A microwave-liquid nitrogen multi-pre-splitting hard rock roadway partition mechanical rock breaking method

Abstract

The present application relates to the technical field of rock breaking in hard rock roadway of metal mine, and particularly relates to a microwave-liquid nitrogen multi-pre-splitting hard rock roadway partition mechanical rock breaking method, which realizes intelligent partitioning based on drilling parameters and AI inversion: a pre-trained MTL-LSTM network is used to process drilling thrust force, torque and other sequences, to synchronously invert lithology and rock integrity index, and to dynamically calculate equivalent dielectric constant in combination with mechanical rock breaking specific energy and complex refractive index model, so as to accurately divide weak, medium and strong wave absorption zones. A pre-split hole with a bottom expansion cavity is drilled along the pre-splitting crack trajectory, differential microwave heating and liquid nitrogen rapid cooling are implemented to cause cracking, and a pre-splitting crack is formed by a high-pressure water jet to divide the cross section into an outer ring rock breaking zone and an inner ring rock breaking zone. An umbrella-shaped stress barrier is constructed by outwardly inclined annular pressure relief holes in the outer ring, and a blank hole is arranged in the center of the inner ring as an initial free surface, and finally a milling device is used for partition rock breaking. The present application realizes accurate pre-splitting, and greatly reduces the mechanical rock breaking clamping effect and surrounding rock disturbance.

Description

Technical Field

[0001] This invention relates to the field of rock breaking technology for hard rock tunnels in metal mines, and particularly to a microwave-liquid nitrogen multi-stage pre-splitting mechanical rock breaking method for hard rock tunnels. Background Technology

[0002] In the excavation of deep hard rock tunnels in metal mines, the rock mass typically possesses characteristics such as high strength, good integrity, and high density. Conventional mechanical rock-breaking methods face problems such as severe tool wear, high energy consumption, and significant clamping effects. While auxiliary fracturing methods such as microwave heating and liquid nitrogen cold impregnation can weaken the rock mass to some extent, their fracturing range and effectiveness are limited when used alone.

[0003] More importantly, existing technologies often rely on ground-penetrating radar (GPR) scanning or consulting fixed dielectric constants to delineate absorption zones, which presents two major drawbacks: First, variations in mineral composition and changes in microfractures and pore water content within the same lithology can cause dynamic drift in the actual dielectric constant. Fixed parameters can lead to excessively long microwave heating times, resulting in wasted energy, or insufficient time, leading to failure in fracturing. Second, performing additional high-frequency GPR scanning in deep, confined spaces is a complex process, making continuous drilling and testing difficult. Furthermore, existing methods fail to systematically integrate rock mass weakening with optimal free surface construction and stress isolation, hindering further improvements in mechanical rock breaking efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a microwave-liquid nitrogen multi-stage pre-splitting mechanical rock breaking method for hard rock tunnels, addressing the shortcomings of the aforementioned background technology.

