Yingbin Zhang

Numerical Simulation in Rockfall Analysis: A Close Comparison of 2-D and 3-D DDA

Accurate estimation of rockfall trajectory and motion behaviors is essential for rockfall risk assessment and the design and performance evaluation of preventive structures. Numerical simulation using discontinuous deformation analysis (DDA) is effective and helpful in rockfall analysis. Up to now, there have been many reports on application of two-dimensional (2-D) DDA programs. In this paper, the major advantages of rockfall analysis using 2-D and extensions to three-dimensional (3-D) analysis are presented. A practical 3-D DDA code is demonstrated to be capable of simulating free falling, rolling, sliding, and bouncing with high accuracy. Because rockfall trajectories and motion behaviors can be described as combinations of these four types, this demonstration indicates that the implemented code is capable of providing reliable rockfall analysis. Finally, specific tests are conducted to compare 2-D and 3-D DDA rockfall analysis in predicting trajectory and dynamic behavior. The results indicate that 3-D DDA simulations are more appropriate for rough tree-laden inclined slopes in providing detailed spatial distribution, whereas 2-D DDA simulations have better efficiency for slopes dominated by valleys and ravines. These results can help in selecting the appropriate DDA simulation for rockfall analysis.

Nonlinear Site Models Derived from 1D Analyses for Ground‐Motion Prediction Equations Using Site Class as the Site Parameter

Nonlinear site models are an important part of ground‐motion prediction equations (GMPEs) and can be constructed in a number of ways. If numerous soil‐site strong‐motion records contain the effect of strong nonlinear soil response, the parameters for the nonlinear model can be a part of the regression parameters for GMPEs. It is also possible to derive nonlinear site models by numerical simulation. However, the number of strong‐motion records from Japan that contain the effect of strong nonlinear soil response is still too small to derive nonlinear site terms. We present a model of nonlinear site terms using site class as the site parameter in GMPEs based on a 1D equivalent linear model. The 1D model was constructed based on the shear‐wave velocity profiles from the KiK‐net strong‐motion stations with a wide range of site periods, soil depth, and impedance ratios. The rock‐site strong‐motion records were from different earthquake categories in Japan and the Pacific Earthquake Engineering Research Center dataset. Those records had a wide range of earthquake magnitudes, source distances, and peak ground accelerations. A random effects regression model was fitted to the calculated spectral amplification ratios, accounting for the effect of site impedance ratios, earthquake magnitudes, and source distances of the rock‐site records. We also designed a method to adjust the 1D model so it can be used in a GMPE, accounting for the fact that a 1D model is an overly simplistic assumption for many real strong‐motion recording stations in many parts of the world. Online Material: Tables of nonlinear site model parameters.

Reliability back analysis of shear strength parameters of landslide with three-dimensional upper bound limit analysis theory

It is essential to determine the shear strength parameters c and φ on the sliding surface for stability evaluation and engineering design of a landslide. In this study, a new parameter back analysis method is proposed by combining the 2D/3D upper bound method of limit analysis and reliability theory to accurately determine the shear strength parameters for a 3D slope with a single failure surface. The proposed reliability back analysis method overcomes the shortcomings of the traditional deterministic analysis method of slope stability that cannot take into account the randomness and uncertainty of geotechnical parameters. Based on the reliability theory, two methods were studied: first-order reliability method (implemented by spreadsheet and Matlab, called spreadsheet method and constrained optimization method, respectively, in this paper) and Monte Carlo simulation. The optimized values of c and φ were obtained by establishing only one balance equation with the consideration of the pore water pressure or other complex conditions, which can solve the problem of the back analysis of strength parameters for a single 3D sliding surface condition. The correlation research showed that the negative correlation between c and φ greatly affected the back analysis results, and the reliability index values were conservative without considering such a negative correlation. A case study for the back analysis of shear strength parameters is conducted based on a practical landslide model with a broken line slip surface slope in Zhuquedong village, Luxi town, Xiangxi County, Hunan Province, China, and a suggestion for the selection of landslide cross section is presented. The results show that the back analysis results determined by the reliability theory coincide well with the survey and experimental results. The proposed method is found to be more accurate and effective in determining the values of shear parameters than that of the traditional deterministic inversion method.

