Raise boring technology is widely applied in underground mine shaft excavation, tunnel pilot hole construction, and underground engineering vertical and inclined hole drilling, featuring high efficiency, low disturbance, and safe operation. The raise boring drill rod serves as the core load-bearing and force-transmitting component of the raise boring machine, undertaking complex alternating loads such as axial tension-compression, torsion, bending, and impact during operation. Drill rod fracture is one of the most frequent and destructive failure forms in raise boring construction, which directly leads to construction shutdown, hole abandonment, and even major safety accidents and economic losses. To clarify the essential mechanism of raise boring drill rod fracture, this paper systematically analyzes the fracture failure characteristics of drill rods from four dimensions: mechanical load characteristics, structural design and manufacturing defects, field operation and process parameters, and geological and environmental corrosion factors. Combined with engineering failure cases and mechanical mechanism analysis, the internal correlation between different influencing factors and drill rod fatigue fracture, brittle fracture, and thread connection fracture is revealed. Meanwhile, targeted prevention and control measures are proposed to provide theoretical support and technical reference for improving the service life of raise boring drill rods and reducing construction failure risks.
Raise boring drilling technology has become the mainstream construction method for vertical and inclined raise engineering in underground mining, hydropower tunnels, and urban underground space engineering due to its advantages of full mechanization, no blasting disturbance, and high forming quality. As the key connecting component between the raise boring machine host and the drilling tool, the raise boring drill rod is responsible for transmitting rotational torque, axial feed pressure and lifting tension, and bears comprehensive dynamic loads throughout the drilling process. Different from conventional horizontal drilling rods, raise boring drill rods operate in a long-distance vertical or inclined hole environment, with poor hole wall stability, complex stress state, and severe dynamic load fluctuation, resulting in a much higher failure rate than ordinary drilling tools.
Field engineering statistics show that drill rod fracture failure accounts for more than 60% of all raise boring equipment failures, among which thread connection fracture and rod body fatigue fracture are the dominant failure modes. Most fracture failures are not caused by sudden single load overload, but by progressive damage accumulation under long-term complex alternating loads, accompanied by the coupling effect of structural defects, material performance attenuation and environmental corrosion. At present, most domestic and foreign studies on drill rod failure focus on conventional geological drilling and oil drilling tools, while targeted systematic research on the fracture mechanism of raise boring drill rods under special vertical raise working conditions is relatively insufficient, and the understanding of the coupling failure law of multiple factors is not in-depth enough.
Based on the working principle and load characteristics of raise boring drill rods, this paper comprehensively analyzes the macroscopic and microscopic causes of drill rod fracture failure, expounds the evolution process of crack initiation, expansion and final fracture, and summarizes the key control points in design, manufacturing, operation and maintenance, so as to provide practical guidance for engineering application and fault prevention of raise boring drill rods.
The raise boring drill rod is a hollow cylindrical steel structural part with threaded joints at both ends, which is connected end to end to form a complete drill string. According to the construction process, raise boring is divided into pilot hole drilling and reaming drilling two stages. In the pilot hole stage, the drill string drives the small-diameter pilot bit to drill downward, bearing axial pressure and rotational torque; in the reaming stage, the reaming cutter head is lifted upward through the drill string, and the drill rod mainly bears axial tensile load, torsional shear load and periodic impact load generated by rock breaking.
The drill string system is in a suspended state in the long and narrow borehole for a long time. Affected by borehole deviation, rock stratum fluctuation and equipment vibration, the drill rod cannot maintain an ideal linear stress state. Local bending and eccentric load will inevitably occur in the operation process, forming a complex multi-load coupling stress field, which is the fundamental structural condition for drill rod fracture failure.
Different from single-load mechanical parts, raise boring drill rods bear multiple alternating dynamic loads throughout the whole working cycle, and the load characteristics directly determine the failure form of the drill rod:
First, alternating tension and compression load. In the pilot hole drilling stage, the drill rod is pressed downward to bear axial compressive stress; in the reaming lifting stage, it bears continuous axial tensile stress. The frequent switching of tension and compression forms alternating stress, which is easy to induce fatigue damage of rod body materials. Especially in deep hole construction, the self-weight of the long drill string will produce superposed tensile stress at the root drill rod, resulting in local stress overload.
