Grooving construction method for seepage interception wall of total rock stratum tailing reservoir

Abstract

The invention discloses a grooving construction method for a seepage interception wall of a total rock stratum tailing reservoir, the method is suitable for a fully weathered to slightly weathered rock stratum and a quartz vein development stratum, and the method comprises the following steps: S1, guide wall construction: constructing a reinforced concrete guide wall at the axis position of the seepage interception wall; s2, interlocking type main hole drilling is conducted, specifically, a large-aperture down-the-hole drill is adopted, and partition-wall-free interlocking type main hole drilling operation is conducted in the guide wall; the interlocking type main hole drilling means that no rock-soil partition wall is reserved between every two adjacent main holes; s3, circular pile casing supporting is conducted, specifically, in the drilling process of the down-the-hole drill in the step S2, a plurality of long pile casings are adopted to be matched with drilling operation; a hydraulic square drill bit is arranged, crescent rock mass left on the two sides of the groove wall is cut and trimmed, and the flat groove wall is formed; s5, staged tank cleaning: the tank cleaning operation is divided into a rough cleaning stage and a fine cleaning stage; and S6, wall body concrete pouring is conducted, specifically, wall body concrete pouring is conducted through an under-bare-water pouring technology, and vibration compensation is conducted in the wall top area.

Description

Technical Field

[0001] This invention belongs to the field of underground anti-seepage wall construction technology in water conservancy, hydropower and mine management projects. Specifically, it relates to a construction method for trenching tailings silo anti-seepage walls in complex geological conditions, especially those with high-hardness quartz veins. Background Technology

[0002] In existing diaphragm wall construction techniques, hydraulic grab drilling or rotary drilling rigs are typically used for deep overburden or soft rock formations. For hard rock formations, a combination of impact drilling rigs, rotary drilling rigs, or down-the-hole drilling rigs with rotary drilling rigs is more commonly used. Traditional hard rock trenching employs a skip-drilling pattern of main and auxiliary holes. This involves drilling the main hole first, leaving a certain thickness of rock and soil barrier between the main holes. After the main hole is completed, auxiliary holes (i.e., the rock and soil barrier) are constructed. Then, a drill bit with cutting capabilities is used to shape and repair the trench, ultimately connecting the sections to form a complete trench.

[0003] However, in projects such as the decommissioning and remediation of a certain mine, unique whole-rock geological challenges are encountered. The strata, from top to bottom, consist of completely weathered, strongly weathered, moderately weathered, and slightly weathered schist or granite, with well-developed high-hardness quartz veins, steep rock strength interfaces, and dense fault zones. During the construction of test sections and the application of existing technologies for these geological conditions, the following significant technical defects and construction difficulties were exposed: In granite or schist strata, high-hardness quartz veins are often found, posing significant technical challenges to trenching using traditional retaining wall techniques. Specifically, when the retained rock-soil retaining wall contains quartz vein developments, the drill bit experiences severely uneven stress due to lithological differences during cutting, making it prone to lateral slippage or "drifting." This prevents the drilling rig from applying effective axial pressure, making borehole trajectory control difficult and resulting in significant deviations in trench verticality. Furthermore, experimental and construction data show that the retaining wall excavation alone is extremely time-consuming, and drilling failures in quartz vein development sections often prevent the formation of complete trenches, severely restricting overall construction efficiency.

[0004] Large-diameter down-the-hole (DHH) drills face two major technical challenges in operation without a retaining wall: air leakage and pressure loss, and borehole stability control. Specifically, while this type of drill has high rock-breaking efficiency and is suitable for hard rock formations, its operation relies on a high-pressure pneumatic down-the-hole hammer. If the retaining wall is removed to avoid the difficulties of excavation and interlocking drilling is implemented directly, adjacent boreholes will become interconnected. In this situation, the high-pressure gas generated by the down-the-hole hammer is prone to leakage through the existing boreholes, causing a sharp drop in borehole pressure. This significantly reduces the down-the-hole hammer's work capacity, or even halts drilling altogether. Simultaneously, due to the lack of lateral restraint from adjacent rock walls, the drill bit is highly susceptible to deflection towards the existing free face during drilling, ultimately causing the borehole verticality to exceed the allowable deviation standard.

[0005] Long trench sections are highly prone to collapse in loose, completely weathered rock strata. Traditional trench segment divisions typically aim for long sections (e.g., 6m-8m) to reduce the number of joints. However, in the unique completely to strongly weathered rock strata of this project, the rock mass structure is loose, consisting of sandy or fragmented material, with extremely poor self-stabilizing capacity. Field tests show that when using 7.8m and 6m long trench sections for construction, the large exposed area of ​​the trench walls and the significant mechanical disturbance make the top of the trench walls highly susceptible to collapse, preventing one-time trenching and severely impacting project progress and safety.

[0006] The deep drill cuttings generated by down-the-hole (DH) drilling are difficult to clean. After using a DH rig for interlocking dry drilling without partitions, a certain amount of drill cuttings will fall back to the bottom of the trench due to the interconnection of the main holes, resulting in a deep accumulation of drill cuttings (generally ≥5m). Conventional cutting tool barrel drills or double-bottom sand-retrieving drill bits are extremely slow in terms of cutting speed when dealing with such deep and loose rock cuttings. Furthermore, the "crescent-shaped" rock masses remaining on both sides of the trench wall can easily cause the cutting tools to break or damage the drill bit opening and closing mechanism, resulting in extremely low cutting efficiency.

[0007] The quality of concrete pouring in dry or low-water-content holes is difficult to guarantee. Due to the dry drilling process in whole-rock strata, the holes lack mud slurry for wall protection and are mostly dry or low-water-content. If concrete is poured directly, the huge drop can easily cause concrete segregation, leading to a risk of leakage at the bottom of the wall. In addition, the concrete at the top of the wall, lacking the pressure of its own weight, is prone to forming a loose zone with many pores and poor density, affecting the overall seepage prevention performance of the cutoff wall.

[0008] In summary, under complex conditions such as thick hard rock layers, well-developed quartz veins, and loose overlying strata, existing technologies struggle to achieve a balance between trenching quality, verticality control, and construction efficiency. Therefore, a new trenching construction method is urgently needed to overcome these technical bottlenecks.

[0009] Based on this, this application is hereby submitted. Summary of the Invention

[0010] The purpose of this invention is to solve the problems existing in the prior art.

[0011] To address the aforementioned problems, this application proposes a trenching construction method for tailings silo cutoff walls in whole-rock formations. This method is applicable to completely weathered to slightly weathered rock strata and strata with developed quartz veins, and includes the following steps: S1 Guide Wall Construction: A reinforced concrete guide wall is constructed at the axis position of the cutoff wall. The guide wall is dynamically adjusted based on the terrain and the thickness of the loose surface layer, serving as a drilling rig operation platform. S2 Interlocking Main Hole Drilling: A large-diameter down-the-hole drill is used to carry out interlocking main hole drilling without partition walls within the guide wall; the interlocking main hole drilling refers to drilling continuously without pre-reserving rock and soil partition walls between two adjacent main holes, maintaining a predetermined amount of interlocking, so that the main holes in the trench section are completed in one go. S3 Circulating Casing Support: During the down-the-hole drilling process in step S2, multiple long casings are used in conjunction with the drilling operation; the length of the long casing is greater than the drilling depth of a single hole, and it is used in rotation between each hole in the trench section to create a relatively "closed" working space to suppress air leakage and control hole inclination. S4 Drill Bit Groove Repair: After the main hole is drilled in a chain, a rotary drilling rig equipped with a hydraulic square drill bit is used to cut and repair the crescent-shaped rock mass remaining on both sides of the groove wall to form a smooth groove wall. S5 Phased Trench Cleaning: The trench cleaning operation is divided into two phases: rough cleaning and fine cleaning. In the rough cleaning phase, a rotary drilling rig equipped with a spiral drill bit is used to clean the trench in multiple stages and layers. In the fine cleaning phase, a rotary drilling rig is used in conjunction with a sand-removing drill bit before the trench inspection. S6 Wall Concrete Pouring: After the hole cleaning is inspected and approved, the wall concrete is poured using the clear water underwater pouring process, and the top area of ​​the wall is subjected to additional vibration.

[0012] Preferably, in step S2, the engagement amount between adjacent main holes is controlled to be no less than 7 cm.

