Electric Spark Source Research Articles

Experimental Study on Stability Analysis of Shallow-Buried Metro Station Hard Rock Excavation under Blasting Vibration Effect

Abstract By considering a certain metro station of Qingdao Metro Line 4 as the research object, a three-dimensional geotechnical testing model has been developed to analyze the stress and deformation characteristics of the surrounding rocks under blasting vibration effect. The simulation of the surrounding rock blasting process is achieved through an electric spark source. We conducted simulation tests on the excavation stability of shallow-buried hard rock metro stations with and without considering blasting damage effects, respectively. The stress, displacement, and primary support stress changing laws of the surrounding rock under different overburden thicknesses using the arch cover method and the primary support arch cover method have been revealed. Finally, we analyzed the applicability of the primary support arch cover method in terms of vault settlement, clearance convergence, and floor heave. Results show that the surrounding rock of the arch has roughly gone through three deformation stages, namely a slow deformation stage, rapid deformation stage, and stable deformation stage. Whether it is blasting excavation or non-blasting excavation, the settlement of the arch is within a safe and controllable range. Because of the refinement of the excavation process by the primary support arch cover method, the stress release is relatively stable throughout the entire excavation process, but the significant impact of blasting vibration on the surrounding rock cannot be ignored. The excavating of the left and right guide holes is the key process for the displacement and stress variation. The deformation and stress generated by the arch cover method have a small difference compared to the deformation and stress generated by the primary support arch cover method, indicating that the latter excavation method can be used under current engineering geological conditions. No large-area collapse and instability occurred in the experimental process, which provides a theoretical basis for similar projects.

Effects of Unidirectional In Situ Stress on Crack Propagation of a Jointed Rock Mass Subjected to Stress Wave

This paper proposes a large-scale experiment combined with multiple cement mortar blocks to simulate stress wave propagation across a jointed rock mass under unidirectional in situ stress. Two identical mortar block models with smooth, dry, and unfilled joints were poured. The stress waves in Model 1 and Model 2 were generated by an electric spark source and a blast-induced source, respectively. The effects of these two excitation sources on stress wave propagation were compared through crack propagation experiments. The experimental results show that the peak value of the transmitted strain wave decreases as the in situ stress increases. The unidirectional in situ stress has a certain inhibitory effect on the stress wave propagation. It also indicates that for Model 1 with the electric spark source, no cracks on the upper surface, but a Livingstone blasting crater at the bottom is generated. For Model 2 with the blast-induced source, cracks on the upper surface and a blasting crater at the bottom are produced. The results verify the similarity between the electric spark source and the explosive source. The two-dimensional finite element program (ANSYS/LS-DYNA) was applied to further simulate the crack propagation of a jointed rock mass under different in situ stresses. The results of numerical simulation verify that the in situ stress has a clear guiding effect on the crack propagation.

A towed-type shallow high-resolution seismic detection system for coastal tidal flats and its application in Eastern China

Abstract Restricted by detective equipment and tidal influence seriously, coastal tidal flats are often treated as the transition belt of paralic zone in terms of seismic detection, causing low exploration degree and accuracy in these areas. What is worse, conventional shallow seismic method has a complex system and low acquisition efficiency, which cannot meet the requirements for high-efficiency acquisition in tidal flats. To solve these problems, a towed shallow seismic detection system is specially established for coastal tidal areas, mainly including electric spark source and towable ship-type geophone strings. After comparisons, some advantages of this method are obtained, such as high efficiency, low cost and reliability. Furthermore, its detective accuracy and depth can reach the meter level and more than 800 m, respectively. Then, a series of shallow seismic processing methods are improved for coastal tidal flats. Deconvolution and velocity analysis are emphatically introduced to multiple-wave attenuation. A short array of this towable detection can keep a far more effective wave from being cut off because of its small NMO stretch. Last but not least, the towable method in coastal tidal-flat zones can successfully identify the stratigraphic structure, interface, palaeo-channel, concealed active fault and submarine shallow gas in Jiangsu Coast, Eastern China, which has great practical results and significance for geological and environmental surveys, as well as scientific geo-hazard prevention and mitigation in these areas.

Model test study on vibration blasting of large cross-section tunnel with small clearance in horizontal stratified surrounding rock

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.

Irregular reflection of spark-generated shock pulses from a rigid surface: Mach-Zehnder interferometry measurements in air.

