Overbreak and underbreak are the crucial problems during the blasting excavation of underground tunnels owing to their effects on the construction costs and operational safety. A critical challenge ...facing overbreak and underbreak control is the difficulty in developing guidelines with respect to various and complex engineering conditions. In this study, a series of field measurements of overbreak and underbreak using the FocusS 150 laser scanner were performed in a deep roadway of the Kaiyang phosphate mine, China. The distribution and extent of the overbreak and underbreak surrounding the roadway contour were accurately analyzed in accordance with the collected point cloud data. Subsequently, a simplified three-dimensional model was established to simulate the blasting excavation of pre-stressed roadway using the explicit dynamic analysis code LS-DYNA. A comparison of numerical and measurement results revealed that the proposed model was a reliable tool to simulate the overbreak and underbreak induced by blasting excavation. Thereafter, the influences of uncontrollable geological factors such as in situ stress conditions and controllable blasting factors including contour hole spacing (S), charge concentration (β) and decoupled coefficient (ζ) as well as stemming were further numerically investigated. The simulation results indicated that the lateral pressure coefficient significantly affected the distribution pattern of the overbreak and underbreak, while the stress magnitude contributed to their extents. Moreover, a comparison of the simulation findings and the field measurement data indicated that the minimal extents of the overbreak and underbreak corresponding the optimal contour blasting results were obtained at S = 0.70 m, β = 0.9 kg/m and ζ = 2.5, respectively. Furthermore, the contour blastholes stemmed with sand created smaller damage to the periphery rock mass of roadway and enhanced the utilization efficiency of explosive energy. The research findings of this study provide important implications for similar blasting excavation projects.
Due to different tensile and compressive properties of rock material, the corresponding tensile and compressive damage evolution show major differences. To investigate the tensile and compressive ...damage evolution in deep cut blasting with different in-situ stresses, an improved Holmquist-Johnson-Cook (HJC) material model considers the tensile and compressive damage separately is developed. The improved HJC model is implemented into LS-DYNA via a user-defined subroutine in this study. Then, a numerical model with different in-situ stresses loading schemes is modelled. Numerical simulation results show that in-situ stress can inhibit the development of tensile damage evolution, while promote the development of compressive damage evolution. The overall damage zone presents a decreasing trend with the increase of in-situ stress, because the tensile damage is more sensitive than the compressive damage for rock material. In addition, the maximum principal stress can determine the development of the direction of damage. Further, for a field test of blind cut raise in deep, the actual in-situ stress values are loaded on the numerical model. Then, in order to overcome the difficulties caused by in-situ stress, the cut blasting design is optimized by reducing hole spacing. Subsequently, the optimized cut parameters are applied in the blind cut raise. However, the one-step raise excavation method is adjusted to two steps to ensure success due to a serious borehole deviation between drilling and design drawing. After these steps, the formation of the blind cut raise with 8.7 m depth is met the requirements of design.
•One dimensional finite element model incorporating direct shear behavior.•Member and direct shear response of RC slabs subjected to blasts.•Parametric study on direct shear PI curves for RC ...slabs.•Investigation of flexural member behavior on direct shear response.
In this paper, both the moment–curvature flexural behavior and the direct shear behavior are incorporated into a numerically efficient one dimensional finite element model, utilizing Timoshenko Beam Theory, to determine the member and direct shear response of one-way reinforced concrete slabs subjected to blasts. The model is used to undertake a case study to demonstrate the flexural member response behavior during the direct shear response and is then used to carry out a parametric study to better understand the interaction of the flexural member response and the direct shear response. This is done by comparing pressure impulse curves corresponding to direct shear failure for one-way reinforced concrete slabs with varying depth, span and support conditions. The results aim to provide insight to facilitate the development of more accurate simplified methods for determining the direct shear response of blast loaded reinforced concrete members, such as the single degree of freedom method.
•A novel approach to simplify the fully confined blast loads was developed in this manuscript.•Centroid of the confined blasts is the key factor affecting structural response.•This simplification ...method can be used for analysing response of ductile structural members.
A blast in a fully confined environment magnifies the blast loading resulting in more serious damage to surrounding structural members. To protect critical infrastructure against confined blasts, the confinement effects need to be fully taken into consideration. In this paper, a procedure to simplify the highly irregular nature of fully confined blast loads was developed for the purpose of being incorporated into a structural response analysis tool such as a Pressure Impulse Diagram. It was found that when the centroid of the simplified confined blast load was close to the centroid of the actual confined blast load, whilst keeping the total impulse the same, the response of a member caused by the simplified confined blast load was similar to that produced by the actual confined blast load. This concept to accurately simplify an irregular confined blast based on structural response was then used to construct the method of simplification. This method of simplification was then tested using a structural response model for the purposes of validation and suitability. A comparison was then made against the current UFC guidelines predictions to assess its accuracy.
