Ground Shock Research Articles

Influence of Ground Impedance on Explosive Shock Wave Test Accuracy

Different propagation media have different effects on explosion shock wave attenuation during the propagation process. Studying the attenuation effect of dielectric impedance on shock wave pressure is of great significance to accurately measure shock wave pressure and evaluate the ammunition damage power. This study analyzes the measured shock wave overpressure peak data of TNT bare charge (60 kg) on soil, sand soil, and concrete grounds. Dimensional analysis was used to establish the medium impedance expressed by material densityρ, burst center distancer, equivalent TNT quality ω, and impedance coefficient η. Based on this, the mapping relationship between the propagation medium impedance coefficientηand the shock wave overpressure peak value was established. The test data were analyzed, and the results showed that in the same explosive environment, different ground propagation media had different impedance effects on the shock wave pressure, which is specifically expressed as ηsoil>ηsandsoil>ηconcrete; the modified calculation model of shock wave pressure established using material impedance coefficient has a fitting accuracy of 93.39% to the measured data, which can well represent the attenuation law between different ground propagation media and shock wave overpressure peak. This provides a theoretical basis and method for obtaining shock wave pressure data in explosion field and verifying validity of the test data.

Experimental and Numerical Studies on Ground Shock Generated by Large Equivalent Surface Explosions

Lately, explosions with a large TNT equivalent have occurred with alarming frequency causing severe structural damage. The damage suffered by these structures has been exacerbated by the ground shock generated during these large equivalent explosions. The aim of this work is to study the ground-shock propagation behaviors, the areas affected by them, and determine the minimum safe distance for various structures. To measure ground shock data at different distances from the epicenter of the blast, actual 1 t and 10 t TNT surface explosion experiments were performed. The velocity and attenuation coefficient of the ground shock generated by the 1 t TNT surface explosion were determined, and the empirical equations provided by the UFC 3-340-01 standard were validated. Additionally, numerical analyses were performed to analyze the effects of ground shocks on buildings around an explosion. The maximum particle vibration velocities and attenuation behaviors of a 10 t TNT surface explosion as well as the minimum safe distances for a variety of structures were obtained from the numerical analyses.

Shock Environment Assessment of Underground Arch–Wall Structures Subjected to Ground Shock

Shock Environment Assessment of Underground Arch–Wall Structures Subjected to Ground Shock

A Calculation Method for Ground Shock Effect Induced by Hypervelocity Impact

In the hypervelocity impact event, the strong ground shock effect is induced and may cause damage to underground engineering. The existing experimental methods and calculation procedures have difficulties in estimating the ground shock under complicated and large-scale impacts. Analytical and experiential methods independently prove that the shock energy radiating from a certain region underground is proportional to the mass of the materials in the region. Based on this relationship, an equivalent method is proposed to estimate the impact-induced ground shock with the help of abundant research results for underground explosions: The ground shock of two events can be seen as being equivalent as long as their crater sizes are the same. The calculated ground shock effect is compared with the measured values in impact experiments, and the results show great agreement.

Prediction method for ground shock parameters of explosion in concrete

The ground shock caused by the explosion is the main load causing the destruction of underground engineering. To study the propagation law of ground shock parameters in concrete, free-field tests of explosions at seven different scaled depth of burial (SDOB) have been carried out by using self-constructed concrete blocks with large-sizes. The results indicate that when the SDOB ranges from −0.25 to 1.2 m/kg1/3, the pressure and acceleration at the same scaled distance from the explosion center increase with the SDOB. When the SDOB reaches 1.2 m/kg1/3, the explosion energy is completely enclosed into underground, and the coupling coefficient reaches 1.0. Combined with the dimensional analysis theory, it is found that when the SDOB of an explosion is determined, the propagation law of pressure and acceleration presents a power function attenuation with the increase of the scaled distance from the explosion center. For the three different strength grades of C30, C50, C70 concrete under semi-buried explosion, the attenuation gradient of the pressure or acceleration is inconsistent with an increase in the scaled distance from the explosion center. The higher the concrete strength, the slower the gradient changes. By introducing the depth of burial (DOB) coefficient of equivalent energy, the prediction method of ground shock parameters at different SDOB of explosion and explosions with different mass of charge can be obtained, and it has high accuracy for calculating the ground shock parameters of explosion with large mass charge which verified by numerical simulation.

