Hard rock trenching construction method based on small-bore drilling blasting pre-ripping

Abstract

This application relates to the field of geotechnical engineering and underground engineering construction technology, and discloses a method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting. The method includes: conducting a geological survey of the construction area to determine whether it belongs to hard rock strata that are difficult to directly trench mechanically; if so, determining the locations of two small-diameter boreholes for a section of the wall to be trenched; drilling small-diameter boreholes at the locations and collecting rock cores during the drilling process; identifying the target hard rock section based on the analysis of the rock cores; blasting explosives at the corresponding borehole depth to create pre-splitting fractures in the hard rock mass between the two small-diameter boreholes; and excavating the hard rock mass with pre-splitting fractures between the two small-diameter boreholes to complete the trenching of the wall section. This application achieves significant optimization of construction economy and operational efficiency while ensuring trenching quality.

Description

Technical Field

[0001] This application relates to the field of geotechnical engineering and underground engineering construction technology, and in particular to a method for trenching construction of hard rock ground walls based on small-diameter borehole blasting pre-splitting. Background Technology

[0002] In underground engineering projects such as water conservancy and transportation, it is often necessary to construct shafts or foundation pits in areas with hard rock, using diaphragm walls as retaining structures. When the diaphragm wall needs to penetrate intact hard rock (such as granite or quartzite) with a uniaxial compressive strength exceeding 100 MPa, the conventional "two-drill-one-excavation" trenching process faces significant challenges. For example... Figures 1-2 As shown, this process typically involves using a large-diameter rotary drilling rig to drill full-depth holes (with a hole diameter of 500-1000 mm) at both ends of a trench section to create a free face, and then using a grab bucket or trenching machine to excavate the intermediate rock mass. However, this process has the following prominent problems.

[0003] First, regardless of the uniformity of the rock mass's vertical integrity, large-diameter drilling across the entire trench depth is required, resulting in a significant amount of ineffective drilling and a severe waste of equipment time and resources. Second, in highly intact hard rock, the limited free face provided by only two large-diameter boreholes offers very limited stress release and rock weakening effects, making it difficult to efficiently excavate the intermediate rock mass, leading to low trenching efficiency and severely restricting the construction schedule. Furthermore, large-diameter rotary drilling rigs are expensive and energy-intensive, especially when only locally high-strength, intact rock strata exist, requiring substantial investment to complete full-hole drilling, resulting in poor construction economics.

[0004] Current technologies lack sophisticated methods for identifying the spatial distribution characteristics of hard rock strata in the construction area, and are unable to precisely weaken rock sections that are truly difficult to trench mechanically. This results in over-construction, energy waste, and efficiency bottlenecks during the trenching process. Therefore, a targeted method is urgently needed to solve these technical problems. Summary of the Invention

[0005] In view of the above-mentioned shortcomings and deficiencies of the prior art, this application provides a method for trenching construction of hard rock walls based on small-diameter borehole blasting pre-splitting, which solves the technical problems of high equipment cost, serious ineffective full-hole drilling operations, low trenching efficiency, and poor construction economy and long construction period caused by the lack of precise identification and targeted weakening methods for the spatial distribution of rock mass integrity when traversing intact hard rock strata.

[0006] To achieve the above objectives, this application provides the following technical solution:

[0007] A method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting includes:

[0008] S1. Conduct a geological survey of the construction area to determine whether it belongs to hard rock strata that are difficult to directly form trenches mechanically. If so, determine the location of two small-diameter boreholes for a section of the ground wall to be trenched.

[0009] S2. Drill small-diameter holes at the designated locations and extract rock cores during the drilling process;

[0010] S3. Based on the analysis of the rock core, the target hard rock section is identified, and explosives are loaded into the borehole depth corresponding to the target hard rock section for blasting, so as to form pre-fractured fissures in the hard rock mass between the two small-diameter boreholes.

[0011] S4. Excavate the hard rock mass with pre-split fractures between the two small-diameter boreholes to complete the trenching of the ground wall section.

[0012] Preferably, the diameter of the small-diameter drill hole is 76mm to 110mm, and the drilling depth is adapted to the design depth of the trench section of the ground wall.

[0013] The layout location meets the following requirements: the two small-diameter boreholes are located 30-50mm inside the design boundary line at both ends of the ground wall trench section, the center of the boreholes coincides with the horizontal center line of the ground wall trench section, and the line connecting the two small-diameter boreholes is parallel to the length direction of the ground wall trench section with a horizontal deviation of ≤0.2%.

[0014] Preferably, when taking core samples during drilling, core samples are extracted once every 1 to 1.5 meters of drilling, with a core recovery rate of ≥90%. The extracted core samples are numbered, packaged, and preserved in sequence according to the drilling depth, and the integrity, joint development, and fracture closure status of the core samples are recorded simultaneously.

[0015] Preferably, the step of identifying the target hard rock segment based on the analysis of the rock core includes: performing segment-by-segment testing on the rock cores packaged according to depth number, and determining the continuous rock core segments that meet the requirements of uniaxial compressive strength of rock mass ≥100MPa and rock quality index ≥0.9 as the target hard rock segment.

[0016] Preferably, the step of loading explosives into the borehole at the corresponding depth of the target hard rock section for blasting includes:

[0017] For each identified target hard rock section, calculate the required amount of explosives and prepare a concentrated explosive charge;

[0018] Using a guide rod, the concentrated explosive charge is precisely lowered to the corresponding borehole depth for each target hard rock section. At least 200mm of buffer material is filled at the top and bottom of the charge. After loading, the borehole opening is sealed and subjected to vibration damping. When multiple discontinuous target hard rock sections exist in a single small-diameter borehole, the explosive charge sections corresponding to each target hard rock section are detonated with a delayed initiation time, with an initiation time interval of 10ms to 25ms between adjacent sections. Target hard rock sections at the same elevation in two small-diameter boreholes are detonated synchronously to ensure that the pre-splitting fractures induced by blasting are effectively connected along the direction of the ground wall trench.