Second, periodic torsional shear load. The rotational torque of the host machine is transmitted step by step through the drill rod thread joints. In the process of rock breaking, the uneven hardness of rock mass causes instantaneous fluctuation of drilling resistance, resulting in periodic torsional vibration of the drill string. The torsional shear stress is concentrated at the thread tooth root and the rod body transition section, which is the main cause of thread shear fracture.
Third, bending and eccentric load. Borehole deviation is unavoidable in raise boring construction. When the drill string is deflected, the drill rod is squeezed by the hole wall, resulting in bending deformation and additional bending stress. Even a small deflection angle of 1-2° will produce significant cyclic bending load during high-speed rotation, leading to fatigue crack initiation at the bending stress concentration position.
Fourth, instantaneous impact load. When the drill bit encounters hard rock, broken rock mass or fault fracture zone during drilling, the drilling resistance changes abruptly, resulting in instantaneous impact torque and impact pressure. The impact load far exceeds the design allowable load of the drill rod in a short time, causing instantaneous plastic deformation or crack expansion of the rod body.
Through statistical analysis of field failure cases of raise boring drill rods, the fracture failure modes are mainly divided into three categories: thread connection fracture, rod body fatigue fracture, and impact brittle fracture. Each failure mode has distinct macroscopic morphological characteristics, corresponding to different failure mechanisms and inducing factors.
Thread fracture is the most common failure mode of raise boring drill rods, accounting for more than 70% of total fracture failures. The fracture position is mostly concentrated at the thread tooth root and the transition shoulder of the drill rod joint. Macroscopically, the fracture section is flat, with obvious shear slip marks and thread tooth wear and galling phenomena. In the early stage of failure, thread adhesion and tooth surface peeling occur, and with the accumulation of damage, local tooth root cracks expand circumferentially, eventually leading to integral thread shear fracture.
This type of failure is mainly caused by stress concentration at the thread structure and poor matching accuracy of threaded joints. The thread tooth root belongs to the structural mutation position, which is prone to stress concentration under torsional and tensile loads. In addition, thread surface abrasion, insufficient thread lubrication and mixed assembly of old and new drill rods will aggravate the uneven stress of the thread joint and accelerate fracture failure.
Rod body fatigue fracture mostly occurs in the middle section of the drill rod or the wall thickness transition zone, with no obvious macroscopic deformation before fracture, belonging to typical brittle fatigue failure. The fracture section presents obvious zoning characteristics, including fatigue source zone, fatigue crack expansion zone and instantaneous fracture zone. The fatigue source is mostly located at the surface scratches, corrosion pits and micro-defects of the rod body. Under the action of long-term alternating load, micro-cracks initiate and expand radially, and when the effective bearing section of the drill rod is reduced to the critical value, instantaneous fracture occurs.
Fatigue fracture is a progressive damage process, which is the main failure form of drill rods in long-term service. It is mostly induced by the coupling of alternating dynamic load, material fatigue performance attenuation and surface micro-defects, and has strong concealment and cumulative characteristics.
Impact brittle fracture mostly occurs in the process of drilling through hard rock strata or sudden jamming of the drill bit. The macroscopic characteristic is that the fracture section is rough and irregular, with obvious impact fracture traces, and the rod body may have local deformation and crack bifurcation. This failure mode occurs suddenly without obvious early warning, and the fracture speed is fast, which is easy to cause drill string falling into the hole and major construction accidents.
The fundamental cause of impact fracture is that the instantaneous impact load exceeds the ultimate bearing capacity of the drill rod material. Unreasonable construction parameter setting, sudden change of geological conditions and improper operation are the main external inducing factors.
Structural design defects and manufacturing process errors are the inherent root causes of drill rod fracture failure. In terms of structural design, the thread transition fillet, wall thickness transition zone and joint matching structure of some raise boring drill rods have unreasonable design problems. The too-small thread root fillet leads to serious stress concentration effect; the abrupt change of rod body wall thickness causes uneven stress distribution in the transition section, which becomes the weak link of load bearing. In addition, the excessive length-diameter ratio of the drill rod increases the flexibility of the drill string, aggravates bending deformation and vibration in operation, and further increases the risk of fatigue fracture.
