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Study on the Safety of Tunnel Structure Vibration in Dry Ice Powder Thermal Shock Rock Breaking
Abstract
The application of explosive blasting for rock breaking works was not possible in densely populated areas, which caused problems in urban tunnel construction. In the application of dry ice powder thermal shock breaking in tunnel excavation, a vibration monitoring system was employed to monitor the vibration response of the tunnel structure during the breaking process, and it was used to extract the signal characteristics by Hilbert-Huang transformation. Experimental results are as follows: 1) The peak vibration speed was below 50 mm/s for concrete structures safe as long as it was farther than 10 m from the burst hole in the support concrete. 2) Aggregate decay of vibration velocities caused by thermal shock breaking of rock with dry ice powder corresponded to the decay law of the power function. The range of influence of the vibration was considerably smaller compared to that of drilling and blasting the rock; 3) The new rock-breaking technology induced damage can be divided into three regions, with radial fractures generated by high-energy fluids dominating. 4) Hilbert Huang transform extracted signal features more objectively and accurately, with excellent reference for the safety monitoring of tunnel structures.
... Currently, engineers in North America have developed an extraction technique based on horizontal well casing termination, cluster injection, and large-scale slip fracturing and simultaneous fracturing of water sections. This novel fracturing technique (volumetric fracturing) has greatly improved the recovery rate and production of shale gas 5,6 . Many scholars conducted experimental studies and field application tests on hydraulic fracturing of shale gas, and the research results are of significance for the improvement of the technique [7][8][9][10] . ...
A new fluid alternative to slick water for fracturing shale gas can reduce the waste of water resources and improve the extraction efficiency, enabling volumetric fracturing. For the new fracturing technique, the experiments of different release pressures under pre-injection and for pre-injection were conducted using a self-designed true triaxial experimental system, and the pressure pulse curves were plotted to analyze the fracturing principle. The experimental results showed that: (1) the pressure rise curve in the reactor can be divided into five stages: initial reaction, linear pressure rise, rate slowdown, instantaneous pressure release, and residual pressure stages; (2) Pre-filling fracturing requires a smaller expansion ratio, weaker pressure degradation, resulting in better fracturing effect; (3) The increase in the initial fracture length leads to an increase in the pressure required to extend the fracture, and high-pressure subcritical water impact fracturing achieved fracture extension at a lower fluid pressure; (4) The fractal dimension has a strong linear relationship with fracture complexity, which is a new option when evaluating the fracturing effect. Volumetric fracturing allows for the creation of more tiny trenches that increase reservoir permeability, leading to better recovery of the reservoir’s energy resources.
The novel dry ice powder static blasting rock breaking lacked working standards, especially for the vibration safety assessment of the construction site for the protection of the building structure, in comparison with the traditional drill and blast method which had a proven operational process and safety specifications. The Sadovsky vibration velocity prediction formula could only predict the vibration velocity and was project specific. Oscillation parameters that needed to be considered in the vibration safety assessment, such as the dominant frequency of vibration, could not be obtained through empirical formulas. Using the five parameters of hole depth, blast center distance, dry ice powder mass and rock classification as the main influencing factors, BP and RBF neural network models were constructed by Matlab software to predict the peak vibration velocity, main frequency and maximum displacement of dry ice powder blasting. Projection results revealed that it is structurally simpler than the BP neural network and that the RBF was more accurate in predicting the target than the BP network. The results of the study had significant implications for the safe application of the new technology, and more samples of field data need to be obtained in the future, along with the use of more advanced predictive modelling.
Liquid carbon dioxide blasting technology has a wide range of applications and is characterized by sound fracturing effects, low vibration hazards, and high safety. In order to investigate the characteristics and mechanism of CO2 phase change rock breaking, liquid CO2 blasting tests on rock-like specimens were carried out in this paper. The results show that 130 MPa is the threshold value at which a CO2 blasting system moves from dynamic tensile stress damage to dynamic pressure stress damage. When blasting pressures of 100 MPa and 70 MPa are used, the lumpiness ratio of the fragments does not change much as the strength of the rock changes, so a suitable blasting pressure should be chosen to improve the blasting effect. Under the impact of blast stress and high-pressure gas flow, cracks develop to form a rough failure surface.