[0019] Preferably, the sealing and vibration damping treatment of the borehole opening includes:

[0020] A composite structure combining a flexible damping pad and a rigid cap is used to seal the borehole opening; the flexible damping pad is laid close to the rock surface at the borehole opening, and the rigid cap is covered on the pad and fixed to the surrounding rock mass by expansion bolts; and the total thickness of the sealing body is not less than 300mm.

[0021] Preferably, when calculating the required amount of explosives and setting the delayed detonation parameters, dynamic corrections are made based on the recorded core integrity, joint development, and fracture closure status, including:

[0022] If the core sample corresponding to the target hard rock section shows open fractures or weak interlayer filling, the calculated charge for that section should be reduced by 10% to 20%.

[0023] If the angle between the dip of the core joint surface corresponding to the target hard rock section and the direction of the line connecting the two small-diameter boreholes is less than 30°, then the detonation time interval between adjacent target hard rock sections will be shortened to 8ms to 15ms.

[0024] If the joint spacing of the rock core corresponding to the target hard rock section is less than 0.3m, and the rock core shows dense closed joints that cause the rock block to have a potential fragmented structure, then segmented low-energy explosive charges are used instead of concentrated explosive charges. The explosive amount of a single segment of the segmented low-energy explosive charge does not exceed 1 / 3 of the explosive amount of the original concentrated explosive charge. The spacing between adjacent segmented low-energy explosive charges is 0.8m to 1.2m. At the same time, the thickness of the buffer material at the top and bottom of the segmented low-energy explosive charge is increased to 300mm to 400mm.

[0025] Preferably, after the blasting in S3 and before the start of S4, a step is included to check the pre-splitting effect: by cross-hole sonic CT scanning or borehole panoramic imaging, it is determined whether a continuous pre-splitting fracture has been formed between the two small-diameter boreholes; if the fracture penetration rate is less than 85%, a low-energy micro-delay blast is added to the original target hard rock section, and the amount of explosives added to the blast does not exceed 30% of the original design amount of explosives for that section.

[0026] Preferably, when the trench section to be formed is connected to an adjacent trench section that has been completed, at one end closer to the adjacent trench section, the placement of the small-diameter borehole is adjusted by 100mm to 200mm from the conventional design position at that end towards the inside of the trench section. During blasting, the detonation time of the target hard rock section in the small-diameter borehole is advanced by 5ms to 10ms compared to the detonation time in the other small-diameter borehole.

[0027] Preferably, before blasting the section of the ground wall to be constructed, the joint surface of adjacent completed sections is protected, including:

[0028] A flexible isolation layer is laid on the outside of the joint surface of adjacent trench sections. The isolation layer is made of rubber sheet with a thickness of 10~15mm. The rubber sheet is fixed to the joint surface by anchors with a fixing spacing of 300~400mm. The edge of the rubber sheet extends 50~80mm beyond the boundary of the adjacent trench section.

[0029] A rigid protective frame is installed on the outside of the rubber sheet. The rigid protective frame is formed by welding angle steel. The distance between the rigid protective frame and the joint surface is 150~200mm. The outside of the rigid protective frame is covered with wire mesh.

[0030] Vibration damping holes with a depth of ≥1.5m and a spacing of 500mm are arranged around the small-diameter boreholes near the adjacent trench section. The diameter of the vibration damping holes is 40~60mm, and the holes are filled with crushed stone with a particle size of 5~10mm.

[0031] The hard rock trenching method based on small-diameter borehole blasting pre-splitting provided in this application achieves refined identification of the spatial distribution characteristics of rock mass integrity in the vertical direction of the trench section by introducing small-diameter boreholes and combining core sampling and analysis. This allows for precise identification and subsequent operations only on target hard rock sections that are difficult to directly excavate mechanically, avoiding the ineffective work of large-diameter drilling across the entire depth and cross-section regardless of lithology in traditional processes, significantly saving equipment shifts and operation time. Furthermore, this method performs directional charging blasting within the identified target hard rock section, using small-diameter boreholes as guides and free faces to concentrate blasting energy on specific hard rock masses between two boreholes, effectively pre-splitting and weakening them, overcoming the shortcomings of traditional large-diameter boreholes with limited free faces and insufficient weakening effects. Finally, the intermediate rock mass, whose integrity has been significantly reduced after blasting pre-splitting, is excavated, greatly improving the excavation efficiency of the trenching machine. This method replaces the expensive large-diameter rotary drilling rig with low-cost exploration drilling rigs and small-dose blasting for full-hole drilling. Through the core logic of precise identification and directional weakening, it achieves significant optimization of construction economy and operation efficiency while ensuring trenching quality.

[0032] This application has the following significant advantages: First, it boasts high construction efficiency. Lightweight exploration drilling rigs are used for small-diameter drilling, allowing multiple units to operate simultaneously. Directional blasting can instantly weaken the rock mass, avoiding prolonged mechanical fracturing in hard rock and effectively shortening the construction period. Second, it is highly targeted. Through core sampling and analysis, the hard rock sections requiring treatment can be accurately identified, enabling targeted pre-splitting and avoiding a large amount of ineffective drilling in traditional processes. Third, it offers high resource utilization. Small-diameter boreholes serve multiple functions, including exploration and sampling, creating free faces, providing blasting channels, and providing guide markers for trenching machines. Fourth, it offers flexible construction. The lightweight equipment is highly adaptable to different construction sites and can quickly respond to complex and changing geological conditions. Attached Figure Description

[0033] Figure 1 This is a top view of the layout of the two-drill-one-excavation process in the existing technology.