[0013] Preferably, in step S3, the long casing is a seamless steel pipe, and its length exceeds the designed hole depth by 50cm to 100cm. The specific operation of the circulating casing support is as follows: three long casings are configured and placed into the current borehole and adjacent completed holes respectively; after each main hole is drilled, the last casing is pulled out using a drilling and pulling machine, transferred and placed into the newly opened hole position, and the cycle is followed up with the drilling progress.

[0014] Preferably, in step S5, the specific process of layered slag removal is as follows: a rotary drilling rig equipped with a spiral drill bit is used to perform preliminary cleaning of the thick layer of drilling slag in the trench in stages and layers; after the thick layer of drilling slag is cleaned, a toothless double-bottom sand-retrieving drill bit is used to perform fine cleaning of the residual sediment at the bottom of the trench until the thickness of the sediment at the bottom of the hole meets the design requirements.

[0015] Preferably, the length of a single trench segment in the trenching construction is set to 3m; in steps S2 and S4, the construction machinery mainly operates at the end of the trench segment or in the backfilled area.

[0016] Preferably, in step S6, the underwater grouting process includes: grouting concrete using a guide pipe method; after the underwater grouting of the wall concrete is completed and the laitance on the top of the wall is removed, and before initial setting, immersion vibration is applied to the concrete within a 3m range from the top of the wall downwards.

[0017] Preferably, the method further includes a connection step between adjacent trench sections: using a drilling and casing method; after the concrete of the first-stage trench section is poured and reaches final setting, a drilling rig is used to perform full-hole casing drilling at both ends of the first-stage trench section to form joint holes; the casing drilling time is controlled as follows: when using a rotary drilling rig, it is carried out 48 hours after the concrete of the first-stage trench is poured; when using a down-the-hole drilling rig, it is carried out 96 hours after the concrete of the first-stage trench is poured.

[0018] Preferably, the guide wall structure in step S1 adopts " The reinforced concrete structure is shaped like a "25cm thick panel, 15cm thick sidewall, and 73cm clear distance within the guide wall. When encountering loose soil layers in the guide wall trench excavation area, the excavation depth must penetrate the loose layer and enter the stable stratum by at least 30cm.

[0019] In summary, the present invention has the following beneficial effects: By employing a wallless interlocking main hole drilling process, combined with a bite depth of no less than 7cm, the difficult-to-drill rock walls encountered in traditional processes are eliminated. This avoids slippage or drifting of the drill bit due to uneven force when contacting differently hard materials. Simultaneously, the use of a circulating long casing support system, with rigid casing forcibly guiding and laterally restraining the drill bit, effectively overcomes the problem of down-the-hole drilling rigs easily deviating towards the free face when operating without walls. This ensures the verticality of the trench and improves the efficiency and quality of trenching in hard rock formations.

[0020] Addressing the characteristics of loose structure and poor self-stabilization ability in completely weathered to strongly weathered rock strata, this invention innovatively limits the length of trenching units to short trench segments of 3 meters and stipulates that construction machinery operates only at the ends of these segments. This trench segment length significantly reduces the free surface area of ​​a single excavation, eliminates lateral dynamic load disturbance from heavy machinery on the most vulnerable part of the trench wall, thus achieving zero-collapse borehole construction in completely weathered strata and ensuring construction safety and progress.

[0021] To address the issue of thick, loose drill cuttings generated during dry drilling with down-the-hole (DHH) rigs, this invention proposes a staged cuttings removal process. Utilizing the low resistance and rapid transport characteristics of auger bits in loose media, coarse cleaning of thick layers of drill cuttings is quickly completed. Subsequently, a flat-bottomed drill bit without cutting teeth is used for fine cleaning, avoiding the risk of bit jamming and tooth loss, and ensuring that the thickness of the sediment at the bottom of the hole is strictly controlled within 10 cm. This combined process increases the cuttings removal efficiency of a single section by more than 50% and significantly reduces drill bit wear.

[0022] By employing a combined underwater concrete pouring and top-mounted vibration reinforcement process, the problem of easy segregation of dry-hole poured concrete was solved using a water-based buffer pad. Top-mounted vibration reinforcement eliminated porosity and looseness defects in the low-pressure zone at the top of the wall, ensuring consistent strength throughout the wall. Furthermore, the use of drilling and chiseling techniques, along with a rigorous joint cleaning process, utilized the mechanical interlocking and microscopic bonding between the new and old concrete to completely block seepage channels at the joints, ensuring the overall seepage prevention coefficient of the cutoff wall meets the high design standards.

[0023] By thickening the wing plates to distribute heavy machinery loads and dynamically adjusting the guide wall depth according to the thickness of the loose layer, the bottom opening is ensured to be embedded at least 30cm into stable strata. This design provides a stable platform for heavy equipment, reduces the risk of trench wall collapse, and effectively seals off the infiltration path of surface water. It is particularly suitable for tailings pond remediation projects with large rock surface undulations and complex geological conditions. Attached Figure Description

[0024] Figure 1 A construction flowchart for a trenching construction method of a tailings silo cutoff wall in a whole-rock formation; Figure 2 A schematic diagram of the construction process for a trenching construction method for a tailings silo cutoff wall in a whole-rock formation; Figure 3 This is a schematic diagram of the interlocking borehole layout for a trenching construction method of a tailings silo cutoff wall in a whole-rock stratum. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This embodiment is applied to the construction of a tailings dam cutoff wall in a mine decommissioning and remediation project. The geological conditions of this project are extremely complex, with the strata consisting of, from top to bottom, completely weathered schist or granite, strongly weathered schist or granite, moderately weathered and slightly weathered rock layers. High-hardness quartz veins are well-developed in the rock mass, fault zones are dense, and the rock strength interface has a steep slope. The cutoff wall is approximately 1170m long, 0.6m thick, and has an average bottom depth of approximately 16.1m, requiring it to be embedded 2m below the slightly weathered bedrock.

[0027] In response to the aforementioned geological structure, this invention provides a trenching construction method for a tailings silo cutoff wall in whole-rock strata. The specific process flow is as follows: Please see Figure 1-3 This embodiment provides a trenching construction method for a tailings silo cutoff wall in whole-rock formations. The specific process flow is as follows: Step S1: Guide wall construction A reinforced concrete guide wall was constructed at the axis of the cutoff wall. To accommodate the sloping terrain of the tailings silo and the uneven thickness of the surface loose layer, this embodiment features a specific design for the guide wall structure: the guide wall adopts a… The guide wall is a reinforced concrete structure. The concrete strength is C30, the panel thickness is 25cm, and the sidewall thickness is 15cm to withstand heavy machinery loads. The net clearance within the guide wall is set at 73cm (i.e., a design wall thickness of 60cm + 13cm allowance), and the width of both side panels is 1.2m. The guide wall height is not specifically limited to a fixed 1.0m, but is dynamically adjusted based on the thickness of the surface loose layer. When encountering loose soil layers in the guide wall trench excavation area, the excavation depth must penetrate the loose layer and enter the underlying stable stratum (such as undisturbed soil) by at least 30cm to ensure the stability of the guide wall foundation and prevent orifice collapse. After the guide wall construction is completed, excavators are used to backfill soil and rock until the top surface of the guide wall is level, forming a horizontal working platform for subsequent heavy machinery operations. For steep slope sections, such as drainage slopes >10%, the guide wall is segmented into multi-level stepped sections.

[0028] Understandably, the surveying and layout, along with the topographic verification, are based on the axis coordinates of the cutoff wall to accurately mark the excavation boundary of the guide wall trench. Simultaneously, the topographic slope along the route is verified. For steep slopes with a drainage slope >10%, the positions and elevations of the stepped guide walls are planned in advance to ensure that the length of each step meets the minimum horizontal stopping requirements for drilling operations. Excavation is carried out layer by layer along the layout line using an excavator, and excessive depth excavation in a single operation is strictly prohibited. During excavation, the geological engineer continuously monitors the exposed strata. When the surface loose soil layer (such as completely weathered rock, tailings fill layer, etc.) is penetrated and the underlying relatively dense soil layer is exposed, it is identified as a potential bearing layer. After confirming the bearing layer, excavation continues downwards by at least 30cm to form the root trench for the guide wall, ensuring that the bottom of the guide wall is completely embedded in stable strata. If the loose layer is too thick, such as >2m, local deepening or pre-grouting reinforcement measures are required based on on-site approvals.

