A composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers

Abstract

This invention discloses a composite drilling construction method for traversing water-rich hard rock and its mud-sand fractured interlayers. The method involves pre-drilling with pipe roof grouting along the outer edge of the tunnel excavation face at the tunnel arch to form a long-distance water-stopping curtain; using the tunnel's vertical centerline as a reference, drainage holes are alternately drilled on both sides of the tunnel; and geological exploration boreholes are drilled to the designed depth to form an exploration channel for geological exploration operations. This invention, by constructing a long-distance water-stopping curtain through pre-drilling with pipe roof grouting along the outer edge of the excavation face at the tunnel arch, effectively blocks the inflow of fractured water from the outside. Furthermore, the alternating drilling of drainage holes on both sides of the tunnel, stopping only after traversing the corresponding mud-sand fractured interlayers, and the subsequent drilling of geological exploration boreholes to the designed depth after the geological boundary conditions have improved, can reduce the impact of mud-sand fractured strata on construction progress and improve construction efficiency.

Description

Technical Field

[0001] This invention belongs to the field of advanced drilling technology for tunnels with complex and adverse geological conditions, and particularly relates to a composite drilling construction method for traversing water-rich hard rock and its mud-sand fracture interlayers. Background Technology

[0002] Advanced geological drilling is an important method of geological forecasting in tunnel construction. Using drilling equipment, boreholes are drilled forward from the tunnel face to obtain rock cores and monitor the drilling progress, thereby identifying the lithology, geological structure, groundwater distribution, and any adverse geological features of the surrounding rock. This method provides direct and accurate results, allowing for the early prediction of safety hazards such as water inrush, mudslides, and collapses. It also verifies previous geological survey data, providing on-site geological data for adjusting construction plans, determining support parameters, and developing disaster prevention and water control measures. This effectively mitigates geological risks and ensures the smooth and safe progress of tunnel excavation.

[0003] For tunnels traversing water-rich hard rock and mud-sand fractured layers, a combination of horizontal long-core drilling and shallow water exploration drilling is primarily employed. Drill holes are arranged in a fan shape at the tunnel face, and specialized drill bits are used to core sample the hard rock, determining the boundaries of rock strata and the location of fracture zones. Additional water exploration boreholes are then installed to investigate groundwater pressure and flow channels within the interlayers. During construction, data such as drilling rate and sand content in the extracted water are recorded in real time. Combined with in-hole exploration methods, the extent, thickness, and water content of the mud-sand interlayers are clarified, accurately delineating the boundaries of different strata and comprehensively understanding the complex geological conditions ahead, meeting the practical needs of advanced exploration in complex strata.

[0004] This combined drilling method is relatively slow, and drilling in hard rock is time-consuming, which will slow down the construction progress to some extent. The borehole layout has blind spots, failing to fully cover the strata and easily overlooking small mud-sand accumulation areas and hidden water-bearing fractures. Drilling operations easily disturb the fractured interlayer structure, greatly increasing the risk of water and sand inrush into the borehole and increasing the risk of surrounding rock instability. At the same time, the investment in drilling consumables and equipment is relatively high, and the on-site operation is difficult, requiring a high level of practical skills from the construction personnel. Summary of the Invention

[0005] The purpose of this invention is to provide a composite drilling method for traversing water-rich hard rock and its mud-sand fractured interlayers, so as to reduce the impact of mud-sand fractured strata on the construction progress and improve construction efficiency.

[0006] This invention adopts the following technical solution: a composite drilling construction method for traversing water-rich hard rock and its mud-sand fracture interlayers, comprising the following steps: Advanced pipe roof grouting is performed along the outer edge of the tunnel excavation face in the tunnel arch to form a long-distance water-stop curtain; Using the vertical centerline of the tunnel as a reference, drainage holes are alternately constructed on the left and right sides of the tunnel; each drainage hole corresponds to a mud-sand fractured interlayer. When the drainage hole passes through the mud-sand fractured interlayer hidden in the stratum, the advance is stopped, and the construction of the drainage hole is completed. Drill geological exploration boreholes to the designed depth to form an exploration channel to facilitate geological exploration operations.

[0007] The beneficial effects of this invention are as follows: By constructing a long-distance water-stop curtain through advanced pipe roof grouting along the outer edge of the excavation face in the tunnel arch, the invention blocks the inflow of fractured water from the outside. Additionally, drainage holes are alternately constructed on the left and right sides of the tunnel, stopping only after passing through the corresponding mud and sand fractured interlayers. Furthermore, geological exploration boreholes are constructed to the designed depth after the geological boundary conditions are improved. This can reduce the impact of mud and sand fractured strata on the construction progress and improve construction efficiency. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of advanced pipe roof grouting in an embodiment of the present invention; Figure 2 This is a schematic diagram showing the distribution of drainage holes and geological exploration holes in an embodiment of the present invention; Figure 3 These are schematic diagrams illustrating two drilling methods in embodiments of the present invention; Figure 4 This is a schematic diagram of the drilling progress of the drainage hole in an embodiment of the present invention; Figure 5 This is a schematic diagram of geological exploration operations in an embodiment of the present invention. Detailed Implementation

[0009] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0010] This invention effectively reduces the risk of sudden inrush during subsequent drilling by using a pre-grouted pipe roof to form a water-stop curtain. Based on the design of drainage holes that are alternately arranged on the left and right and targeted to penetrate the interlayer, high-pressure fracture water and mud and sand can be released in stages and in a controlled manner, avoiding stress imbalance caused by concentrated drainage on one side. With the help of alternating operation of pneumatic drills and water drills and the flushing and washing process after the pre-set drilling depth, efficient crushing of hard rock sections and flexible slag removal of mud and sand sections are achieved, preventing hole blockage and drill bit entanglement.