[0034] Figure 2 This is a three-dimensional schematic diagram of the two-drill-one-excavation process in existing technology.

[0035] Figure 3 This is a top view of the arrangement of the method in this embodiment.

[0036] Figure 4 This is a three-dimensional schematic diagram of the method in this embodiment.

[0037] Figure 5 This is a schematic diagram of the blasting pre-splitting of a wall trench section in the method of this embodiment.

[0038] Figure 6 This is a flowchart illustrating the method of this embodiment.

[0039] In the diagram: 1. Ground wall trench section; 2. Large-diameter borehole; 3. Free face; 4. Small-diameter borehole; 5. Target hard rock section; 6. Blasting pre-splitting zone; 7. Guide rod; 8. Explosive charge; 9. Excavation area. Detailed Implementation

[0040] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0041] In the description of this application, 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 this application and 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 this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0042] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed 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 between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0043] Diaphragm walls are reinforced concrete walls constructed in sections within rock and soil, commonly used in water conservancy, transportation, and other engineering projects for deep foundation pit support, water interception, and seepage prevention. The core construction step is trenching, which involves excavating vertical trenches of the designed dimensions at predetermined locations, placing reinforcing cages, and pouring concrete to form a continuous retaining or water-retaining structure. When the trench section needs to traverse hard rock strata with high integrity and strength (such as granite and quartzite), direct mechanical excavation using grab buckets or trenching machines is extremely difficult, inefficient, and results in severe equipment wear. Therefore, a two-drill-one-excavation process is often used in these projects. Figures 1-2 As shown, large-diameter boreholes 2 are first drilled at both ends of the trench section 1 to provide a free face 3 (i.e., a free surface) for excavation of the intermediate rock mass, thereby reducing excavation resistance. However, this process still faces problems such as high cost, low drilling efficiency, and limited weakening effect of the free face in intact hard rock. This application addresses these technical bottlenecks by proposing a hard rock trenching construction method based on small-diameter borehole blasting pre-splitting. Through precise lithology identification and directional blasting weakening, the trenching efficiency and economy are significantly improved.

[0044] The method described in this application has been successfully applied in trenching construction in hard rock formations, such as the Hanjiang-Rongjiang-Lianjiang connecting project. Practice has shown that using the method described in this application for pre-splitting significantly improves the excavation efficiency of the trenching machine in the rock mass, substantially reduces overall construction costs, and significantly shortens the construction period, achieving remarkable economic and technical benefits. This verifies the practical effectiveness of the method described in this application in improving trenching efficiency and economy.

[0045] Example

[0046] This embodiment provides a method for trenching construction of hard rock walls based on small-diameter borehole blasting pre-splitting, including:

[0047] S1. Exploration and judgment, including: conducting a geological survey of the construction area to determine whether it belongs to hard rock strata that are difficult to directly form trenches mechanically. If so, determine the location of two small-diameter boreholes for a section of the ground wall to be trenched.

[0048] S2. Core drilling, including: drilling small-diameter holes at the designated location and taking rock cores during the drilling process;

[0049] S3. Blasting pre-splitting, including: based on the analysis of rock cores, identifying the target hard rock section, loading explosives into the borehole depth corresponding to the target hard rock section for blasting, so as to form pre-splitting fractures in the hard rock mass between two small-diameter boreholes.

[0050] S4. Trenching and excavation, including: excavating the hard rock mass with pre-split fractures between two small-diameter boreholes to complete the trenching of the ground wall section.

[0051] See Figure 3 It shows the layout of the two small-diameter boreholes 4 and their relative relationship with the ground wall trench section 1. Figure 4 The diagram visually illustrates the spatial morphology of a small-diameter borehole extending along the depth direction, providing a free face and guiding function for the intermediate rock mass, while also showing the final outline of the trench formed by the ground wall. Figure 5 The diagram illustrates the blasting pre-splitting process. After the target hard rock section 5 (i.e., the high-strength intact section) is blasted with explosives, a continuous blasting pre-splitting zone 6 is formed between two small-diameter boreholes. The integrity of the rock mass in this area is significantly reduced due to the development of fissures, which creates favorable conditions for subsequent trenching and excavation.

[0052] Step S1, "conducting a geological survey of the construction area to determine whether it belongs to hard rock strata that are difficult to directly form mechanical trenches," refers to confirming the distribution and strength of hard rock strata in the construction area based on the preliminary geological survey report, drilling data, and on-site reconnaissance. This involves macroscopically determining whether the area contains high-integrity, high-strength hard rock strata (such as granite, quartzite, etc.) and assessing whether they might affect the efficiency and economy of conventional mechanical trenching. This step is a preliminary screening step, designed to determine whether the method of this embodiment should be used for trenching construction of the ground wall.

[0053] In step S2, a conventional simple exploration drilling rig is used to construct small-diameter boreholes, which is much cheaper than large-diameter rotary drilling rigs. Furthermore, rock cores are collected throughout the drilling process to provide physical samples for subsequent detailed identification.

[0054] Based on this, step S3 identifies the target hard rock section that needs to be treated and performs directional blasting pre-splitting. This blasting utilizes small-diameter boreholes as free faces and charge channels. Through the impact and splitting action of the directional blasting, effective pre-splitting fractures are formed in the hard rock between the two small-diameter boreholes, thereby specifically weakening the hard rock between the two boreholes and significantly reducing its integrity and strength. In step S4, a trenching machine is used, guided by the two small-diameter boreholes, to excavate the weakened intermediate rock mass, achieving efficient trenching, significantly reducing the difficulty and cost of trenching construction, and improving construction efficiency and flexibility.

