Seismic Elastic Response of Earth Dams with Canyon Interaction

This paper deals with the seismic response of earth and rockfill dams, using elastic models that consider the deformability of the surrounding medium and effects of spatial variation of the seismic excitation. A finite-element-based method has been developed in which the dam is idealized as a shear beam and the surrounding medium as a halfspace. The model can simulate canyons of arbitrary shape, oblique SH-wave seismic excitation, and inhomogeneous distribution of elastic and damping properties of the dam and the surrounding medium. The methodology is illustrated by simulating the upstream-downstream response of the La Villita Dam, an earth and rockfill dam located near the epicenter of the 1985 Mexico earthquake, for various conditions of the surrounding foundation material. Results show that for models that include both foundation flexibility and radiated energy, the earthquake response of the dam is, overall, only a fraction of that obtained under the assumption of a rigid canyon. Exploiting these results may have a potentially beneficial effect in the retrofit of existing dams and design of new ones.     

Effects of Environmental Action on Thermal Stress Analysis of Karaj Concrete Arch Dam

A three-dimensional finite element analysis is carried out to determine the thermal stresses of a concrete arch dam. Appropriate heat transfer boundary conditions in the dam body are used for air and reservoir temperature as well as solar radiation variations. A finite element model is used to determine annual variation of temperature and thermal stress in the body of Karaj arch dam in Iran as a case study. The rate of convergence of the numerical solution is examined. The temperatures predicted by the model are satisfactorily compared with the instrumentation records at Karaj Dam. Results of the finite element analysis show that probable cracks occur in a very narrow region of the downstream face. Thermal loads have the most significant effects for causing downstream cracks in comparison with self-weight and hydrostatic loads. The cracked areas of the downstream face conform to the regions that have the highest temperature in the downstream face. This can be associated with the solar radiation, which shows that two-dimensional analysis of an arch dam cannot yield accurate results and three-dimensional analysis is necessary.     

Replacement of Pontesei Bridge, Italy

Pontesei Dam is a major concrete dam built after World War II across the canyon of Maè Creek (Valle di Zoldo) in the Italian Eastern Alps. Just upstream from the dam, a gully discharges water from steep mountains. The only road serving the dam and the entire valley upstream of the dam runs along the mountainside and crosses the gully. In 1959, an exceptionally rainy season caused a flood, which destroyed the bridge. A temporary Bailey bridge was subsequently built by the army, but in 1990 it was decided to design a new bridge. The main challenges posed to the designer included building the new deck and the abutments underneath the Bailey bridge without disrupting traffic, hoisting the deck just to the bottom of the Bailey bridge, and finally substituting the new deck for the Bailey bridge in one day. Other problems included the instability of the rock mass at the abutments and the gravitative convergence of the two sides of the gully. This paper describes the design, construction, and testing of the bridge replacement and the bridge abutments.     

Composite Element Analysis of Gravity Dam on a Complicated Rock Foundation

This paper presents the formulation and application of a composite element method, which is intended for numerical modeling of discontinuous rock masses. This method allows analysis of fractured rock masses using regular meshes that do not need to rigorously respect the orientations and positions of discontinuities. It can be incorporated in conventional finite-element programs. The performance of this method are illustrated through its use for the analysis of the mechanical behavior of the Baozhusi gravity dam which is constructed on a complex rock foundation.     

Embankment Dam on Liquefiable Foundation—Dynamic Behavior and Densification Remediation

Earthquake-induced liquefaction is a major concern for embankment dam safety. Many liquefaction-induced earth embankment failures or near failures have been reported around the world during various earthquakes. Such embankment damages were particularly destructive when the underlying saturated granular soils liquefied, resulting in cracking, settlement, lateral spreading, and slumping of the embankment. Through a series of four highly instrumented geotechnical centrifuge model tests, seismic behavior of a zoned embankment dam with saturated sandy soil foundation was studied under moderate earthquake conditions. The beneficial effects of foundation densification were investigated. Valuable insights into the dynamic behavior of the employed embankment–foundation systems are provided. Test results suggest that there may be an optimum depth of densification treatment beneath an earth dam beyond which the reduction of the earthquake-induced deformations are relatively minor and that relatively small and isolated zones (e.g., at depth) of loose material within a densified volume of soil may not impair the overall effectiveness of treatment and do not necessarily result in damaging displacements.     

地下开采条件下干堆尾矿库稳定性分析

湖北三鑫金铜股份有限公司(简称"三鑫公司")拟建干堆尾矿库位于生产采场之上,影响其稳定性的主要因素有不良工程地质条件、早期采矿遗留下的采空区、地下开采引起的岩层移动。在工程地质分析的基础上,提出了地基改良和工程优化方案。通过建立的三维有限元数值分析模型,对干堆尾矿库设计方案进行评价,提出适合三鑫公司干堆尾矿库的第三种工况,并建议尾矿库堆积高度为20 m。     

Earth Dam on Liquefiable Foundation and Remediation: Numerical Simulation of Centrifuge Experiments

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress–strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.     