[0011] Based on this, after drilling through the interlayer, a casing with drainage perforations was installed to stabilize the borehole wall, transforming the temporary borehole into a stable channel with continuous pressure relief. Subsequently, geological exploration boreholes were constructed and the entire borehole section was flushed in a stable environment, ensuring the integrity and cleanliness of the exploration channel. This effectively solved the problems of borehole collapse, stuck drill, and detection failure caused by sudden changes in water pressure and sediment accumulation in complex alternating strata, thus avoiding construction interruptions and equipment damage, and significantly improving the accuracy of advanced geological prediction, the stability of the construction process, and the overall safety of the operation.

[0012] By employing a spatial zoning and hierarchical control logic of outer ring water sealing, middle ring targeted pressure relief, and inner ring fine exploration, a complete technical closed loop from risk pre-control to precise detection is constructed. This enables the transformation of high-risk and adverse strata from a dynamically unstable state to a statically controllable state, providing a systematic solution for safe tunnel construction under complex geological conditions.

[0013] This invention discloses a composite drilling construction method for traversing water-rich hard rock and its mud-sand fracture interlayers, comprising the following steps: pre-grouting of pipe roofs along the outer edge contour of the tunnel excavation face at the tunnel arch to form a long-distance water-stopping curtain; using the tunnel's vertical centerline as a reference, alternately constructing drainage holes on the left and right sides of the tunnel; wherein, as... Figure 4 As shown, each drainage hole corresponds to a mud-sand fractured interlayer. When the drainage hole passes through the mud-sand fractured interlayer hidden in the stratum, the advance stops, and the construction of the drainage hole is completed. Geological exploration boreholes are constructed to the designed depth to form an exploration channel to facilitate geological exploration operations.

[0014] This invention constructs a long-distance water-stop curtain by performing advanced pipe roof grouting along the outer edge of the excavation face in the tunnel arch, which blocks the inflow of fractured water from the outside. It also alternately performs drainage holes on the left and right sides of the tunnel, which stop only after passing through the corresponding mud and sand fractured interlayers. After the geological boundary conditions are improved, geological exploration boreholes are then performed to the designed depth. This can reduce the impact of mud and sand fractured strata on the construction progress and improve construction efficiency.

[0015] Advanced pipe roof grouting refers to a series of pipe roof drilling and grouting operations carried out in a ring along the outer edge of the tunnel arch in front of the tunnel excavation face. This step aims to create a relatively dry and stable working environment for subsequent drilling by constructing an outer barrier to prevent external fissure water from flowing into the tunnel face at the source.

[0016] In specific implementation, such as Figure 1As shown, drilling is first performed along the outer edge of the excavation face in the tunnel arch. The drilling depth is determined according to design requirements, typically forming a ring-shaped reinforcement zone covering a certain area around the tunnel. Subsequently, steel pipe roofs are lowered into the drilled holes, and grouting is carried out. Under pressure, the grout diffuses into the surrounding geological fissures, solidifying to form a long-distance water-stopping curtain with certain strength and water-stopping properties. This water-stopping curtain not only reinforces the surrounding rock but, more importantly, effectively blocks water-rich areas outside the excavation face, significantly reducing the probability of sudden water inrush during subsequent internal drilling. For example, in actual construction, the total length of the pipe roof can be designed to be 30m, with each pipe roof being 6m long. Threaded overlaps are used between the pipe roofs to ensure continuity, and several drainage holes are pre-drilled on the surface of the pipe roofs to facilitate grout entry and exit and diffusion.

[0017] When encountering muddy and sandy interlayers during drilling, drilling should be stopped immediately. A small-scale grouting should be performed to reinforce the affected geological area. After reinforcement, drilling should resume, repeating this process until the designed depth is reached. Finally, overall grouting should be performed to form a complete water-stop curtain. This outer-ring water-stopping measure significantly improves the hydrogeological conditions of the working face, providing a safe foundation for the subsequent construction of drainage holes and exploration holes.

[0018] Drainage holes are drilled specifically to release pressurized water and discharge sediment within the mud-sand fractured interlayer. The core of this step lies in targeted depressurization and alternating placement, meaning that the goal is not to achieve the depth of a single drill, but rather to precisely target the specific risk source—the mud-sand fractured interlayer.

[0019] Specifically, the construction process uses the tunnel's vertical centerline as a symmetrical reference, with drilling operations alternating between the left and right sides of the tunnel. This left-right mirror distribution and intermittent skip-hole method effectively avoids stress imbalance in the surrounding rock caused by concentrated water discharge on one side, ensuring the stability of the tunnel structure. The final position of each drainage hole is strictly controlled after penetrating a corresponding mud-sand fractured interlayer. Once it is confirmed that the interlayer has been penetrated, the drilling is immediately stopped, and no further drilling is allowed. The determination of whether an interlayer has been penetrated can be based on a comprehensive assessment of characteristics such as changes in the color of the discharged water (from clear to turbid), the presence of backfill (fragmented granular hard rock particles), increased drilling resistance, and abnormal sounds.

[0020] For example, if geological forecasts indicate the presence of three layers of mud and sand interlayers ahead, the first drainage hole is constructed at the location of the first interlayer. Drilling is stopped once the first interlayer is penetrated, making it a dedicated drainage and mud removal channel for the first layer. After the water flow from the hole stabilizes and the mud content decreases, a second drainage hole is constructed on the other side to deal with the second interlayer, and so on.

[0021] This design, which stops drilling at the interlayer, releases the pressurized water and sediment within the interlayer, reducing formation pore water pressure, while avoiding excessive damage to deep, undisturbed formations. Lowering a casing with drainage perforations further stabilizes the borehole wall, ensuring continuous and effective drainage and pressure reduction. This step, combined with the aforementioned water-stop curtain, forms a synergistic mechanism of external blocking and internal drainage, gradually transforming the high-risk, dynamically unstable formation into a static and controllable state.