The irregular reflection of weak acoustic shock waves, known as the von Neumann reflection, has been observed experimentally and numerically for spherically diverging waves generated by an electric spark source. Two optical measurement methods are used: a Mach-Zehnder interferometer for measuring pressure waveforms and a Schlieren system for visualizing shock fronts. Pressure waveforms are reconstructed from the light phase difference measured by the interferometer using the inverse Abel transform. In numerical simulations, the axisymmetric Euler equations are solved using finite-difference time-domain methods and the spark source is modeled as an instantaneous energy injection with a Gaussian shape. Waveforms and reflection patterns obtained from the simulations are in good agreement with those measured by the interferometer and the Schlieren methods. The Mach stem formation is observed close to the surface for incident pressures within the range of 800 to 4000 Pa. Similarly, as for strong shocks generated by blasts, it is found that for spherical weak shocks the Mach stem length increases with distance following a parabolic law. This study confirms the occurrence of irregular reflections at acoustic pressure levels and demonstrates the benefits of the Mach-Zehnder interferometer method when microphone measurements cannot be applied.

Irregular reflection of weak acoustic shock pulses on rigid boundaries : Schlieren experiments and direct numerical simulation based on a Navier-Stokes solver

The local interactions occurring between incident and reflected shock waves in the vicinity of rigid surfaces are investigated. Both regular and irregular — also called von Neumann — regimes of reflection are studied, via experimental and numerical simulations. Shock waves are produced experimentally with a 20 kV electrical spark source which allows the generation of spherically diverging acoustic shocks. The behaviour of the resulting weak acoustic shocks near rigid boundaries is visualized with a Schlieren optical technique which allows the spatial structure of the shocks to be studied. In particular, the evolution of the Mach stem forming above a flat surface is examined, and its height is observed to be directly linked to the angle of incidence and the pressure amplitude of the incident shock. The propagation of an acoustic shock between two parallel rigid boundaries is also studied. It is shown that the strong interactions between the Mach stems emerging from the two boundaries can lead to a drastic modification of the morphology of the acoustic field in the waveguide. Experimental results are compared to numerical results obtained from high-order finite-difference based simulations of the 2D Navier-Stokes equations. The good agreement between the experimental distribution of the acoustic field and numerical results suggests that numerical simulations are promising as a predictive tool to study nonlinear acoustic propagation of acoustic waves in complex geometrical configurations with rigid boundaries.

Characterization of spark-generated N-waves in air using an optical schlieren method.

Accurate measurement of high-amplitude, broadband shock pulses in air is an important part of laboratory-scale experiments in atmospheric acoustics. Although various methods have been developed, specific drawbacks still exist and need to be addressed. Here, a schlieren optical method was used to reconstruct the pressure signatures of nonlinear spherically diverging short acoustic pulses generated using an electric spark source (2.5 kPa, 33 μs at 10 cm from the source) in homogeneous air. A high-speed camera was used to capture light rays deflected by refractive index inhomogeneities, caused by the acoustic wave. Pressure waveforms were reconstructed from the light intensity patterns in the recorded images using an Abel-type inversion method. Absolute pressure levels were determined by analyzing at different propagation distances the duration of the compression phase of pulses, which changed due to nonlinear propagation effects. Numerical modeling base on the generalized Burgers equation was used to evaluate the smearing of the waveform caused by finite exposure time of the high-speed camera and corresponding limitations in resolution of the schlieren technique. The proposed method allows the study of the evolution of spark-generated shock waves in air starting from the very short distances from the spark, 30 mm, up to 600 mm.

Mach-Zehnder interferometry method for acoustic shock wave measurements in air and broadband calibration of microphones.

A Mach-Zehnder interferometer is used to measure spherically diverging N-waves in homogeneous air. An electrical spark source is used to generate high-amplitude (1800 Pa at 15 cm from the source) and short duration (50 μs) N-waves. Pressure waveforms are reconstructed from optical phase signals using an Abel-type inversion. It is shown that the interferometric method allows one to reach 0.4 μs of time resolution, which is 6 times better than the time resolution of a 1/8-in. condenser microphone (2.5 μs). Numerical modeling is used to validate the waveform reconstruction method. The waveform reconstruction method provides an error of less than 2% with respect to amplitude in the given experimental conditions. Optical measurement is used as a reference to calibrate a 1/8-in. condenser microphone. The frequency response function of the microphone is obtained by comparing the spectra of the waveforms resulting from optical and acoustical measurements. The optically measured pressure waveforms filtered with the microphone frequency response are in good agreement with the microphone output voltage. Therefore, an optical measurement method based on the Mach-Zehnder interferometer is a reliable tool to accurately characterize evolution of weak shock waves in air and to calibrate broadband acoustical microphones.

A focused electric spark source for non-contact stress wave excitation in solids.

