Gaohui Wang

Comparative Study of the Dynamic Response of Concrete Gravity Dams Subjected to Underwater and Air Explosions

AbstractThe response of dam structures subjected to explosion shock loading is a key element in assessments for the dam antiknock safety and antiterrorism applications. The physical processes during an explosive detonated in underwater/air and the subsequent response of structures are extremely complex, involving many complex issues such as the explosion, shock wave propagation, shock wave–structure interaction, and structural response. In addition, there exists a significant contrast in wave propagation phenomena in the water and the air medium due to their different physical properties and interface phenomena. In this paper, a fully coupled numerical approach with combined Lagrangian and Eulerian methods is used to simulate the dynamic responses of a concrete gravity dam subjected to underwater and air explosions. The shock wave propagation characteristics from explosions in water and air are simulated and compared. The damage profiles of concrete gravity dams subjected to underwater and air explosions ...

Bayesian-Based Hybrid Simulation Approach to Project Completion Forecasting for Underground Construction

AbstractReal-time simulation is powerful in forecasting the completion probability of long-term projects with repetitive tasks but fails to consider the time-varying uncertainty of inputs caused by construction process variabilities. In this paper, an improved method is introduced for predicting the time-varying probability of project completion of ongoing underground cavern group projects using Bayesian updating techniques. Within a tailor-made hierarchical simulation model, the Bayesian approach is adopted to constantly update duration distributions of unfinished project activities according to onsite data. The probability of project completion can therefore be increasingly refined during the process. The methodology is further explained in a case study where its feasibility and advantage over traditional approaches are verified. The success may also be replicated in addressing other similar time-varying uncertainty issues inherently present in almost all construction projects.

Coring damage mechanism of the Yan-tang group marble: combined effect of stress redistribution and rock structure

Micro-cracks induced by the coring process in rock samples retrieved from high in situ stress conditions may seriously influence the evaluation of rock properties. This paper presents work performed to study the coring damage mechanism of marble samples containing a kind of inclined grey (or brown) ribbon-like stripes, whose mineral components are calcite and dolomite. These samples were obtained from the Jin-ping Second Stage Hydropower Station (JPII) in China, and the coring site is located in the strata of the 5th layer of the Yan-tang group marble in the Middle Triassic system (T2y5). A special stress-relief coring scheme was carried out to collect rock samples under different stress levels at one position, then the coring damage in these samples was examined by sonic wave testing and acoustic emission monitoring under uniaxial loading conditions, and a numerical simulation was also conducted to investigate the stress path experienced by a rock core during sampling. The results indicate that for the specific Jin-ping Yan-tang group marble (T2y5) sample, the coring damage can be attributed to two kinds of micro-cracks: horizontal micro-cracks induced by stress redistribution, and vertical or sub-vertical micro-cracks mainly derived along the special rock structure. The horizontal cracks, generated by the tensile stress induced during coring, are mostly trans-granular ones, and can be regarded as the main reason for coring damage. In contrast, the vertical (or sub-vertical) micro-cracks are mainly caused by the combined effect of coring-induced high deviatoric stress and the inclined grey-brown ribbons, and it usually occurs along grain boundaries. At a relatively low stress level, the shear failure may preferentially occur along special rock structures. However, the impact of the micro-structure on cracking reduces significantly with the increase of stress level, and then the induced tensile stress may become the dominant factor leading to horizontal cracks or disking.

Experimental and numerical investigation of the effect of blast-generated free surfaces on blasting vibration

In rock blasting, the blast of the former delay always creates new free surfaces for the latter. And the blast-generated free surfaces play a critical role in blasting vibration. A quantitative assessment of the blast-generated free surfaces on blasting vibration was carried out through experimental and numerical studies. Blasting vibration from the first and followed delays in the same row was compared and analysed in two field experiments. A coupled numerical approach with smooth particle hydrodynamics and dynamic finite-element methods was also conducted to simulate the fragmentation process and the blasting vibration from different delays in the same row. The results show that with the increasing number of free surfaces, the peak particle velocity decreases nonlinearly, and if the blasting geometry parameters and charge weight are the same in each blast delay in the same row, the explosive detonated in a first delay always produce higher vibration compared to those followed delays. The charge weight of the first delay in every row should be reduced by certain percentage if the same peak particle velocities are expected from followed delays in the same row.

Frequency-Dependent Attenuation of Blasting Vibration Waves

The dominant frequency, in addition to the peak particle velocity, is a critical factor for assessing adverse effects of the blasting vibration on surrounding structures; however, it has not been fully considered in blasting design. Therefore, the dominant frequency-dependent attenuation mechanism of blast-induced vibration is investigated in the present research. Starting with blasting vibration induced by a spherical charge propagating in an infinite viscoelastic medium, a modified expression of the vibration amplitude spectrum was derived to reveal the frequency dependency of attenuation. Then, ground vibration induced by more complex and more commonly used cylindrical charge that propagates in a semi-infinite viscoelastic medium was analyzed by numerical simulation. Results demonstrate that the absorptive property of the medium results in the frequency attenuation versus distance, whereas a rapid drop or fluctuation occurs during the attenuation of ground vibration. Fluctuation usually appears at moderate to far field, and the dominant frequency generally decreases to half the original value when rapid drop occurs. The decay rate discrepancy between different frequency components and the multimodal structure of vibration spectrum lead to the unsmooth frequency-dependent attenuation. The above research is verified by two field experiments. Furthermore, according to frequency-based vibration standards, frequency drop and fluctuation should be considered when evaluating blast safety. An optimized piecewise assessment is proposed for more accurate evaluation: With the frequency drop point as the breakpoint, the assessment is divided into two independent sections along the propagating path.