Previous molecular characterization of Mycoplasma pneumoniae in China focused only on one or two cities. In this study, we characterized 835 samples from patients suspected to be infected with M. ...pneumoniae; these samples were collected in 2016 from pediatric patients from different regions of China. Multiple locus variable number tandem repeat analysis (MLVA), P1-restriction fragment length polymorphism (RFLP) analysis, and sequencing of the domain V of 23S rRNA were performed to compare genotype distribution across different locations. Two-hundred-and-thirteen samples tested positive for M. pneumoniae by PCR. P1 types were identified in 154 samples: 78.6% were type I and 21.4% were type II. Type I was the most prevalent genotype in five locations, except Nanjing where type II was the most common type (p < 0.01). Five distinct MLVA types were identified in the 172 samples. Genotype M4-5-7-2 was predominant at all locations, except Nanjing where type 3-5-6-2 was the most common (p < 0.01). Macrolide resistance-associated mutations were identified in 186 (76.3%) samples. The resistance rate differed with the location. This study showed that genotypes and macrolide resistance rate differed across China. The most prevalent genotype in China remains M4-5-7-2/P1-1. The resistance rate decreased, along with changes to the M4-5-7-2 genotype.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In structural engineering practice, understanding the behaviour of steel under extreme loading conditions is essential for accurate prediction of material response when subjected to a combination of ...severe load scenarios such as collision by heavy objects and a following fire. Hitherto, the combined effects of high strain loading and subsequent elevated temperature have not been widely investigated on the mechanical properties of structural steel. A comprehensive test program is carried out to investigate the post-impact fire properties of Grade 350 steel under well-defined conditions, the results of which are reported in this paper. Coupon specimens have undergone interrupting high strain rate (HSR) tensile loading at impact level, controlled locally at different levels of elongation, to account for different deformation states. Three different damage levels are introduced with respect to the displacement corresponding to the ultimate stress (fu). Subsequently, the partly damaged specimens are subjected to steady-state quasi-static tensile loading to failure at temperatures ranging from ambient to 600 °C. The overall stress–strain relationship, as well as the mechanical properties of pre-damaged steel, are presented at elevated temperatures and compared to those of each individual loading scenario. The test results demonstrate that the effects of these combined actions are profoundly different from those in which the structure is subjected to either high strain rate or thermal loading individually. It is shown that the strength and ductility of mild steel is significantly dependent on the rate of loading, the pre-deformation history and the temperature to which it is subsequently exposed. This necessitates the development of models which take into account the coupled effect of high strain rate and temperature in rational fire analysis and design of steel structures.
Seismic safety is considered to be one of the key design objectives of AP1000 nuclear power plant (NPP) in strong earthquakes. Dynamic behavior, damage development and aggravation effect are studied ...in this study for the three main components of AP1000 NPP, namely reinforced concrete shield building (RCSB), steel vessel containment (SVC) and reinforced concrete auxiliary building (RCAB). Characteristics including nonlinear concrete tension and compressive constitutions with plastic damage are employed to establish the numerical model, which is further validated by existing studies. The author investigates three earthquakes and eight input levels with the maximum magnitude of 2.4 g and the results show that the concrete material of both RCSB and RCAB have suffered serious damage in intense earthquakes. Considering RCAB in the whole NPP, significant damage aggravation effect can be detected, which is mainly concentrated at the upper intersection between RCSB and RCAB. SVC and reinforcing bar demonstrate excellent seismic performance with no obvious damage. Keywords: AP1000 NPP, Plastic damage, Strong earthquake, Simulation, Nonlinear behavior
A surface explosion generates both ground shock and airblast pressure on nearby structures. Although ground shock usually arrives at a structure foundation earlier than airblast pressure because of ...the different wave propagation velocities in geomaterials and in the air, ground shock and airblast might act on the structure simultaneously, depending on the distance between the explosion center and the structure. Even though they do not act simultaneously, ground shock will excite the structure and the structure will not respond to airblast pressure with zero initial condition. Therefore, an accurate analysis of structure response and damage to a nearby surface explosion should take both ground shock and airblast pressure into consideration. But current practice usually considers only airblast pressure. Many empirical relations are available to predict airblast pressure. Most of them, however, only predict peak pressure values. The primary objective of this study is to define simultaneous ground shock and airblast forces that can be easily applied in structural response analysis. Parametric numerical simulations of surface explosions are conducted. Empirical expressions of airblast pressure time history as a function of surface explosion charge weight, distance to structure, structure height, as well as the ground shock time history spectral density function, envelope function and duration are derived. Time lag between airblast pressure and ground shock to structure is also determined. The empirical formulae are all given in analytical forms and they can be used in structural response analysis to surface explosions.
•The geometry centroid is defined for arch dam performance evaluation.•The space-time monitoring model for the centroid deformation is established.•The nonlinear PCA-LSSVM prediction model is ...developed.•Accurate prediction is achieved on the world’s highest arch dam.
The purpose of this study is evaluating the arch dam performance through modelling on the defined centroid deformation with the measured data. The space-time distribution characteristics of the arch dam deformation are firstly identified through the comparative analysis of typical arch dams with height of 200–300 m in China. Subsequently, the geometry centroid of the deflection curve of an arch or a cantilever, composed of multiple monitoring points, is defined to indicate the global structural behavior of the arch dams. In order to evaluate the future performance of the arch dams, the space–time monitoring model for the centroid is established by introducing its initial coordinates based on the traditional statistical models. Afterwards, a novel centroid prediction model is developed based on the least squares support vector machine to balance the empirical risk and generalization ability of the statistical regression models. The input factors of the prediction model are determined in advance by principal component analysis to eliminate the multi-collinearity and reduce the computational complexity. The model construction and validation of the centroid deformation method are implemented on the world’s highest arch dam through evaluating its structural behavior and predicting the development trend. The results can provide strong technical support to better grasp its performance during long-term operation.
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•Ability to model small models cut from SFRC blocks just as the way test specimens are prepared.•Ability to truly reflect the interfacial behaviour of fibre and mortar, which is ...explicitly modelled in this work.•Relatively high computational efficiency achieved while applying meso-scale modelling for high fidelity.
Based on Delaunay triangulation, a 3D meso-scale model is successfully developed and verified. This approach modelling fibre and concrete separately and linking them with slide line contact has the capability to truly reflect the interfacial behaviour of fibre and mortar, and thus achieve high fidelity of numerical simulations. However, meso-scale modelling usually means tremendous complexity and long computational time. This paper proposes a model to achieve relatively high computation efficiency, as well as accuracy. Besides, the model has the potential to deal with small specimens cut from steel fibre reinforced concrete (SFRC) blocks.
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