[0022] Geological exploration boreholes refer to full-depth boreholes drilled after the aforementioned water-stop curtain has been formed and the initial pressure relief and mud removal operations of the drainage holes have been completed, for the purpose of conducting detailed geological exploration. The prerequisite for this step is that the geological boundary conditions have been significantly improved, specifically manifested in stable water output from each drainage hole, a reduction in the mud content of the drainage to a preset threshold (such as below 5%), and no significant seepage pressure at the working face.

[0023] Based on this, using primarily pneumatic drilling or a combination of water drilling, drilling continues to the designed depth (e.g., 50–60 m), thus forming a stable and unobstructed exploration channel. The main function of this channel is to serve as a carrier, facilitating the subsequent insertion of imaging equipment, acoustic testing instruments, and other detection instruments to conduct full-section long-hole wall scanning imaging and geophysical exploration operations, in order to accurately grasp the structural features of the surrounding rock, the location of lithological boundaries, and the spatial distribution of mud-sand fractured interlayers.

[0024] For example, geological exploration boreholes can only be started after confirming that multiple drainage holes on the left and right sides have completed their drainage and mud removal tasks, and that the water flow from the boreholes is stable with no significant pressure fluctuations. If slight signs of borehole collapse occur during drilling due to residual mud and sand, the drill rod can be withdrawn immediately for high-pressure water flushing to restore flow and continue drilling until the borehole is completed. After completion, a full-section borehole flushing operation is required to ensure that the water returning to the borehole remains clear, ultimately forming a stable channel suitable for high-precision geological exploration. This sequence achieves a smooth transition from extensive drainage and mud removal to refined geological exploration, effectively ensuring the accuracy of the detection data and the safety of the operation process.

[0025] This invention constructs a complete spatial zoning and hierarchical control logic encompassing outer ring water sealing, middle ring pressure relief, and inner ring fine exploration. Firstly, the aforementioned advanced pipe roof grouting forms a long-distance water-stopping curtain, cutting off the direct supply of water from the working face to the large-area water-rich area, significantly reducing the risk of sudden inrush during subsequent drilling and providing the first safety barrier for the entire construction process. Based on this, as... Figure 2As shown, the drainage holes, constructed alternately with the tunnel's vertical centerline as a reference, precisely release pressurized water and sediment within each fractured interlayer using a targeted strategy. The alternating left-right arrangement avoids eccentric pressure damage caused by unilateral pressure relief, while the drilling-to-stop mechanism effectively reduces pressure while minimizing disturbance to deep strata. The combination of these two steps gradually transforms the originally high-risk, dynamically unstable, water-rich fractured strata into a statically stable state with low water pressure and low sediment content.

[0026] Ultimately, the geological exploration boreholes constructed under the favorable geological conditions created in the first two steps ensured the quality and stability of the boreholes, enabling the smooth implementation of subsequent high-precision imaging and geophysical exploration operations. This allowed for accurate acquisition of geological information ahead, providing a reliable basis for adjusting support parameters and controlling construction risks. This progressive construction method not only solved the problems of borehole collapse, stuck drill, and sudden water inrush that are prone to occur in traditional processes, but also achieved an organic combination of water drainage and pressure reduction with geological exploration, significantly improving the efficiency and safety of tunnel construction under complex and adverse geological conditions.

[0027] In one embodiment, during the construction of the drainage holes, pneumatic drilling and water drilling are used alternately; pneumatic drilling is used for hard rock sections, while water drilling is used for sections with fractured mud and sand interlayers. It should also be noted that the drainage holes are distributed in a mirror image relative to the tunnel centerline, with a center-to-center spacing of 2–3 m and a borehole diameter >100 mm, forming a ring-like arrangement near the excavation outline at the tunnel face. Geological exploration boreholes on both sides are located below the drainage holes at their centers, with a designed borehole diameter of 100–120 mm and a designed drilling depth of 50–60 m.

[0028] Alternating operation of pneumatic and water drilling refers to a dual-mode drilling process that dynamically switches between the drilling power source and the cuttings removal medium based on changes in the lithology of the formation revealed during drilling. This alternating operation mechanism is designed based on the differences in the physical and mechanical properties of the formation: when the drill bit is in a hard rock section with good integrity and high strength, it switches to pneumatic drilling mode; when the drill bit enters a loosely structured, high-water-content muddy-sand fractured interlayer section, it immediately switches to water drilling mode. This alternation is not based on a fixed time or length, but rather on real-time monitored formation response characteristics (such as the color of the discharged water changing from clear to turbid, an increase in granular rock cuttings in the returned cuttings, a sudden drop in drilling rate, and accompanying abnormal friction sounds) as triggering conditions. Through the coordinated operation of pneumatic and water drilling, the technical challenge of simultaneously achieving both hard rock breaking efficiency and soft rock wall stability using a single drilling process is solved, ensuring efficient and safe drilling progress at different geological interfaces.

[0029] Pneumatic drilling refers to a drilling method that uses compressed air as a power source to drive a down-the-hole impactor to generate high-frequency impact force to break rocks, and then uses high-pressure airflow to expel rock cuttings from the borehole. Specifically, pneumatic drilling involves starting an air compressor to provide high-pressure air, typically controlled at 0.8–1 MPa, with an impact frequency set at 800–1200 times / minute and a rotation speed controlled at 20–40 rpm. In hard rock sections, the drill bit, under high-frequency impact, causes brittle fracture of the intact rock mass, forming fragments. Subsequently, high-pressure air flows at high speed along the gap between the drill rod and the borehole wall or through the central channel of the drill rod, carrying the broken hard rock cuttings out of the borehole. Figure 3 As shown in (a), the front end of the pneumatic drill bit is subjected to high-frequency impact, the surrounding rock mass is fractured, and there is no large amount of water accumulation in the hole. The high-pressure airflow smoothly carries away the rock cuttings, ensuring the dryness and unobstructed flow of the borehole. This step aims to utilize the high energy density of the pneumatic drill to quickly penetrate hard rock layers, avoiding the problem of slow progress caused by the high resistance of the flushing fluid in hard rock when using water drilling, and at the same time preventing the mud slurry phenomenon that occurs when using water drilling into hard rock.