Long-Term Structural Responses of Orifices in Gravity Dams Considering Thermal and Creep Effects

AbstractA numerical simulation method is presented to analyze the thermal and creep effects on structural performance of orifices in gravity dams during construction and operation period. Time-dependent conditions such as the changes of gravity load and water pressure, the concrete viscoelastic material properties, and the variations of environment temperature are all considered for numerical modeling. Compared temperature results obtained from heat transfer analysis with the monitored data around orifice, proposed model is verified to be a reliable tool for thermal-structural coupling analysis. This paper discusses temperature distributions of orifice concrete surfaces in contact with water or air. Tensile stress evolutions arising from temperature variations are investigated correspondingly. Subsequently, cracking risks of orifice with different environments and locations are calculated to evaluate orifice structural performance. Thermal and creep effects on the distributions of tensile stress and crack...

The Influence of Initial Cracks on the Crack Propagation Process of Concrete Gravity Dam-Reservoir-Foundation Systems

A scaled-down 1:40 model of a gravity dam with the initial notch in the upstream wall is analyzed using the XFEM. Furthermore, the cracking process of Koyna dam including dam-reservoir-foundation interaction under the 1967 Koyna earthquake is also simulated numerically by employing the XFEM. The computed distribution of cracking damage is consistent with other numerical methods and the model experiment. Subsequently, a typical concrete gravity dam with different crack cases is employed as a numerical example. The effects of the initial crack position and length on the crack propagation and seismic response of dam-reservoir-foundation systems are studied.

Feasibility of Applying Phase-Based Video Processing for Modal Identification of Concrete Gravity Dams

Hydraulic structures have been considered as one of the most essential civil infrastructures, and play a critical role in developing countries throughout the history for water storage and electricity generation. Due to the importance and the catastrophic consequences of unexpected failures in hydraulic infrastructures, monitoring and maintenance of dams should be handled very meticulously and with high precision. Among several measurement techniques as a specific modern non-contact sensing technology, optical/video information is getting more and more attention to interpret structural responses and system status awareness. By means of processing the acquired video, a full-field system information is available which may be applied later to Experimental Modal Analysis, Structural Health Monitoring (SHM), System Identification, etc. Such non-contact full-field sensing technologies avoid the installation of a gigantic number of conventional sensors in the occasions of large dimension. Within this context, the feasibility of applying Phase-Based Motion Estimation (PME) and video magnification has been studied for structural identification purposes on the concrete gravity dam subject to white noise excitations. Firstly, the PME and motion magnification algorithms are validated by the comparison of a lab-scale cantilever beam test and the numerical simulation. Next, the modal dynamic procedure in ABAQUS is carried out and the time history response of the dam is obtained. Then the simulated motion video of the dam is exported and processed using PME and magnification. The video processing results are finally compared with the results from frequency procedure in ABAQUS. The results obtained prove the concept of using PME and video magnification as a successful methodology in the modal identification of large-scale concrete gravity dams.

Combined effects of penetration and explosion on damage characteristics of a mass concrete target

With the development of the precision guidance technology, the earth-penetrating weapon (EPW) is a huge threat to infrastructures. The objective of this research is to investigate the damage characteristics of mass concrete targets under the combined action of penetration and explosion. The validity of the penetration model is discussed by reproducing a previous experimental test reported in the literature. Meanwhile, a field test about the internal explosion in a concrete cube is conducted to verify the validity of the internal explosion model. Subsequently, damage characteristics of a mass concrete target from the upper part of a concrete gravity dam are discussed when subjected to the combined action of the penetration and explosion. In order to improve the structural performance of the mass concrete target to penetration and explosion loadings, high strength concrete material is selected as a preventive measure. Penetration processes and damage patterns of the mass target with normal and high strength concrete material are compared. Then, three internal explosion models are presented to investigate the influence of the initial penetration damage on the failure characteristics of the mass concrete target. The results show that the resistance of the mass target to the combined action of the penetration and explosion can be enhanced significantly by using the high strength concrete material. The initial penetration damage has a significant influence on the damage processes of the mass concrete target subjected to internal blast loading.

Earthquake Direction Effects on Seismic Performance of Concrete Gravity Dams to Mainshock–Aftershock Sequences

Abstract The aim of the present study was to examine the effects of earthquake direction on seismic performance of concrete gravity dams subjected to seismic sequences. For these purposes, two seismic incident directions are applied along horizontal axes to assess the maximum structural demands. Nonlinear dynamic analyses of the dam-reservoir-foundation system subjected to as-recorded mainshock–aftershock sequences are conducted to investigate the effects: 1) direction of single earthquake events; 2) direction of seismic sequences; 3) aftershock polarity; and 4) earthquake intensity. The results indicate that earthquake direction can significantly change the damage propagation processes of concrete gravity dams.