[0005] To achieve the above objectives, the present invention provides a microwave-liquid nitrogen multi-stage pre-splitting mechanical rock breaking method for hard rock tunnels, comprising the following steps: Step 1: During the construction preparation phase, the drilling parameter sequence from historical boreholes is used as input, and the corresponding lithology label and rock quality index are used as dual task labels to train a multi-task learning long short-term memory network. Detection holes are drilled in the cross-section of the roadway to be excavated to collect the drilling parameter sequence in real time. The sequence is then input into the multi-task learning long short-term memory network to invert and output the lithology category and equivalent rock integrity index. Based on the lithology category, the pre-calibrated basic dielectric constant and benchmark rock breaking energy are retrieved, and the equivalent dielectric constant is dynamically calculated. Based on this, the cross-section is divided into a weak absorption zone, a medium absorption zone, and a strong absorption zone. Step 2: Determine the layout trajectory of the pre-cracks within the tunnel cross-section, determine the spacing of the pre-crack holes according to the divided wave-absorbing areas, and drill multiple pre-crack holes into the rock mass along the trajectory. At least some of the pre-crack holes have radially enlarged cavities at their bottom. Step 3: Microwave heating is applied to the pre-fractured holes, and a differentiated heating method is used for the pre-fractured holes located in the weak absorption zone, the medium absorption zone, and the strong absorption zone. After heating, liquid nitrogen is injected into the pre-fractured holes for rapid cooling, and thermal shock is used to weaken the rock mass around the pre-fractured holes. Step 4: Use a high-pressure water jet to cut along the radial direction of the pre-splitting hole, connect adjacent pre-splitting holes on the same trajectory, and form at least one pre-crack. Divide the roadway cross section into an outer rock-breaking zone and an inner rock-breaking zone using the pre-crack as the boundary. Step 5: Construct an annular pressure relief hole within the outer ring rock breaking zone. The axis of the annular pressure relief hole forms an outward inclined angle with the tunnel excavation direction. Open a blanking hole in the center of the inner ring rock breaking zone. The blanking hole serves as the initial central free surface for mechanical rock breaking. Step Six: Using a milling-type rock breaking equipment, the pre-cracks and the reserved holes are used as free surfaces for zoned mechanical rock breaking operations; for the inner ring rock breaking area, the reserved holes are used as the initial free surfaces for central grooving, and then milling is carried out layer by layer from the inside out; for the outer ring rock breaking area, the pre-cracks and the excavated areas are used as free surfaces for radial trimming and cutting.

[0006] Preferably, in step one, the training of the multi-task learning long short-term memory network employs a joint loss function. : (1) in, The predicted lithology probability is the output of the network; To predict the integrity index; and This is the weighting balance coefficient; Before training, synthetic minority oversampling technology was used to enhance extreme geological samples such as fault fracture zones.

[0007] Preferably, in step one, the equivalent dielectric constant Dynamic calculations include: Calculate the specific energy of actual mechanical rock breaking : (2) in, For propulsion; Torque; Rotational speed; This refers to the drilling speed; This refers to the cross-sectional area of ​​the drill bit. The stiffness correction factor is determined based on the ratio of torque to propulsion force. Correcting the fundamental dielectric constant : (3) Inversion equivalent gap slightly : (4) in, This is the empirical attenuation coefficient; Based on rock breaking energy; Calculating the equivalent dielectric constant using the complex refractive index model : (5) in, is the dielectric constant of the void fluid.

[0008] Preferably, in step one, the weak absorption region, the medium absorption region, and the strong absorption region are defined according to the following equivalent dielectric constant ranges. Division: For granite, <5 indicates a weak absorption region, 5≤ <6 indicates a moderate absorption region. ≥6 indicates a strong absorption region; For diorite, <6.5 is the weak absorption region, 6.5≤ <7.5 is the medium absorption region. ≥7.5 indicates a strong absorption region; For gabbro, <9 indicates a weak absorption region, 9≤ <11 indicates a moderate absorption region. ≥11 indicates a strong absorption region.

[0009] Preferably, in step two, the spacing between the pre-cracked holes is determined according to the differences in microwave absorption capacity of the region, with the spacing ratio of the pre-cracked holes in the weak absorption region, medium absorption region, and strong absorption region being 3:4:6; the spacing of the pre-cracked holes... d Determined by the following formula: (6) in, The radius of microwave radiation; This refers to the microwave penetration depth. This is a correction factor, with a value ranging from 0.8 to 1.2.

[0010] Preferably, in said step 3, the differential heating method is: for the pre-splitting holes in the weak microwave absorption area, medium microwave absorption area and strong microwave absorption area, the proportional relationship of microwave heating time is 15:6:2.5, and the heating time is determined by the shortest time required for the weak microwave absorption area obtained in advance to reach the cracking temperature.