Investigating rock fragmentation mechanisms under blasting and developing new blasting technologies are important and challenging directions for blast engineering. Recently, with the development of experimental techniques, the fundamental theory of rock blasting has been extensively studied in the past few decades and has made important achievements in the full understanding of the rock fracturing process under blast loading. It is thus imperative to systematically review the progress in this direction. This paper mainly focuses on the experimental study of rock blasting, including the distribution characteristic of blast energy, evolution of the blast stress field, propagation mechanism of cracks, interaction mechanism between blast waves and cracks, and influence of geostatic stress on rock fragmentation. In addition, some newly developed blasting technologies and their applications are briefly presented. This review could provide comprehensive insights to guide the study on the rock fracturing mechanism under blasting and further provide meaningful guidance for optimizing blast parameters in engineering.
Bozhong 19-6 gas field is the first discovered large-scale gas condensate field in eastern China, which is also one of the largest metamorphic rock gas condensate fields in the world. It is a buried hill type, low permeability reservoir, with ultra-high condensate content where the fluid is nearly at its dew point pressure. No similar experience with such reservoirs have previously been reported in the context of gas field development in China and step-by-step progresses is been made to characterize this reservoir. Overall, documentation concerning this type of reservoir is rarely seen worldwide. This paper includes key successful results from multiple perspectives including experiments correlations, numerical modeling and the significance of incorporating certain details. Based on a fluid-solid coupling method, the simulations consider several factors including the fracture distribution, low permeability, medium deformation, and condensate characteristics, as well as their effects on the gas productivity. In the laboratory experiments, the stress sensitivity of the rock was tested using representative core samples. Here, experiment-based correlations of the starting pressure gradient of the gas condensate reservoir are proposed. The starting pressure gradient of different fluid types, such as black oil and gas condensate are highlighted as accurately simulating the reservoir. As a result, the numerical model to predict the dynamic productivity of a single well was successfully established considering all those factors. This paper can serve as a reference for studying other studies of metamorphic, fractured gas condensate reservoirs.
Displacement-based seismic design methods support the performance-based seismic design philosophy known to be the most advanced seismic design theory. This paper explores one common type of irregular-continuous bridges and studies the prediction of their elastoplastic displacement demand, based on a new nonlinear static procedure. This benefits to achieve the operation of displacement-based seismic design. Three irregular-continuous bridges are analyzed to advance the equivalent SDOF system, build the capacity spectrum and the inelastic spectrum, and generate the new nonlinear static analysis. The proposed approach is used to simplify the prediction of elastoplastic displacement demand and is validated by parametric analysis. The new nonlinear static procedure is also used to achieve the displacement-based seismic design procedure. It is tested by an example to obtain results which show that after several combinations of the capacity spectrum (obtained by a pushover analysis) and the inelastic demand spectrum, the simplified displacement-based seismic design of the common irregular-continuous bridges can be achieved. By this design, the seismic damage on structures is effectively controlled.
Underground seismic exploration is a technical assurance for the geological transparency of the mining face. However, seismic exploration with explosive seismic source has shortcomings such as acquisition difficulties and serious safety hazards. Besides, dynamic detection of the mining face with explosive source is challenging. To this end, the study proposes a supercritical CO2 source based on the underground seismic geological condition of the roadway. The phase change blasting mechanism of the supercritical source was revealed, and the equivalent energy of the source was calculated. Additionally, a field test was also carried out. The results show that the seismic signals of the CO2 source have clear P-waves, S-waves, and channel waves; the signal excited by the source has strong energy and rich frequency bands; the signal-to-noise ratio of single CO2 source is high; and the attenuation images of transmitted channel wave excited by the source shows high resolution and the result is consistent with field validation data. CO2 source can be used as a new source of underground in-seam seismic exploration and it also helps in reducing the dependence of explosive seismic source. Besides, the study can achieve early warnings of dynamic geological disasters in the working face and has the advantages of high efficiency, safe, low cost, and environmentally friendly. Keywords: Seismic exploration, Seismic source, CO2 source, Spectrum, Transmitted channel wave
Control of damaged zone caused by impact load is the challenging issue in blasting engineering. In this paper, a new rock breaking method using carbon dioxide ice powder is developed, which is characterized by weak disturbance. Then experiments on the \(\text {CO}_{2}\) pneumatic fracturing of concrete specimens were conducted. Generally, the \(\text {CO}_{2}\)-driven fracturing process generates medium strain rate (\(\varepsilon ^\cdot = 10^{0}\) to \(10^{1}\) 1/s), the large concrete specimen was fractured into 3–5 blocks, and no crushing damage occurred. The transient failure modes in the shock wave event were discussed, and the fragment size-strain rate relationship was established based on the cusp mutation theory. The theoretical calculation results are consistent with the experimental results. The research achievement may provide a new and safe rock-fracturing method for geological engineering.