[0029] After the trench is formed, the reinforcement binding and formwork are completed, a bedding layer is laid, and reinforcement binding is carried out. The guide wall reinforcement mesh must be arranged in a double layer and bidirectional manner to enhance overall rigidity. Additionally, for irregularly shaped formwork: inverted L-shaped formwork is used. For vertical sidewall formwork: ensure the internal clear distance is strictly controlled at 73cm (±10mm) to allow for subsequent installation of short casing with openings. For horizontal panel (wing plate) formwork: ensure the wing plate width reaches 1.2m and the thickness reaches 25cm, and compact and level the soil under the wing plate to ensure close contact with the soil. Concrete pouring and curing: C30 concrete is used for integral pouring. During pouring, attention should be paid to vibration and compaction, especially at the internal corners where the sidewall and wing plate meet, to prevent honeycomb and pitting that could affect structural strength. After pouring, cover and cure promptly, and remove the formwork after the strength reaches the design requirements. After the guide wall formwork is removed, backfill with earth and stone on the outside of the guide wall. The backfill soil is compacted in layers using excavators or road rollers, with the backfill height level with the top surface of the guide wall wing plate. This ultimately creates a wide, horizontal heavy machinery operating platform, allowing heavy drilling rigs (such as 80-ton down-the-hole drilling rigs) to operate directly on the guide wall wing plate.

[0030] Compared to traditional rectangular guide walls, this design addresses the stability challenges of heavy machinery operating on loose foundations by employing a thickened horizontal wing plate, 1.2m wide and 25cm thick. This wing plate effectively functions as a strip raft foundation. When down-the-hole drills or rotary drilling rigs weighing over 80 tons operate on the guide wall, the enormous point loads and dynamic construction loads are evenly distributed across a larger area of ​​soil beneath through the wide wing plate. This effectively reduces the compressive stress at the base, preventing guide wall subsidence, fracture, or overturning caused by localized stress concentration, and providing a stable physical reference for high-precision trenching.

[0031] This process specifies that the excavation depth must penetrate the loose layer and embed at least 30cm into the stable stratum. The root-like design allows the bottom of the guide wall to be embedded into the undisturbed soil like tree roots. On one hand, the constraint of the undisturbed soil locks the guide wall's position; on the other hand, the concrete sidewall completely severs the connection between the surface loose soil and the trench interior, sealing off the channel for surface water to seep into the trench along the bottom interface of the loose layer. This eliminates the risk of borehole collapse caused by surface water soaking or loose particle shedding, achieving the goal of zero borehole collapse in the test section. Furthermore, by dynamically adjusting the guide wall height based on the loose layer thickness and employing a multi-stage stepped design in steep slope areas, dynamic depth adjustment avoids the waste caused by blindly digging deep in areas with thin loose layers and prevents the guide wall from being suspended in areas with thick loose layers. The stepped design resolves the elevation difference problem caused by the steep slope terrain of the tailings silo, reducing a significant amount of slope cutting while ensuring that each drilling rig can drill vertically on a horizontal plane, balancing economy and construction quality. The internal clearance was set at 73cm (design wall thickness 60cm + 13cm allowance). This 13cm allowance avoids both excessive clearance leading to difficulty in casing positioning and insufficient clearance causing the drill bit to scrape against the guide wall sidewall during drill bit raising and lowering. These parameter settings and process controls ensured smooth operation of the φ530mm long casing and 600mm drill bit, improving construction efficiency.

[0032] Step S2: Interlocking main hole drilling without partitions In this step, the XR150DV large-diameter down-the-hole drill rig is selected as the main equipment, equipped with two high-pressure air compressors (such as the XRVS1188PRO model, with an exhaust volume of 33.5 m³ / min) to provide power. The length of a single trench segment is set at 3m. Compared with traditional long trench segments, the 3m short trench segment can significantly reduce the lateral disturbance of the middle of the trench wall by the machinery, effectively solving the problem of easy collapse of completely weathered rock strata. It can be seen that this method abandons the traditional main hole and auxiliary hole skipping process and adopts a non-partitioned interlocking hole layout. That is, when constructing two adjacent main holes, no rock and soil partition wall is reserved, but drilling is carried out continuously according to the preset interlocking amount until the design depth is reached. Among them, the interlocking amount between adjacent main holes is controlled to be no less than 7cm. The bite-off design creates the necessary working conditions for subsequent trench wall trimming using a hydraulic square drill bit. It can effectively eliminate rock ridges between holes and avoid traditional process problems such as excessive time consumption and drill bit slippage caused by clearing the partition wall. This enables efficient and high-quality trenching operations in thick hard rock.

[0033] More specifically, to ensure the drilling accuracy and efficiency of large-diameter down-the-hole (DTH) drills in a guideless (without partition) state, the single drilling action is broken down into the following standard operating procedures: An XR150DV large-diameter DTH drill is selected, with a torque of 150 kN·m, which can effectively drive a large-diameter DTH hammer. Simultaneously, to ensure the impact frequency of the DTH hammer at a hole depth of approximately 16.1 m and in high-hardness rock formations such as slightly weathered rock with a strength of 30-53 MPa, two Atlas XRVS1188PRO high-pressure air compressors are connected in parallel for air supply. Each compressor has an exhaust capacity of 33.5 m³ / min; the two in parallel provide sufficient air volume and pressure to ensure smooth slag removal and prevent drill jamming. Before the DTH drill begins drilling, a pilot hole with a depth of 5–6 m is drilled at the first hole position of the trench section using an XR240E or XR150 rotary drilling rig. Leveraging the advantage of rotary drilling rigs in achieving high verticality in loose overburden, a vertical physical guide hole is provided for the subsequent down-the-hole hammer. This effectively limits the hammer's entry trajectory into the rock from the outset, preventing deviation immediately upon drilling. Within a 3m trench section, no rock ridges are left between adjacent main holes, ensuring a minimum engagement of 7cm between adjacent holes. Blindly applying pressure is strictly prohibited throughout the drilling process. The bottom pressure should be controlled within 80% of the total weight of the drill string (excluding buoyancy). At this point, the drill rod is under tension, utilizing the hammer's own weight to maintain verticality and effectively preventing drill rod bending and hole deviation caused by excessive pressure.

[0034] During the drilling phase, low speed and light pressure are employed, with frequent checks of verticality. Any deviations are immediately corrected using the drilling rig's automatic correction system. During dynamic slag removal and dust suppression drilling, a wet drilling method is used, continuously injecting an appropriate amount of clean water into the hole. This effectively suppresses dust, meets environmental protection requirements, and wets the drill cuttings into a sticky slag, making it easier to adhere to the auger bit, thus simultaneously improving dust suppression and slag removal efficiency.

[0035] This solution addresses the issues of drill bit drift and slippage caused by insufficient lateral constraint in hard rock formations, such as those with quartz veins, by setting a minimum engagement distance of 7cm between adjacent boreholes and combining this with the rigid guiding effect of a long casing. The mechanism is as follows: compared to traditional methods that use insufficient engagement or retain soil-rock barriers, this solution's large engagement distance design structurally eliminates the need for separate removal of these barriers, thus avoiding the efficiency bottleneck caused by this step. More importantly, this engagement distance creates a working space with sufficient width and depth within the rock mass, providing effective operating conditions for the square drill bit. Simultaneously, the rigid long casing provides continuous and precise verticality control for the drilling system, forcing the drill bit to cut strictly along the designed axis. The synergistic effect of both constitutes a forced constraint on the drill bit's posture, ensuring continuous and stable drilling in hard rock formations and fundamentally suppressing stress deviation and trajectory instability caused by uneven lithology.

[0036] Furthermore, the trench length was forcibly shortened from the conventional 6-8m to 3m. This was primarily due to the loose structure of the fully weathered schist / granite in the upper part of the trench wall, resulting in a short self-stabilization time. A 7.8m or 6m long trench section means a large exposed area of ​​the trench wall, and the machinery would need to repeatedly move around the trench opening, generating dynamic loads that could easily induce a top collapse. The 3m short trench section not only significantly reduces the free face during a single excavation, but more importantly, it matches the operating radius of rotary drilling rigs and down-the-hole drilling rigs. Construction machinery only needs to be parked at the end of the trench section or on the backfilled ground, without needing to move the tracks to cover the entire hole. This method eliminates the shear damage to the most vulnerable soil in the middle of the trench wall caused by the heavy machinery tracks, thus achieving a 100% one-time trenching success rate in the test section.