[0011] Preferably, in step 3, the spraying pressure when injecting liquid nitrogen is 1000KPa~2000KPa, and when it is detected that the difference between the temperature after cooling in the pre-splitting hole and the temperature reached by microwave heating reaches 300°C, the liquid nitrogen injection is stopped.

[0012] Preferably, in step 4, the cutting pressure of the high-pressure water jet is 100MPa~300MPa, and the depth of the cut slot is 0.8 times the depth of the pre-splitting hole.

[0013] Preferably, in step 4, a plurality of pre-splitting cracks are formed, and the plurality of pre-splitting cracks divide the roadway section from outside to inside into a plurality of outer ring rock breaking areas and one inner ring rock breaking area; the spacing between the pre-splitting cracks L is determined by the following formula: (7) wherein, is the safety margin coefficient, with a value range of 1.1~1.3; is the expansion coefficient, with a value range of 2~4; the number of pre-splitting cracks X is determined by the following formula: (8) wherein, is the horizontal distance from the vertical central axis of the roadway to the edge of the roadway; is the geological condition correction coefficient, which is determined according to the equivalent rock quality index RQD output by the inversion of the multi-task learning long short-term memory network in step 1: When RQD>75% or RMR>80, =1; When 50%<RQD≤75% or 60<RMR≤80, =1.1; When 25%<RQD≤50% or 40<RMR≤60, =1.2; When RQD≤25% or RMR≤40, =1.3.

[0014] Preferably, in step five, the depth of the annular pressure relief hole is 1.2 to 1.4 times the depth of the pre-cracked hole, the drilling angle is 6°, and the inclination directions of adjacent pressure relief holes are symmetrically arranged. The depth of the blank hole is 1.1 to 1.5 times the depth of the pre-cracked hole, and the diameter of the blank hole is 1.2 to 1.4 times the diameter of the pre-cracked hole.

[0015] The above-described solution of the present invention has the following beneficial effects: This invention integrates drilling AI inversion with physical mechanisms to precisely delineate the microwave absorption zone and achieve differentiated pre-fracture for each borehole, significantly improving energy utilization. Microwave-liquid nitrogen thermal shock combined with high-pressure water jet cutting creates multiple weakening effects, greatly reducing mechanical rock breaking energy consumption and tool wear. Annular pressure relief holes construct umbrella-shaped stress barriers, while blank holes release central clamping forces, effectively suppressing surrounding rock disturbance. Zoned milling uses pre-fractures as precise boundaries, resulting in high-quality forming, no over-excavation, and overall efficient, low-disturbance, and highly adaptable construction.

[0016] Other beneficial effects of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process of the present invention; Figure 2 This is a schematic diagram showing the location of the pre-splitting holes in the roadway cross-section according to the present invention; Figure 3 This is a schematic diagram of the inner and outer rock-breaking zones and pre-crack distribution of the present invention; [Explanation of Labels in the Attached Image] 1. Pre-splitting hole; 2. Pre-crack; 3. Outer ring rock-breaking zone; 4. Inner ring rock-breaking zone. Detailed Implementation

[0018] To make the technical problems, solutions, and advantages of this invention clearer, a detailed description will be provided below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Furthermore, the technical features involved in the different embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0019] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a locking connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] like Figure 1 As shown, an embodiment of the present invention provides a microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method, comprising the following steps: Step S1: Offline training and deployment of a multi-task model based on historical engineering data Before actual tunnel excavation, a training sample set is constructed using historical borehole data accumulated in the mining area and adjacent areas to train the Multi-Task Learning Long Short-Term Memory Network (MTL-LSTM) offline.

[0022] First, the drilling parameter sequence for each borehole across its entire depth is extracted from the historical borehole database. These parameters are along the borehole depth. Feature vectors in time series form : (9) in, For propulsion; This refers to the rotational torque; This refers to the drill bit rotation speed; This refers to the drilling speed; Drilling time; Each complete drilling sequence is used as a training sample, and the corresponding lithology labels obtained through core analysis are matched with them. and the true values ​​of rock quality indicators obtained through on-site testing. , a dual-task label is formed.

