Rock Breaking Mechanism Research Articles

Study on Optimization of Delay Method of Wedge Cut Blasting in Tunnel

Relying on the entrance section of a high-speed railway tunnel blasting project, the fluid-solid coupling algorithm based on ANSYS/LS-DYNA was used to optimize the parameters of wedge cut blasting, and the vibration could be reduced on the basis of ensuring the blasting effect. Through the combination of visual numerical simulation results and rock-breaking mechanism of wedge cut blasting, the maximum vibration velocity of different monitoring points in the model under different segmented time delay was analyzed. The results show that the best method for detonation is dividing the blastholes into three segments from upper to lower and dividing the left and right symmetrical blastholes into one segment. When the delay time is 10 ms, the average vibration reduction ratio is the best, which is reduced by 18% compared with the six-hole simultaneous blasting. In addition, the actual surrounding rock stress has a clamping effect on the cut blasting area. The wedge cut blasting footage obtained by numerical simulation was basically consistent with the field results, which proved that the model is reasonable and effective. This study intuitively and accurately demonstrated the process of cut blasting, the superposition curve of vibration velocity and the vibration reduction results under different delay times, and the effect of cut blasting. The results can be directly applied to similar projects, and the optimal blasting parameters and related issues can be solved more accurately with the help of this engineering analysis method.

Explosion wave and crack field of an eccentric decoupled charge.

To investigate the characteristics of the explosion damage from an eccentric decoupled charge, the rock-breaking mechanism of an eccentric decoupled charge is revealed from the perspectives of the explosion wave field and crack field through theoretical and experimental analyses. The ratio of the maximum to minimum pressure on an eccentric decoupled hole wall increases exponentially with an increase in the decoupling coefficient, but it does not change with a change in the explosive density or explosive detonation velocity. The explosive energy on the eccentric charge side has a certain accumulation effect, the velocity of the reflected shock wave at the blasthole wall is greater than that of the incident shock wave, and the incident velocity of the explosion wave on the eccentric side is greater than that on the non-eccentric side. The expansion range of the explosion gas is significantly better than that on the non-eccentric side, indicating that the eccentric decoupled charge can strengthen the action of the explosion gas product on the eccentric side. In addition, the overpressure of the explosion wave on the eccentric side is greater than that on the non-eccentric side. The fractal dimension and damage degree of the explosion crack on the eccentric side are larger than those on the non-eccentric side. The explosion cracks can be divided into intensive, transition, and sparse areas, and the fractal dimension and damage degree decrease successively in the three areas. The explosion cracks formed on the non-eccentric side outside the range of three times the blasthole diameter concentrate in the sparse area, and the destructive effect of the explosion on the non-eccentric side medium is effectively reduced.

Rock breaking mechanism during TBM tunnelling: insight from DEM simulations of rotary cutting tests

Tunnel boring machine (TBM) has been widely applied to various tunnel construction projects in hard rocks due to its high efficiency and safety. A number of theoretical or semi-empirical models have been proposed to predict TBM performance under different geological conditions. Cutter forces in these theoretical models are derived mainly based on idealized assumptions, related to the cutting pattern and rock breaking mechanism. However, there is no consensus on the rock breaking as well as the cutter-rock interaction mechanisms. In this study, DEM simulations of the rotary cutting tests are conducted using PFC3D. The cutter-rock contacting behaviour is monitored and the cracking events during the cutting process are recorded. The variation of cutting forces is found to be linked to both the occurrence rate and the total number of cracks. The spatial orientation of cracks is also examined, which seems to vary from horizontal-dominant to vertical-dominant with an overall isotropic pattern. Lastly, the distribution of contact forces between the disc cutter and the rock sample is analysed and its potential implication on the development of TBM performance model is discussed.