[0037] Of course, the design without a partition wall, combined with rotary drilling for pilot holes and down-the-hole rock breaking, results in a situation where the geological strata are soft on top and hard underneath. Using only a down-the-hole drill would be costly; using only a rotary drill would be time-consuming and prone to wear and tear on the cutting tools in the lower hard rock (30-53 MPa). This solution utilizes a rotary drill to quickly treat the upper loose layer and create a pilot hole, then leverages the high efficiency of the down-the-hole drill's pneumatic impact rock breaking to focus on the lower hard rock. Simultaneously, the most difficult rock partition wall to handle (traditional methods for clearing partition walls take >12 hours) is eliminated, significantly improving the efficiency of single-slot construction, averaging about two slots completed per day, ensuring the verticality quality of ultra-deep holes. Furthermore, a pressure-reducing drilling process is strictly implemented, with the bottom drilling pressure <80% of the drill bit's own weight. In deep-hole operations at 16m depth, the flexible deformation of the drill rod is the main cause of hole deviation. Pressure-reducing drilling keeps the drill rod in a suspended, tensile, and straight state, similar to the principle of a plumb line. With the guidance of a 5-6m rotary drilling pilot hole, the borehole inclination caused by the anisotropy of the strata is effectively overcome, and the borehole inclination rate is stably controlled within 0.4%, meeting the high design standards.

[0038] Step S3: Circulating casing support To address the risks of air leakage affecting down-the-hole hammer performance and borehole deviation caused by interlocking drilling without partition walls, a long casing support system was implemented throughout the entire process. Specially made seamless steel pipe casings with an outer diameter of φ530mm and a wall thickness of 12mm were used. The casing length was customized according to the designed borehole depth, exceeding the depth by 50cm to 100cm. An EC700CL type drilling and pulling machine (with a pulling height of 21m) and at least three long casings were deployed on-site, forming a "drill one, support two, pull three" cyclical pattern: specifically, three long casings were placed in the current borehole and adjacent existing boreholes to provide lateral "near-closure" and guidance to the borehole walls; after each main borehole was completed, the last casing was pulled out using the drilling and pulling machine; after cleaning, the pulled-out casing was transferred and pressed into the newly drilled borehole, and this cycle continued with the drilling progress. This process ensured that the down-the-hole hammer always operated in a relatively closed and constrained pipe environment, solving the problem of air pressure loss caused by air leakage during dry drilling.

[0039] More specifically, to ensure the continuity and stability of the interlocking drilling, high-strength seamless steel pipes were selected to make the casing, with an outer diameter of φ530mm and a wall thickness of 12mm. This diameter is slightly smaller than the hole diameter (600mm), facilitating installation and removal and ensuring a minimum 7cm engagement between adjacent main holes. This provides working conditions for the square drill bit to create grooves and effectively seals any engagement gaps between holes. The casing length needs to be customized according to the designed hole depth, requiring it to exceed the hole depth by 50cm to 100cm to prevent blocks from falling into the hole and to facilitate clamping by the drilling and pulling machine. An EC700CL drilling and pulling machine (maximum lifting weight 15T, lifting height 21m) was deployed on-site, specifically responsible for the vertical hoisting, vibratory pressing, and extraction of long casings.

[0040] To further clarify the implementation of the "drill one, pull one" cyclical operation, the initial setup is as follows: At least three long casings are configured during the initial drilling phase of the trench section. Once the Nth main hole is completed, the drilling and pulling machine immediately hoists a spare casing to that hole location. Using a level in the drilling and pulling machine's cab and manual verification outside the machine, the casing is ensured to be vertically aligned. The casing is then slowly lowered into the hole using its own weight; if resistance is encountered, a hydraulic vibratory hammer is activated for low-amplitude vibratory sinking until the bottom of the casing reaches the bottom of the hole and the top is above ground level.

[0041] When drilling the N+1th main hole, casings are installed in the Nth, N-1th, and N-2th holes to create a nearly enclosed working environment.

[0042] After the N+1th hole in the same trench section is completed, or according to the fourth hole sequence node determined on site, the drilling and pulling machine moves to the last hole N-2, clamps the top of the casing, turns on the vibration to reduce frictional resistance, and pulls upward at a uniform speed.

[0043] After the removed casing is cleaned of surface mud and sand and its integrity is checked, it is immediately transported to the latest drilling position (the N+1th hole) for placement, forming a dynamic cycle of "three removals and one replacement".

[0044] If the casing is blocked by drill cuttings and cannot sink during installation, a rotary drilling rig must be used to remove the cuttings before installation. If it is difficult to pull out, it is strictly forbidden to force it out.

[0045] This circulating casing support technology is designed for dry trenching construction in whole-rock strata of a certain mine, and can solve the problems of air leakage and borehole inclination in drilling without partition walls. In open-hole drilling, adjacent holes are interconnected. Down-the-hole hammers rely on high-pressure gas for cuttings removal. Without obstruction, this high-pressure gas would instantly flood into the existing holes on the lateral free face, causing a sharp drop in borehole pressure, making it difficult to remove cuttings and resulting in low drilling efficiency. By embedding φ530mm steel casings in adjacent holes, the space between them is physically sealed, while also providing some guidance during drilling. This forces the high-pressure gas to rise only along the annular gap of the current borehole, maintaining sufficient cuttings removal velocity and bottom hole pressure, ensuring normal drilling and borehole verticality in an open environment.

[0046] The steel casing within the adjacent borehole provides exceptional bending stiffness. When the drill bit attempts to deviate from the adjacent borehole, the casing wall provides a rigid lateral reaction force, limiting the drill bit's deviation trajectory. This rigid guide action forces the drill bit to remain vertical during drilling, ensuring the flatness and perpendicularity of the trench wall.

[0047] Step S4: Drill bit grooving After the down-the-hole drill rig completes the interlocking main hole, serrated crescent-shaped rock masses will remain on both sides of the trench wall.

[0048] The down-the-hole drill rig was removed, and an XR240E rotary drilling rig was brought in. Equipped with a specially designed hydraulic square drill bit, in one embodiment measuring 300cm x 60cm, or a rectangular trenching drill bit customized to a width of 60cm based on the wall thickness. Using the square drill bit, the drill cuts vertically down along the trench wall contour, layer by layer removing the remaining crescent-shaped rock mass, thus shaping the originally wavy trench wall composed of circular boreholes into the designed straight trench wall. During the trenching process, the operator monitors the entire process via the cab's verticality display system to ensure the trench's verticality.

[0049] More specifically, to ensure the accuracy of the trenching operation and prevent trench wall collapse due to improper operation, an XR240E rotary drilling rig of suitable size was selected to provide sufficient torque for cutting hard rock. A hydraulic square drill bit, 300cm x 60cm in size corresponding to the wall thickness, was used. This drill bit has guide plates on the side and cutting teeth on the bottom. Before trenching, an XR150 rotary drilling rig with a spiral drill bit was used to initially clear the large amount of drill cuttings accumulated in each interlocking main hole, providing working space for the square drill bit. The square drill bit was then lowered to the trench opening, ensuring its centerline was precisely aligned with the trench section axis. Using an electronic level in the drilling rig cab and external manual double-checking, the drilling rig mast was precisely straightened according to the guide wall and the initial guide hole to ensure the initial verticality of the drill bit when it entered the trench.

[0050] Specifically, for the upper trenching layer (0-8m) of loose rock, a light-pressure, slow-speed drilling mode is adopted. Utilizing the four guide plates on the side of the square drill bit, it slowly descends along the existing borehole wall. Because the completely weathered rock in this depth range is relatively loose, light pressure prevents the drill bit from swinging excessively, thus avoiding damage to the trench wall stability and preventing borehole collapse. When the drill bit cannot descend further due to the accumulation of cut debris, trenching is immediately stopped, and the auger bit is switched back for process cleaning. Once the debris is mostly removed, trenching continues.

[0051] When entering the middle and lower layers of the trench, 8m down to the bottom hard rock layer: Guided by the already prepared upper trench wall, appropriately increase the drilling pressure to improve cutting efficiency. When encountering hard rock (such as quartz veins), slow down and drill slowly to reduce impact and vibration on the machinery and prevent excessive vibration from causing instability of the upper trench wall. After layering the trench to the designed trench bottom elevation, use the verticality display system again to fully check the verticality of the trench wall to ensure that there are no "probe" errors or dead corners where the cutting is not completed throughout the entire depth.

[0052] By adopting the above process steps, the crescent-shaped blind zone of interlocking drilling is eliminated, ensuring the effective wall thickness.