[0023] Extremely broken rock masses such as fault fracture zones often occur in deep roadways, and the number of such samples accounts for a very low proportion in the overall data set. This embodiment introduces the synthetic minority oversampling technique (SMOTE), which interpolates and synthesizes minority-class samples such as fracture zones and extremely low RQD in the statistical feature space extracted from the measurement while drilling sequence, and generates virtual samples close to the distribution of real samples, so that the proportion of each category in the training set tends to be balanced.

[0024] The structural design of the MTL-LSTM network is as follows: the input layer receives the measurement while drilling parameter sequence of the entire borehole ; two shared LSTM hidden layers are used to extract common deep features of rocks in terms of stress-strain response, rock breaking energy consumption, etc., and a hidden state matrix is obtained ; above the shared layer, it bifurcates into two parallel task branches. The first task branch is the lithology classification branch, which outputs the predicted lithology probability of encountering various lithologies through the fully connected layer and the Softmax activation function ; the second task branch is the RQD regression branch, which outputs the predicted equivalent rock quality designation through a fully connected layer .

[0025] A joint loss function is used during training back-propagation optimization is performed on the network parameters. The joint loss function is obtained by weighted summation of the cross-entropy loss for lithology classification and the mean square error loss for RQD regression: (1) wherein, and are weight balance coefficients, both of which are taken as 0.5 in this embodiment. The Adam optimizer is used for training, the initial learning rate is set to 0.001, and the early stopping strategy is used to prevent overfitting. After training is completed, the pre-trained model including network weights and structure is deployed to the edge computing terminal matched with on-site tunneling equipment.

[0026] Step S2: In-situ calibration of pilot hole and acquisition of basic parameters On the tunnel face of the roadway to be excavated, a pilot geological borehole is firstly constructed, the depth of which is consistent with the design depth of the subsequent pre-split hole, and the pore diameter is 75 mm. Drilling parameters are strictly controlled during the drilling process, and the core is completely extracted. Laboratory tests are carried out on the obtained core to obtain the basic permittivity of the lithology (taking granite as an example) of the specific excavation section in an absolutely dense state . At the same time, combined with the drill bit parameters recorded during the drilling of the pilot hole, substitute them into the mechanical specific energy of rock breaking formula to calculate the reference mechanical specific energy of rock breaking of the dense granite .

[0027] Mechanical specific energy of rock breaking The energy consumed in breaking a unit volume of rock is defined as the energy required to break up rock. Taking into account the work done by the propulsion force and the rotational torque, the calculation formula is: (2) in, For propulsion; Torque; Rotational speed; This refers to the drilling speed; This refers to the cross-sectional area of ​​the drill bit. Step S3: Real-time acquisition and feature construction of cross-section drilling parameters To obtain the dielectric property distribution of the entire section to be excavated, multiple probe holes are arranged in a grid pattern at the working face. The diameter and depth of the probe holes are the same as those of the pre-splitting holes, but no cavity enlargement is performed. Using a rock drilling rig or drilling system, the hole depth is collected in real time at high frequency by sensors mounted on the drilling rig during the drilling process of each probe hole. propulsion at the point Rotational torque Drill bit rotation speed and drilling speed The data acquisition system records a set of data every few centimeters of depth, forming a sequence of drilling parameters for each probe hole. A complete probe hole will yield a drilling parameter matrix arranged by depth. .

[0028] Step S4: Real-time online inversion based on pre-trained MTL-LSTM network The drilling parameter matrix X collected from each borehole in step S3 is input one by one into the MTL-LSTM network deployed in the edge computing terminal in step S1. A shared LSTM layer traverses the entire depth sequence, generating a set of high-level shared features. Subsequently, the lithology classification branch outputs the posterior probability distribution of the lithology encountered in the borehole, and the lithology category corresponding to the maximum probability is selected as the inversion result; in this embodiment, it is determined to be granite. Meanwhile, The regression branch directly outputs a continuous value. , which serves as the equivalent rock integrity index at the location of the probe hole.