Ground‐Motion Prediction Equations for Subduction Interface Earthquakes in Japan Using Site Class and Simple Geometric Attenuation FunctionsGMPEs for Subduction Interface Earthquakes in Japan

The frequency content of strong ground motions from subduction slab earthquakes differs significantly from that of ground motions produced by other categories (tectonic locations: shallow crustal, upper mantle, and subduction interface) of earthquakes in subduction zones. In the last two decades, a large number of records from subduction slab events have been obtained in Japan. We present a ground‐motion prediction equation (GMPE) for this category of earthquakes. We used a large dataset from reliably identified slab events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, site classes were used as site terms, and nonlinear site amplification ratios were adopted. A bilinear magnitude‐scaling function was adopted for large earthquakes with moment magnitude M w≥7.1, with the scaling rates for large events being much smaller than for the smaller events. A magnitude‐squared term was used for events with M w<7.1 as well as the bilinear magnitude‐scaling function. We also modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within possible volcanic zones. We found that excluding the records from sites with inferred site classes improved the model goodness of fit. The within‐event residuals were approximately separated into within‐site and between‐site residuals, and the corresponding standard deviations were calculated using a random effects model. The separation of within‐event residuals into within‐site and between‐site components allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired. Online Material: Figures showing the distribution of between‐event residuals with respect to magnitude and fault‐top depth and the distribution of within‐event residuals with respect to magnitude and source distance.

The slope modeling method with GIS support for rockfall analysis using 3D DDA

Rockfall is the most frequent major hazard in mountainous areas. For hazard assessment and further countermeasure design, realistic and accurate prediction of rockfall trajectory is an important requirement. Thus, a modeling method to represent both geometrical parameters of slope and falling rock mass is required. This study, suggests taking the advantages of discontinues deformation analysis (DDA) and geographical information system (GIS). In this study, after developing a three dimensional (3D) DDA program, firstly a special element named contact face element (CFE) was introduced into 3D DDA; secondly, effectively modeling tools with GIS support were developed. The implementation of CFE also improves the efficiency of both the contact searching and solution process. Then a simple impact model was devised to compare the 3D DDA implemented directly with a sliding model with theoretical analysis to verify the reliability of the modified 3D DDA program and investigate the parameter settings. Finally, simulations concerning rock shapes and multi-rocks were carried out to show the applicable functions and advantages of the newly developed rockfall analysis code. It has been shown that the newly developed 3D DDA program with GIS support is applicable and effective.

Numerical Analysis of the Largest Landslide Induced by the Wenchuan Earthquake, May 12, 2008 Using DDA

The Daguangbao landslide, with an estimated affected area of about 7.3–10 million m2 and a volume of 750–840 million m3, is the largest landslide induced by the 2008 Wenchuan earthquake. The sliding mass travelled about 4.5 km and blocked the Huangdongzi valley, forming a landslide dam nearly 600 m high. In order to investigate the landslide progression and reproduce the post-failure configuration, the kinematic behavior of sliding mass was simulated by a dynamic discrete numerical analysis method called DDA that has been widely applied for geotechnical engineering problems due to its superiority in modeling the discontinuous material. In this simulation, based on the shape of failure surface and the character of slope topography, the whole slope was divided into three parts: base block, upper sliding mass, and lower sliding mass. Then two sliding masses were divided into the smaller discrete deformable blocks based on pre-existing discontinuities. Corrected real horizontal and vertical ground motion records were applied as volume forces act to the base block. The simulation results of landslide progression, sliding distance, and shape of post-failure were in good agreement with those obtained from post-earthquake investigation, description from the survivors. Therefore, the methodology applied in this paper is able to capture essential characteristics of the landslide and give a post-failure configuration.