[0053] The interlocking holes drilled by down-the-hole drills are circular. After adjacent holes interlock, serrated "crescent-shaped" rock masses (small walls) inevitably remain on both sides of the trench wall. This leads to a reduction in the effective thickness of the wall, and the sharp rock ridges can easily cause stress concentration or hinder concrete flow. By using a customized 300cm×60cm square drill bit, whose shape is equivalent to a vertical planer, it cuts vertically down along the axis of the trench section, physically removing the remaining crescent-shaped rock ridges. This not only transforms the wavy hole wall into a straight and smooth trench wall, ensuring that the entire cross-section of the cutoff wall meets the design thickness of 0.6m, but also provides regular space for subsequent lowering of the reinforcing cage or pouring of concrete.

[0054] To address the issue that even with casing guidance, slight deviations may still occur in deep holes (>15m) during down-the-hole drilling.

[0055] By incorporating long guide plates on the sides of the square drill bit, the smoothed groove walls after the upper 0-8m trenching is completed provide guidance for the lower trenching. The square drill bit can correct minor local bends generated during main hole drilling, further improving the overall verticality of the trench.

[0056] To address the vulnerability of completely weathered strata to collapse due to severe vibrations, this method utilizes a rotary square drill bit to cut and trench through a continuous main borehole, resulting in less disturbance compared to the intense vibrations caused by retaining soil and rock barriers during trenching. Furthermore, a layered trenching approach is employed, strictly limiting drilling pressure and speed in the upper loose strata to effectively protect the fragile borehole wall. Combined with long casing support, this ensures the safety of trenching in loose strata.

[0057] In summary, by adopting the process of first drilling the main hole to release stress and then finishing the square groove to remove the remaining material, the square drill bit only needs to cut a small amount of residual rock, rather than breaking the entire rock face. Compared with full-face milling or impact grooving, this significantly reduces the amount of rock broken and shortens the grooving cycle of a single section.

[0058] Step S5: Staged slag removal To address the problem of excessively thick drill cuttings accumulation at the bottom of the trench (generally ≥5m) caused by chain-type dry drilling, a multi-stage, layered, and phased slag removal strategy is adopted to improve efficiency.

[0059] First stage: Coarse cleaning of thick slag. When the slag layer thickness detected in the trench exceeds 0.5m (especially exceeding 5m), an XR150 rotary drilling rig equipped with a φ600mm auger bit is used for operation. The auger bit has low drilling resistance in loose, thick slag, and can quickly rotate a large amount of drill cuttings out of the trench opening. Actual measurements show that this process reduces the slag cleaning time for a single trench section from 15 hours to 7-8 hours, and a single slag removal only takes 1.5 minutes.

[0060] Level Two: Fine Cleaning of Sludge. When the sludge layer thins to the point where the auger bit can no longer effectively remove sludge, switch to a toothless double-bottom sand-removing drill bit. Utilize its flat-bottom structure to perform final cleaning of the trench bottom until the sludge thickness at the bottom of the hole is less than 10cm. The use of drill bits with cutting teeth is strictly prohibited here to prevent the cutting teeth from getting stuck in bedrock fissures or dead corners of crescent-shaped rock masses, leading to tooth breakage.

[0061] A triple method was employed to determine the penetration depth into the bedrock: cuttings comparison, drilling rate monitoring, and, when necessary, core sampling, ensuring that the penetration depth into slightly weathered bedrock was no less than 2 meters. The trenching quality control standards were: borehole position deviation ≤ 3 cm, borehole inclination < 0.4%.

[0062] To address the issue of excessively thick drill cuttings accumulation (generally ≥5m) at the bottom of the trench caused by chain-drilling, primarily consisting of loose rock cuttings, a phased, layered, and staged cuttings removal strategy was adopted to improve efficiency. The specific process is as follows: The first step is primary coarse cleaning: For thick layers of drill cuttings, rotary drilling rigs (such as the XR150 model) equipped with auger bits are preferred for rapid cutting removal. Auger bits offer low drilling resistance in loose, thick cuttings, allowing them to quickly remove large amounts of cuttings from the trench opening.

[0063] The second step is secondary fine cleaning: After the thick layer of drill cuttings has been removed, a toothless double-bottom sand-retrieving drill bit is used to finely clean the remaining sediment at the bottom of the trench. The toothless drill bit is used to adapt to the uneven, slightly weathered rock surface at the bottom of the trench, avoiding the cutting teeth getting stuck in bedrock fissures or crescent-shaped rock dead corners, which could lead to tooth breakage. This process is repeated until the sediment thickness at the bottom of the hole meets the design requirements (≤10cm).

[0064] Step S6: Concrete pouring for the wall After the borehole cleaning is inspected and approved, the wall concrete is poured. Given that the dry-forming process involves no mud slurry for wall protection and the boreholes are mostly dry or low-water, direct concrete pouring is prone to segregation. Specifically, the first step is water buffering: before pouring, clean water to a depth of at least 3.0m is injected into the trench as a buffer medium. The clean water layer acts as a hydraulic buffer, preventing the first batch of concrete from directly impacting the bottom of the rock trench and causing segregation. Next is underwater pouring: underwater concrete is poured using a tremie pipe method. The tremie pipe diameter is φ260mm. After the first pour, the tremie pipe burial depth is controlled at more than 1.0m, and subsequently maintained at 2m-6m, with the concrete surface rising at a speed greater than 2m / h. Finally, wall top vibration: after the concrete is poured to the designed wall top elevation, the laitance on the wall top is removed. Before the concrete initially sets, an immersion vibrator is used to manually vibrate the concrete within a 3m depth from the wall top downwards. The main purpose is to eliminate air pockets and looseness that may be caused by insufficient self-weight pressure of the upper concrete, and to ensure the waterproof performance of the top of the wall and the density of the interface between the new and old concrete.

[0065] It should be noted that in this embodiment, the connection between adjacent slot sections adopts the drilling and chisel sleeve method.

[0066] After the first-stage trench concrete is poured and reaches its final set, a drilling rig is used to drill full-hole casings at both ends of the first-stage trench to form joint holes, creating a mechanically interlocking structure with convex and concave surfaces. To prevent vibration damage to the newly poured wall, the casing drilling time is strictly controlled: if a rotary drilling rig is used, casing drilling must be carried out 48 hours after the first-stage trench concrete is poured; if a down-the-hole drilling rig is used, casing drilling must be carried out 96 hours after the first-stage trench concrete is poured. Before the second-stage trench concrete is poured, the end faces of the joint holes are repeatedly scrubbed until no obvious silt or mud adheres to the washing water returning from the bottom of the hole.

[0067] After adopting the above construction method, in the test section of the tailings dam of Mine 753, the problems of slow trenching in hard rock, easy hole collapse, difficulty in cleaning slag and difficulty in ensuring joint quality were successfully solved in the complex whole rock strata of the tailings dam, and high-quality cutoff wall construction was achieved.

[0068] Preferably, in step S2, the engagement amount between adjacent main holes is controlled to be no less than 7 cm, so as to eliminate rock ridges between adjacent holes and prevent the drill bit from slipping in hard quartz vein formations.

[0069] Specifically, during the interlocking main hole drilling process in step S2, in order to overcome the constraints on trenching quality imposed by the thick hard rock strata and high-hardness quartz veins unique to this project, this embodiment limits the spatial geometric relationship between adjacent main holes. The specific implementation is as follows: When performing interlocking hole layout, the interlocking amount between two adjacent main holes (circular cross-section) in the tangential direction is strictly controlled to be no less than 7cm. That is, the drill bit trajectory of the subsequent main hole must cover the edge of the previously completed main hole by at least 7cm, so that no rock and soil barriers remain between the two main holes, but rather a continuous spatial connection is formed.

[0070] In traditional main hole and partition wall techniques, or when the bite depth is too small (e.g., only tangential contact), when a down-the-hole hammer drill bit encounters hard rock with significant variations, the stress at the bottom of the drill bit is extremely uneven due to the presence of rock partitions or only a small free surface on the side of the drill bit. Hard rock formations generate enormous lateral reaction forces on the drill bit, causing it to slip towards the softer rock side or the partition wall side, leading to excessive borehole inclination and preventing the drilling rig from applying effective axial pressure. By setting the bite depth to no less than 7cm, a bite width of no less than 7cm can completely remove any remaining triangular or crescent-shaped sharp rock ridges at the cutting points of adjacent holes. This means that during the drill bit's descent, it is always cutting a relatively wide and clearly defined cross-section, preventing instability caused by sharp rock ridges. Furthermore, a larger bite depth ensures that one side of the current borehole remains connected to the large space of the existing borehole, providing working conditions for grooving operations with square drill bits.