Influences of different crossing types on dynamic response of underground cavern subjected to ground shock

An intersecting cavern is a common structural form used in underground engineering, and its safety and stability performance directly control the service performance of the whole project. The dynamic responses of the three kinds of crossing type (＋-shaped, T-shaped, L-shaped) caverns subjected to ground shock were studied by numerical simulation. The velocity plus force mode boundary setting method was proposed in the coupled static and dynamic analysis of a deep underground cavern. The results show that, among the three types of crossing caverns, the ＋-shaped cavern is the most significantly affected by the dynamic action, followed by T-shaped, and then L-shaped caverns. The vault settlement, straight wall deformation, vault peak particle velocity, effective plastic strain of surrounding rock, and maximum principal stress and strain at the bottom of the lining of the straight wall increase with the increase of cavern span. The vault settlement, straight wall deformation, effective plastic strain of surrounding rock, and the maximum principal stress and strain at the bottom of lining to the straight wall decrease with the increase of lateral pressure coefficient, and the peak particle velocity at the vault increases. The variation is small compared with the change of cavern span. The influence range of the underground cavern intersection is two cavern diameters from the intersection centre. The bottom of the straight wall at the intersection is the weak part. It is suggested to thicken the support locally to improve the stability of the cavern.

Global response of underground structures subjected to ground shock with consideration of rise time

The global response of underground structures subjected to ground shock is investigated theoretically, experimentally and numerically. Non-zero rise time of ground shock is taken into account in the theoretical prediction. A scale model field test is conducted to examine the prediction accuracy, suggesting that the conventional method without ground shock rise time may result in over-design, while the present approach with rise time more reasonably predicts the global acceleration response which is the excitation to the affiliated devices inside the structure. Subsequently, shock response spectra of inside devices with mounting are established to evaluate the shock environment within the underground structure. Numerical simulation further complements the study. With combined approach, the key factors influencing the global response and in-structure shock of underground structures are discussed in detail, whose findings facilitate more reasonable design of underground structures in terms of shock environment assessment and mitigation.

Surface and Buried Explosions: An Explorative Review with Recent Advances

Partially or fully buried explosive on detonation releases a large amount of kinetic energy a part of which gets dissipated during creation of crater and the rest gets converted into ground shock. Both phenomena are of complex nature with involvement of non-linearity in both loading and material characteristics. This review aims at providing an insight into mechanisms involved during an event of buried explosion with varying degree of confinement. Factors affecting crater formation and ground shock propagation in media are discussed in detail. An overview of various prediction methods developed over the years based on dimensional analysis and theory of similarity to estimate crater dimensions and magnitude of ground shock and ground motion along with their limitations is presented. Prediction models used to define and optimise rock fragmentation distribution in surface mining operations are additionally reviewed and discussed. Various state-of-the-art experimental and numerical techniques are discussed in brief. Finally, it discusses the challenges involved in both experimental and numerical analysis and thereby provides alternative solutions and suggestions for further investigations in specific areas of lacuna.

Numerical study on the behavior of utility tunnel subjected to ground surface explosion

Over the past few decades, underground utility tunnels have become increasingly popular in many countries, especially in North America and China. Utility tunnels host multiple services’ infrastructures inside an accessible underground space that allows regular inspection, maintenance, and easy replacement. Previous studies on the safety of underground utility tunnels were mainly focused on the seismic performances, their dynamic behaviors under accidental/hostile blast loads were seldomly investigated. TM 5-855-1 is frequently employed to calculate the ground shock load and responses of buried structures. However, it is only applicable for the cases with a scaled distance larger than 0.4 m/kg1/3. The present study performs a comprehensive numerical investigation on the blast performance of utility tunnel subjected to ground surface explosion with small scaled distance. With validated material and structural models, a FEM model based on a utility tunnel project is established. The influence of charge weight on blast resistance of the utility tunnel is studied numerically. The effect of the reinforcement ratio, shear reinforcement arrangement, buried depth and wall thickness are investigated under the scenario of a sedan car bomb overhead explosion. It is found that over 200 kg TNT ground surface explosion will cause a great threat to the utility tunnel with 2 m burial depth (scaled distance smaller than 0.4 m/kg1/3). The shear reinforcement arrangement is of great importance to the blast resistance of the utility tunnel. The failure mode of the roof can be changed from combined shear and flexural failure to flexural failure, with an increased reinforcement ratio. Increased wall thickness and buried depth can improve the blast resistance of the utility tunnel as expected. Safety buried depth, and wall thickness for a sedan car bomb overhead explosion are discussed in the present study.