[0029] Step S5: Dynamic calculation of equivalent permittivity based on physical mechanisms After obtaining the lithology category, the system automatically retrieves the basic dielectric constant corresponding to that lithology from the local physical reference library saved in step S2. Rock breaking energy compared to benchmark .

[0030] To account for variations in microscopic mineral composition and the influence of microcracks and pores on dielectric properties, the following physical mechanism inversion and calculation are performed: (1) Calculate the actual mechanical rock-breaking specific energy at each depth of the probe borehole. Take the stable values ​​of thrust, torque, rotational speed, and drilling speed averaged over depth during the drilling process and substitute them into Formula 2 for calculation.

[0031] (2) The basic dielectric constant is fine-tuned based on the ratio of torque to propulsion force, T / F. This ratio comprehensively reflects the hardness and abrasiveness of the rock, and changes in mineral composition will alter the overall dielectric behavior of the rock. The fine-tuning coefficient is obtained by fitting experimental data. : (10) For example 0, where The torque-propulsion ratio is used as the reference granite. This coefficient is used to correct the fundamental dielectric constant, yielding the equivalent compact dielectric constant considering mineral variations. : (3) (3) Equivalent porosity is inverted by rock breaking energy. The actual rock Compared to the benchmark The reduction can be attributed to the presence of microfractures and pores in the rock mass, which increases drillability and reduces energy consumption. An empirical attenuation index is introduced. (Use 0.5 for granite) Establish equivalent porosity The calculation formula is as follows: (4) (4) Calculate the equivalent dielectric constant using the complex refractive index model (CRIM). The classic CRIM model treats rock as a mixture of framework particles and porous fluid medium, whose effective dielectric constant satisfies: (5) in, Let be the dielectric constant of the fluid in the pores; for groundwater, take a value of 80. Substitute... , and The equivalent dielectric constant of the probe hole location was calculated. .

[0032] Step S6: Absorbing Zone Division and Pre-crack Trajectory Planning The calculations obtained from each probe hole The absorption grades are classified according to the lithology-based standards: for granite, <5 indicates a weak absorption region, 5≤ <6 indicates a moderate absorption region. ≥6 indicates a strong absorbing zone. Spatial interpolation is used to generate a distribution map of the absorbing zone across the entire cross-section for the area between the probe holes.

[0033] Based on the tunnel cross-sectional dimensions, excavation outline, and distribution of the wave-absorbing zone, several pre-crack patterns are planned on the cross-section. The pre-cracks are arranged radially along the tunnel cross-section, divided into multiple layers from the outline edge towards the center. The pre-crack spacing... Determined by the following formula: (7) in, This is the safety margin factor, with a value ranging from 1.1 to 1.3. This is an expansion factor, with a value ranging from 2 to 4; and These are the microwave radiation radius and microwave penetration depth measured in the pilot hole pre-test of step S2, respectively. Take the larger of the two, d.

[0034] The number of pre-cracks, X, is determined by the following formula: (8) In the formula, It is the horizontal distance from the vertical centerline of the tunnel to the edge of the tunnel. This is a correction factor for geological conditions. Here, the average equivalent rock integrity index of the inner ring rock-breaking zone obtained through network inversion in step S4 is directly used. Dynamic determination: when 50% < When ≤75%, Take 1.1. In this embodiment, the cross-section is divided into an outer rock-breaking zone and an inner rock-breaking zone from the outside in, as follows: Figure 3 As shown.