[0030] Water-cooled drilling refers to a drilling method that uses high-pressure water as a medium to soften mud and sand through hydraulic flushing and remove loose debris from the borehole, while simultaneously using water pressure to balance formation pressure and maintain borehole stability. Specifically, upon identifying the entry into a muddy, fractured interlayer, pneumatic drilling is stopped, the water supply line is switched, and high-pressure water at 2–4 MPa is injected into the drill pipe. The water flow forms a high-speed jet at the drill bit, flushing and stripping away loose mud, sand, and fractured rock fragments. The resulting mud mixture flows out of the borehole with the return water. During this process, a slow-advance strategy of flushing while drilling is employed, controlling the drilling rate at 0.5–0.8 m / h. Every 50 cm of drilling constitutes a cycle, during which drilling is paused and a dedicated borehole flushing operation is performed until the mud content in the drainage water decreases to a preset threshold (e.g., below 5%) before continuing to the next cycle. Figure 3 As shown in (b), this figure illustrates the structural diagram of drilling through a fractured mud-sand interlayer. The drill bit is surrounded by a high-pressure water flow, and the loose mud-sand is carried out in a suspended state under the action of the water flow. The borehole wall does not collapse under the support of water pressure. This step replaces mechanical impact with fluid dynamics, effectively avoiding borehole collapse, stuck drill, and drill bit entanglement accidents caused by mechanical disturbance, thus ensuring a high success rate of borehole formation in extremely unstable strata.

[0031] This invention achieves precise matching between drilling technology and formation characteristics through the alternating operation of pneumatic and water drilling. In hard rock sections, the high-frequency impact and pneumatic slag removal characteristics of pneumatic drills significantly improve rock breaking efficiency and avoid energy loss caused by water media; while in muddy and sandy interlayer sections, the scouring and slag-carrying characteristics of water drills and hydraulic wall protection effectively suppress the risk of borehole instability. The combination of the two not only overcomes the adaptability defects of a single technology in complex formations, but also ensures that the drainage hole can smoothly penetrate the hidden muddy and sandy interlayers through a real-time process switching mechanism, laying a solid foundation for the subsequent formation of a stable drainage-sludge removal channel.

[0032] In another embodiment, constructing the drainage hole includes: after the advance length reaches the preset advance length, flushing and washing the hole is carried out until the mud content of the drainage is reduced to the mud content threshold, and then construction continues until the current mud and sand fracture interlayer is drilled through.

[0033] The preset drilling length refers to the maximum distance that can be continuously drilled in a single operation during drilling through fractured mud and sand interlayers to prevent excessive cuttings and mud from clogging the borehole or causing drill bit jamming. This preset drilling length is determined based on the cementation degree, water content, and cuttings removal capacity of the fractured mud and sand interlayer, and is preferably set to 50 cm. By decoupling long-distance drilling through mud and sand layers into multiple short cycles of the preset drilling length, the amount of sediment accumulated in a single operation can be effectively limited. The flushing and washing operation refers to the process of pausing drill bit rotation and advance after completing a unit of drilling, and using a high-pressure fluid medium to powerfully flush the bottom and walls of the borehole.

[0034] Specifically, in the mud-sand interlayer section, high-pressure water is used as the flushing medium, with the water pressure controlled at 2-4 MPa. The high-pressure water is delivered to the drill bit through the internal channel of the drill rod. The impact and carrying force of the water flow are used to flush the mud clumps attached to the surface of the drill bit, the debris wrapped around the drill rod, and the silt deposited at the bottom of the hole to the outside of the hole.

[0035] The mud content threshold is a key indicator for determining whether borehole cleaning operations have met standards, used to quantify the degree of cleanliness within the borehole. This threshold is typically set at a mud content of less than 5% in the drainage. During the cleaning process, the color and turbidity of the return water at the borehole opening are monitored in real time. When the return water changes from turbid to clear, and sampling tests show that the content of particulate mud and sand has decreased to below the mud content threshold, the current cycle of borehole cleaning is considered complete, the borehole is clear again, and conditions are met for continued drilling. If the threshold is not met, high-pressure flushing continues to prevent residual mud and sand from accumulating and worsening in the next cycle, which could lead to stuck drill bit.

[0036] For example, when traversing a 2.0m wide mud-sand interlayer, the preset drilling depth is set to 50cm. The drilling rig first slowly drills 50cm in water drilling mode, generating a large amount of mud-sand mixture in the hole. Then, the drilling is stopped, and a high-pressure water pump is started to flush and wash the hole. The return water at the hole opening is observed. When the mud content in the return water drops from the initial 30% to 4% (i.e., below the 5% mud content threshold), the flushing is stopped, and the next 50cm drilling cycle begins. This process of advancing 50cm and flushing until the mud content is <5% is repeated four times until a cumulative drilling depth of 2.0m completely penetrates the mud-sand interlayer. This segmented, cyclical operation significantly reduces the mud-sand concentration in the borehole, preventing the drill bit from being completely encased in mud and losing its cutting ability, while also preventing abnormal pressure increases in the hole due to poor slag removal.

[0037] This step aims to address the problems of borehole blockage, drill bit entanglement, and jamming that easily occur during continuous drilling in fractured mud-sand interlayers. By discretizing the long-distance drilling process into controlled short-cycle units and using a quantitative mud content threshold as the basis for process switching, precise control of the drilling process is achieved. This not only ensures the sustainable operation of high-pressure water drilling technology in soft and fractured formations but also guarantees the relative stability of the drainage hole walls, laying a solid foundation for subsequent casing installation and the formation of stable drainage channels.