Drillability characteristics of sandstone with different pores under simulated bottom hole conditions

Rock drillability has an important application value in oil and gas well engineering. Accurate evaluation of rock drillability characteristics is the theoretical basis of improving the rate of penetration (ROP) in the drilling process of the deep well. Up to date, few scholars have studied the degree of rock break difficulty according to the rock’s pore structure characteristics. In order to study the influence of pore structure characteristics on sandstone drillability under the condition of the bottom hole and realize the accurate prediction of sandstone drillability. Sandstones with different pore structures were collected as the research object. The pore structure characteristics of the collected sandstone were studied by casting thin section identification. The porosity, permeability, acoustic wave, and Richter hardness of sandstone with different pore structures were tested, and the sandstone drillability experiment was carried out under simulated bottom hole conditions. The relationship between the characteristics of pore structure and drillability was analyzed by statistical method. The experiment results show that sandstone’s pore structure characteristics have an important effect on the drillability of sandstone. The porosity, permeability, and acoustic wave of sandstone have a very high correlation with the Richter hardness. The drillability of sandstone decreases exponentially with the increase of porosity and permeability. With the increase of borehole pressure, sandstone’s drillability with different pore structures generally increases exponentially. When the porosity is less than 0.19, the drillability of rock decreases gradually with the increase of porosity. When the porosity is more than 0.19, the drillability of rock decreases significantly under different borehole pressures. The characteristics of pore structure are an important factor affecting the drillability of rock, which should be paid more attention to by related scholars. The study is of great significance for revealing the mechanism of rock breaking, objectively evaluating the characteristics of drillability, and establishing the prediction model of drillability.

Numerical Study on Free Face-Assisted Rock Fragmentation Induced by a TBM Disk Cutter

This paper presents a numerical study on the rock fragmentation process induced by a tunnel boring machine disk cutter under different free face conditions. A grain-based discrete element method is incorporated into the particle flow code to synthesize appropriate rock samples, considering macroscopic mechanical properties and fracturing mechanisms. The influences of free face structures (height H, spacing S, and direction) on rock breaking mode, normal force, and penetration specific energy are investigated via numerical simulations. Results show that the rock breaking force and PSE of the side free face (STFF) model are significantly lower than those of the top free face (TFF) model. Based on the rock fragmentation analyses in rock breaking mode, four critical values were obtained, which can be used as the basis to design free face structures considering the rock breaking performance. The normal force significantly improves with free face spacing while the PSE is primarily influenced by free face height and rock breaking mode. These findings may be useful to understand the rock breaking mechanism of a disk cutter under free face conditions and can be used as guidance for the design of free face structures.

Investigating the mechanism of rock fracturing induced by high-pressure gas blasting with a hybrid continuum-discontinuum method

To study the rock-breaking mechanism resulting from high-pressure gas blasting, a hybrid continuum-discontinuum method considering the actual breakage of rocks was presented in this study. The mechanical parameters were calibrated against the uniaxial compression test and Brazilian disc test, and the capabilities of simulating blast-induced fractures were verified by laboratory tests. After that, the differences in rock-breaking mechanism between explosive blasting and high-pressure gas blasting were compared from the perspectives of peak pressure and loading rate. The results indicated that the crack patterns, fracture mode and vibration attenuation induced by high-pressure gas blasting were significantly different from that of explosive blasting: (a) no crushing zone and fewer radial cracks were formed in high-pressure gas blasting and the spalling cracks were absent near the free face in high-pressure gas blasting; (b) the difference of rock fracture mode in high-pressure gas blasting and explosive blasting indicated that the tensile fracturing was the dominant fracture mode in high pressure gas blasting which is different from the catastrophic fragments due to compressive-sheared fracturing surrounding the borehole in explosive blasting; (c) the intensity and attenuation factors of peak particle velocity both increased with the increase of peak pressure or loading rate, which indicated that the peak particle velocity induced by explosive blasting results in more intense and faster attenuated vibration disasters than that of high-pressure gas blasting.

Study on Rock Breaking Mechanism and Field Test of Prismatic Cutter

The rock breaking mechanism and effect of prismatic cutter is not clear, which leads to errors in the application and design of prismatic cutter, and often fails to achieve the expected effect. Therefore, the finite element method is used to carry out numerical simulation analysis of prismatic cutter. The change law of cutting force in the process of rock breaking is analyzed. Rock breaking mechanism is studied. What’s more, single cutter straight-line scraping experiment and field application are carried out to verify correctness. The results of numerical simulation and experiment show that the stress state of rock is obviously different between prismatic cutter and conventional plane cutter in the process of rock breaking. Under the same conditions, the tangential force and the fluctuation of tangential force required by prismatic cutter in cutting sandstone are less than those of conventional plane cutter. The field test of PDC bit with prismatic cutter shows that it has high rock breaking efficiency and good impact resistance in specific formation. The results show that the prismatic cutter destroy the rock mainly through the action of tension and shear. The tangential force and torque are smaller. The impact resistance and wear resistance are higher, which can effectively improve the rock breaking efficiency and increase the service life of the bit.