[0035] Step S7: Differentiated Drilling of Pre-split Holes Along each pre-crack trajectory, pre-crack holes 1 are drilled at differentiated intervals according to the absorption level at their location, such as... Figure 2 As shown. The ratio of the pre-cracked hole spacing d in the weak absorption zone, medium absorption zone, and strong absorption zone is 3:4:6. The foundation spacing d is determined by the following formula: (6) in, The radius of microwave radiation. This represents the microwave penetration depth. The correction factor ranges from 0.8 to 1.2. This is used to obtain the spacing between the weak absorbing zones. The spacing between medium absorbing zones is The spacing between the strong absorbing regions is .

[0036] All pre-splitting holes maintain a consistent depth and a diameter of 75 mm. The front section of each pre-splitting hole is a hole of equal diameter, and the bottom is machined into a radially enlarged trumpet-shaped cavity. The maximum diameter of the enlarged cavity is 1.8 times that of the equal diameter section, and the length of the enlarged cavity section accounts for 1 / 5 of the hole depth.

[0037] Step S8: Microwave-liquid nitrogen alternating thermal fracturing A microwave thermal conductivity meter was sequentially inserted into equal-diameter sections of each pre-cracked hole for microwave radiation heating. The heating time was strictly controlled according to the differences in the absorbing zones: the base heating time for the weak absorbing zone was set as... (Determined by pilot hole microwave fracturing pre-test to ensure the rock mass reaches the effective fracturing temperature), then the heating time in the medium microwave absorption zone is... The heating time for the strong absorbing region is The ratio is approximately 15:6:2.5. The heating sequence is first the weak absorption zone, then the medium absorption zone, and finally the strong absorption zone.

[0038] Immediately after microwave heating, the thermal conductivity meter was removed, and a liquid nitrogen sprayer was inserted into the same hole, with the spraying pressure controlled at 1500 kPa. The hole wall temperature was monitored in real time using a temperature sensor inserted with the sprayer. Liquid nitrogen injection was immediately stopped when the temperature difference between the cooled state and the highest temperature reached during the heating phase reached 300°C. Under the immense thermal shock, a dense network of microcracks formed in the rock mass surrounding the pre-splitted hole, significantly weakening its strength.

[0039] Step S9: High-pressure water jet cutting to form a pre-crack After microwave-liquid nitrogen thermal shock fracturing, a high-pressure water jet is used for kerf cutting. The water jet nozzle moves segment by segment along the pre-crack trajectory, with the jet pressure selected between 1000 kPa and 2000 kPa, and the kerf depth controlled at 0.8 times the depth of the pre-cracked hole. The high-pressure jet, along the path weakened by microcracks between the holes, completely cuts through adjacent pre-cracked holes on the same trajectory radially, forming a continuous pre-crack 2.

[0040] After all the pre-cracks have formed, the tunnel cross-section is divided into an outer ring rock-breaking zone 3 and a central inner ring rock-breaking zone 4, as shown below. Figure 3 As shown.

[0041] Step S10: Construction of the annular pressure relief hole and the blanking hole Within each outer ring of the rock-breaking zone, a set of annular pressure relief holes is arranged circumferentially. The depth of the annular pressure relief holes is 1.3 times the depth of the pre-splitting holes, and the borehole axis forms a 6° outward tilt angle with the tunnel excavation direction. Adjacent pressure relief holes are symmetrically arranged in tilt direction, i.e., the first hole tilts outward at 6°, the second hole tilts inward at 6°, and so on. In the bottom area of ​​the holes, the overlapping influence zones caused by the tilt of adjacent holes will connect with each other, forming an umbrella-shaped continuous annular weakening zone surrounding the excavation area.

[0042] Meanwhile, a blanking hole is drilled at the geometric center of the inner rock-breaking zone 4. The blanking hole is not subjected to any heating, freezing or cutting treatment, and its depth is 1.3 times the depth of the pre-splitting hole and its diameter is 1.3 times the diameter of the pre-splitting hole.