[0038] By setting a preset drilling length and a mud content threshold in synergy, a closed-loop control mechanism for limited drilling and quantitative cleaning was constructed. The preset drilling length spatially limits the total amount of mud and sand generated in a single operation, preventing the instantaneous slag discharge load from exceeding the borehole's slag-carrying capacity. Meanwhile, the mud content threshold establishes the termination standard for borehole cleaning operations from a qualitative perspective, ensuring that the borehole environment is in a low mud content state before each cycle begins. The combination of these two factors enables the water-drilling flushing drilling process to adapt to extreme geological conditions with high water content and high mud content, effectively breaking the vicious cycle caused by mud and sand accumulation, and ensuring that the drainage hole can safely and efficiently drill through the target mud and sand fractured interlayer, thereby achieving the stable implementation of the alternating operation mode described in the above embodiments.

[0039] In one embodiment, constructing the drainage hole further includes: after drilling through the current mud and sand fracture interlayer, withdrawing the drill rod and lowering a casing with drainage perforations to stabilize the hole wall.

[0040] Drilling through the current mud-sand fractured interlayer refers to the drill bit penetrating the mud-sand fractured zone and entering a relatively intact hard rock layer or reaching the preset termination depth. At this point, the water and soil pressure within the mud-sand fractured interlayer has been initially released. Removing the drill rod means that after confirming the drilling is complete, rotation and impact are immediately stopped, and the drill string is completely withdrawn from the hole to make room for subsequent wall protection work. A casing with drainage perforations refers to a tubular structure specifically designed to support the borehole wall. Several through holes, i.e., drainage perforations, are distributed along the axial and circumferential directions on its wall. These drainage perforations are key channels for hydraulic exchange between the casing and the surrounding strata. Their diameter is typically designed to be 25mm-30mm, ensuring that fracture water and fine mud particles in the mud-sand fractured interlayer flow smoothly into the pipe and are discharged under pressure difference, achieving continuous drainage and pressure reduction. They also effectively prevent large pieces of collapsed rock and soil from entering the borehole and causing blockage. Stabilizing the borehole wall refers to using the physical strength of the casing to resist the creep, narrowing, or collapse tendencies caused by the loss of original structural support in muddy and sandy interlayers, transforming a temporary drilling channel into a permanent or semi-permanent engineering structure with long-term stability. For example, when using a 108mm diameter, 6mm thick steel pipe as casing, the surface of the steel pipe is pre-machined with 120 perforations approximately 25mm in diameter. After installation, the gap between the outer wall of the casing and the borehole wall can be fixed by injecting a small amount of quick-setting grout or by relying directly on the friction of the formation, thus forming a robust protective barrier. The tight connection between the drill pipe and the installed casing avoids the risk of borehole collapse due to prolonged exposure of the bare hole, ensuring the unobstructed flow and safety of the drainage channel. This step aims to solve the technical problem of borehole wall instability after drilling through muddy and sandy interlayers. Through the synergistic effect of the casing's support and the perforations' guiding function, a unity of structural stability and functional continuity is achieved, enabling the drainage hole to perform its pressure reduction and mud removal functions for a long time, providing stable boundary conditions for subsequent geological exploration operations.

[0041] Specifically, at the critical moment of drilling through the fractured mud-sand interlayer, the drill pipe was promptly withdrawn to eliminate further impact of drilling disturbance on the fragile formation. A casing with drainage perforations was then lowered, utilizing its rigid support to lock the borehole wall morphology and prevent instantaneous collapse caused by stress release. Simultaneously, the drainage perforations on the casing served as the sole fluid channel, guiding the orderly discharge of water and mud from the interlayer. This reduced formation pore water pressure and prevented borehole blockage caused by disorderly mud inflow. This combination of rigidity and flexibility in the structural design ensured that the drainage perforations not only physically formed the borehole but also possessed the engineering attributes for continuous operation, effectively guaranteeing the safety and effectiveness of the composite drilling method in complex water-rich hard rock and mud-sand fractured interlayer environments.

[0042] In one embodiment, the advanced pipe roof grouting process includes: when encountering a muddy and sandy interlayer during drilling, stopping drilling and performing grouting reinforcement; after reinforcement is completed, continuing drilling until drilling reaches the designed depth.

[0043] Encountering a mud-sand fractured interlayer refers to a complex geological area where the drill bit enters during advanced pipe roof drilling operations, where water-rich hard rock and mud-sand fractured interlayers alternate. Indications for encountering a mud-sand fractured interlayer include: the water discharged from the borehole changes from clear to turbid, and its color deepens; the return cuttings at the borehole opening contain fragmented hard rock particles; drilling resistance increases significantly and is accompanied by abnormal friction sounds; and drilling efficiency decreases significantly. If any or more of these signs are detected, drilling operations should be immediately stopped to prevent drill pipe deviation, stuck drill bit, or borehole collapse due to blindly forcing progress.

[0044] Grouting reinforcement refers to the localized high-pressure grouting treatment of identified mud-sand fractured interlayer sections. Specifically, keeping the drill rod in place or slightly retracting it to the starting position of the interlayer, cement grout or cement-water glass dual-liquid grout is injected into the unfavorable geological section through the drill rod. The grouting pressure is controlled at 1-2 MPa. Utilizing the penetration and filling effect of the grout, the loose mud-sand particles are cemented and solidified, filling the rock fissures, thereby forming a reinforcement ring with a certain strength. This reinforcement process aims to improve the self-stabilizing capacity of the interlayer area, seal seepage channels, and create a stable borehole environment for subsequent drilling.

[0045] For example, when drilling to a depth of 12.5m, if the backwater becomes turbid and a large amount of fine sand is discharged, it is determined that the first layer of mud and sand fracture interlayer has been entered. At this time, drilling should be stopped immediately, and the grouting pump should be started to inject modified cement grout at a pressure of 1.5MPa. Grouting should continue until the pressure at the borehole opening is stable and the grouting volume is significantly reduced, indicating that the interlayer voids have been effectively filled.

