Yaqun Liu

Numerical Modeling of Wave Transmission Across Rock Masses with Nonlinear Joints

Rock masses usually consist of intact rocks and discontinuities such as faults, joints and bedding planes. The discontinuities not only govern the mechanical behaviors of rock masses but also influence wave propagation in rock masses (Goodman 1976; Pyrak-Nolte 1996). Studying wave propagation across joints is the basis for the analysis of dynamic responses and stability of jointed rock masses, which is of great interest to geophysics, mining and underground construction. Currently, a number of theoretical analyses or numerical simulations have been carried out on stress wave propagation across rock joints. The typical analytical method is the displacement discontinuity method (DDM) (Schoenberg 1980; Pyrak-Nolte et al. 1990). The DDM was coupled with the characteristic method (CM) to study normal wave propagation across linear and nonlinear joints (Zhao and Cai 2001; Zhao et al. 2006a, b). Based on DDM, the interaction between blast waves and a single rock joint with arbitrary incident angle was effectively analyzed by Li and Ma (2010) and Li et al. (2011). In addition, Zhao et al. (2012), Li et al. (2012), Zhu and Zhao (2013) proposed the propagator matrix method, the time-domain recursive method and the virtual wave source method, respectively, and applied them to study wave propagation obliquely across a set of parallel joints. Later, Li (2013) adopted and extended the time-domain recursive method to analyze the effect of nonlinear joints on wave propagation. As an alternative, numerical methods have been more popularly applied with the development of computer technology nowadays. Cai and Zhao (2000), Fan et al. (2004), Zhao et al. (2006a), Barla et al. (2010) and Sun et al. (2013) conducted a series of numerical studies on normally incident wave propagation across a single or a set of parallel rock joints. Lei et al. (2007), Deng et al. (2012), Zhu et al. (2013) and Deng et al. (2014a, b) studied oblique incidence across a set of parallel joints and intersecting rock joints with linearly elastic behavior. In addition, Lemos (1987), Brady et al. (1990), Zhao and Cai (2001), Zhao et al. (2006b), and Zhao et al. (2008) carried out numerical studies on wave transmission across a single joint or a set of parallel joints with nonlinear deformation behavior. & Haibo Li hbli@whrsm.ac.cn

Shear Wave Propagation Across Filled Joints with the Effect of Interfacial Shear Strength

The thin-layer interface model for filled joints is extended to analyze shear wave propagation across filled rock joints when the interfacial shear strength between the filling material and the rocks is taken into account. During the wave propagation process, the two sides of the filled joint are welded with the adjacent rocks first and slide on each other when the shear stress on the joint is greater than the interfacial shear strength. By back analysis, the relation between the shear stress and the relative tangential deformation of the filled joints is obtained from the present approach, which is shown as a cycle parallelogram. Comparison between the present approach and the existing method based on the zero-thickness interface model indicates that the present approach is efficient to analyze shear wave propagation across rock joints with slippery behavior. The calculation results show that the slippery behavior of joints is related to the interfacial failure. In addition, the interaction between the shear stress wave and the two sides of the filling joint influences not only the wave propagation process but also the dynamic response of the filled joint.

Experimental Study on the Seismic Efficiency of Rock Blasting and Its Influencing Factors

The seismic efficiency of a blast is the percentage of seismic energy in the total energy delivered by the explosives. It is a key indicator of the blast effects in civil engineering and seismic exploration. A method to determine seismic efficiency has been proposed based on the assumption of spherical wave radiation in an indefinite elastic medium and has been used in a series of blast tests performed at the construction site of a nuclear power plant. Analysis of the influencing factors of seismic efficiency shows that seismic efficiency increases with an increasing explosive charge and stemming length of the blastholes, while it decreases with an increasing decoupling coefficient. Generally, seismic efficiency is markedly lower in bench blasts than in paddock blasts due to free surface effects. Under any circumstances, the seismic energy only accounts for a few percent of the explosive energy. A comparison with theoretical solutions proves that the errors in the present method are low and acceptable in engineering. Therefore, some practical measures have been proposed to improve or lower the seismic efficiency according to the specific requirements of the blast operations.

Numerical evaluation of topographic effects on seismic response of single-faced rock slopes

This paper investigates the seismic responses of homogenous single-faced rock slopes subjected to vertically propagating shear waves by numerical simulations in order to explore the topographic amplification of ground motion. The horizontal and vertical topographic amplification factors both on the free surface and in the slope are evaluated using parametric studies focusing on slope geometry, rock material, and input motion with the two-dimensional finite element code LS-DYNA. Comparison of the results obtained in this study with those of previous numerical analyses available in the literature and with the provisions of the existing seismic codes shows good agreement. Both qualitative and quantitative insights into the topographic amplification effects on the seismic responses of single-faced slopes are presented in this study. The results show that both slope geometry and rock material have great influences on the horizontal and vertical amplification factors. As for input motion, the magnitude and duration have negligible effects on the amplification factors when rock materials are homogeneous and elastic. However, the frequency extent of input motions has great impact on the amplification factors. It is also indicated that the modern seismic codes may underestimate the amplification effects of ground motion. Nevertheless, modification of the provisions of the codes may require more convincing evidence from reliable field experiments.