INVESTIGATION ON THE ROCK CUTTING MECHANISM OF HETEROGENEOUS GRANITE USING A GRAIN-BASED MODELING APPROACH

A good understanding of rock cutting mechanism is of great importance for improving the cutting efficiency and reducing the costs in hard heterogeneous formation drilling. This paper implements the grain-based modeling approach to mimic the granite considering the highly heterogeneous mechanical behaviors of grains in granite. The rock cutting mechanism of granite, accounting for the influence of hydrostatic pressure, mineral size, random distribution, rake angle, cutting depth and velocity of cutter, are investigated. The research results demonstrate that the inter-grain shear crack and intra-grain tensile crack are the two main crack types generated during rock cutting process. The cutting force responses show strong vibration with respect to the cutting distance. The magnitude of cutting force sharply drops and abruptly rises during the cutting process, which becomes more obviously with the increasing hydrostatic pressure, cutting depth and rake angle of cutter. The hydrostatic pressure, grain size and random distribution of grains have great influence on chip morphology. In contrast, comparing with the above three factors, the cutting depth, rake angle of cutter and cutting velocity have little influence on the chip morphology. These results lead to an enhanced understanding of rock breaking mechanisms in granite, and provide a basis for improving the design of rock-breaking bits.

Numerical simulation study on rock-breaking process and mechanism of compound impact drilling

Improving the rock-breaking efficiency of hard rock formation has always been of concern in drilling. Compound impact drilling technology is a new method to achieve efficient crushing of hard rock by exerting axial and torsional impacts on the drilling bit. In this study, to investigate the dynamic rock-breaking process and mechanism under compound impact, a numerical model was developed based on cohesive elements. Dynamic impact tests on sandstone were conducted to verify the effectiveness of the method. Then, the dynamic rock-breaking process under compound impact was simulated using the proposed model. Finally, the effects of impact parameters and cyclic loading on the rock-breaking performance were systematically studied. The results indicate that the stereoscopic crushing effect of rock can be achieved under compound impact. The amount of broken rock increases with the increase in the impact load amplitude and impact loading times, whereas both the impact angle and duration have an optimal range. When the impact angle is 30°, the impact duration is 2.5 ms, the amount of broken rock is maximum. A novel compound impact drilling tool was designed based on the analysis of the rock-breaking mechanism, the laboratory and field tests were carried out and the parameters were optimized.

Numerical analysis for rock cutting force prediction in the tunnel boring process

Study of rock cutting force prediction is of great significance for improving the efficiency of rock breaking and optimizing the tunnel boring process. A new prediction model named RCC (the Roxborough Model and the CSM Model and the Colorado School of Mines’ Data Integration) model is proposed to predict vertical, rolling and lateral forces acting on the disc cutter based on the theoretical model developed by Roxborough and Phillips (1975) (Roxborough model) and the theoretical model developed by Rostami (1997) (CSM model). The suggested model is based on the improvement that the shape of extrusion surface is reasonably assumed to be trapezoid and the arc segment at the edge of the cutter is reasonably simplified to a straight-line segment. Results of the full-scale linear cutting experiments by the Colorado School of Mines are used to validate the accuracy and reliability of the suggested model, some additional calculation results from the linear cutting prediction formula developed by Ozdemir et al. (1978) (Linear Cutting model) and the CSM model are also used in this purpose. The results indicate that the formulas for calculating vertical force and rolling force presented in this paper are more accurate than the Linear Cutting model and the CSM model. Although the calculation results of the lateral rock breaking force conforms to the variation law of lateral rock breaking force with the penetration and cutter spacing, they are smaller than the experimental data. Finally, through theoretical analysis of the suggested RCC model and numerical simulation by using ANSYS/LS-DYNA, the rationality of the simplified disc cutter used in the RCC model is verified and the influence of penetration depth on the rock cutting force is obtained, which provides a theoretical reference for the study of rock breaking mechanism and optimization of tunneling process.