[0046] By stopping drilling in a timely manner and carrying out targeted grouting reinforcement, the risk of hole collapse caused by weak interlayers can be effectively avoided, ensuring the accuracy of the pipe roof drilling trajectory, and initially improving the water-stopping performance of the area.

[0047] Reinforcement completion refers to the end of grouting operations and the necessary initial setting time for the grout to allow the strength of the soil and rock mass in the mud-sand fractured interlayer area to recover to a drillable state. The judgment criteria are usually that the grouting pressure reaches the design final pressure and is stabilized for a certain period of time, or the shortest waiting time determined based on field tests.

[0048] Hole sweeping drilling refers to re-entering a reinforced borehole section using the original drilling rig and drill bit to clean and break up the solidified slurry and residual loose material to restore the borehole's patency. During hole sweeping, a low-speed rotation combined with moderate impact can be used to carefully pass through the original interlayer area until the drill bit touches the undisturbed intact rock mass behind it.

[0049] Until drilling reaches the designed depth, the cycle of stopping drilling upon encountering an interlayer, grouting for reinforcement, and then cleaning the hole and resuming drilling is repeated until the pipe roof borehole reaches the preset total length (e.g., 30m). Throughout the process, if a new muddy and sandy interlayer is encountered again, the same reinforcement process is executed again, adopting a strategy of point reinforcement and segmented advancement, breaking down the long-distance borehole into multiple controllable construction sections.

[0050] For example, after completing the first interlayer reinforcement and passing through the borehole, drilling continued to 18.2m and encountered a similar geological anomaly again. Drilling was then stopped again for grouting. After consolidation, the borehole was passed through. This process was repeated until all the 30m deep pipe roof drilling operations were completed.

[0051] This step ensures that the advanced pipe roof can successfully pass through multiple layers of mud and sand, and guarantees that the pipe roof is accurately positioned at the designed angle and depth along its entire length, thus laying a solid foundation for the subsequent formation of a continuous and complete long-distance water-stop curtain.

[0052] By identifying and immediately stopping drilling through broken mud and sand interlayers during the drilling process, accidents such as stuck drills, deviations, and borehole collapses, which are prone to occur in traditional continuous drilling methods, are avoided. Subsequent localized grouting reinforcement not only stabilizes the weak interlayers but also acts as a localized water plug, reducing the interference of water pressure within the borehole on the drilling operation. After reinforcement, the subsequent sweeping drilling ensures the continuity of the passage, allowing the pipe roof to successfully penetrate the adverse geological area to reach the designed depth. This dynamic construction mode of detection-reinforcement-through significantly improves the success rate and quality of pre-drilling grouting, ensures the integrity and reliability of the water-stop curtain, and provides stable preconditions for the subsequent construction of drainage holes and geological exploration boreholes. It effectively solves the technical challenges of difficult pre-drilling and low borehole formation rates when deep-buried tunnels traverse complex and adverse geological bodies.

[0053] When encountering muddy and sandy interlayers and signs of borehole collapse or drill jamming during geological exploration drilling, the drill rod is removed and replaced with a water drill to carry out high-speed drilling operations. After the borehole is cleared, drilling can continue.

[0054] Signs of borehole collapse or stuck drill pipe include abnormally increased drilling resistance, a sudden drop in drill bit speed, a sudden interruption of water return at the borehole opening, or a sharp increase in turbidity accompanied by a large amount of cuttings. These signs indicate that the rigid impact of the current pneumatic drilling operation has caused borehole instability or that the drill bit is encased in broken rock and soil. Removing the drill pipe is intended to immediately cut off the power source from the complex environment inside the borehole, preventing the drill bit from being permanently buried or damaging expensive detection equipment. For example, when drilling to the designed depth of 45m, if the drill torque is detected to suddenly increase to 1.5 times the rated value and no cuttings are discharged from the borehole opening, the operator should immediately stop the feed and reverse the drill bit to remove the entire drill pipe from the borehole. This step provides the necessary operating space and safety for subsequent borehole repair work.

[0055] Replacing with a water drill involves dismantling the pneumatic down-the-hole hammer drill bit and high-pressure air pipeline originally used for hard rock breaking, and replacing them with a water drill bit and corresponding drill rod assembly equipped with a high-pressure water pump supply system. High-pressure flushing is a process that uses high-pressure water flow as a flushing medium to powerfully wash away and suspend the collapsed deposits inside the hole.

[0056] Specifically, high-pressure water at 2–4 MPa is injected into the drill pipe using a high-pressure pump. The water jets out at high speed from the drill bit nozzle, stripping away mud, sand, gravel, and collapsed soil adhering to the borehole wall and suspending them in the water. The mixture is then carried out of the borehole by the upward flow velocity. For example, after withdrawing the pneumatic drill pipe, a water drill bit with a diameter slightly smaller than the borehole diameter is lowered in. The high-pressure water pump is turned on, and the borehole is swept back and forth at a slow speed of 0.5–0.8 m / h until the water returning from the borehole opening changes from turbid to clear, and the particulate content in the returned slag is less than 5%, indicating that the borehole patency has been restored. Based on this, the pneumatic drilling mode can be switched back, or the water drill can continue to advance slowly until the unfavorable geological section is traversed. This mechanism of withdrawing the drill in case of danger and flushing the borehole with water effectively avoids stuck drill accidents caused by forced drilling, ensuring that the geological exploration borehole can successfully reach the designed depth and preserving a complete physical channel for subsequent imaging and geophysical exploration operations.

[0057] By monitoring drilling parameters in real time to identify signs of borehole collapse or stuck drill pipe, and closely coordinating with drill rod removal, water drill replacement, and high-pressure flushing operations, a shift from passive obstruction to proactive obstacle removal was achieved. The flexible flushing capability of the high-pressure water flow compensated for the tendency of pneumatic drills to cause secondary collapses in fractured strata, rapidly restoring the geometry and fluid connectivity of the borehole. This process conversion not only protected drilling equipment from damage, but more importantly, ensured the continuity and stability of the exploration channel. Even when encountering unforeseen muddy and sandy interlayers, a stable borehole meeting the requirements of high-precision geological exploration can still be formed, thereby ensuring the accuracy and reliability of advanced geological prediction data.