Numerical study on oblique incidence across rock masses with linear and nonlinear joints

The two-dimensional discrete element program Universal Distinct Element Code (UDEC) is applied to simulate stress wave propagation across linear and nonlinear rock joints with arbitrary incident angles. The numerical study for stress wave obliquely impinging upon a single linearly elastic joint is firstly conducted. For this case, the wave-type transformation is analyzed and the variations of transmission and reflection coefficients with joint stiffness and incident angle are investigated numerically. It is found that numerical results agree well with those from existing theoretical methods, which demonstrates the feasibility of using UDEC to simulate the propagation of obliquely incident stress wave across a single joint. Furthermore, the transmission of obliquely incident waves across a set of parallel joints is investigated and compared with analytical solutions when the joints are linearly and nonlinearly elastic, respectively. The numerical results indicate that the parameters, such as the joint number, the joint spacing and the mechanical property of joints, have great influence on wave propagation through joints. The results in the present study may provide a reference for revealing stress wave propagation across jointed rock masses and the responses of rock masses subjected to dynamic loads.

A New Method for Determining Thickness of River Incision Layers in High Mountain and Deep River Valley Areas

In high mountain and deep river valley areas, geological materials of river valley evolution are often missing or incomplete. To address this problem, tectonic movements at project site are analyzed using mathematical statistics based on the analysis of formation and evolution history of river terraces, and then a new method to determine the thickness of river incision layers is proposed. Taking Jiata dam area at the Western Route of South-to-North Water Transfer Project in China for an example, the reliability and reasonability of the proposed method are validated through a case study.

Pile Foundation In Situ Test in Seaside Soil-Squeezing Backfill Zone

Design and construction of pile foundation in seaside zone face the specific geotechnical conditions. The in-situ geotechnical tests need to be conducted. The quality feature of the piles is assessed according to the results of in-situ small strain test, coring test and pile stress test. For different depth of rock and soil stratum and pile bearing layer, the trial pile stress distribution results and follow-up stress and settlement monitoring provide engineering parameters for design and pile-driving.

Failure Mechanism of an Idealized Layered Rock Slope Subjected to Seismic Loads

The dynamic response of an idealized layered rock slope with a single joint subjected to seismic loads is investigated using the three dimensional distinct element code in the present study. Based on the numerical modeling, the variations of the stresses of the blocks close to the joint and the deformation of the joint are discussed, and the progressive failure mechanism of the slope is analyzed. It is found that, with the increasing excitations, the tensile stresses and the areas of tension zones in the upper part of the slope near the joint have increased gradually. In addition, the normal displacement at the upper part of the joint also becomes larger and larger, which leads to the gradual split of the upper part of joint. Hence the contact area for blocks at both sides of the joint has decreased, which gradually results in the decrease of the cohesion of the joint. When the induced shear stress for the joint under the applied excitations exceeds its shear strength, the potential sliding blocks will slip along the joint. The results in this paper may provide references for the study on failure mechanism of complicated layered rock slopes subjected to dynamic loads.

Parametric Study on Cylindrical P-Wave Propagation

Considering the energy variation, cylindrical P-wave propagation across a linear elastic rock joint is analyzed. Then parametric studies are carried out to investigate the effects of the wave source distance,the joint stiffness and the incident waveforms on wave propagation across a jointed rock mass.

Influence of Slope Gradient on Distribution Rule of Geostress Field in River Valleys

During the construction of hydropower station, the change of slope gradient in river valleys often takes place. In order to study influence of slope gradient change on distribution rule of geostress field, the three dimensional unloading models under different slope gradients were established by finite difference software (FLAC3D). After numerical simulation, the results were as follows: (1) The phenomenon of stress concentration at the bottom of river valleys was obvious, which appeared the typical stress fold. Both the depth of stress concentration zone and the principal stress values significantly increased with the increment of slope gradient. (2) Maximum principal stress values increased less in shallow part of upper bank slope (low stress zone) but increased more in the nearby slope foot with the increment of slope gradient, causing great difference in geostress field of bank slope. (3) There was some difference in released energy of bank slope due to slope gradient change in river valleys. In order to distinguish the difference, stress relief zone was further divided into stress stably released zone and stress instability released zone. Finally, take Ada dam area of the western route project of South-to-North Water Transfer as an example, the results by numerical simulation were reliable through comparing the distribution rule of geostress field for the dam, which could provide important reference for stability of the design and construction of steep and narrow river valleys.