[0055] The method in this embodiment, through a process of first macroscopic identification and then precise pre-splitting, replaces expensive large-diameter rotary drilling with low-cost small-diameter drilling and replaces full-section mechanical crushing with targeted blasting. While ensuring the quality of trenching, it significantly improves the construction efficiency and economy in hard rock formations.

[0056] Specifically, the diameter of the small-diameter boreholes is 76mm to 110mm, and the borehole depth is adapted to the designed trench depth of the ground wall section. The layout position meets the following requirements: the two small-diameter boreholes are located 30 to 50mm inside the design boundary line at both ends of the ground wall section, the center of the boreholes coincides with the horizontal center line of the ground wall section, and the line connecting the two small-diameter boreholes is parallel to the length direction of the ground wall section with a horizontal deviation of ≤0.2%.

[0057] The use of small-diameter boreholes (76mm to 110mm) aims to completely eliminate the reliance on large-diameter (500-1000mm) rotary drilling rigs in traditional processes. This borehole diameter range meets the requirements of efficient and low-cost construction by conventional exploration drilling rigs, while also providing sufficient space for subsequent guide rod placement of explosive charges and accumulation of blasting energy, achieving an optimal balance between feasibility and economy.

[0058] Placing small-diameter boreholes 30-50mm inside the design boundary line, rather than on or outside the boundary line, ensures that the small-diameter boreholes function as free surfaces and charging channels. At the same time, it provides necessary operating space for the grab bucket or milling head of the subsequent trenching machine to perform final contour trimming along the design boundary, effectively avoiding the risk of trenching accuracy exceeding tolerances due to borehole damage to the boundary.

[0059] Strict control (≤0.2%) of the coincidence between the borehole center and the horizontal centerline of small-diameter boreholes and the horizontal deviation of the connecting line is to ensure that the pre-set fracture surface formed by the two small-diameter boreholes can accurately fit the designed trench wall position of the ground wall, and ensure that the blasting energy is concentrated on the core area of ​​the rock mass to be excavated, so that the pre-splitting fracture surface formed subsequently is neat and accurately guided, laying a solid foundation for efficient and high-precision trenching excavation.

[0060] Specifically, during the drilling process, core samples are extracted every 1 to 1.5 meters of drilling, with a core recovery rate of ≥90%. The extracted core samples are numbered, packaged, and preserved in sequence according to the drilling depth, and the integrity, joint development, and fracture closure status of the core samples are recorded simultaneously.

[0061] The core sampling interval of "extracting a core every 1-1.5m of drilling" was determined based on the scale of potential lithological changes in hard rock formations and the positioning accuracy requirements of subsequent blasting and pre-splitting. This interval captures the continuity of the intact rock mass while effectively identifying relatively broken or weak interlayers, providing a sample basis with sufficient spatial resolution for subsequent precise segmentation of the target hard rock section and avoiding the omission of key change points due to excessively large core sampling intervals. Simultaneously, strictly controlling the core recovery rate to ≥90% ensures the maximum in-situ representativeness and information integrity of the obtained cores. A high recovery rate means that the actual length of intact cores extracted from the borehole is close to the drilling depth of that run. This directly ensures the accuracy and reliability of subsequent analyses such as Rock Quality Designation (RQD) calculations and uniaxial compressive strength tests, thus avoiding misjudgments of rock mass integrity due to extensive core breakage or loss. Based on this, the rock cores are numbered at depth, sealed and preserved, and their integrity, joint and fracture characteristics are recorded in detail. This series of operations not only constructs a systematic and traceable rock core geological archive, but more importantly, it provides direct and visualized geological basis for dynamically adjusting blasting parameters (such as charge amount and detonation sequence) in step S3.

[0062] Specifically, based on the analysis of rock cores, the target hard rock segments were identified as follows: the rock cores encapsulated by depth number were tested segment by segment, and the continuous rock core segments that met the requirements of uniaxial compressive strength of rock mass ≥100MPa and rock quality index ≥0.9 were identified as the target hard rock segments.

[0063] The strength criterion in the above-mentioned judgment criteria is mainly used to screen out high-strength rock masses that are extremely inefficient to excavate directly with conventional grab buckets or trenching machines, and where tool wear is severe. For example, the typical strength range of granite is 100~250MPa, and that of quartzite can reach 150~300MPa. The rock quality index ≥0.9 aims to limit the integrity and homogeneity of the rock mass. This value indicates that more than 90% of the core samples are intact segments longer than 10cm, excluding rock masses that, although strong, have well-developed joints and fissures and can be mechanically broken. This dual judgment mechanism ensures that the identified target hard rock segments are truly difficult-to-construct rock formations that combine high strength and high integrity. For example, for granite layers with good integrity, the RQD value obtained from core sampling is often above 0.95, and the uniaxial compressive strength generally exceeds 120MPa, fully meeting the judgment criteria for target hard rock segments. Through the above precise identification, the limited blasting energy and operation time can be concentrated on the sections that truly need pre-splitting, completely avoiding the drawbacks of indiscriminate and costly treatment of the entire hole depth in traditional processes. This fundamentally achieves optimized allocation of construction resources and targeted improvement of operation efficiency.

[0064] Specifically, explosives are loaded into the borehole at the depth corresponding to the target hard rock section for blasting, including:

[0065] For each identified target hard rock section, calculate the required amount of explosives and prepare a concentrated explosive charge;

[0066] Using guide rods, the concentrated explosive charge is precisely lowered to the corresponding borehole depth for each target hard rock section. At least 200mm of buffer material is filled at the top and bottom of the charge. After loading, the borehole opening is sealed and subjected to vibration damping. When multiple discontinuous target hard rock sections exist in a single small-diameter borehole, the explosive charge sections corresponding to each target hard rock section are detonated with a delayed initiation time, with an initiation time interval of 10ms to 25ms between adjacent sections. Target hard rock sections at the same elevation in two small-diameter boreholes are detonated synchronously to ensure that the pre-splitting fractures induced by blasting are effectively connected along the direction of the ground wall trench.