[0058] Full-section borehole washing refers to a systematic and continuous flushing process carried out along the entire length of the borehole, from the borehole opening to the bottom, after the geological exploration borehole has been drilled to the designed depth and the preliminary borehole opening has been completed. This step aims to thoroughly remove mud cake, adhering materials, and fine-grained rock cuttings and cement deposited at the bottom of the borehole, thus solving technical problems such as blurred imaging, attenuated geophysical signals, and jamming of detection equipment caused by a turbid borehole environment.

[0059] Specifically, the full-section borehole washing operation uses high-pressure water as the flushing medium. Utilizing fluid dynamics principles, the high-pressure, high-flow-rate water impacts the borehole wall and carries suspended particles out of the borehole. During this step, the flushing medium is injected from inside the drill pipe, reaches the bottom of the borehole, and then returns, flowing back up along the annular space of the borehole wall to the borehole opening, forming a fully covered circulating flushing path to ensure no blind spots in the cleaning process. For example, when the drilling depth of a geological exploration borehole is 55m, the washing operation needs to be continuous until the fracture water returning from the borehole opening changes from turbid to consistently clear, and the particulate matter content in the returned slag decreases to below 3%. Simultaneously, the water flow from the borehole opening is monitored to be stable with no significant pressure fluctuations. Only then can the full-section borehole washing be considered successful. This deep cleaning of the entire borehole section effectively removes weak interlayer residues adhering to the borehole wall, significantly improving the straightness and cleanliness of the borehole. This provides an ideal operating environment for subsequent installation of in-hole imaging equipment and acoustic testing, avoiding data distortion caused by borehole wall collapse or foreign object interference.

[0060] Based on this, the aforementioned drainage hole construction has significantly reduced formation water pressure and removed large particles of sediment, alleviating the load on the entire borehole washing process. This step, as the final quality control checkpoint before geological exploration, further refines the treatment of residual fine particles within the borehole, ensuring the stability of the borehole wall structure. This continuous cleaning strategy not only restores the borehole's patency but also creates a homogeneous fluid medium environment, enabling subsequent borehole wall scanning imaging to clearly identify lithological boundaries and structural plane development characteristics. Simultaneously, it ensures the signal penetration quality and inversion accuracy of geophysical operations such as single-hole acoustic testing or seismic wave reflection. This achieves a closed-loop technology process from coarse sediment removal to fine cleaning and high-precision detection, effectively improving the reliability and safety of advanced geological prediction under complex water-rich hard rock geological conditions.

[0061] The following is an example of a tunnel under construction encountering a significant amount of granite strata rich in fissure water during excavation. Expert on-site investigation and assessment revealed multiple fractured mud-sand interlayers within the strata, necessitating advanced geological drilling. The relevant construction methods implemented on-site are as follows: (1) Advanced pipe roof grouting: Key design parameters are shown in Table 1 below.

[0062] Table 1 If a muddy or sandy interlayer is encountered during the drilling of the pipe roof, drilling should be stopped immediately. Then, grouting should be carried out in a small area to reinforce the unfavorable geological area. Then, the drilling should be resumed and repeated until the designed depth is reached. Finally, grouting should be carried out to reinforce the area and form a water-stop curtain.

[0063] (2) Hole layout design: The key design parameters are shown in Table 2 below.

[0064] Table 2 The actual number of drainage holes needs to be determined based on the site's geological conditions. For example, if the previous advanced geological survey revealed only three distinct mud-sand fractured interlayers within the current 50m section of strata, then only three drainage holes are required. The drainage holes should be installed "from the outside in," using an intermittent "skip-hole" pattern. Figure 2 As shown, the first "drainage" borehole is drilled at the outermost opening on the left half of the working face, the second "drainage" borehole is drilled at the outermost opening on the right half of the working face, and the third "drainage" borehole is drilled at the second opening from the outside in on the left half... (3) Construction of drainage holes.

[0065] A dual-mode drilling process is adopted: for hard rock, down-the-hole percussion drilling based on pneumatic drills is used. The on-site drilling parameters are controlled as follows: impact frequency of 800-1200 times / minute, rotation speed of 20-40 revolutions / minute, and high-pressure air pressure of 0.8-1MPa. The drill bit breaks the hard rock under high-frequency impact, and the rock cuttings are blown out of the hole with the high-pressure air. When encountering muddy and sandy interlayers, a slow, flushing drilling approach based on water drilling is used to drill into the muddy and sandy interlayers. The design is to complete one cycle every 50cm of drilling progress. Specifically: Water drill slowly (mud, sand, and debris are discharged with the water from the surrounding rock fissures, with a drilling rate of 0.5–0.8 m / h) → Stop drilling, connect the high-pressure pump to the drill rig's water supply pipe → Inject high-pressure water into the drill rod, controlling the water pressure at 2–4 MPa → "wash" the hole → The water output is stable, the water is clear, and the mud content is less than 5% → Drill the next 50 cm section.

[0066] Drilling can only stop after each drainage hole has penetrated the corresponding mud and sand interlayer. The drill rod is then lowered into a "drainage perforation" casing of the corresponding length to stabilize the hole wall. For example, Figure 4 As shown in Table 3, the final advance length of each drainage hole and the length of each interlayer traversed are as follows.

[0067] Table 3 During on-site construction, the transformation of drilling technology is determined by the characteristics of the surrounding rock in different strata. The judgment criteria are: 1-the quality of the discharged water; 2-the change in the color of the discharged water; 3-the situation of backfilling; 4-the drilling efficiency; 5-the presence of abnormal noise, etc.