[0043] Step S11: Zoned Mechanical Rock Breaking Operation Mechanical rock breaking is carried out using milling rock breaking equipment such as cantilever milling machines, following a sequence of advancement from the inside out and in layers.

[0044] First, rock breaking is carried out in the inner ring rock breaking zone 4. The milling head is directly inserted into the blanking hole, and rotating excavation is performed with the blanking hole as the initial free surface to quickly form a conical groove cavity with a diameter several times that of the blanking hole, releasing the high clamping stress in the core area of ​​the working face. Subsequently, using this groove cavity as the free surface, the entire inner ring rock mass is milled back and forth layer by layer from the inside to the outside.

[0045] After the inner ring excavation is completed, the outer ring rock-breaking zones 3 are then trimmed and cut. Using the innermost pre-crack 2 and the already excavated area of ​​the inner ring as free surfaces, the rock mass of the outer ring rock-breaking zones is milled radially from the inside out. The pre-cracks provide a clear termination boundary for the milling head, preventing over-excavation. At the same time, the annular pressure relief holes have isolated the deep surrounding rock stress, minimizing disturbance to the surrounding rock. This ultimately results in a tunnel with precise cross-sectional dimensions and a smooth profile.

[0046] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0047] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A microwave-liquid nitrogen multi-stage pre-splitting mechanical rock breaking method for hard rock tunnels, characterized in that, Includes the following steps: Step 1: In the construction preparation stage, the drilling parameter sequence of historical boreholes is used as input, and the corresponding lithology label and rock quality index are used as dual task labels to train a multi-task learning long short-term memory network. Detection holes are drilled in the cross-section of the roadway to be excavated, and the drilling parameter sequence is collected in real time. The sequence is input into the multi-task learning long short-term memory network to output the lithology category and equivalent rock integrity index in parallel. Based on the lithology category, the pre-calibrated basic dielectric constant and benchmark rock breaking energy are retrieved, and the equivalent dielectric constant is dynamically calculated. Based on this, the cross-section is divided into weak absorption zone, medium absorption zone and strong absorption zone. Step 2: Determine the layout trajectory of the pre-cracks within the tunnel cross-section, determine the spacing of the pre-crack holes according to the divided wave-absorbing areas, and drill multiple pre-crack holes into the rock mass along the trajectory. At least some of the pre-crack holes have radially enlarged cavities at their bottom. Step 3: Microwave heating is applied to the pre-fractured holes, and a differentiated heating method is used for the pre-fractured holes located in the weak absorption zone, the medium absorption zone, and the strong absorption zone. After heating, liquid nitrogen is injected into the pre-fractured holes for rapid cooling, and thermal shock is used to weaken the rock mass around the pre-fractured holes. Step 4: Use a high-pressure water jet to cut along the radial direction of the pre-splitting hole, connect adjacent pre-splitting holes on the same trajectory, and form at least one pre-crack. Divide the roadway cross section into an outer rock-breaking zone and an inner rock-breaking zone using the pre-crack as the boundary. Step 5: Construct an annular pressure relief hole within the outer ring rock breaking zone. The axis of the annular pressure relief hole forms an outward inclined angle with the tunnel excavation direction. Open a blanking hole in the center of the inner ring rock breaking zone. The blanking hole serves as the initial central free surface for mechanical rock breaking. Step Six: Using a milling-type rock breaking equipment, the pre-cracks and the reserved holes are used as free surfaces for zoned mechanical rock breaking operations; for the inner ring rock breaking area, the reserved holes are used as the initial free surfaces for central grooving, and then milling is carried out layer by layer from the inside out; for the outer ring rock breaking area, the pre-cracks and the excavated areas are used as free surfaces for radial trimming and cutting.

2. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step one, the training of the multi-task learning long short-term memory network employs a joint loss function. : （1） in, The predicted lithology probability is the output of the network; To predict the integrity index; and This is the weighting balance coefficient; Before training, synthetic minority oversampling technology was used to enhance extreme geological samples such as fault fracture zones.

3. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step one, the equivalent dielectric constant Dynamic calculations include: Calculate the specific energy of actual mechanical rock breaking : （2） in, For propulsion; Torque; Rotational speed; This refers to the drilling speed; This refers to the cross-sectional area of ​​the drill bit. The stiffness correction factor is determined based on the ratio of torque to propulsion force. Correcting the fundamental dielectric constant : （3） Inversion equivalent gap slightly : （4） in, This is the empirical attenuation coefficient; Based on rock breaking energy; Calculating the equivalent dielectric constant using the complex refractive index model : （5） in, is the dielectric constant of the void fluid.

4. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step one, the weak absorption region, the medium absorption region, and the strong absorption region are defined according to the following equivalent dielectric constant ranges. Division: For granite, <5 indicates a weak absorption region, 5≤ <6 is the medium absorption region. ≥6 indicates a strong absorption region; For diorite, <6.5 is the weak absorption region, 6.5≤ <7.5 is the medium absorption region. ≥7.5 indicates a strong absorption region; For gabbro, <9 indicates a weak absorption region, 9≤ <11 indicates a moderate absorption region. ≥11 indicates a strong absorption region.

5. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step two, the spacing between the pre-cracked holes is determined based on the differences in microwave absorption capacity of the region. The ratio of the pre-cracked hole spacing to the weak absorption region, medium absorption region, and strong absorption region is 3:4:

6. d Determined by the following formula: （6） in, The radius of microwave radiation; This refers to the microwave penetration depth. This is a correction factor, with a value ranging from 0.8 to 1.

2.

6. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step three, the differentiated heating method is as follows: for the pre-cracked holes in the weak absorption zone, medium absorption zone, and strong absorption zone, the ratio of microwave heating time is 15:6:2.5, and the heating time is determined by the shortest time required for the weak absorption zone to reach the cracking temperature.

7. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 6, characterized in that, In step three, the spraying pressure during liquid nitrogen injection is 1000KPa~2000KPa. When the temperature difference between the pre-cracked hole after cooling and the temperature reached by microwave heating reaches 300℃, the liquid nitrogen injection is stopped.

8. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step four, the cutting pressure of the high-pressure water jet is 100MPa~300MPa, and the cutting depth is 0.8 times the depth of the pre-cracked hole.

9. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 8, characterized in that, In step four, multiple pre-cracks are formed, which divide the roadway cross-section into multiple outer rock-breaking zones and one inner rock-breaking zone from the outside in; the spacing of the pre-cracks... L Determined by the following formula: （7） in, This is the safety margin factor, with a value ranging from 1.1 to 1.

3. This is an expansion factor, with a value ranging from 2 to 4; Number of pre-cracks X Determined by the following formula: （8） in, It is the horizontal distance from the vertical centerline of the tunnel to the edge of the tunnel. The geological condition correction factor is determined based on the equivalent rock integrity index (RQD) output from the multi-task learning long short-term memory network inversion step one. When RQD>75% or RMR>80 =1; When 50% < RQD ≤ 75% or 60 < RMR ≤ 80, = 1.1; When 25% < RQD ≤ 50% or 40 < RMR ≤ 60, = 1.2; When RQD≤25% or RMR≤40 =1.

3.

10. The microwave-liquid nitrogen multi-stage pre-splitting hard rock tunnel zonal mechanical rock breaking method according to claim 1, characterized in that, In step five, the depth of the annular pressure relief hole is 1.2 to 1.4 times the depth of the pre-cracked hole, the drilling angle is 6°, and the inclination directions of adjacent pressure relief holes are symmetrically arranged. The depth of the blank hole is 1.1 to 1.5 times the depth of the pre-cracked hole, and the diameter of the blank hole is 1.2 to 1.4 times the diameter of the pre-cracked hole.

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