Estimation of the effect of surface blast on buildings

A surface blast generates both air pressure and ground shock on structures. Studies on the relative effects of the air pressure and ground shock on the responses of structures are meagre. In the present study, a preliminary estimate of the relative contributions of the ground shock and air pressure on building responses due to a surface blast is obtained by using an equivalent single-degree-of-freedom (SDOF) model. For this purpose, both linear and non-linear analyses of SDOF models of four reinforced concrete building frames of different heights are performed under different blast scenarios by varying the standoff distance and charge weight. In addition, the criticality of the surface blast in comparison to the free air blast is investigated with the help of the same SDOF models. The study reveals that the contribution of the ground shock is less than that of the air pressure on the responses of low-rise buildings. For high-rise buildings, the opposite trend is observed for smaller standoff distances (R < 15 m). Further, it is shown that the surface blast is more critical as compared to the free air blast for shorter buildings (fewer than six storeys), but the opposite holds good for taller buildings.

Analysis of Nonlinear Dynamic Characteristics in Saturated Soil with Blast Wave Diffusion and Damage to Its Effective Strength

Explosions release energy and generate shock waves. Analysis of how shock waves damage transmission media is crucial in protection engineering. Soil liquefaction due to explosions occurs in a highly nonlinear manner. The time point at which soil receives stress waves has yet to be determined. Therefore, this study analyzed the dynamic reaction and liquefaction phenomenon that protects soil after it has received a shock wave. A numerical analysis model was developed and verified through on-site explosion experiments for measuring ground acceleration and excess water pressure. Finite element analysis was employed to establish a numerical analysis model based on experimental site conditions. The LS-DYNA finite element program was used for numerical simulations. The analysis model had a three-dimensional structure with fluid–solid coupling and consisted of eight-node elements. To analyze the dynamic reaction and liquefaction phenomenon that occurs after soil has received a shock wave, a multimaterial arbitrary Lagrangian–Eulerian calculation model was combined with an element erosion algorithm. Meshes were overlapped to achieve fluid–solid coupling for subsequent analysis. The nonlinear dynamic reaction in soil triggered by shock wave transmission and the liquefaction of the material due to excess water pressure were analyzed. The research results showed that the relative errors between numerical analysis and experimental results were within 10%, verifying that the fluid–solid coupling numerical model developed in this study can effectively be used to analyze the dynamic reactions of materials receiving shock waves. Shock waves from both multisite time-delayed explosions and from continual explosions at a single nearby site lead to soil liquefaction. Compared with single-site explosions, multisite time-delayed explosions more easily trigger liquefaction in saturated sandy soil. Analysis of shock wave transmission characteristics and ground shock effects revealed that at a distance of 600 cm from the explosion center, the shock wave was substantially attenuated. The results of shear strain failure analysis indicated that soil liquefaction had an approximate area of scale distance Z = 30.81–64.89 cm/g1/3 and approximate depth of Z = 17.62 cm/g1/3. The results verify the validity of the numerical model and algorithm developed in this study, which can describe the liquefaction process of saturated soil under explosion loading.

Performance of masonry heritage building under air-blast pressure without and with ground shock

ABSTRACT In general, the blast-induced ground shock excites the foundation of the structure prior to the air-blast pressure because of the obvious reasons of difference in wave propagation velocities. However, the ground shock and air-blast pressure might act simultaneously on the structure for some standoff distance with a particular quantity of the explosive charge and cause severe damage than by air-blast pressure. Therefore, the ground shock and air-blast pressure both should be considered for estimating the structural response by such a near-field detonation. The objective of the present study is to assess the performance of the masonry heritage building under air-blast pressure without and with ground shock generated from on the ground spherical-shaped TNT explosive charge at varying standoff distances. The considered building is a heritage brick masonry structure in lime-surkhi mortar. The time lag between the ground shock and air-blast pressure for different scaled distances is evaluated. Stresses have been computed and are compared with the provisions of the Indian and Australian Standard Codes of Practice for Structural Use of Unreinforced Masonry. Also, the damage assessment has been carried out using the concrete-damaged plasticity (CDP) model available in the ABAQUS finite element program.