One of the most adverse effects encountered during blasting in open-pit mines is ground vibration. The peak particle velocity (PPV) is a measure used for ground vibrations; however, accurate prediction of PPV is challenging for blasters as well as managers. Herein, boosted generalised additive models (BGAMs) were applied for estimating the effects of blast-induced PPV. An empirical equation, support vector machine (SVM) and artificial neural network (ANN) were also adapted and used to approximate the blast-induced PPV for comparison. Herein, a database covering 79 blasting cases at Nui Beo’s open-pit coal mine, Vietnam, were used as a case example. Several performance indicators such as the coefficient of determination (R²), root-mean-square error (RMSE) and mean absolute error (MAE) were used to evaluate the quality of each predictive model. According to the results, the proposed BGAM performed better than the other models, yielding the highest accuracy with an R² of 0.990, RMSE of 0.582 and MAE of 0.430. ANN and SVM models exhibited only slightly lower performance, while the empirical technique had the worst performance. Two testing blasts were performed to validate the accuracy of the developed BGAMs in practical engineering and the results showed that the BGAMs provided high accuracy than other models. Results also revealed that the elevation difference between the blasting site and monitoring point is one of the predominant parameters governing the PPV predictive models.
High energy gas fracturing is a simple approach
of applying high pressure gas to stimulate wells by generating several radial cracks without creating any other
damages to the wells. In this paper, a numerical algorithm
is proposed to quantitatively simulate propagation of these
fractures around a pressurized hole as a quasi-static phenomenon. The gas flow through the cracks is assumed as a
one-dimensional transient flow, governed by equations of
conservation of mass and momentum. The fractured
medium is modeled with the extended finite element
method, and the stress intensity factor is calculated by the
simple, though sufficiently accurate, displacement extrapolation method. To evaluate the proposed algorithm,
two field tests are simulated and the unknown parameters
are determined through calibration. Sensitivity analyses are
performed on the main effective parameters. Considering
that the level of uncertainty is very high in these types of
engineering problems, the results show a good agreement
with the experimental data. They are also consistent with
the theory that the final crack length is mainly determined
by the gas pressure rather than the initial crack length
produced by the stress waves.
When the traditional drill and blast method is applied to rock crushing projects, it has strong vibration, loud noise and dust pollution, so it cannot be used in densely populated areas such as urban public works. We developed a supercritical CO2 true triaxial pneumatic rock-breaking experimental system, and conducted laboratory and field tests of dry ice powder pneumatic rock-breaking. The characteristics of the blast-induced vibration velocity waveform and the evolution of the vibration velocity and frequency with the focal distance were analyzed and discussed. The fracturing mechanism of dry ice powder pneumatic rock breaking is studied. The research results show that: (1) The vibration velocity induced by dry ice powder pneumatic rock breaking decays as a power function with the increase of the focal distance; (2) The vibration frequency caused by dry ice powder pneumatic rock breaking is mainly distributed in 1–120 Hz. Due to the dispersion effect, the dominant frequency of 10–30 Hz appears abnormally attenuated; (3) The traditional CO2 phase change fracturing energy calculation formula is also applicable to dry ice pneumatic rock breaking technology, and the trinitrotoluene (TNT) equivalent of fracturing energy is applicable to the Sadovsky formula; (4) Dry ice powder pneumatic rock breaking is shock wave and high-energy gas acting together to fracture rock, which can be divided into three stages, among which the gas wedge action of high-energy gas plays a dominant role in rock mass damage.
Excessive blast-induced vibration may cause damage and even the collapse of underground tunnels, and the monitoring and prediction of vibration have an important reference role for the control of these disasters. In this paper, the propagation characteristics and attenuation prediction equation of blast-induced vibration on closely spaced rock tunnels were discussed based on field monitoring and numerical simulation. First, the propagation and attenuation characteristics of blast-induced vibration (e.g. particle peak velocity, PPV) on rock tunnels were analyzed and the applicability of some commonly used empirical equations to predict the PPV on lower bench and sidewall of closely spaced adjacent tunnel was discussed and compared. Then, considering the diffraction characteristics of the wave around tunnels, the scaled distance (SD) of the empirical equations was improved to predict the PPV on the whole space of the rock tunnels. Finally, a new equation was proposed to predict the PPV on adjacent tunnel section in the blasting centre plane considering the diffraction and reflection amplification effects of blast-induced vibration around adjacent tunnel. The research results show that the vibration velocities of surrounding rock around the tunnels are different from those in the free field surrounding rock without the influence of tunnel cavities due to the reflection and diffraction of blasting vibration on the tunnel wall. The improved and proposed empirical equations (5) and (6) are more accurate in predicting the PPV on the closely spaced rock tunnels because the diffraction and reflection amplification effects of blast-induced vibration around tunnels are considered.