[0068] The water discharged from the borehole changed from clear to turbid (color deepening), and contained backflow debris, mostly broken, granular hard rock particles. Drilling encountered significant resistance and unusual noises (mostly the sound of the drill bit rubbing against the broken rock when the drill bit got stuck). These signs indicate that the drilling has reached the mud-sand interlayer zone, requiring a timely change in drilling techniques.

[0069] (4) Construction of geological exploration boreholes.

[0070] Geological exploration boreholes can only be constructed after the drainage holes have been depressurized and sludge removed. Before construction, it must be confirmed that the water output from each drainage hole is stable, the sludge content of the drainage is less than 5%, and the seepage pressure at the working face is <0.5MPa.

[0071] If slight hole collapse or drill bit jamming occurs during drilling due to the influence of mud and sand interlayers, the drill rod should be removed in time and replaced with a water drill to carry out high-pressure water washing operation until the hole is mainly filled with clear water and very little particulate backflow. Only after the hole is clear can the drilling continue.

[0072] After the geological exploration boreholes are drilled, the entire borehole section must be flushed until the water returning from the borehole is consistently clear, there is no obvious particulate slag, the water flow from the borehole opening is stable, and there are no significant pressure fluctuations. Only then can it be considered a stable exploration channel, ready for borehole imaging and geophysical exploration operations. The final lengths of the two geological exploration boreholes on site were 54.2m and 57.5m, respectively.

[0073] (5) Geological exploration operations.

[0074] Before conducting geological exploration borehole operations, it is necessary to confirm that the water output from the borehole is stable, the mud content of the backfill is less than 3%, and there are no signs of continuous collapse of the borehole wall before the exploration equipment can be lowered.

[0075] The geological exploration borehole on the left is used for in-bore imaging. The corresponding equipment is used to carry out full-length borehole wall scanning imaging to obtain the development characteristics of the surrounding rock structural plane, the location of lithological boundaries, and the spatial distribution of mud and sand fractured interlayers. Subsequently, based on the imaging, geophysical exploration was carried out in the geological exploration borehole on the right side. Single-hole acoustic testing or seismic wave reflection methods were used to obtain the wave velocity parameters of the surrounding rock mass in front of the borehole, and to invert the integrity and water-bearing state of the surrounding rock.

[0076] After geophysical exploration is completed, the results of imaging and geophysical exploration are combined to comprehensively determine the surrounding rock grade and adverse geological distribution of the current section. The geological boundary conditions within a range of 30-50m ahead are also detected using boreholes to provide a basis for adjusting support parameters and controlling construction risks in the next stage.

[0077] (6) Hole cleaning operation.

[0078] High-pressure water flushing is used during water drilling, with water pressure controlled at 2-4 MPa and drilling speed controlled at 0.5-1.0 m / h. High-pressure flushing is performed every 50 cm of drilling until the return water changes from turbid to clear and the content of granular rock cuttings in the return slag is less than 3%. Only then can the next cycle continue or the return air drilling mode be switched.

[0079] like Figure 5As shown, geological exploration includes using in-hole imaging equipment to conduct full-length borehole wall scanning imaging to obtain the development characteristics of surrounding rock structural surfaces, lithological boundaries, and spatial distribution of mud-sand fractured interlayers; then, based on the imaging, in-hole geophysical exploration is carried out, using methods such as single-hole acoustic testing or seismic wave reflection to obtain wave velocity parameters of the rock mass around the borehole, and to invert the integrity and water-bearing state of the surrounding rock.

[0080] After geophysical exploration is completed, the surrounding rock grade and adverse geological distribution of the current section are comprehensively determined by combining imaging and geophysical results. The geological boundary conditions within a 30-50m range ahead are also explored using boreholes to provide a basis for adjusting support parameters and controlling construction risks in the next stage. If a sudden surge or localized collapse occurs during exploration, exploration is immediately terminated and the equipment withdrawn. Work can only resume after the borehole has been re-washed and stabilized.

Claims

1. A composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers, characterized in that, Includes the following steps: Advanced pipe roof grouting is performed along the outer edge of the tunnel excavation face in the tunnel arch to form a long-distance water-stop curtain; Using the vertical centerline of the tunnel as a reference, drainage holes are alternately constructed on the left and right sides of the tunnel; each drainage hole corresponds to a mud-sand fractured interlayer. When the drainage hole passes through the mud-sand fractured interlayer hidden in the stratum, the advance is stopped, and the construction of the drainage hole is completed. Drill geological exploration boreholes to the designed depth to form an exploration channel to facilitate geological exploration operations.

2. The composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in claim 1, characterized in that, During the construction of the drainage holes, pneumatic drills and water drills were used alternately. Pneumatic drilling was used for hard rock sections, while water drilling was used for scouring and advancing through muddy and sandy interlayer sections.

3. The composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in claim 2, characterized in that, The construction of drainage holes includes: Once the drilling depth reaches the preset depth, flushing and washing operations are carried out until the mud content in the drainage is reduced to the mud content threshold. Then, drilling continues until the current mud and sand fracture interlayer is drilled through.

4. The composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in claim 3, characterized in that, The construction of drainage holes also includes: After drilling through the current mud and sand fracture interlayer, remove the drill rod and lower a casing with drainage holes to stabilize the borehole wall.

5. A composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in any one of claims 2-4, characterized in that, The construction of advanced pipe roof grouting includes: When encountering muddy and sandy interlayers during drilling, stop drilling and perform grouting reinforcement; after reinforcement, continue drilling until the designed depth is reached.

6. The composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in claim 5, characterized in that, When encountering muddy and sandy interlayers and signs of borehole collapse or drill jamming during geological exploration drilling, the drill rod is removed and replaced with a water drill to carry out high-speed drilling operations. After the borehole is cleared, drilling can continue.

7. The composite drilling method for traversing water-rich hard rock and its mud-sand fracture interlayers as described in claim 6, characterized in that, After the geological exploration boreholes are drilled, the entire borehole section is washed.

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