[0071] This parameter addresses the problem of drill bit drift, effectively overcoming borehole deviation caused by differential hard rock in composite strata of fully weathered to slightly weathered granite or schist. The verticality of the borehole is consistently controlled within 1 / 300. It also eliminates the bottleneck of partition wall excavation. Compared to pre-reserved partition wall solutions (where partition wall excavation typically takes over 12 hours, sometimes even up to 24 hours), this solution directly eliminates the partition wall through an interlocking depth of at least 7cm. While increasing the cutting volume per hole, it eliminates the highly complex partition wall excavation process. This improves the construction efficiency of 3m long trench sections from less than 12 days to an average of 2 sections per day, significantly increasing construction efficiency and eliminating drilling problems such as drill bit drift.

[0072] Preferably, in step S3, the long casing is a seamless steel pipe, and its length exceeds the designed hole depth by 50cm to 100cm. The specific operation of the circulating casing support is as follows: at least three long casings are configured and placed into the current borehole and adjacent completed holes respectively; after each main hole is drilled, the last casing is pulled out using a drilling and pulling machine, transported and pressed into the newly opened hole position, and cyclically followed up with the drilling progress.

[0073] Specifically, in step S3, in order to solve the two major problems of air leakage and lack of borehole wall constraint in the interlocking drilling process without partition walls, this embodiment adopts a long casing circulation support system, and the specific implementation details are as follows: High-strength seamless steel pipes were selected for the casing to withstand the severe vibrations and hammer pressure during down-the-hole drilling. The casing outer diameter was set at φ530mm to fit the main borehole diameter of this project, and the wall thickness was 12mm to ensure rigidity. The casing length did not just cover the surface but had to extend throughout the entire borehole. Its length was designed as follows: L 护筒 =H 设计孔深 +(0.5m-1.0m). The 0.5-1.0m elevation above the orifice serves as a lifting operation section and a barrier to prevent surface debris from falling in. Additionally, each working section must be equipped with at least three casings, labeled Casing A, Casing B, and Casing C, to form the minimum circulation unit.

[0074] This process adopts a rolling operation flow of drilling one hole, protecting two holes, and pulling three holes. It is assumed that the drilling direction is a hole position sequence N, N+1, N+2... Initial state: Before drilling the current hole (let's call it the Nth hole), the adjacent preceding hole N-1 and the even earlier hole N-2 have already had their casings embedded.

[0075] Drilling Stage (Nth Hole): The down-the-hole drill rig drills at hole N. At this time, the casing inside hole N-1 acts as the artificial hole wall for hole N. Because there is an engagement (connection) of at least 7 cm between hole N and hole N-1, the casing almost "blocks" the connection gap, forcing the high-pressure gas generated by the down-the-hole hammer to be discharged upwards along the annular gap of hole N, with only a small amount escaping to hole N-1.

[0076] After drilling hole N to the designed depth and completing the hole, the drilling and pulling machine (model EC700CL can be selected) moves to the position of hole N-3. The casing inside hole N-3 is pulled out. At this point, hole N-3 has completed its sealing and guiding functions and is far from the current work point, in a safe and stable zone. After cleaning the surface soil of the pulled-out casing, it is hoisted into the newly completed hole N and lowered into place. Now, hole N has a casing, ready to provide sealing support for the upcoming drilling of hole N+1.

[0077] More specifically, down-the-hole (DH) drills rely on high-pressure air to blow drill cuttings from the bottom of the hole to the surface through the annular space between the drill rod and the borehole wall. In interlocking drilling, without a casing, the high-pressure air would instantly flood into adjacent holes (low-pressure areas), causing a short circuit in the air path. This embodiment physically isolates the lateral escape channel of the airflow by inserting a full-length casing into adjacent holes, forcibly maintaining the upward air velocity within the current borehole, ensuring smooth cuttings removal, and avoiding drilling inefficiencies caused by sediment accumulation. It is understandable that during interlocking drilling, one side of the drill bit is approximately 7cm wide in a free-fall state. When encountering high-strength rock, the drill bit will instinctively deflect towards the free-fall side, i.e., the weaker side. The steel casing in the adjacent hole has extremely high bending stiffness, similar to the structure of a guide rail. When the drill bit attempts to deflect towards the adjacent hole, it is constrained by the rigid reaction force of the casing wall, thereby forcibly correcting the drilling trajectory and ensuring the verticality of the trench.

[0078] The aforementioned process allows down-the-hole drilling rigs to maintain the slag removal efficiency originally only found in intact rock masses, even in an open environment without partition walls. Compared to embedding casing along the entire length, this three-pipe circulation method reduces steel input by more than 95%, requiring only three steel pipes to complete the trenching construction of multi-section cutoff walls, resulting in significant economic benefits. Furthermore, the extra-long casing also provides a certain degree of constraint, eliminating the risk of excessive borehole inclination.

[0079] Preferably, in step S5, the specific process of phased and layered slag removal is as follows: for thick layers of drill cuttings with a thickness greater than 5m generated by interlocking drilling, rotary drilling rigs equipped with spiral drill bits are preferred for rapid slag removal; after the thick layer of drill cuttings is cleaned, a toothless double-bottom sand-removing drill bit is used to finely clean the residual sediment at the bottom of the trench until the thickness of the sediment at the bottom of the hole meets the design requirements.

[0080] Specifically, in step S5, considering the unique characteristics of the dry drilling process without partition walls, such as the large thickness of drill cuttings accumulation inside the hole (generally ≥5m) and the loose rock cuttings, this embodiment abandons the traditional approach of using only a sand-retrieving drill bit and innovatively adopts a layered slag removal process with spiral slag lifting as the main method and sand retrieval as a supplementary method. The specific implementation details are as follows: After the down-the-hole drill rig completes the 3m trench section in a chain-like drilling process, although most of the rock cuttings are blown out by the high-pressure air, a certain amount of drill cuttings will still remain at the bottom of the trench due to the interconnection of the main holes in the trench section, with an average thickness of more than 5m.

[0081] If a conventional double-bottom sand-retrieval drill bit with cutting teeth is used directly, in deep, loose slag layers, the downward pressure of the cylindrical drill bit will generate huge sidewall friction, and the internal slag is very prone to causing a pipe blockage effect, resulting in extremely slow progress; at the same time, the drill bit cutting teeth are very likely to get stuck on the crescent-shaped rock mass remaining on both sides of the trench wall during the lifting and lowering process, which will cause the cutting teeth to break or the drill bit opening and closing mechanism to be damaged.

[0082] In implementing this step, the highly maneuverable XR150 rotary drilling rig was selected, equipped with a φ600mm single-head, single-screw, double-head auger drill bit. Utilizing the blade structure of the auger drill bit, the cuttings removal process is transformed from enclosed soil extraction to helical conveying. When the drill bit cuts into a thick layer of loose cuttings, the cuttings, guided by the rotation of the blades, rise continuously along the helical track and fill the gaps between the blades. This structure exhibits low penetration resistance in loose media and eliminates the vacuum adsorption effect caused by a closed cylinder wall.

[0083] The drilling rig quickly lowers the drill rod to the cuttings surface and rotates it into the cuttings layer at a relatively fast speed of about 20-30 r / min. Due to the loose cuttings, each drilling cycle is extremely fast. After the drill string is pulled up, the cuttings are quickly unloaded using the drilling rig's soil-throwing function.

[0084] Actual test data shows that using a spiral drill bit to process a 5m thick slag layer requires only about 1.5 minutes for a single slag removal cycle. The total slag removal time for a 3m section is significantly reduced from over 15 hours using the original double-bottom sand scoop process to 7-8 hours. The spiral drill bit has a simple structure, no moving parts, and strong wear resistance, effectively avoiding mechanical failures of complex drill bits under heavy loads of thick slag.

[0085] When the amount of cuttings carried by the auger bit decreases significantly after multiple drilling operations, or when the thickness of residual cuttings at the bottom of the borehole is less than 0.5m, the secondary cutting process should be initiated. More specifically, the rotary drilling rig bit should be replaced with a toothless, double-bottom sand-removing bit. At this point, a flat-bottomed bit without cutting teeth must be removed or replaced. This is understandable, as the flat scraper at the bottom of the bit is used for final leveling and scavenging of the trench bottom. The toothless design is to adapt to the uneven, slightly weathered rock surface of the trench bottom, preventing alloy cutting teeth from getting stuck in bedrock fissures or in the crescent-shaped corners of the trench wall, thus avoiding tooth breakage accidents.

[0086] During the process, after the drill bit reaches the bottom of the hole, it rotates slowly, using its own weight and slight pressure to collect the last remaining sediment into the bucket, and the bottom of the bucket is then leveled. This method ensures that the thickness of the sediment at the bottom of the hole is strictly controlled within 10cm, meeting the requirements for end bearing and seepage prevention, while also guaranteeing the safety of the cleaning process.