In order to promote the development of the new dry ice powder pneumatic rock breaking technology and the safety and environmental pollution assessment of the application site, this paper measures the vibration and noise generated by the dry ice powder pneumatic rock breaking process and extracts the vibration through the Hilbert-Huang transform analysis method. The test results show that (1) the noise measurement found that the noise propagation is directional. The pre-splitting of the first shot makes the second shot of the noise spread more quickly and the sound pressure level is higher. (2) Compared with the Fourier transform, HHT transform is a better tool to extract the characteristics of vibration waveform. (3) When the focal distance is 7 m, the distribution of energy in frequency is scattered, and the damage of vibration to buildings can be considered more comprehensively through the distribution of energy in time and frequency.
The rapid development of the city has brought the traffic serious problems, the subway construction greatly alleviates the heavy traffic. Due to the limitation of geology and mechanical equipment, mining method is often used in subway construction. However, the vibration caused by blasting rock is the most obvious disaster of this method. For solving this problem, this paper, taking a metro station as a case, verified the effectiveness of the pre-splitting crack on the damping above the tunnel face by simulation. Furthermore, the article examines the effect of material of the medium, pre-splitting crack width and the number of blasting holes on the damping vibration. The results show that the increase in diameter improves the energy and vibration absorption performances of damping hole better than the decrease in the distance. In addition, damping performance in gas and liquid medium is better than that in solid medium. The results show that the particle vibration velocity in Z direction increased near the tunnel vault (within 5 m), which does not conform to the vibration attenuation law of Sadovsky's formula. This suggests that the vibration velocity at the ground measuring point is not the maximum. The pre-splitting crack interfered with the propagation of seismic wave, and its barrier greatly weakened the vibration velocity of ground measuring points. Compared with the ground of the tunnel without pre-splitting crack, the vibration velocity is reduced by about 40%. In addition, damping performance in gas and liquid medium is better than that in solid medium. Increasing the width of pre-splitting crack would not enhance the damping performance. What's more, the number of blasting holes does not affect the damping performance of the pre-splitting crack.
This paper proposes a parameter calculation method for the mixed initiation circuit with both nonel detonators (NDs) and electronic detonators (EDs) to increase the detonator delay for reducing the blast-induced vibration in tunnel blasting and to ensure safe circuit operation. The circuit involves the outside-hole delay for increasing the delay numbers under the premise of safe circuit operation. The time calculation and vibration-reduction effects of the circuit were quantitatively determined through mathematical modeling and field experimental studies. A calculation model of mixed detonator groups was established based on three constraint conditions: no delay irregularly, no misfire, and an increased delay number. The main parameters obtained were the initiation sequence of the EDs, the delay number of the NDs and the time interval of the EDs inside the blastholes. The factor considered in the mathematical modeling was the delay error of the NDs. The calculation results agreed well with experimental results, the latter confirming that the mixed initiation circuit was feasible and safe when using a 90 ms delay outside the blasthole. In addition, the delay numbers of the mixed-detonator group were increased by 40% relative to those of NDs. Compared with conventional blasting that reduces the advance per round, blasting with the mixed initiation circuit using an outside-hole delay produced 82.3% less blast-induced vibration. It was also found that the blasting cost could be reduced by 54.3% compared with that of using EDs only. Overall, the mixed initiation method had a positive effect on tunnel controlled-blasting. The research methods and results are expected to be useful in guiding related engineering efforts.
In the tunneling of frozen soil section of shaft sinking by freezing method, the traditional air pick excavation has large consumption of pick and drill rod, low excavation efficiency, and high labor intensity. The drilling explosive blasting method has a poor working environment and potential safety hazards. The safe and efficient excavation of frozen soil section is a difficult problem faced by freezing method shaft sinking. In this paper, a directional fracturing method by liquid CO2 phase change for freezing shaft sinking is proposed. Based on the traditional CO2 phase change fracturing device, a liquid CO2 phase change fracturing device with directional function is designed. It can not only fully crush and weaken the frozen soil in the shaft, but also can not disturb the shaft wall structure. The liquid CO2 phase change directional fracturing technology is firstly applied to the assisted tunneling in freezing shaft sinking. The field test results indicate that the directional liquid CO2 phase change generator has a good directional fracturing function. There are many obvious cracks formed around the directional fracture drilling hole, and the cracks are all extending toward the center of the protection circle. The angle between the two outermost cracks is close to the phase angle of directional discharge hole of liquid CO2 phase change generator. According to the statistics of field construction, the average daily excavation speed of frozen soil section is increased from 1.67 m/d to 2.45 m/d after directional fracturing by phase change of liquid CO2. Compared with explosive blasting weakening, liquid CO2 phase change fracturing has the advantages of no spark in the phase transformation process, easy control of pressure, convenient storage and transportation, simple operation, and relatively safe construction process. Liquid CO2 phase change fracturing shows a good application prospect in some fields which have high safety requirements. It is expected to be popularized and applied in slope excavation, tunnel chamber excavation, and other fields in the future.
