting excavation conditions was established. The water inrush under this condition is the result of the combined action of static load (water pressure and

After mining or tunnel engineering enters deep [

In coal mining, hydraulic fracturing water inrush mainly includes two types: floor fracturing water inrush and karst collapse column fracturing water inrush. When the water pressure in the floor is high, the water pressure in the confined aquifer has a fracturing and expansion effect on the water-resistant rock mass, which can cause the primary fissures in the water-resistant rock mass to open and expand [

In tunnel engineering, a large amount of water inrush has occurred under high water pressure with hydraulic fracturing effect (Table

Typical tunnel water inrush accidents with hydraulic fracturing effect [

Number | Location | Bad geology | Water source | Outlet position | Water inrush characteristics |
---|---|---|---|---|---|

1 | Bagualing Tunnel DK132 + 340 | A large cave on the left arch | Karst water | Advanced geological exploration | Water inflow was 15,000 m^{3}/h |

2 | Yuanliangshan Tunnel DK354 + 879 | Cave in front | High-pressure karst water | Face | Peak water inrush volume was 3000 m^{3}/h |

3 | Yuanliangshan Tunnel DK361 + 764 | A karst pipeline on the right side and developed bedding cracks | Karst water head is about 200 m | Karst pipeline | Maximum water inflow was 216 m^{3}/min |

4 | Bieyancao Tunnel DK406 + 422 | The junction of soluble rock and nonsoluble rock | High-pressure karst water | Rock blast hole | The initial water pressure was over 1.0 MPa |

5 | Bieyancao Tunnel DK406 + 680 ∼+ 710 | Fault zone, a dark river on the right | Karst water | Right wall | Peak water inflow was 2100 m^{3}/h |

6 | Pishuangao Tunnel RK63 + 094∼+102 | Developed karst | Karst water | Construction joints at the bottom of the tunnel | Gushing |

7 | Yesanguan Tunnel DK124 + 602 | Fault fracture zone | Karst water | Face | Water inflow in the first 30 minutes was 151,000 m^{3} |

8 | Wuzhishan Tunnel K29 + 543 | Fault fracture zone | Karst water | Face | Peak water inflow was 16,000 m^{3}/d |

Based on the experimental law of true triaxial hydraulic fracturing, this paper established a mechanical calculation model for high-pressure water inrush during tunnel excavation by blasting. Then, the established mechanical model was solved. Finally, an example was used to verify the correctness of the theoretical derivation.

After hydraulic fracturing with red dye water on a true triaxial experimental system, the hydraulic morphology in the rock can be divided into “four zones and three fronts” (Figure

Macroscopic hydraulic cracks and microcracks at the tip of the crack after the hydraulic fracturing experiment [

“Four zones and three fronts” around hydraulic fractures [

The hydraulic fracturing of high-pressure water in the source of water inrush disaster forms macroscopic hydraulic cracks in the surrounding rock. The macrohydraulic fracture zone is formed from the high-pressure water source to the tip of the macrohydraulic fractures. As the high-pressure water gradually flows to the tip of the macrohydraulic fracture and seeps around the macrohydraulic fracture, the water pressure at the tip of the hydraulic fracture gradually increases. This reduces the hydraulic gradient in the hydraulic cracks. Since the rock is composed of mineral particles [

Based on the above analysis, a plane mechanical model of the high-pressure water inrush during tunnel excavation is established, as shown in Figure

Mechanical model of high-pressure water inrush during excavation in tunnels.

Drilling and blasting are the main method of tunnel excavation. When the tunnel is excavated by drilling and blasting, the excavation disturbance stress is caused by the explosion stress wave on the surrounding rock. The blasting excavation redistributes the stress in the surrounding rock and disturbs the high-pressure water at the source of the water inrush hazard. The original equilibrium state between high-pressure water and surrounding rock is broken. Hydraulic cracks begin to crack and expand [

The rock mass is subjected to the static load of in situ stress (

Calculation model of water inrush in complete rock mass during blasting excavation.

Calculation model of water inrush in fractured rock mass during blasting excavation.