Blast-induced ground motion effect on dynamic response of a cylindrical vertical water tank with piled raft foundation

This paper studies to estimate the dynamic behavior of a demineralized water tank with a piled raft foundation system considering soil-pile-structure-fluid interaction to shock-ground motion. A three-dimensional finite element model of a coupled system is constituted in ANSYS software. Interaction between pile and soil is represented with the frictional contact element. The frictionless contact elements are utilized to model between the water and tank shell to allow for displacement of the free surface adjacent to the tank wall. Shell elements are used for the tank body and its vault. The dynamic analyses of the tank including soil-pile-structure-fluid interaction are presented by using shock response spectra. Ground shock acceleration time histories, generated by using a developed computer program based on Fortran programming language, produce shock response spectra. The effects of the different charge weights and distances from the charge center are examined in the analyses. Also, the effect of the water fill level in the tank and the number of piles is also investigated. The results of the research are presented with the directional displacements and equivalent stresses. It seen from the analyses that the dynamic responses of the tank increase with the charge weight, while decreasing with the charge center distance. Moreover, the water fill level and the number of piles extremely affect the displacement and stress values of the coupled interaction system.

Drift Response Evaluation of Buckling-Restrained Braced Frames (BRBFs) under Sequential Seismic Disturbances

Buckling Restrained Braced Frames (BRBFs) are reliable lateral load resisting systems featured with Buckling Restrained Braces (BRBs) to safeguard building frames against earthquake-induced damages. BRBs are axial yielding type metallic dampers characterised with stable and balanced hysteretic behavior along with excellent low-cycle fatigue capacity. However, BRBFs are susceptible to relatively large drift responses mainly due to low axial stiffness of BRBs resulted from the higher seismic response modification factor, R. In the event of sequential seismic actions immediately after mainshock may induce additional drift demands which may adversely affect the reusability and retrofitting or in the worst case, it may lead to premature collapse of the BRBFs. The present study encompasses on drift response demand evaluation of BRBFs under the combined action of the main shock and sequential secondary ground shocks. For the study, a medium-rise BRBF used in the past investigation is selected and re-designed as per the recommended prevailing seismic provisions. Nonlinear time history analysis of BRBFs under the action of a suit of sequential ground disturbances is carried out and the results are analysed in terms of drift responses.

Accurate measurement of ground shock with cellular solid

Ground vibration induced by subsurface detonations in geological media such as soil, termed as ground shock, has long been measured for decades, since the free field time history and peak intensity of the shock is the incident load to underground structures and important for design. To address the affected measurement accuracy due to acoustic impedance mismatch, a simple but effective approach is proposed: A sensor is embedded into a cellular solid casing before being installed in soil, whose acoustic impedance is designed to be the same as that of the surrounding soil to eliminate the reflection between the soil-casing interface while the relative motion between the sensor and casing is minimized. A field test and theoretical analysis are conducted to investigate the proposed approach, and it is found that the accuracy of ground shock measurement is improved.