The rock breaking mechanism analysis of axial ultra-high frequency vibration assisted drilling by single PDC cutter

Axial ultra-high frequency vibration (UHFV)-assisted drilling technology that combines polycrystalline diamond compact bits and rotary ultrasonic machining technology is expected to improve rock-breaking efficiency and reduce the drilling costs of deep and ultra-deep wells. To better understand the rock-breaking mechanism under a UHFV load and provide a theoretical basis for practical engineering, this paper presents numerical modelling with the finite element method of dynamic cutting processes under different UHFV loads. The cutting force, mechanical specific energy (MSE), rock failure mode, and crack propagation process of a heterogeneous rock with natural cracks are analysed, and then compared with the steady load cutting condition. The simulation results indicate that when the excitation frequency is close to the natural frequency of the rock (approximately 25–30 kHz), the MSE and cutting force reach the minimum value. Compared with the steady load, the average cutting force and MSE under a 25 kHz UHFV load decreased by 49.4% and 28.5%, respectively. The number of cracks increased by 24.9%, while the crack volume and area reached the maximum. In addition, the shear crack ratio can be reduced under UHFV loads. The proportion of shear cracks in 25 kHz UHFV cutting was 5.3% lower than that in steady load cutting. Thus, axial UHFV-assisted drilling can effectively improve the rock-breaking efficiency by adjusting the magnitude of the excitation frequency. The results provide a theoretical basis for the new rock-breaking technology of PDC bit drilling.

Study on rock-breaking mechanism of axe-shaped PDC cutter

Polycrystalline diamond compact (PDC) cutter is the key component of a drill bit. Recently the shaped PDC cutters, especially the axe-shaped cutter (ASC), have been warmly welcomed by the drilling engineers. However, the rock-breaking mechanism of ASC is rarely studied. In order to investigate the cutting action of ASC, comparative experiments of ASC and conventional cylinder-shaped cutter were carried out based on a vertical turret lathe (VTL). VTL tests' procedure were modified to mimic the initial state, steady state of PDC cutter in the real down hole drilling. The cutting forces and temperature were measured using a dynamometer load cell (3-component force sensor) and a thermocouple, respectively. The results show that ASC has better wear resistance and cutting efficiency than the conventional cylinder-shaped cutter. ASC is easier to press into the rock with its pointed geometry, leading to lower normal force at the initial state of cutting. The cutter shape has large influence on the cutters’ resistance to impact. In addition, both the normal force and tangential force required by ASC during stable cutting are less than that of conventional cylinder-shaped cutter under the same test conditions. Although the cuttings produced by both ASC and conventional cutter have pretty similar particle size distributions, ASC consumes less energy to break the rock based on the fractal analysis of cuttings. • Two states of PDC cutters were simulated on vertical turret lathe. • Consider fractal analysis into cuttings analysis of PDC cutters. • The wear resistance of PDC cutters were compared. • A new experimental attempt to explore the impact resistance of PDC cutters.

Effects of the chamfer and materials on performance of PDC cutters

Polycrystalline diamond compact (PDC) cutters play a key role in the performance of PDC bits in the oil & gas drilling. The study of single cutter on the rock-breaking mechanism is very important for PDC bits to deal with challenging formations, especially when the wells become increasingly longer and complex. Both cutting efficiency and durability are always the focuses of attention for the researches on single PDC cutter. Of all the factors influencing the cutting efficiency, the chamfer is always ignored in theoretical and experimental studies. In this work, the rock-cutter interaction model was modified with considering the effect of the chamfer, which could be used to predict the cutting forces and cutting efficiency of PDC cutters while shearing the rocks. All cutters have been tested on a vertical turret lathe to verify the modified theoretical model. The accuracy of the theoretical prediction agreed well with the experimental results. It was found that the cutting efficiency could be greatly enhanced by reducing the chamfer size for the cylinder-shaped PDC cutters. Unfortunately, the small chamfer size made PDC cutters vulnerable to the premature failure. In order to improve the durability of small-chamfered PDC cutters while retaining their cutting efficiency, an optimized mix of diamond grains was developed and experimentally studied. The results illustrated that the cutter with optimized diamond mix exhibited much better wear resistance. On the other hand, the chamfer size has relatively weak effect on the wear resistance of PDC cutters. The increased chamfer size contributes to protect the cutter edge from chipping but will cause more cumulative friction heat and impair the cutter's durability. The findings in this work will provide a valuable guideline for the manufacturers of PDC bits. • A theoretical prediction model is modified by considering chamfer. • Obtain a relationship between MSE and chamfer size. • Explored the relationship between the diamond grains and wear resistance. • Both wear resistance and thermal durability of PDC cutters are more influenced by material composition than chamfer size.