As shown in Figure

When

When the water pressure is greater than the sum of the tangential stress and the tensile strength of the rock, the rock cracks. Therefore, the critical water pressure

When the tunnel is excavated by blasting, the rock cracking around the water inrush source is subjected to not only the static load of the water pressure and ground stress but also the dynamic load of the blast stress wave. If the disturbing effect of the explosion stress wave on the rock is

The instability propagation of hydraulic cracks during blasting excavation is the result of the superposition of static loads (water pressure and in situ stress) and dynamic loads (explosive stress waves). According to the superposition principle, when multiple loads are applied to a crack in the linear elastic range, the total stress intensity factor at the crack tip is equal to the sum of the stress intensity factors under the individual loads that produce the same crack propagation mode. The stress intensity factor of the crack under the conditions of blasting excavation should be the superposition of the stress intensity factor generated under the combined action of static and dynamic loads, namely,

As shown in Figure

The normal stress

For I-II tensile-shear composite cracks, the approximate fracture judgment criteria in engineering can be used [

According to fracture mechanics, the stress intensity factors of type I and type II crack tips are, respectively,

The rock is permeable. After hydraulic fracturing, there are “four zones and three frontiers” in the rock. There are no cracks in the osmotic hydraulic zone, but there is the pore water pressure. The pore water pressure gradient is formed from the hydraulic crack to the water pressure front in the osmotic hydraulic zone. Pore pressures of different sizes have a splitting effect at the crack tip. The pore water pressure splitting factor

Subsequently, the effective normal stress

Namely,

For type I cracks under the influence of the pore water pressure gradient, the stress intensity factor at the tip of the hydraulic crack is

For type II cracks, the stress intensity factor at the tip of the hydraulic crack is

When tunnels are excavated by blasting, the blast stress waves propagating in the rock mass are mainly P waves and SV waves. The type I and type II dynamic stress intensity factors generated by the P wave and SV wave at the tip of a water-bearing crack are analyzed [

From equations (

The dynamic stress intensity factor generated by the SV wave at the tip of the hydraulic crack [

Among them,

To analyze the maximum impact of the dynamic stress intensity factor generated by the SV wave on the crack growth,

Substituting formulae (

According to the change law of dynamic stress intensity factors

Therefore, the critical water pressure when the surrounding rock undergoes hydraulic fracturing effect of water inrush with combined tensile and shear failure is

When the normal stress on the surface of the hydraulic crack is compressive stress, the crack propagation mode is the I-II compression-shear compound type. Based on the maximum circumferential stress theory [

The crack propagation proceeds along the section of maximum circumferential stress

If the effects of crack thickness and tip curvature radius are neglected, the propagation angle of the compression-shear compound crack is a fixed value [

When the normal stress is compressive stress, the hydraulic crack is closed. The hydraulic crack surface is expanded under the action of shearing force. Compressive stress also has frictional force on the hydraulic crack surface to resist the movement of the crack surface. The effective shear stress can be expressed as

The static stress intensity factor is still

Namely,

The calculation method of the dynamic intensity factor of compression-shear crack propagation is the same as that of tension-shear composite crack propagation. The dynamic stress intensity factor takes the maximum value and does not consider its change with phase and time, so it can be calculated by equation (

Substituting formulae (

If the intensity of the incident wave does not change, the angle between the incident wave and the long axis of the hydraulic crack is about 90° when

Consequently, the critical water pressure when the surrounding rock undergoes hydraulic fracturing effect of water inrush with compression-shear failure is

A typical hydraulic fracturing effect of high-pressure water inrush occurred in the Jinping deep-buried (2848 m) exploration cave. According to the measured data of Huang Runqiu et al. [_{IC} value of the mode I crack in marble was 15.2 MN/m^{3/2}. It can be seen from equation (_{IC} is a negative value, so the calculation should be −15.2 MN/m^{3/2}. The fracture toughness _{IIC} of type II crack was 11.2 MN/m^{3/2}, which should be a negative value during calculation. The internal friction angle

Substituting the above parameters into equation (

Based on the experimental law of true triaxial hydraulic fracturing, a mechanical calculation model for high-pressure water inrush during tunnel blasting and excavation was established.

When the pore water pressure, skeleton stress, and blasting disturbance stress at the tip of the hydraulic crack exceed the bond strength of the mineral particles, the mineral particles are split and microcracks are formed at the tip of the hydraulic crack.

For fracturing water inrush of fractured rock mass, two types of water inrush were proposed: tension-shear composite type and compression-shear composite type. Considering the pore water pressure fracturing factor, these two types of critical water pressure were calculated.

The calculated critical water pressure formula was used to verify the water inrush in the Jinping deep-buried tunnel. The calculated critical water pressure was similar to the measured water inrush pressure, which proved the correctness of the deduced theory.

The data used to support the study can be obtained from the corresponding author upon request.

The authors declare that there are no conflicts of interest regarding the publication of this article.

The authors appreciate the financial support of this work provided by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX21_2368).