A focused electric spark is used as a non-contact acoustic source to excite stress waves in solids. The source consists of an electric spark source located at the near focus of an ellipsoidal reflector that focuses the acoustic disturbance generated by the spark source to the far focal point. Experimental studies using both contact and non-contact sensors indicate that the source has the capability to excite the Rayleigh surface wave and impact-echo mode (S1-zero-group-velocity Lamb mode) in a 250 mm thick concrete slab and to enable fully air-coupled testing of concrete specimens.

Characterization of a spark source focused by an ellipsoidal reflector

Air-coupled ultrasonic non-destructive testing (NDT) provides an ideal solution for rapid scanning of large specimens. Unfortunately, despite decades of research, many challenges remain to render air-coupled ultrasonic methods a broadly effective sensing modality for high impedance materials due to low energy transmission between air and the solids being inspected. In this paper, we present experimental results and theoretical analysis of an electrical spark source focused by an ellipsoidal reflector. This acoustic source, which generates a short duration, high amplitude signal in air, is of high interest for air-coupled NDT for high impedance materials and has been shown to excite wave motion in concrete without contact. Theoretical modeling using weak shock theory and the KZK equation is used to predict the temporal and spatial features of the pressure field in the region of the geometric focus. We also present a series of experimental studies that characterize the spark generated acoustic wave in both free-field and the focused conditions. The bandwidth and directivity of the focused spark source are shown to be adjustable by changing the spark gap size and the reflector geometry. Finally, experimental results from three reflectors made of different material and geometries are presented.

Propagation of N-waves in a turbulent and refracting atmosphere with ground effects (laboratory-scale experiment)

Sound propagation in a refracting atmosphere leads to the formation of a geometrical shadow zone close to the ground, and of an illuminated zone above a limiting ray. Without turbulence, acoustic waves can be diffracted into the shadow zone close to the ground. When propagating through a turbulent atmosphere, it is known that waves can be distorted, scattered, or focused. In order to investigate how turbulence modifies the pressure field into the shadow zone, a well controlled laboratory-scale experiment has been performed. An electrical spark source is used to generate short duration (20 μs) and high pressure (1500 Pa) N-waves. A convex surface models the effect of an upward refracting atmosphere, and a heating grid generates thermal turbulence (1% fluctuations of refraction index). To compute statistics of wave parameters variation, seven 1/8 in. microphones have been used to record 2000 waves at each position after propagation through the turbulent field. Wave parameters (peak pressure, rise time) obtained with turbulence are compared to data obtained without turbulence. Results show that turbulence scatters sound into the shadow zone, which increases significantly the noise level.

Random focusing of nonlinear acoustic N-waves in fully developed turbulence: Laboratory scale experiment

A laboratory experiment was conducted to study the propagation of short duration (25 μs) and high amplitude (1000 Pa) acoustic N-waves in turbulent flow. Turbulent flows with a root-mean-square value of the fluctuating velocity up to 4 m/s were generated using a bidimensional nozzle (140 × 1600 mm(2)). Energy spectra of velocity fluctuations were measured and found in good agreement with the modified von Kármán spectrum for fully developed turbulence. Spherical N-waves were generated by an electric spark source. Distorted waves were measured by four 3 mm diameter microphones placed beyond the turbulent jet. The presence of turbulence resulted in random focusing of the pulse; more than a threefold increase of peak pressures was occasionally observed. Statistics of the acoustic field parameters were evaluated as functions of the propagation distance and the level of turbulence fluctuations. It is shown that random inhomogeneities decrease the mean peak positive pressure up to 30% at 2 m from the source, double the mean rise time, and cause the arrival time about 0.3% earlier than that for corresponding conditions in still air. Probability distributions of the pressure amplitude possess autosimilarity properties with respect to the level of turbulence fluctuations.

Laboratory studies of near-grazing impulsive sound propagating over rough water

Acoustic impulses due to an electrical spark source (main acoustic energy near 15 kHz) have been measured after propagating near to the water surface in a shallow container resting on a vibrating platform. Control of the platform vibration enabled control of water wave amplitudes. Analysis of the results reveals systematic variations in the received acoustic waveforms as the mean trough-to-crest water wave amplitude is increased up to 7 mm. The amplitudes of the peaks corresponding to specular reflections are reduced and the variability in the tails of the waveforms is increased.