In terms of manufacturing technology, the quality fluctuation of drill rod steel materials and processing defects directly affect the service performance. The ununiform internal grain structure, excessive inclusion content and residual casting forging stress of the steel will reduce the fatigue strength and impact toughness of the material. In the thread processing process, tool marks, burrs and uneven tooth surface roughness formed by precision errors will form surface micro-defects, which become the initiation source of fatigue cracks. Moreover, improper heat treatment process will lead to inconsistent hardness and toughness of the drill rod, local over-hardening will cause brittle fracture, and insufficient hardness will cause thread wear and deformation failure.
Unstandardized field operation and unreasonable drilling parameter matching are the main external inducements of drill rod fracture. In the drilling process, excessive feed pressure and rotational speed are the most common operational problems. When the feed pressure is too high, the axial load of the drill rod exceeds the design range, and the rod body is in a long-term over-pressure state, resulting in plastic deformation and crack initiation. Excessively high rotational speed will intensify the torsional vibration and lateral swing of the drill string, increase the alternating bending stress of the drill rod, and accelerate fatigue damage accumulation.
Improper operation such as forced drilling through hard strata and blind lifting when the drill bit is stuck will produce instantaneous overload impact load. When the drill bit encounters hard rock or fault fracture zone, the drilling resistance increases sharply, and the operator fails to adjust the parameters in time, resulting in sudden increase of drill rod torque and axial load, which easily causes thread shear fracture and rod body impact fracture. In addition, irregular assembly operation, such as insufficient thread tightening torque and mixed use of new and old worn drill rods, will lead to loose connection and uneven stress of the drill string, causing local stress concentration and accelerated failure.
Long-term overload operation and lack of daily maintenance will also aggravate drill rod damage. The residual rock slag and sediment on the thread surface will wear the thread tooth surface during operation, resulting in reduced connection precision and increased load fluctuation. The lack of special thread lubricant will cause thread galling and adhesive wear, further reducing the connection strength of the drill rod joint.
The complex underground geological environment is an important factor inducing drill rod fracture failure. Underground boreholes are often filled with humid air, mine water and corrosive mediums such as sulfide and carbonate. The raise boring drill rod made of carbon alloy steel is prone to electrochemical corrosion in humid and corrosive environments, forming corrosion pits on the rod body and thread surface. These corrosion pits destroy the integrity of the material surface structure, produce local stress concentration under load action, and become the priority initiation position of fatigue cracks.
In the strata containing hydrogen sulfide and high-salinity mine water, the drill rod is prone to sulfide stress corrosion and hydrogen embrittlement failure. Corrosion will continuously erode the internal structure of the material, reduce the mechanical properties such as tensile strength and fatigue toughness of the drill rod, and make the drill rod prone to brittle fracture under lower load than the design allowable value. In addition, the unstable rock stratum and broken hole wall will cause continuous extrusion and friction on the drill string, resulting in surface wear and scratch damage of the drill rod, which further cooperates with corrosion and alternating load to accelerate fracture failure.
The working state of the raise boring machine host and the vibration characteristics of the drill string also have a crucial impact on drill rod fracture. The wear of the host guide rail, the deviation of the rotating head and the failure of the centralizer will cause the drill string to be misaligned, resulting in continuous bending deformation of the drill rod in the rotating process. Even tiny assembly misalignment will produce cyclic alternating bending stress in the whole working cycle, leading to cumulative fatigue damage of the rod body.
Drill string vibration includes axial vibration, torsional vibration and lateral swing vibration. Under the coupling effect of multiple vibrations, the dynamic load of the drill rod fluctuates violently, and the instantaneous peak load is far higher than the static design load. Resonance will occur when the vibration frequency of the drill string is close to the natural frequency of the drill rod, resulting in instantaneous sharp increase of stress, which easily causes rapid expansion of existing micro-cracks and sudden fracture of the drill rod.
Optimize the structural design of drill rod joints, appropriately increase the thread root transition fillet, eliminate structural sudden change, reduce stress concentration effect, and improve the uniform stress distribution of threaded joints and rod body transition sections. Select high-strength and high-toughness alloy steel materials suitable for raise boring working conditions, strictly control the content of internal inclusions and grain fineness of materials, and improve the comprehensive mechanical properties of drill rods. Optimize the thread processing technology, improve the machining precision of thread tooth surface and transition zone, eliminate surface tool marks and burrs, and reduce structural micro-defects.