[0067] See Figure 6 This demonstrates the complete construction sequence of the method in this embodiment from preliminary preparation to final trenching. Among them, process (1) corresponds to step S1, that is, to determine the hard rock strata through geological survey and to lay out the boreholes; process (2) is the borehole number, which corresponds to step S2, that is, to drill small-diameter boreholes and take cores at the same time; process (3) and (4) together correspond to step S3, where process (3) is to put in explosives, which shows that the explosive charge 8 is lowered to the design depth of the target hard rock section 5 using the guide rod 7; process (4) is blasting, which represents the formation of a continuous blasting pre-splitting zone 6 between the two holes after detonation; process (5) is excavation, which corresponds to step S4, that is, to mechanically excavate the rock mass (excavation zone 9) that has formed the blasting pre-splitting zone 6, and finally complete the formation of the ground wall trench section.

[0068] The use of guide rods for precise lowering of explosive charges and the filling of buffer material ensures that the explosive energy is concentrated on the target hard rock section. Simultaneously, the buffer material effectively suppresses the damage of the blast shock wave to non-target sections of the borehole and the borehole wall, guaranteeing the integrity of the small-diameter borehole and maintaining its function as a guide marker for subsequent trenching. When multiple target hard rock sections exist in a single small-diameter borehole, millisecond delay detonation (10ms–25ms interval) is employed. The purpose is to utilize the stress wave and fractures generated by the first detonation to create new free surfaces for subsequent detonations, achieving phased and regional energy release. This effectively reduces the amount of explosive charge and vibration intensity per stage while ensuring pre-splitting effects. Most importantly, the target hard rock sections at the same elevation in two small-diameter boreholes are detonated simultaneously. This aims to superimpose and collide the stress waves generated by the blasts on both sides along the line connecting the two boreholes (i.e., the direction of the trench section), thereby preferentially forming and penetrating tensile fractures in the weakest surface (between the two boreholes). This is crucial for forming a regular pre-splitting surface.

[0069] The amount of explosive Q can be calculated based on the following formula: .

[0070] Where Q is the designed charge quantity for a single target hard rock segment, in kg; k is the comprehensive charge quantity coefficient, in kg / (m²). 2 • MPa), its value is calibrated through field tests, comprehensively reflecting factors such as explosive performance, rock breaking energy, free face conditions, and blasting efficiency; 'a' is the distance between two small-diameter boreholes, i.e., the design width of the pre-splitting surface, in meters; 'H' is the length of the target hard rock section, in meters; σ c d represents the uniaxial compressive strength of the target hard rock section, in MPa; d represents the diameter of the small-diameter borehole, in meters; d0 represents the diameter of the reference borehole, in meters.

[0071] When determining the value of the comprehensive coefficient k for the charge quantity, the differences in rock type and its physical and mechanical properties must be fully considered. Specifically, for rock masses with dense mineral composition and high wave impedance, the value of k usually needs to be appropriately increased to provide sufficient energy to overcome their fracture strength; while for rock masses with high strength but relatively well-developed micro-joints, the value of k can be adjusted accordingly. For example, for medium-coarse-grained granite with extremely high integrity, the experimental calibration range of its comprehensive coefficient k is approximately 1.0 × 10⁻⁶. -5 ~1.4×10 -5 kg / (m 2 • MPa); For quartzite with higher strength and denser texture, the range of k-values ​​may shift upwards to 1.3 × 10 MPa. -5 ~1.8×10 -5 kg / (m 2 (MPa)

[0072] Accordingly, the reference borehole diameter d0 in the formula corresponds to the typical borehole diameter of the large-diameter rotary drilling used in the traditional two-drill-one-excavation process, and its value ranges from 0.8m to 1.2m.

[0073] Therefore, in practical applications, the rock type should be identified based on core test results, and a preliminary k value should be selected within the corresponding range. This selection should then be verified and adjusted through small-scale test blasts. Simultaneously, by combining a clearly defined d0 benchmark value, the above formula can be used to achieve refined and predictable calculation of the charge amount Q, thereby ensuring the technical reliability and economic rationality of the pre-splitting effect.

[0074] Furthermore, the borehole opening is sealed and subjected to vibration reduction treatment, including: sealing the borehole opening with a composite structure combining a flexible vibration-damping pad and a rigid cap; wherein, the flexible vibration-damping pad is laid close to the rock surface of the borehole opening, and the rigid cap is covered on the pad and fixed to the surrounding rock mass by expansion bolts; and the total thickness of the sealing body is not less than 300mm.

[0075] The use of a composite structure of flexible damping pad and rigid cap, instead of a single concrete or sandbag seal, is to address the two main negative effects of blasting. The flexible pad (usually made of rubber or foam) effectively absorbs and attenuates the high-frequency vibration energy transmitted from the blast shock wave to the ground surface and surrounding rock mass, reducing vibration hazards. The rigid cap (usually made of steel plate) mainly restricts the reverse ejection of explosive gases and debris, prevents the borehole from being destroyed, and maintains the airtightness of the charge chamber, ensuring that the blast energy is transmitted as much as possible to the bottom and sides of the borehole.

[0076] The total thickness of the sealing material is set at no less than 300mm, based on a comprehensive consideration of blasting mechanics and engineering practice. Insufficient thickness will result in inadequate shock absorption and sealing performance, while excessive thickness will cause construction difficulties and may excessively inhibit the effective transmission of blasting energy to the lower rock mass. This specific thickness ensures that the blast shock wave is sufficiently attenuated within the sealing material, while providing sufficient quality assurance to resist the upward force of the blasting gas.