Fluid-solid-thermal coupling analysis of rock failure under the joint action of cutting and jet impingement

Mechanical cutting assisted by high-pressure jet is a common and efficient drilling method in the exploration of hot dry rock (HDR). The drilling efficiency can be further improved with an in-depth understanding in the rock-breaking mechanism, which is a complex process involving the coupling of thermal stress, jet impact pressure and cutting force. To address such a multi-physics coupling issue, a loosely coupling method was adopted in this work to accomplish the exchange of data, including temperature, heat flux, nodes deformation and impinging pressure of jet in every time increment. In the fluid subdomain, the liquid nitrogen (LN2) jet and water jet were modeled and compared to demonstrate the potential of the novel jet in improving the drilling rate. In the solid subdomain, cutting force monitored in the experiment was loaded, and the concrete damage plastic model was employed to evaluate damage behaviors of the rock. Relative experiments were conducted to validate the reliability of simulation results. Results indicate that a great temperature difference develops on the coupling surface under the impingement of LN2 jet due to its cryogenic feature and lower heat transfer efficiency. Thereby, irreversible damage of the rock can be induced even at room temperature (298 K). As for the water jet, tensile damage is not observed until the rock temperature increases to 473 K. The assistance of jet impact on rock breaking is mainly reflected in two aspects: (1) the occurrence of tensile damage on the coupling surface facilitates the subsequent rock cutting; (2) the enhancement of tensile damage around the cutter promotes the initiation of cracks. Higher rock temperatures significantly improve the heat transfer rate and enhance the advantage of jet assistance. The present study can provide a new sight for the exploration of HDR.

3D numerical simulation study of rock breaking of the wavy PDC cutter and field verification

The structure of polycrystalline diamond compact (PDC) bits is a key factor in improving their rock-breaking efficiency. To further study the rock-breaking mechanism of the wavy PDC cutter, a 3D finite element model of the dynamic rock breaking of the PDC cutter was established, and a comparative study on the rock-breaking process of the wavy PDC cutter and the conventional PDC cutter was conducted. The results show that the wavy PDC mainly breaks the rock by ploughing and shearing, which results in a higher rock-breaking efficiency. The cutting force of the wavy PDC cutter is 6.64% lower than that of the conventional PDC cutter, and its fluctuation is obviously smaller. With an increase in back-rake angle, the cutting force, its fluctuation, and the mechanical specific energy (MSE) increase, and the optimal back-rake angle is 10°–15°.With an increase in cutting depth, the cutting force and its fluctuation in the wavy PDC cutter increase, the MSE decreases, and the optimal cutting depth is 2 mm. As the number of waves increases, the cutting force and MSE of the PDC cutter increase first and then decrease, and the greater the number of waves, the larger the fluctuation of the cutting force. The cutting temperature of the wavy PDC cutter is lower than that of the conventional cutter, it decreases with an increase in the back-rake angle and number of waves, and gradually increases with an increase in cutting depth. The main results of this work will help to reveal the process of rock breaking and the mechanism of the wavy PDC cutter, thus contributing to the achievement of a high rate of penetration and efficiency in the drilling process of hard formations with PDC bits.

Research on rock breaking mechanism and load characteristics of TBM cutter based on discrete element method

Tunnel boring machine (TBM) is a large-scale tunnel engineering equipment, which has unparalleled advantages in safety and work efficiency. The cutter is subjected to complex and variable random impact loads, resulting in damage to bearings, cutter rings, and cutter shafts. Therefore, based on the discrete element simulation platform and experiment, this paper established the cutter rock simulation model based on the experimental data, and analyzed the rock breaking process, cutter load magnitude, and variation law. The research results show that when the cutter pole diameter decreases from 200 mm, 100 mm, and 0 mm in sequence, the rolling force increases accordingly, and the maximum increase of the rolling force is 56%. Meanwhile the lateral force decreases and the maximum average decrease value is 26%. Moreover, the rolling force showed a downward trend with the increase of the cutter spacing. When the cutter spacing is increased from 80 mm to 120 mm, the average rolling force decreases by 51%, and the maximum value rises slightly, increasing by 2%. And, the lateral force tends to increase, with an increase of 17% and the maximum lateral force value increase was 29%. The results can provide load input reference for TBM cutter layout, cutterhead structure design, and performance evaluation.