Drilling and blasting play an important role in operation cycle of a mine or quarry. Optimum blast design plays a pivotal role to achieve maximum utilization of explosive energy and blast fragmentation. Only 20 to 30% of explosive energy is utilized for fracturing and fragmentation of rock, and the rest of the energy is converted in noise, air overpressure, ground vibration, etc. It has been observed that using the low-density explosive may reduce the deleterious effect of engineering blasting with desired blast fragmentation. This paper substantiates this fact wherein distinct explosive energy utilization enhancement has been achieved with the use of low-density explosive and reduces ground vibration. This paper outlines the on-field assessment of low-density explosive in Quarry AB of Tata Steel West Bokaro Division. It is a solid sensitized emulsion blend. It comprises products designed for blasting in dry, dewatered, and wet blast-hole applications. Low-density explosive provides the capability to better match explosive performance to ground conditions. Being a low-density, low-energy explosives, it provides an added benefit of reducing the environmental effects of blasting. The key objective of the field assessment was to test the on-field performance of the new low-density bulk products and its effects on blast-induced ground vibration and air overpressure.
Based on the directional fracture blasting experiment of polymethyl methacrylate (PMMA) specimens and combined with an analysis of the strain field around the dynamic crack tip, a strain gauge method with a specific acute (obtuse) orientation is proposed to record the dynamic fracture process of mode I cracks. A time evolution of the strain of the whole test process is employed to establish a binomial expression to represent the crack tip position and surrounding strain field and calculate the crack propagation velocity (CPV) and dynamic stress intensity factor (DSIF). Compared with the dynamic caustics method, the feasibility of the strain gauge method is analyzed. The strain gauge method is employed to research the dynamic fracture process of mode I cracks in rock directional fracture blasting. The results show that when the normalized strain value is 0 in the acute orientation or the maximum in the obtuse orientation, the position of the propagating crack tip is directly below the strain gauge. The calculations of the CPV and DSIF are consistent with those of the dynamic caustics method, which verifies the feasibility of the strain gauge method. The acute orientation has a large error dispersion. Compared with the dynamic caustics method, the strain gauge method can approximately record the whole process of rock crack propagation, reveal the evolution law of the CPV and DSIF and provide an experimental basis for research on the directional fracture process of nontransparent materials. Compared with the PMMA specimens, the rock specimens have a larger oscillation of the CPV and a smaller oscillation of the DSIF.
In recent years, CO2 phase transition jet (CPTJ) coal-breaking technology has been reported to increase coal seam permeability. However, the structural evolution effect of coal subjected to CPTJ technology is unclear, which restricts widespread CPTJ technology application. In this study, a laboratory experimental system and field technical equipment were developed, and coal-breaking experiments under different CPTJ pressure conditions were conducted. We investigated pore structure changes by combining scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) to better understand the coal pore structure evolution characteristics under different CPTJ pressures. Furthermore, an enhanced coalbed methane (ECBM) recovery experiment using CPTJ coal-breaking technology was conducted with the self-developed technical equipment. The results show that notable damage occurs in the coal body influenced by CPTJ, and the damage area increases with increasing jet pressure. The SEM results revealed that more pores and cracks are produced due to liquid CO2 CPTJ, and the porosity and crack size increase with increasing jet pressure. MIP analysis indicates that the pore structure of the breaking coal samples mainly includes macropores, and coal sample macroporosity increases significantly with increasing jet pressure. The field ECBM experiment showed that CPTJ technology can reduce the coalbed methane (CBM) drainage decay coefficient and increase the CBM extraction pure flow rate and recovery efficiency 8.3–10.4 and 20.4 times, respectively.