[0077] The rigid cap is directly anchored to the surrounding original rock mass using expansion bolts. The purpose is to form a stable overall force-bearing system, prevent the cap from shifting or bouncing under explosive loads, and thus ensure the long-term effectiveness of the composite sealing structure.

[0078] Specifically, when calculating the required explosive charge and setting the delayed detonation parameters, dynamic corrections are made based on the recorded core integrity, joint development, and fracture closure status. These corrections include: if the core corresponding to the target hard rock section shows open fractures or weak interlayer filling, the calculated explosive charge for that section is reduced by 10%–20%; if the angle between the joint surface of the core corresponding to the target hard rock section and the direction of the line connecting the two small-diameter boreholes is less than 30°, the detonation time interval between adjacent target hard rock sections is shortened to 8ms–15ms; if the joint spacing of the core corresponding to the target hard rock section is less than 0.3m, and densely closed joints in the core result in a potentially fragmented rock structure, segmented low-energy explosive charges are used instead of concentrated explosive charges. The explosive charge in a single segment of the segmented low-energy explosive charge does not exceed 1 / 3 of the original concentrated explosive charge, the spacing between adjacent segmented low-energy explosive charges is 0.8m–1.2m, and the thickness of the buffer material at the top and bottom of the segmented low-energy explosive charges is increased to 300mm–400mm.

[0079] The aforementioned dynamic correction rules based on core geological characteristics embody the fundamental shift in the method of this embodiment from homogenized experience-based construction to geologically adapted precision control.

[0080] For rock sections with open fractures or weak interlayers, these geological defects themselves act as natural stress concentration zones and energy dissipation channels, with blasting energy preferentially released along these paths. Using the calculated charge amount for intact rock masses would not only waste explosives but could also lead to excessive fragmentation or localized over-excavation. Therefore, reducing the charge amount for this section by 10%–20% can ensure effective fracture propagation and penetration while avoiding ineffective energy release.

[0081] When the angle between the dip of the core joint surface and the borehole line is less than 30°, it means that the main joint surface is nearly parallel to the pre-splitting direction. In this case, shortening the detonation interval between adjacent sections (to 8ms–15ms) aims to utilize the stress wave generated by the first detonation section to propagate rapidly along this dominant structural surface, thereby more effectively guiding the fractures from subsequent blasts to extend along the predetermined direction and enhancing the planarity and continuity of the fractures. Using a conventional interval might lead to misalignment of the fractures generated by different blasting sections, affecting the overall integrity of the pre-splitting surface.

[0082] For rock sections with dense joints (spacing < 0.3m) and potentially fragmented rock blocks, their mechanical properties are close to those of a fracture medium. Using concentrated explosive charges can easily lead to excessive energy concentration, excessive rock fragmentation, or even localized scattering. Instead, segmented low-energy explosive charges (each segment containing ≤ 1 / 3 of the original charge, spaced 0.8m–1.2m apart) with thickened buffer material to 300mm–400mm effectively redistribute and soften the blasting energy in space. This ensures sufficient energy to fracture weak surfaces between rock blocks while effectively controlling vibration and fragmentation, protecting the borehole morphology, and providing a relatively regular fracture surface for subsequent excavation.

[0083] The above-mentioned modified rules enable the blasting design in this embodiment to intelligently respond to subtle geological changes. While improving the reliability of the pre-splitting effect, it maximizes the precision of the charge and the controllability of blasting hazards, which is something that traditional uniform blasting design cannot achieve.

[0084] Specifically, after the blasting in S3 and before the start of S4, there is a step to check the pre-splitting effect: by cross-hole sonic CT scanning or borehole panoramic imaging, it is determined whether a continuous pre-splitting fracture has been formed between the two small-diameter boreholes; if the fracture penetration rate is less than 85%, a low-energy micro-delay blast is added to the original target hard rock section, and the amount of explosives used in the supplementary blast does not exceed 30% of the original design amount of explosives for that section.

[0085] Cross-hole acoustic CT scanning and borehole panoramic imaging were selected as complementary inspection methods. Cross-hole acoustic CT can intuitively reflect the degree of weakening and continuity of the rock mass between two boreholes through the wave velocity change profile, while borehole panoramic imaging can directly observe the opening, extension direction and density of fractures on the borehole wall. The combination of the two realizes a comprehensive quantitative evaluation of the pre-fracture effect from macro to micro.

[0086] Setting the acceptable threshold for fracture penetration rate at 85% is a key value that balances technical feasibility and economic rationality. This threshold allows for a small number of incompletely penetrated rock bridges in certain areas, which aligns with the uneven nature of fracture propagation under blasting. Simultaneously, it ensures that the vast majority of pre-splitting surfaces have been effectively weakened, meeting the requirements for efficient mechanical excavation in subsequent trenching. If the threshold is too low, the pre-splitting quality will be insufficient; pursuing 100% complete penetration often requires extremely high blasting costs.

[0087] The regulation stipulates that the amount of explosives used in supplementary blasting should not exceed 30% of the original design amount, with the aim of achieving precise reinforcement. Since the target rock section has already undergone the initial blasting, it already contains significant internal damage and potential fissures. At this point, only a small amount of energy is needed to further expand and connect the existing fissures. Limiting the amount of supplementary explosives effectively avoids excessive fragmentation of the already loosened rock mass and prevents the cumulative blasting vibrations from adversely affecting borehole maintenance and the stability of the surrounding rock mass.

[0088] By introducing the aforementioned quantitative testing and targeted reinforcement mechanisms, the method in this embodiment overcomes the technical bottlenecks of difficulty in verifying pre-splitting effects, reliance on experience for quality, and large fluctuations in construction results, significantly improving the overall controllability and final trenching quality of trenching construction in hard rock strata.