Standardize the heat treatment process of drill rods, adopt integral quenching and tempering + local thread strengthening treatment to ensure matching hardness and toughness of rod body and thread parts, avoid local brittleness and excessive wear, and improve the fatigue resistance and impact resistance of drill rods. Strictly implement factory quality inspection, carry out nondestructive testing on drill rod bodies and threaded joints to eliminate defective products with internal cracks and residual stress overload.
Formulate scientific drilling parameter matching standards according to different geological conditions. For hard rock strata, appropriately reduce rotational speed and feed pressure to avoid impact overload caused by excessive drilling resistance; for broken and soft rock strata, reduce drilling speed and strengthen hole wall protection to avoid drill string jamming and deflection. Real-time monitor drilling torque, axial pressure and vibration parameters during construction, and adjust working parameters dynamically to keep the drill rod working within the rated load range.
Standardize field assembly and operation processes, use special thread lubricant for assembly to ensure full thread fitting and stable connection torque, and prohibit mixed assembly of new and severely worn drill rods. It is forbidden to carry out forced drilling and impact operation when the drill bit is stuck or encounters hard strata to avoid instantaneous impact load damage. Arrange special personnel to inspect the drill string state regularly during construction, and stop the machine in time for inspection and replacement when abnormal vibration and torque fluctuation are found.
Establish a full-cycle maintenance system for raise boring drill rods. After each construction operation, clean the rock slag and sediment on the drill rod surface and thread teeth thoroughly, check for surface scratches, corrosion pits and thread wear, and carry out maintenance and repair in time. Regularly detect the fatigue damage and residual stress of drill rods by nondestructive testing technology, and eliminate old and defective drill rods regularly to avoid cumulative damage failure.
Strengthen corrosion protection of drill rods in humid and corrosive geological environments. Apply special anti-corrosion coating on the surface of drill rods, use anti-corrosion thread lubricant, and regularly replace the protective layer. For working environments with high salinity and sulfide content in mine water, adopt corrosion-resistant drill rod materials or carry out surface strengthening anti-corrosion treatment to reduce electrochemical corrosion and stress corrosion damage. Keep the drill rod storage environment dry and ventilated to avoid long-term idle corrosion.
Regularly maintain and calibrate the raise boring machine host, check the wear state of the guide rail, rotating head and centralizer, eliminate equipment assembly deviation and operation clearance, ensure the linearity and stability of drill string operation, and reduce bending stress caused by misalignment. Install vibration monitoring and damping devices on the drill string to suppress axial, torsional and lateral vibration of the drill rod, avoid resonance phenomenon, and reduce dynamic load fluctuation and fatigue damage.
Optimize the drill string assembly scheme, reasonably match the number and specification of drill rods according to the drilling depth and hole inclination, reduce the self-weight load and flexibility of the long drill string, and improve the overall rigidity and stability of the drill string system.
The fracture failure of raise boring drill rod is the result of the coupling of inherent structural defects, dynamic load characteristics, field operation errors and geological environmental factors. Thread stress concentration and fatigue crack propagation under complex alternating loads are the core mechanisms of most drill rod fracture failures, while unreasonable construction parameters, equipment vibration and environmental corrosion are the key inducing and accelerating factors. Different from ordinary drilling rods, raise boring drill rods bear the composite load of tension, compression, torsion and bending in vertical long-hole working conditions, with more complex stress state and more prominent failure characteristics.
To effectively solve the problem of drill rod fracture failure, it is necessary to establish a full-process prevention and control system covering design and manufacturing, field construction, daily maintenance and equipment operation. By optimizing drill rod structural design and manufacturing quality, standardizing drilling parameter matching and operation behavior, strengthening corrosion protection and vibration control, the cumulative damage and instantaneous overload failure of raise boring drill rods can be fundamentally reduced, the service life of drilling tools can be prolonged, and the safety and efficiency of raise boring engineering construction can be guaranteed. In the future, finite element numerical simulation and field real-time monitoring technology can be further combined to explore the dynamic damage evolution law of drill rods under multi-factor coupling conditions, so as to provide more accurate and intelligent technical support for drill rod failure prediction and prevention.