The damage prediction and protective measures of tunnels subjected to blast loads have gained importance in recent years due to increasing accidental events and terrorist attacks. This paper performs numerical modeling to assess the damage characteristics of an underwater tunnel subjected to blast loads and explore the potential mitigation measures. Firstly, a three-dimensional numerical model of an underwater tunnel based on coupled Lagrange and Euler (CLE) method is developed. The accuracy of the CLE method and material models is verified with blast testing data. Subsequently, the dynamic response and damage evolution of the tunnel subjected to underwater explosions are investigated. Potential failure patterns due to various underwater blast loads are explored. Intensive numerical simulations are then carried out to investigate the damage levels of the tunnel. Based on numerical data, empirical relations are suggested to predict the tunnel damage levels based on charge weight and standoff distances. CFRP (carbon fiber reinforced polymer) cloth is used to improve the resistance of the tunnel to underwater explosions. Mitigation effects of different strengthening schemes are investigated, and the optimum strengthening thickness of the CFRP cloth is recommended. The results show that the rigidity and load carrying ability of the tunnel are significantly improved by bonding the CFRP cloth. The recommended thickness of the CFRP cloth is 0.5–0.835 mm.
During the construction process of excavation blasting in small clearance tunnel, the blasting vibration inevitably disturbs and damages the interlaid rock and influences the stability. A large-scale three-dimensional tunnel excavation simulation test system is designed and developed. The system includes the monitoring system and the test bench which is composed of steel members, reaction frame and hydraulic loading device. Based on the system and portable electric spark source, the dynamic response disciplinarian of the interlaid rock of large cross-section tunnel with small clearance during excavation is studied. The results indicate that the blasting process during the backward tunnel excavation has a greater influence on the stability of the interlaid rock, yet the surrounding rock at the skewback of the tunnel had a low sensitivity to excavation blasting vibration and is affected by the blasting process for a long time. The research methods and results will guide similar engineering.
This paper presents a 3D coupled Smoothed Particle Hydrodynamics (SPH) and Finite Element Method (FEM) model, which was developed to investigate the extent of damage zone and fracture patterns in rock due to blasting. The RHT material model was used to simulate the blasting-induced damage in rock. The effects of discontinuity persistence and high in-situ stresses on the evolution of blasting-induced damage were investigated. Results of this study indicate that discontinuity persistence and spatial distribution of rock bridges have a significant influence on the evolution of blasting-induced damage. Furthermore, high in-situ stresses also have a significant influence on the propagation of blasting-induced fractures, as well as the patterns of fracture networks. It is also shown that the blasting-induced cracks are often induced along the direction of the applied high initial stresses. Moreover, additional cracks are normally generated at the edges of the rock bridges probably due to the relatively high stress concentration.
Nitrogen gas drive and foam drive are commonly adopted for practical production in fractured carbonate reservoirs. A systematic study on nitrogen gas drive and foam assisted nitrogen gas drive in varying-aperture fractures is a practical and essential method to understand the EOR mechanism. In this study, we designed and fabricated similarity models and microscopic visual models. The fluid flow mechanisms were discussed combining the pressure response and dynamic displacement phenomenon. Experimental results show that the effective time of nitrogen gas drive was longer in decreasing-aperture fractures than that in increasing-aperture fractures. After water flooding, 0.8 PV nitrogen gas injection lead to gas channeling in decreasing-aperture fracture of 1 mm–0.2 mm while 0.6 PV nitrogen gas injection by reverse displacement direction. Both production rate and average pressure appeared discrete in narrow fracture compared to wide fracture. Enhanced oil recovery efficiency by foam drive was average 5% higher than that by nitrogen gas drive in single fracture. The displacement front of foam varies in four forms: micro-scale foam accumulation, small-scale foam accumulation, mesoscale foam slight deformation and large-scale foam serious deformation. This study could provide a theoretical basis for enhance oil recovery and further dig the potential of remaining oil in fractured carbonate reservoirs.
Using drilling and blasting method to construct the Mountain tunnel, it is important to monitoring and analysis of blasting seismic wave. The research of simple characteristics of blasting vibration wave such as amplitude, frequency, duration in the previous study, it is lack the detailed characteristics of blasting seismic waves such as frequency distribution characteristics of blasting seismic wave, blasting energy distribution. Thus, Fourier transform and wavelet packet transform are both used to analysis the time - frequency characteristics of measured vibration signal, and the characteristics of its multi-frequency band and energy distribution are discussed. The blasting vibration with the blasting charge of 79.2 kg are did, and the wave are received distance blast source 10 m, 25 m, 40 m. The maximum three vector resultant velocity at 10 m is 5.84 cm/s, which meets the specification safety requirement. Analysis of blasting vibration signals of vertical axis which distance blast source 10 m, the main frequency of this signal is 101.9 Hz, and the relative amplitude is 2382 use by Fourier transform. The Wavelet packet analysis show that, blasting vibration signal has obvious energy distribution characteristics of multi-frequency band. The dominant frequency band (31.25 Hz~125.00 Hz) contains more than 70% of the energy of vibration signal.