Modeling and measurements of nonlinear spherically divergent N waves in air

Propagation of nonlinear N pulses in relaxing medium was studied experimentally and numerically. An electric spark source was used in laboratory‐scale experiments in ambient air to obtain spherically divergent shock pulses of 30‐s duration and 1000‐Pa peak pressure at 15‐cm distance from the source. Pressure waveforms were measured by the broadband 1/8‐in. microphone at the distances varying from 15 cm to 2 m from the spark source. A numerical solution was obtained for an initial analytic N waveform at the same distances based on the modified Burger’s equation for nonlinear spherical waves in dissipative medium with relaxation. The duration of the initial N wave was defined by matching the zero values of the corresponding analytic measured spectra; the amplitude was obtained by matching the experimental and theoretical nonlinear propagation curves for the N‐wave duration. The results of modeling were found to be in good agreement with the experimental data. A frequency response of the microphone was obtained at various distances as a ratio between the measured and modeled spectra and showed a good reproducibility. The relative effects of relaxation, nonlinearity, and microphone filtering on the N‐waveform parameters are discussed. [Work is supported by RFBR and INTAS grants.]

Model experiments to study sonic boom propagation through turbulence

The aim of this paper is to present data from laboratory‐scale experimental simulations of sonic boom propagation through a turbulent atmosphere. The source used to simulate sonic boom N waves is an electric spark source. In order to model turbulence, two setups have been used: a turbulent jet for kinematic turbulence and a heated grid for thermal turbulence. Acoustic measurements have been conducted for three configurations: free field; close to a plane surface in order to simulate ground effects; and close to a curved surface in order to investigate the propagation into a shadow zone. The surfaces were either smooth or rough. With turbulence, many waveforms have been recorded, then the statistics of various parameters, including the peak pressure and the rise time, have been analyzed. In the free field, data show that the increase of the mean rise time and the decrease of the mean peak pressure are linked to the probability of random focusing. The experiments have enabled separate assessment of the effects of turbulence and roughness in the shadow zone caused by the curved surface. Comparison with numerical simulations will be discussed. [This work was supported by European Commission Contract N:G4RD‐CT‐2000‐00398 and French Ministere de la Recherche (Decision 00T0116).]

Characteristics and application of laser-generated acoustic shock waves in air

When a high-power laser beam is focused at a point, the air at the focal point is heated to temperatures of thousands of degrees within several nanoseconds and breaks down. This generates a spark that, in turn, is accompanied by an acoustic shock wave. The acoustic shock waves generated by focussing the beam from a pulsed laser with a 1064 nm wavelength and a power of 800 mJ per pulse have been measured using 1/4″ and 1/8″ B&K microphones. Nonlinear sound levels are observed up to 1.5 m from the laser-induced sparks. Beyond a certain region close to the source, levels are found to decrease in a manner consistent with spherical spreading plus nonlinear hydrodynamic losses. Analysis of the waveforms shows that the acoustic pulses associated with the laser-induced sparks are more repeatable and have higher intensity than those from an electrical spark source. Laser-generated acoustic shock waves are ideal for simulating a blast wave or a sonic boom in the laboratory and for studying the associated propagation effects. To illustrate this application, the propagation of the laser generated shock waves over a series of different hard, rough surfaces has been investigated. The results show the distinctive influences of ground roughness on the propagation of the shock wave.

Determination of ultra-low carbon and nitrogen contents in steel: combustion versus electrical spark source optical emission spectrometry for steelmaking process control

Electrical spark optical emission spectrometry (spark-OES) was evaluated for carbon and nitrogen trace (<30 µg g−1) analysis in steel in comparison with traditional combustion methods associated with infrared detection. The development of an easily practicable process control method is described, including steel sample quality requirements, sample preparation optimization, and spectrometer operating conditions definition. In comparison with traditional combustion methods, spark-OES can decrease analysis delays by 2 min, with a corresponding reduction in the number of potential sources of error, and reduced analytical costs. Concerning the analytical figures of optimized spark-OES, limits of detection of 1 µg g−1 and 4.5 µg g−1 for C and N, respectively, have been demonstrated, with a precision in the range 1–2 µg g−1 for both elements at a level of 25 µg g−1 in steel. These figures are comparable to those obtained by use of combustion methods. Mean analytical differences between spark-OES and combustion analysis were evaluated to 0.6 µg g−1 and 1.8 µg g−1 for C and N, respectively, typically within both methods’ scatter. A successful transfer of ultra-low carbon spectrometric determination to five industrial process control laboratories has been carried out and is discussed. The organization of a round-robin for C determination in these laboratories resulted in an inter-laboratory scattering of 2.6 µg g−1.

A fas electrical detection spark source mass spectrometer with low elemental detection limits

An electrical detection spark source mass spectrometer has been developed which is faster, and which has lower elemental detection limits, than a photoplate detection instrument. The system is described, and the required sample analysis time is compared to that of a photoplate detector. Detection limits are given for 17 elements in the iron matrix. Many elements are detectable to less than 0.1 p.p.b. in this matrix.