Acoustic Overpressure Signals from Fully Confined and Vented Chemical Explosions

ABSTRACT We analyzed acoustic overpressure signals generated by overburied underground chemical explosions conducted in hard rock in New Hampshire in 2018. The explosions had comparable yields between 62.8 and 82.6 kg trinitrotoluene equivalent and were buried at depths between 12 and 13 m. Two explosions resulted in crater formation and gas venting, whereas the remaining explosions were fully confined and did not result in ground failure. Acoustic signals from the confined explosions were produced by the ground shock near ground zero. Acoustic signals from cratered explosions represent a combination of a ground shock signal and a time-delayed high-amplitude signal generated by gas venting. The cratering and venting occurred during the free-fall phase observed on the near-source accelerograms. We argue that the main reason for the cratering in this experiment is the low-rock porosity, preventing postexplosion pressure relief in the cavity and promoting long fracture formation during the unloading phase and subsequent containment failure. The ground-shock-induced signals were modeled using the Rayleigh integral of the near-source ground acceleration. The equations of nonlinear acoustics were used to model the observed gas venting signals produced by the gas flow from the explosion cavity to the surface. By comparing the near-source signals produced by venting to theoretical signals from surface blasts we have shown that the venting signals have significantly lower peak pressures and longer signal durations compared to surface blasts of the equivalent impulse. The observed amplitudes of the acoustic signals produced by the venting are significantly higher than the ground-shock related signals expected from overburied explosions. This is important to consider because higher acoustic amplitudes may potentially lead to errors in yield estimation.

Meltdown at Browns Ferry

Browns Ferry nuclear power plant (abbreviated BF NPP - three reactors) is located in Northwest Alabama within the New Madrid Seismic Zone (NMSZ) of the North American intraplate fault system. It has the same plan as Fukushima-Daiichi NPP in Japan, meaning it has the same design errors which led to the latter's meltdown in 2011 from flooding. Next to BF NPP is the very large Wheeler Reservoir. The NMSZ is capable of generating an 8+ class moment magnitude earthquake. BF NPP would then be at extreme risk from meltdown due to flooding from Wheeler of the backup diesel generators, both from a turbulent kinetic energy incoherent response, and the far more serious coherent tsunami-seiches response. It is strongly advocated that the Nuclear Regulatory Commission (NRC) create a mitigation plan to safeguard BF NPP, including a possible levee. However, liquefaction flooding with possible Wheeler Reservoir run-in causing inundation, is expected and so the flooding hazard may be an intractable problem. The NRC should include in its mitigation plan facility shutdown. The BF NPP antiquated design is built to only withstand Peak Ground Acceleration (PGA) of 0.1 g (g is the acceleration of gravity). Seismic study of the NMSZ anticipates PGA at BF NPP of around 0.25 g. Thus, ground shocks may disrupt the emergency cooling system from operating, even if the diesels are not flooded.

Analytical Solution of the Dynamic Response of Piles under Blast Waves

Several studies have addressed the effect of blasting on piles, but none of them has used an analytical method to find the piles blast-induced peak particle velocity (PPV). In this paper, the response of piles to ground shocks induced by surface blast is presented using mathematical analysis. In order to construct mathematical model, pile was modeled as beam and its displacements were calculated under blast wave equivalent dynamic load. Mathematical modeling of the problem resulted in a fourth-order nonhomogeneous partial differential equation which had to be solved so that the pile displacements and PPV could be found. Since it was not possible to solve the problem in time domain, first, time domain was converted to frequency domain by Fourier transform, and then, the differential equation was solved. Finally, the solution was transformed to time domain and blast-induced PPV was found using a computer code. In addition to verify results of analytical solution, a finite element modeling was generated, and its results are compared with analytical solution results. The techniques applied in this paper and its findings provide valuable information to find a safe distance between blast source and piles.

Feasibility Study on a Damage Assessment of Underground Structures by Ground Shock Using the Fast Running Model

Feasibility Study on a Damage Assessment of Underground Structures by Ground Shock Using the Fast Running Model

Numerical Analysis of Reinforced Concrete Piles under Blast Loads

Pile foundations are commonly used as foundation systems for high-rise buildings and bridges. This paper uses a fully coupled three dimensional numerical modelling procedure to study the performance of pile foundations subjected to ground shocks induced by surface explosions. The comprehensive numerical model includes the pile, surrounding soil, air and the explosive. Appropriate material models are incorporated and dynamic non-linear analysis is carried out using finite element techniques. The soil in which the pile is buried could influence the blast performance of the pile. A parametric study is hence carried out to evaluate the effects of soil properties of density, friction angle, cohesion and Poisson’s ratio on the blast performance of the pile. It is found that density and cohesion of soil have significant effects on the deflection of the pile under blast loading. Poisson’s ratio has some effect, but effect of the soil friction angle is not very significant. The findings of this study will serve as a benchmark reference for future analysis and design of pile foundations to blast loading.