Almost all the coal seams in China are characterized by high gas content, extremely low permeability, complicated geological structure, thus, natural disasters such as gas explosive and outburst easily occur. At present, gas extraction is an effective approach for preventing such disasters. In this paper, in order to improve gas extraction efficiency, the Multi-discharge CO2 fracturing system (Multi–CO2–Frac) was proposed and tested in Changping coal mine. Through analysis of the waveforms from the explosions of 0.5 and 1 kg TNT dynamites and 1 kg liquid CO2, it can be obtained that the TNT equivalence of 1 kg CO2 is about 400–430 g, that is, the explosion of 1 kg liquid CO2 has same fracturing ability as 400–430 g TNT dynamites. Besides, field test shows that the effective drainage radius caused by Multi-CO2-Frac technique is about 12.5 m, regardless of the number CO2 discharging sets. However, the gas extraction concentration increases with the number of CO2 discharges. This phenomenon indicates that more cracks will be created along the axial direction in coal seam drilling boreholes by Multi–CO2–Frac techniques. By comparing gas concentration before and after fracturing, it can be concluded that the Multi–CO2–Frac techniques can change and maintain the uptrend of gas concentration at a higher level in a long period of time. Therefore, in order to extend the duration of high efficiency gas extraction, Multi–CO2–Frac techniques can be used periodically at different position of coal seam. Meanwhile, after gas extraction, the maximum gas emission of coal seam was reduced from 5.5 m³/min to 3.48 m³/min and the driving footage was enhanced from 2.4 m/d to 5.7 m/d. In summary, Multi–CO2–Frac techniques can effectively improve the coal seam permeability and further enhance the gas drainage efficiency, which can meet the requirement of gas extraction in high gas content thick and soft coal seam. Therefore, this technique has a promising application prospect because of its advantages of safety, environmental protection and economy.
The efficiency of gas drainage in deep coal seams is generally poor because of their typically low-gas permeability and high-geostress characteristics. Therefore, we conducted a test study on the permeability enhancement of cross-measure boreholes using liquid CO2 phase-transition blasting (LCPTB) to identify an effective method for enhancing coalbed methane reservoir. Through a numerical simulation of LCPTB, the fracture propagation in the coal seam after blasting was analysed. Subsequently, a field test arrangement for boreholes of LCPTB was designed, and the range of enhanced permeability after blasting as well as the efficiency of gas drainage were investigated. The results indicate a significant increase in the permeability of the coal seam and the efficiency of gas drainage following LCPTB. The amount of gas extracted from the blast holes was 1.8–8 times greater than that extracted from boreholes without LCPTB. The practical spacing between boreholes was deemed to be 2.5–3 m. In addition, with decreasing measuring distance from the blast hole, the efficiency of LCPTB was improved by increasing the permeability of the coal mass surrounding the observation hole. The observation holes arranged around the blasting holes could increase the efficiency of gas drainage. In summary, this technology uses the energy released by LCPTB to increase the permeability of the coal surrounding the blast hole, and to displace methane in the coal seam, thereby improving the efficiency of gas drainage and providing economic and environmental benefits.
The cavity effect means the amplifications of blast vibrations when propagating through a cavity inside the rock mass. It has been investigated by a series of blast tests in an underground water tunnel. Monitoring points were set up in pairs on the ground surface and symmetrically about the tunnel face. The amplifying coefficient of blast vibrations was defined as the PPV (peak particle velocity) ratio of the two symmetry points. It is found that the amplifying coefficient first rises to a maximum of 3.5, and then falls down with the distance in the tunnel axial direction. Meanwhile, tunnel radius and its buried depth are two other influencing factors. The coefficient linearly increases with tunnel radius, while decreases with the depth in a power function. These results suggest that the cavity effect only works within a limited scope on the ground, and mainly occurs in shallow tunnel excavations. By introducing a ratio of the tunnel depth to its radius, an empirical formula was proposed to calculate the amplifying coefficient. It can be used to predict the PPV amplifications and therefore define a vibration control area for the safety of ground structures and facilities. It could also be applied to more site conditions by choosing appropriate parameter values in the formula.