[0089] Specifically, when the trench section of the ground wall to be formed is connected to the adjacent trench section that has been completed, at the end closer to the adjacent trench section, the layout position of the small-diameter borehole is adjusted 100mm to 200mm from the conventional design position at that end towards the inside of the trench section. During blasting, the detonation time of the target hard rock section in the small-diameter borehole is advanced by 5ms to 10ms compared to the detonation time in the other small-diameter borehole.

[0090] Adjusting the small-diameter boreholes on the side closest to the adjacent trench section by 100mm to 200mm inwards is primarily aimed at creating a buffer zone between the rock mass to be excavated and the concrete or joint surface of the existing trench section. An offset less than 100mm results in insufficient isolation, allowing blasting stress waves and fractures to easily extend to the existing structure; an offset greater than 200mm unnecessarily increases the workload of subsequent trenching machine trimming and may affect the verticality of the pre-splitting surface. This offset ensures that the main blasting energy acts on the rock mass to be excavated, while effectively preventing direct damage to the joint surface of adjacent trench sections by utilizing the pre-reserved intact rock mass barrier.

[0091] The detonation time of the target hard rock section in the borehole on that side is specified to be 5ms to 10ms in advance. The core principle is to achieve energy guidance by utilizing the spatiotemporal superposition effect of stress waves. When this side is detonated first, the generated stress wave will preferentially propagate towards the free surface (i.e., the borehole on the other side that was detonated later and the area to be excavated) and form pre-damage, thereby guiding the stress wave generated on the later side to superimpose with it, jointly enhancing the fracture propagation on the predetermined path between the two boreholes. This small but crucial time difference ensures that the energy released by the blast is mainly guided and consumed in the area to be excavated, further weakening the explosive energy propagating towards the excavated section, and achieving dynamic energy deflection protection.

[0092] The combined design of spatial inward offset and slight time advance constitutes a dual (static and dynamic) protection mechanism for blasting operations in adjacent existing structural areas. This mechanism ensures the pre-splitting effect of the new trench segment while minimizing potential disturbances to the integrity of the joint surfaces and water-stopping performance of the existing trench segment. It solves the joint protection problem faced by traditional methods in continuous construction and demonstrates the refined control capability of the method in this embodiment under complex boundary conditions.

[0093] Furthermore, before blasting the trench section to be constructed, the joint surfaces of adjacent completed trench sections are protected, including:

[0094] A flexible isolation layer is laid on the outside of the joint surface of adjacent trench sections. The isolation layer is made of rubber sheet with a thickness of 10~15mm. The rubber sheet is fixed to the joint surface by anchors with a fixing spacing of 300~400mm. The edge of the rubber sheet extends 50~80mm beyond the boundary of the adjacent trench section.

[0095] A rigid protective frame is installed on the outside of the rubber sheet. The rigid protective frame is formed by welding angle steel. The distance between the rigid protective frame and the joint surface is 150~200mm. The outside of the rigid protective frame is covered with wire mesh.

[0096] Vibration damping holes with a depth of ≥1.5m and a spacing of 500mm are arranged around the small-diameter boreholes near the adjacent trench section. The diameter of the vibration damping holes is 40~60mm, and the holes are filled with crushed stone with a particle size of 5~10mm.

[0097] The flexible isolation layer and the rigid protective structure form a dual barrier combining static and dynamic elements. Specifically, the 10-15mm thick rubber sheet laid close to the joint surface effectively absorbs and attenuates the mid-to-high frequency vibration energy transmitted from the blast stress wave to the concrete joint surface, reducing the risk of microcracks. Anchors, fixed at 300-400mm intervals, ensure that the sheet does not peel or resonate under impact. On top of this, a rigid angle steel frame positioned 150-200mm away does not directly resist the stress wave, but rather, together with the outer wire mesh, forms a fragment trapping and gas buffer chamber. This spacing allows the blasted rock debris and expanding gases to be sufficiently slowed and diffused before contacting the rigid frame, thus eliminating direct impact and erosion damage to the rubber sheet and joint surface.

[0098] The shock-absorbing holes (depth ≥ 1.5m, spacing 500mm, diameter 40~60mm, filled with gravel) arranged around the borehole constitute the third and most critical near-source energy dissipation system. Its mechanism is as follows: when the explosive stress wave arrives, the dense array of gravel-filled holes causes the gravel particles within the holes to rub against each other, roll, and rearrange, thereby breaking up the originally continuously propagating wavefront and converting the wave energy into heat and frictional energy, achieving efficient in-situ dissipation. A depth ≥ 1.5m ensures its ability to influence the stress field of the main charge section; the 500mm spacing creates effective area coverage; and the 5~10mm gravel particle size optimizes frictional energy dissipation efficiency and filling density.

[0099] The above protective measures are based on the progressive logic of surface damping, aerial interception, and near-source dissipation. They are precisely designed to address the characteristics of small-diameter borehole blasting and the special requirements for joint surface protection. Through multi-parameter coordination, multiple energy attenuation paths are constructed in space from the blast source to the protected body. This achieves comprehensive control over the source, path, and endpoint of blast energy release, significantly surpassing the effect of a single protective measure. It provides a reliable technical guarantee for ensuring the safety and integrity of existing structures in complex construction environments.

[0100] The embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting, characterized in that, include: S1. Conduct a geological survey of the construction area to determine whether it belongs to hard rock strata that are difficult to directly form trenches mechanically. If so, determine the location of two small-diameter boreholes for a section of the ground wall to be trenched. S2. Drill small-diameter holes at the designated locations and extract rock cores during the drilling process; S3. Based on the analysis of the rock core, the target hard rock section is identified, and explosives are loaded into the borehole depth corresponding to the target hard rock section for blasting, so as to form pre-fractured fissures in the hard rock mass between the two small-diameter boreholes. S4. Excavate the hard rock mass with pre-split fractures between the two small-diameter boreholes to complete the trenching of the ground wall section.

2. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 1, characterized in that, The diameter of the small-diameter borehole is 76mm to 110mm, and the drilling depth is adapted to the design depth of the trench section of the ground wall. The layout location meets the following requirements: the two small-diameter boreholes are located 30-50mm inside the design boundary line at both ends of the ground wall trench section, the center of the boreholes coincides with the horizontal center line of the ground wall trench section, and the line connecting the two small-diameter boreholes is parallel to the length direction of the ground wall trench section with a horizontal deviation of ≤0.2%.

3. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 1, characterized in that, When taking core samples during drilling, core samples are extracted every 1 to 1.5 meters of drilling, with a core recovery rate of ≥90%. The extracted core samples are numbered, packaged, and preserved in sequence according to the drilling depth, and the integrity, joint development, and fracture closure status of the core samples are recorded simultaneously.

4. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 3, characterized in that, The process of identifying the target hard rock segment based on the analysis of the rock core includes: conducting segment-by-segment testing on the rock cores packaged according to depth number, and determining the continuous rock core segment that meets the requirements of uniaxial compressive strength of rock mass ≥100MPa and rock quality index ≥0.9 as the target hard rock segment.

5. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting according to claim 3, characterized in that: The step of loading explosives into the borehole at the corresponding depth of the target hard rock section for blasting includes: For each identified target hard rock section, calculate the required amount of explosives and prepare a concentrated explosive charge; Using a guide rod, the concentrated explosive charge is precisely lowered to the corresponding borehole depth for each target hard rock section. At least 200mm of buffer material is filled at the top and bottom of the charge. After loading, the borehole opening is sealed and subjected to vibration damping. When multiple discontinuous target hard rock sections exist in a single small-diameter borehole, the explosive charge sections corresponding to each target hard rock section are detonated with a delayed initiation time, with an initiation time interval of 10ms to 25ms between adjacent sections. Target hard rock sections at the same elevation in two small-diameter boreholes are detonated synchronously to ensure that the pre-splitting fractures induced by blasting are effectively connected along the direction of the ground wall trench.

6. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting according to claim 5, characterized in that, The sealing and vibration damping treatment of the borehole opening includes: A composite structure combining a flexible damping pad and a rigid cap is used to seal the borehole opening; the flexible damping pad is laid close to the rock surface at the borehole opening, and the rigid cap is covered on the pad and fixed to the surrounding rock mass by expansion bolts; and the total thickness of the sealing body is not less than 300mm.

7. A method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 5, characterized in that, When calculating the required amount of explosives and setting the time-delay detonation parameters, dynamic corrections are made based on the recorded core integrity, joint development, and fracture closure status, including: If the core sample corresponding to the target hard rock section shows open fractures or weak interlayer filling, the calculated charge for that section should be reduced by 10% to 20%. If the angle between the dip of the core joint surface corresponding to the target hard rock section and the direction of the line connecting the two small-diameter boreholes is less than 30°, then the detonation time interval between adjacent target hard rock sections will be shortened to 8ms to 15ms. If the joint spacing of the rock core corresponding to the target hard rock section is less than 0.3m, and the rock core shows dense closed joints that cause the rock block to have a potential fragmented structure, then segmented low-energy explosive charges are used instead of concentrated explosive charges. The explosive amount of a single segment of the segmented low-energy explosive charge does not exceed 1 / 3 of the explosive amount of the original concentrated explosive charge. The spacing between adjacent segmented low-energy explosive charges is 0.8m to 1.2m. At the same time, the thickness of the buffer material at the top and bottom of the segmented low-energy explosive charge is increased to 300mm to 400mm.

8. The method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 1, characterized in that, After the blasting in S3 and before the start of S4, there is a step to check the pre-splitting effect: by cross-hole sonic CT scanning or borehole panoramic imaging, it is determined whether a continuous pre-splitting fracture has been formed between the two small-diameter boreholes; if the fracture penetration rate is less than 85%, a low-energy micro-delay blast is added to the original target hard rock section, and the amount of explosives added to the blast does not exceed 30% of the original design amount of explosives for that section.

9. A method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 1, characterized in that, When the trench section of the ground wall to be formed is connected to the adjacent trench section that has been completed, at the end closer to the adjacent trench section, the layout position of the small-diameter borehole is adjusted 100mm to 200mm from the conventional design position at that end towards the inside of the trench section. During blasting, the detonation time of the target hard rock section in the small-diameter borehole is advanced by 5ms to 10ms compared with the detonation time in the other small-diameter borehole.

10. A method for trenching hard rock walls based on small-diameter borehole blasting pre-splitting as described in claim 9, characterized in that, Before blasting the trench section of the ground wall to be constructed, the joint surfaces of adjacent completed trench sections shall be protected, including: A flexible isolation layer is laid on the outside of the joint surface of adjacent trench sections. The isolation layer is made of rubber sheet with a thickness of 10~15mm. The rubber sheet is fixed to the joint surface by anchors with a fixing spacing of 300~400mm. The edge of the rubber sheet extends 50~80mm beyond the boundary of the adjacent trench section. A rigid protective frame is installed on the outside of the rubber sheet. The rigid protective frame is formed by welding angle steel. The distance between the rigid protective frame and the joint surface is 150~200mm. The outside of the rigid protective frame is covered with wire mesh. Vibration damping holes with a depth of ≥1.5m and a spacing of 500mm are arranged around the small-diameter boreholes near the adjacent trench section. The diameter of the vibration damping holes is 40~60mm, and the holes are filled with crushed stone with a particle size of 5~10mm.

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