[0087] In particular, it can be seen that by combining the above-mentioned roughing and refining processes and spiraling and flat-bottoming processes, this embodiment solves the industry problem of low slag removal efficiency in deep slag layers while ensuring that the slag removal quality is ≤10cm. The overall slag removal efficiency is improved by more than 50%, and the drill bit wear rate is reduced by 70%.

[0088] Preferably, the length of a single trench segment during trenching is set to 3m; in steps S2 and S4, the construction machinery mainly operates at the ends of the trench segments or in the backfilled areas to reduce lateral pressure and disturbance to the middle of the trench wall.

[0089] In this embodiment, considering the unique binary geological structure of the engineering site—a loose, fully weathered to strongly weathered rock in the upper part and a hard rock in the lower part—this invention imposes strict mechanical limitations on the division of trench sections and the space for mechanical operations in order to completely solve the problem of frequent top collapse of the trench walls during the construction of long trench sections. The specific implementation is as follows: The optimized definition of trench length abandons the 6m-8m long trench scheme adopted in traditional cutoff wall projects to pursue fewer joints, and locks the length of a single trench unit at 3m.

[0090] Field tests showed that in completely weathered schist / granite strata, the rock mass structure is loose, appearing as sand or fragments, with extremely poor self-stabilizing ability. When using 7.8m and 6m long trench sections for testing, due to the large exposed area of ​​the trench walls and the need for repeated mechanical reciprocating movements to cover the entire trench length, severe top wall collapses occurred in the 7.8m trench section and all four 6m trench sections, making single-stage trenching impossible.

[0091] In steps S2 (down-the-hole drilling) and S4 (rotary drilling and trench clearing), the movement and work locations of down-the-hole and rotary drilling rigs, which can weigh up to 80 tons, are strictly limited. Construction machinery is mainly parked at the ends of 3m trench sections or above adjacent excavated trench sections with concrete backfill support, to avoid the tracks of the machinery directly pressing on the top surface of the loose trench wall in the middle of the currently excavated trench section.

[0092] Because trench wall collapses mostly occur in the middle of the trench section, where the stress is most unfavorable, by controlling the machinery at the end and utilizing the working radius of the rotary drilling rig's telescopic drill rod, the entire 3m trench section can be cleared and repaired without moving the chassis. This eliminates the ground overload and vibration "liquefaction" effect exerted by heavy machinery on the most vulnerable middle section of the trench wall. Furthermore, the 3m short trench section allows the machinery to cover the entire borehole without frequent repositioning on narrow access roads, significantly reducing the shear damage to loose strata caused by tracked compaction.

[0093] Compared to the high collapse rates of the 7.8m and 6m trench sections, the use of 3m trench sections combined with the end-point operation mode resulted in a 100% first-pass trenching success rate for the subsequent 16 test trench sections, with no instances of borehole collapse. This size also matched the XR240 / XR150 rotary drilling rigs on site, ensuring both operational coverage and minimizing disturbance to the surrounding rock.

[0094] Preferably, in step S6, the water-filled grouting adopts a composite process, which specifically includes: using a conduit method to grout concrete, injecting clean water to a depth of not less than 3.0m into the trench as a buffer medium before grouting; and before the concrete at the top of the wall is poured and initially set, performing immersion vibration on the concrete within a 3m range from the top of the wall downwards to eliminate air holes and improve density.

[0095] In step S6, considering the conditions of dry drilling in whole-rock formations resulting in no or little water in the trench and large drop in depth, this embodiment does not use the conventional dry hole grouting or standard mud grouting process, but instead adopts a composite grouting process, which is implemented as follows: In this embodiment, the trench depth generally exceeds 15m. Due to the use of down-the-hole drills for dry drilling, there is no wall-protecting mud in the trench and groundwater is scarce. If concrete is poured directly into the dry trench, the concrete mixture will fall from a height under gravity, and the coarse and fine aggregates will easily separate, resulting in severe honeycomb formation at the bottom of the wall, which will weaken the seepage prevention performance of the cutoff wall to a certain extent.

[0096] Underwater concrete is typically a highly fluid, self-compacting concrete, whose density relies primarily on the self-weight of the upper concrete column for compaction. However, in the final 3-5m section of the wall top, the lack of sufficient upper self-weight pressure, coupled with the accumulation of laitance, easily leads to a loose zone with numerous pores and low strength.

[0097] Before implementation, clean water is injected into the trench using a water truck or water pump before or after the injection conduit is installed, and the water injection depth is strictly controlled to be no less than 3.0m.

[0098] The injection of clean water acts as a hydraulic buffer. When the first batch of concrete flows out of the duct, it no longer directly impacts the hard rock bottom of the trough, but instead enters the fluid medium. The buoyancy and viscous resistance of the water effectively dissipate the kinetic energy of the falling concrete, preventing the coarse aggregate from splashing and segregating. At the same time, this artificially transforms the dry-hole operation environment into a standard underwater concrete pouring environment, ensuring process consistency.

[0099] Next, a sealed conduit with a diameter of φ250mm is used for grouting. After the initial grouting, ensure that the bottom of the casing is buried in the concrete to a depth of not less than 0.8m, and maintain a burial depth of 2m-6m in subsequent processes. Control the concrete surface rising speed to be greater than 2m / h, and use the pressure difference of the concrete column inside the conduit to continuously lift the clean water and sediment at the bottom of the trench to the trench opening for discharge.

[0100] Furthermore, when the concrete is poured to the designed top elevation of the wall and is still in a plastic state (before initial setting), the reinforcement process should be initiated immediately. Understandably, construction workers use an immersion high-frequency vibrator to manually vibrate the concrete within a 3m depth range extending downwards from the top surface of the wall.

[0101] For the low-pressure area at the top of the wall, the excitation force provided by mechanical vibration forces the concrete aggregate to rearrange, destroys the flocculated structure formed inside and forcibly discharges the air bubbles accumulated at the top; thereby promoting the cement paste to float up and fill the gaps between the aggregates, and can also further eliminate the strength stratification caused by excessive laitance layer.

[0102] Core sampling tests conducted on-site revealed that after adopting this reinforcement process, the concrete core sample at the top of the wall had a smooth surface, uniform aggregate distribution, no air holes or loose interlayers, and compressive strength consistent with that of the middle and lower parts of the wall, effectively solving the common problem of weak top sections in seepage interception walls.

[0103] Preferably, the method further includes a connection step between adjacent trench sections: using a drilling and casing method; after the concrete of the first-stage trench section is poured and reaches final setting, a drilling rig is used to perform full-hole casing drilling at both ends of the first-stage trench section to form joint holes; the casing drilling time is controlled as follows: when using a rotary drilling rig, it is carried out 48 hours after the concrete of the first-stage trench is poured; when using a down-the-hole drilling rig, it is carried out 96 hours after the concrete of the first-stage trench is poured.

[0104] To address the challenges posed by the high hardness of the all-rock strata, the difficulty in extracting traditional joint pipes, and the excessive cost of milling, this embodiment employs a drilling and sleeve-driving method to connect adjacent trench sections. Time-based control and cleaning standards have been established, and the specific implementation is as follows: Using a circular drill bit, such as a down-the-hole drill or a rotary drill, the concrete at the end of the completed first-stage trench section is partially cut to create an arc-shaped groove with a fresh concrete cross-section. When the second-stage trench section is poured, the new concrete fills this groove, forming a convex-concave mechanical interlocking structure that blocks the seepage path.

[0105] To prevent drilling vibrations from damaging the internal structure of the newly poured concrete in the first-stage trench section and causing micro-cracks, drilling must be carried out 48 hours after the first-stage trench concrete is poured.

[0106] If a down-the-hole drill is used for casing drilling, due to the significant impact and vibration, it must be carried out 96 hours after the first-stage trench concrete pouring is completed. Additionally, before pouring the second-stage trench concrete, the end face of the joint hole should be repeatedly scrubbed using a special wire brush drill bit or a high-pressure water gun. Continuous flushing should continue until the cleaning water returning from the bottom of the hole is clear and free of obvious silt or mud. This step aims to remove the mud film and rock debris adhering to the old concrete surface, exposing the rough concrete aggregate surface and ensuring the microscopic bonding between the old and new concrete.

[0107] Subsequent cross-hole geophysical tests and core drilling at the joint of the test section showed that the core samples at the joint were continuous and intact, the old and new concrete were tightly bonded, and no obvious mud layer or cold joint was found.