Effect of parameters on threshold pressure of sandstone tested by water jet

Threshold pressure of sandstone is of great importance in water jet drilling, which is regarded as an economical method to enhance the oil productivity. However, the effect of the parameters on threshold pressure jetted by water, which helps decide the pump pressure in the field test and guide the feeding velocity, remains unknown. In this paper, two nozzles are chosen as the jetting tools, after which the structure of the nozzles are revealed. Artificial sandstones with various properties are made as rock samples, and then the threshold pressure of the rock samples is tested comparatively by self-made experimental system. The results are analyzed by dimensionless method and data analysis software, until then a simple regression equation is presented in this paper and the experimental data and prediction data fit well. From the analysis, threshold pressure is proportional to 0.84 power of compressive strength, which indicates its major role of contribution, while the porosity and permeability play a subtle role in threshold pressure There is a little difference of the minor factors of threshold pressure between the nozzles, which is correlated with the rock breaking mechanism by water jet. Rock breaking velocity is also discussed in this paper. The results presented in this paper can provide guidelines for the applications of water jet drilling.

Dynamic Analysis of Percussion Drilling System under Harmonic Impact

As a new efficient rock‐breaking technology, harmonic impact drilling technology has received great attention, but the research on its rock‐breaking mechanism is insufficient, which limits its development. Based on the theory of vibration, a simplified model of high‐frequency harmonic vibration impact system is established in this paper. The numerical model was solved by Matlab and the motion equations of drill bit and rock at different stages of motion are obtained, respectively. Based on the factor analysis of the mathematical model, the dynamic characteristics of the impact system under harmonic excitation are studied. Finally, numerical simulations are carried out to further analyze the drilling effect of harmonic impact drilling and verify the correctness of the simplified model. The results show that when the excitation frequency equals the resonance frequency of rock, the vibration displacement of rock reaches the peak value. The drilling speed is greatly increased by harmonic impact drilling compared with conventional drilling.

Rock Breaking Mechanism and Experimental Research on Axial Tilt PDC Bit Tool

PDC bit has poor adaptability in soft and hard staggered, gravel-bearing and other heterogeneous layers and hard formations. The problem of easy pre-damage of the cutting teeth of the bit has not been solved yet. A shaft-tilted PDC bit tool is proposed. The most obvious feature of the tool is that the bit body is designed as an "axial-tilted" structure, which can realize the alternating trajectory movement of the cutting teeth under the cooperation of supporting tools. Researching and analyzing the corresponding theory of the shaft inclination PDC bit, conducting laboratory experiments to analyze the influence of transmission ratio and shaft inclination angle on drilling efficiency, revealing the rock breaking mechanism of the tool. This technology can effectively enhance the intrusion capability of PDC bits in complex and difficult-to-drill and deep formations, and improve rock breaking efficiency. This can provide beneficial support for the drilling industry, especially for deep and ultra-deep well drilling, to speed up, increase efficiency, and reduce costs. Contribute to the progress of our country's drill bit technology.

Investigation of rock breaking mechanism with TBM hob under traditional and free-face condition

To compare the rock breaking mechanism of the tunnel boring machine (TBM) hob under traditional condition and free-face condition, a series of rock cutting tests and rock cutting simulations of the TBM hob under two cutting conditions are conducted. The rock chip, rock breaking forces and rock breaking efficiency etc. are investigated in this study. The research results denote that the rock chip size under the free-face is far larger than that under the traditional condition. Both the rock breaking forces and the rock breaking efficiency under the free-face condition could be reduced and be enhanced respectively compared with that under the traditional condition when the value of St and Sf ranges from 20 mm to 80 mm. Furthermore, the numerical and theoretical results also confirm the cutting rock under the free-face condition is promising, which can be applied to the water-jet TBM and the two-stage cutterhead TBM to improve the rock breaking performance of the TBM hob significantly.