Coal mine researchers have been studying the subject of improving the effect of gas drainage to control gas accident. This paper focuses on a new permeability-improvement technology, liquid carbon dioxide phase change fracturing technology. This technology can be classified as physical blasting by using the phase energy of liquid carbon dioxide in the drilling underground coal mine. According to the results of field tests, by applying the liquid carbon dioxide phase change fracturing technology, the radius of the damaged area in the coal body around the drilling reached 5 m. Also, compared with the original coal body, the permeability of the damaged area increased approximately six times. In addition, based on the permeability of the damaged area, the influence of the liquid carbon dioxide phase change fracturing technology on the gas drainage radius was analyzed by numerical calculation. It can be observed from the numerical results that the effect of gas drainage was improved greatly by the liquid carbon dioxide phase change fracturing operation. Research results of this paper can contribute to safe and efficient mining of coal mines.
A pre-cut discontinuity, similar to the pre-splitting around a tunnel periphery, is introduced to investigate the attenuation of the wave propagation due to tunnel blasting. For this, two empty holes were drilled to support the sawing jigs and a wire sawing machine with embedded industrial diamond wire was used to cut an artificial discontinuity near the initial blasting holes at the tunnel face. The peak particle velocity with/without pre-cut was measured during tunnel blasting and the effect of the pre-cut method was quantitatively accounted for, although, due to limited time and unfavorable field conditions, the cutting depth was not able to reach the initial design depth. Numerical analysis was also performed in order to model the tunnel geometry and blasting procedure. For this, a commercial package called UDEC was used and the accuracy of UDEC was initially proven by an analytical solution of the reflection and transmission of the incident wave. Various parametric studies including sequential detonations were performed and the reduction factor of attenuation with ideal cut-off discontinuity was shown to be 50%. Future work will concentrate on the improvement of the wire sawing machine to reduce the cutting time and three dimensional modeling of the blasting effect will also be performed.
This paper presents the results of ground vibration analysis induced by blasting during the construction of the Istanbul Kadıköy–Kartal metro tunnel. Different rock formations in this tunnel route were encountered during the excavation with blasting. As a first stage, the test site is divided into 6 main regions with respect to lithology changes in the rock units and Hoek's Geological Strength Index value of these rock units. During the excavation, a total of 659 events were recorded in 260 shots by vibration monitors. Scaled distance and peak particle velocity data pairs belonging to these shots were carefully recorded and analyzed statistically. As a result of this analysis, empirical relationships between scaled distance and peak particle velocity were established with higher correlation coefficients specific to each region. Finally, the particle velocities and frequency values of all blast events were evaluated according to Turkish Environmental Regulation, the United States Bureau of Mines (USBM) and the German DIN 4150 Norms in order to predict the influence level to the neighboring buildings and structures.
The dynamics and diagnostics of a cracked rotor have been gaining importance in the recent years. Vibration monitoring during start-up or shut-down is as important as during steady-state operations to detect cracks especially for machines such as aircraft engines which start and stop quite frequently and run at high speeds. In the present study a relatively new signal processing technique namely Hilbert–Huang transform (HHT) has been applied to transient response of a cracked rotor and few interesting results have been observed. And in all cases it has been found that HHT appears to be a better tool compared to fast Fourier transform and continuous wavelet transform for crack detection in a transient rotor.
Based on numerical experiments on white noise using the empirical mode decompo-sition (EMD) method, we find empirically that the EMD is effectively a dyadic filter, the intrinsic mode function (IMF) components are all normally distributed, and the Fourier spectra of the IMF components are all identical and cover the same area on a semi-logarithmic period scale. Expanding from these empirical findings, we further deduce that the product of the energy density of IMF and its corresponding averaged period is a constant, and that the energy-density function is chi-squared distributed. Furthermore, we derive the energy-density spread function of the IMF components. Through these results, we establish a method of assigning statistical significance of information content for IMF components from any noisy data. Southern Oscillation Index data are used to illustrate the methodology developed here.
As the fractional Fourier transform has attracted a considerable amount of attention in the area of optics and signal processing,
the discretization of the fractional Fourier transform becomes vital for the application of the fractional Fourier transform.
Since the discretization of the fractional Fourier transform cannot be obtained by directly sampling in time domain and the
fractional Fourier domain, the discretization of the fractional Fourier transform has been investigated recently. A summary
of discretizations of the fractional Fourier transform developed in the last nearly two decades is presented in this paper.
The discretizations include sampling in the fractional Fourier domain, discrete-time fractional Fourier transform, fractional
Fourier series, discrete fractional Fourier transform (including 3 main types: linear combination-type; sampling-type; and
eigen decomposition-type), and other discrete fractional signal transform. It is hoped to offer a doorstep for the readers
who are interested in the fractional Fourier transform.
Crack propagation patterns and factors controlling complex crack network formation in coal bodies during tri-axial supercritical carbon dioxide fracturing
- H Yan
- J Zhang
- B Li
- C Zhu