Earthquake Induced a Chain Disasters

A strong earthquake not only cause directly damage on constructs but also can result in a series of natural disasters such as landslide, debris flow and flooding. These secondary disasters occurs as a chain disasters as shown in Fig. 1. A strong earthquake can induce a large amount of landslides. And then, a large scale landslide can create a landslide dam when its debris fill into and stop a river. The water impounded by a landslide dam may create a dam reservoir (lake). While the dam is being filled, the surrounding groundwater level rises and causes back-flooding (upstream flooding). And because of its rather loose nature and absence of controlled spillway, a landslide dam is easy to fail catastrophically

Stability Analysis of Breakwater Under Seepage Flow Using DDA

In order to explain the mechanism of breakwater failure destroyed by the massive tsunami triggered by the 2011 off the Pacific coast of Tohoku Earthquake, a failure mode is presented from the view of geotechnical engineering by considering the seepage force rather than the failure modes by using the existing failure mechanisms released in Coastal Engineering. It is proposed that a pseudo-steady state flow condition inside of the rubble mound can be created by high hydraulic head difference, which becomes possible due to the behaviour of the long period of tsunami wave. At first, a piping initiated local failure model is proposed and the phenomenon of piping initiated failure by laboratory model experiments is re-enacted. Also, the piping-initiated local failure of a breakwater is re-enacted by DDA simulation. And then, DDA is applied to the stability analysis of breakwater under seepage flow with new proposed displacement based failure criteria which can consider both global and local failure. A relationship between the height of a tsunami with the safety factor of a breakwater has been obtained. The results have been validated by both the laboratory model experiments and comparison with simplified Bishop Method.Copyright

Distinct Element Modelling of a Landslide Triggered by the 5.12 Wenchuan Earthquake: A Case Study

The Donghekou landslide, which was triggered by the 2008 Wenchuan earthquake, was estimated to have a volume of 24 million cubic meters and resulted in numerous properties and lives lost. The landslide is considered a typical rapid long run-out earthquake-induced event, but the kinematic processes are not well understood. The main objectives of this study were to numerically model the landslide progression and to reproduce the post-failure configuration. We first built a physical model of the slope based on the topography and geology of the source area from field investigations. The corrected baseline and filtered actual ground motions were then used as the volume force acting on the base block, and the kinematic process and mechanics of the throwing phenomenon were modelled using a 2-D discrete element code. We used the non-linear Barton–Bandis criterion to accurately simulate the behaviour of joints. The size effect of the enormous landslide was also considered. The results of the simulations agreed well with those obtained from post-earthquake field investigations. A sensitivity analysis of several related parameters that may control the dynamic movements of the Donghekou landslide was discussed in detail. The results show that: the seismic force and the residual friction angle were main factors that affect the run-out of landslides.

The performance improvement of MgB2 prepared by the Mg diffusion method with the MgB4 addition

The Mg diffusion method with the MgB4 addition is carried out to improve the MgB2 application performance. The precursor B powders and MgB4 powders are uniformly mixed as the ratio of (1- 4x) : x mol %, (x = 0, 1, 2, 5, 10 and 20). The mixed powders are pressed into bulks and the excess of Mg powders are added into the iron pipe. Then the heat treatment is carried out under the pure argon atmosphere. Finally, the high-quality MgB2 bulks are achieved with high density. The superconducting critical temperature of the MgB2 bulks decreases slightly with the increase of MgB4 addition content. However, the critical current density increases significantly at the high-fields. It is perhaps due to the small-sized MgB4 impurities in MgB2 bulk function as effective pinning centers and generates significant lattice distortion in the MgB2 lattice to yield improved J c performance in the applied field. It suggests that further improvement in the superconducting properties of the MgB2 added with the MgB4 will be possible via a better control of the composition and processing parameters. This also provides a reference for improving the preparation of MgB2 wire.