[0108] Permeability tests conducted on core samples showed that the permeability coefficient at the joints was on the same order of magnitude as that of the wall itself (K≤1×10⁻⁶). -7 (cm / s), with no seepage channels, verifying the high reliability of this drilling and casing technique in the construction of anti-seepage walls in hard rock formations.

[0109] Preferably, the guide wall structure in step S1 adopts The reinforced concrete structure has a panel thickness of 25cm, a sidewall thickness of 15cm, and a guide wall net distance of 73cm. When encountering loose soil layers in the guide wall trench excavation area, the excavation depth must penetrate the loose layer and enter the stable stratum by at least 30cm.

[0110] Specifically, in step S1, in order to construct a stable heavy machinery operation platform on the loose overburden layer and undulating terrain of the tailings silo, this embodiment abandons the traditional rectangular shallow guide wall design and adopts an inverted L-shaped or 7-shaped dynamic rooted guide wall, as implemented below: The structure employs an inverted L-shaped or 7-shaped, wing-plate reinforced concrete structure. This structure consists of horizontal wing plates and vertical sidewalls. The horizontal wing plates are selected as the load-bearing layer: designed with a width of 1.2m and a thickness increased to 25cm. The vertical sidewalls serve as retaining layers: with a thickness of 15cm and an internal clear distance of 73cm (slightly larger than the trench width, reserving space for casing installation). The concrete strength grade is no less than C30; a double-layer, bidirectional steel mesh is internally configured. Given that the XR150DV down-the-hole drilling rig and XR240E rotary drilling rig to be brought in later both weigh over 80 tons and will generate severe impact and vibration during operation, the wide and thickened horizontal wing plates act as a raft foundation, evenly distributing the huge point and dynamic loads across a larger area of ​​soil, preventing uneven settlement or breakage of the guide wall during drilling. The guide wall height must not be mechanically set to a fixed value (such as the conventional 1.0m). During construction, dynamic adjustments must be made based on the actual geological conditions revealed during the excavation of the guide wall trench. Specific criteria are as follows: the bottom of the guide wall must penetrate the surface loose soil layer, including completely weathered rock and tailings fill layers, and embed at least 30cm into the underlying relatively stable undisturbed soil layer of strongly weathered or moderately weathered rock. If the thickness of the loose layer exceeds the conventional guide wall depth, such as >2m or even 5m, the guide wall should not be deepened indefinitely. Instead, pre-grouting reinforcement should be carried out on the base area of ​​the guide wall first, and the guide wall construction should only proceed after the soil strength meets the self-stabilization requirements.

[0111] The specific construction process is as follows: Surveying and Setting Out: Based on the axis of the cutoff wall, the excavation line of the guide wall trench is set out. Trench Excavation: The excavator excavates layer by layer until a stable bearing layer that meets the bearing capacity requirements is exposed, which is then confirmed as the bottom elevation of the guide wall. Formwork Erection and Casting: The reinforcing cage is tied, an inverted L-shaped formwork is erected, and the concrete is poured as a whole. Backfilling and Platform Construction: After the guide wall concrete reaches the design strength, earth and rock are backfilled on the outside of the guide wall and above the wing plate and compacted. The backfill height is flush with the top surface of the wing plate, forming a wide and horizontal heavy machinery operating platform.

[0112] The aforementioned techniques, such as the 30cm root-planting into the rock, ensured that the guide wall interlocking section completely sealed the contact surface between the loose layer and the trench, thoroughly cutting off the path for surface water infiltration and loose soil particle detachment. This resulted in a zero collapse rate at the borehole opening during the subsequent 3m trench construction. The thickened wing plate structure successfully withstood months of frequent down-the-hole drilling operations without cracking or overturning, providing a physical benchmark for high-precision trenching.

[0113] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A trenching construction method for a cutoff wall of a tailings silo in whole-rock strata, characterized in that, The method is applicable to completely weathered to slightly weathered rock strata and strata with developed quartz veins, and includes the following steps: S1 Guide Wall Construction: A reinforced concrete guide wall is constructed at the axis position of the cutoff wall. The guide wall is dynamically adjusted based on the terrain and the thickness of the loose surface layer, serving as a drilling rig operation platform. S2 Interlocking Main Hole Drilling: A large-diameter down-the-hole drill is used to carry out interlocking main hole drilling without partition walls within the guide wall; the interlocking main hole drilling refers to drilling continuously without pre-reserving rock and soil partition walls between two adjacent main holes, maintaining a predetermined amount of interlocking, so that the main holes in the trench section are completed in one go. S3 Circulating Casing Support: During the down-the-hole drilling process in step S2, multiple long casings are used in conjunction with the drilling operation; the length of the long casing is greater than the drilling depth of a single hole, and it is used in a cyclical manner between each hole in the trench section to create a relatively "closed" working space to suppress air leakage and control hole inclination; S4 Drill Bit Groove Repair: After the main hole is drilled in a chain, a rotary drilling rig equipped with a hydraulic square drill bit is used to cut and repair the crescent-shaped rock mass remaining on both sides of the groove wall to form a smooth groove wall. S5 Phased Trench Cleaning: The trench cleaning operation is divided into two phases: rough cleaning and fine cleaning. In the rough cleaning phase, a rotary drilling rig equipped with a spiral drill bit is used to clean the trench in multiple stages and layers. In the fine cleaning phase, a rotary drilling rig is used in conjunction with a sand-removing drill bit before the trench inspection. S6 Wall Concrete Pouring: After the hole cleaning is inspected and approved, the wall concrete is poured using the clear water underwater pouring process, and the top area of ​​the wall is subjected to additional vibration.

2. The trenching construction method for a tailings dam cutoff wall in whole-rock strata according to claim 1, characterized in that, In step S2, the engagement amount between adjacent main holes is controlled to be no less than 7 cm.

3. The trenching construction method for a tailings dam cutoff wall in whole-rock strata according to claim 1, characterized in that, In step S3, the long casing is a seamless steel pipe, and its length exceeds the designed hole depth by 50cm to 100cm. The specific operation of the circulating casing support is as follows: three long casings are configured and placed into the current borehole and adjacent completed holes respectively. After each main hole is drilled, the last casing is pulled out using a drilling and pulling machine, transferred and placed into the newly opened hole position, and the cycle is followed up with the drilling progress.

4. The trenching construction method for a tailings silo cutoff wall in whole-rock strata according to claim 1, characterized in that, In step S5, the specific process of layered slag removal is as follows: a rotary drilling rig equipped with a spiral drill bit is used to perform preliminary cleaning of the thick layer of drilling slag in the trench in stages and layers; after the thick layer of drilling slag is cleaned, a toothless double-bottom sand-removing drill bit is used to perform fine cleaning of the residual sediment at the bottom of the trench until the thickness of the sediment at the bottom of the hole meets the design requirements.

5. The trenching construction method for a tailings silo cutoff wall in whole-rock strata according to claim 1, characterized in that, The length of a single trench segment in the trenching construction is set to 3m; in steps S2 and S4, the construction machinery mainly operates at the ends of the trench segments or in the backfilled areas.

6. The trenching construction method for a tailings dam cutoff wall in whole-rock strata according to claim 1, characterized in that, In step S6, the underwater grouting process includes: grouting concrete using a guide pipe method; after the underwater grouting of the wall concrete is completed and the laitance on the top of the wall is removed, and before initial setting, immersion vibration is applied to the concrete within a 3m range from the top of the wall downwards.

7. The trenching construction method for a tailings dam cutoff wall in whole-rock strata according to claim 1, characterized in that, The method also includes a connection step between adjacent trench sections: using a drilling and casing method; after the concrete of the first-stage trench section is poured and reaches final set, a drilling rig is used to perform full-hole casing drilling at both ends of the first-stage trench section to form joint holes; the casing drilling time is controlled as follows: when using a rotary drilling rig, it is carried out 48 hours after the concrete of the first-stage trench is poured; when using a down-the-hole drilling rig, it is carried out 96 hours after the concrete of the first-stage trench is poured.

8. The trenching construction method for a tailings silo cutoff wall in whole-rock strata according to claim 1, characterized in that, The guide wall structure described in step S1 adopts " The reinforced concrete structure is shaped like a "25cm thick panel, 15cm thick sidewall, and 73cm clear distance within the guide wall. When encountering loose soil layers in the guide wall trench excavation area, the excavation depth must penetrate the loose layer and enter the stable stratum by at least 30cm.

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