Numerical Simulation of Street Canyon Flows with Simple Building Geometries

The velocity and pressure fields of the flow over street canyons formed by groups of buildings are studied numerically. The flow fields are computed by solving the time-dependent incompressible Navier–Stokes equations using the fractional step method. The numerical model is validated by simulating flows over a square cylinder at different Reynolds numbers. The Strouhal numbers, which reflect the dynamic flow characteristics, agree well with published experimental data over a wide range of Reynolds numbers. The wind field model is then applied to two street canyon configurations. First, flows inside street canyons formed by four identical buildings are simulated. The incidental flow is raised by the most upstream building and becomes parallel to the ground at the rooftop level of the fourth building downstream, resulting in a clockwise rotating vortex in downstream street canyons with an inflow from left to right. Second, flows inside street canyons formed by two identical buildings are simulated. In this case, a primary eddy that is counterclockwise rotating may be formed due to flow separation at the front corner of the upstream building. A clockwise rotating primary eddy is formed in the wake area of the separate zone above the street canyon, which drives the counterclockwise rotating eddy in the street canyon. The result indicates that rooftop level flows cannot be assumed parallel to the ground as some modelers have done in their studies. Studies also show that flow regimes in the street canyon will remain unchanged while the inflow velocity is greatly increased from 0.1 to 6.0?m/s. In addition, the wind velocities in the street canyon have a linear response to the inflow velocity.     

Analysis of Arch Dams Using Coupled Trial Load and Block Element Methods

This paper presents the use of the trial load method and the block element method with elastoviscoplastic discontinuities for analysis of arch dams. The arch dam is considered as an arch-cantilever system and the foundation as a block element system. With the displacement compatibility condition at the contact surface of the dam and the foundation (including abutment), the governing equations of the arch dam and foundation are established. These methods are used for the analysis of the double curvature arch dam with complex geology conditions of the Xiaowan Hydroelectric Project in China. The deformation and stress states in both the dam body and the foundation are determined. Furthermore, the stability safety factors of the foundation and the abutment are calculated at the same time, which allows for an optimal design of the arch dam considering the strength, the deformation and the stability of the dam and foundation.     

某尾矿坝三维动力响应分析

针对尾矿坝的动力响应问题,基于完全非线性动力分析理论,采用Flac3D软件,分析计算了某尾矿库1#副坝地震响应,获得了该尾矿坝在地震作用下的加速度放大系数、动位移及库区液化的动力响应特性。结果表明:在动力荷载作用下,坝体竖直方向和水平方向加速度放大系数分别为1. 47和2. 91,具有较大的安全储备;坝体水平方向位移变化不大,且液化范围较小,不影响尾矿坝的整体稳定性。     

Ground motion spatial variability effects on seismic response control of cable-stayed bridges

The spatial variability of input ground motion at supporting foundations plays a key role in the structural response of cable-stayed bridges (CSBs); therefore, spatial variation effects should be included in the analysis and design of effective vibration control systems. The control of CSBs represents a challenging and unique problem, with many complexities in modeling, control design and implementation, since the control system should be designed not only to mitigate the dynamic component of the structural response but also to counteract the effects of the pseudo-static component of the response. The spatial variability effects on the feasibility and efficiency of seismic control systems for the vibration control of CSBs are investigated in this paper. The assumption of uniform earthquake motion along the entire bridge may result in quantitative and qualitative differences in seismic response as compared with those produced by uniform motion at all supports. A systematic comparison of passive and active system performance in reducing the structural responses is performed, focusing on the effect of the spatially varying earthquake ground motion on the seismic response of a benchmark CSB model with different control strategies, and demonstrates the importance of accounting for the spatial variability of excitations.     

Dynamic Response of Plates on Elastic Foundation to Moving Loads

A procedure incorporating the finite strip method and a spring system has been developed and applied to treat the dynamic response of plate structure resting on an elastic foundation to moving loads. The response to a single moving concentrated load is first investigated and then the effects of velocity, elastic foundation stiffness, moving path, and distance between multiple moving loads are studied. The response under a moving harmonic load with constant velocity is finally treated and the effect of the load frequency is investigated. Results indicate that the foundation stiffness and the velocity and frequency of the moving load have significant effects on the dynamic response of the plate and on resonant velocities. Some of these findings might find use in practical applications.     

Influence of In Situ Factors on Dynamic Response of Piedmont Residual Soils

The in situ chemical and physical weathering of igneous and metamorphic rocks, indentified as the process of formation of Piedmont residual soils, is a fairly well understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock∕soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to mid-level amplitudes of vibration. The paper presents laboratory test results and correlations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from different depths were tested in a Resonant Column (RC) device. The specimens tested were SM and ML soils according to the USCS classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses due to foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G∕Gmax, fall within the same range as those recently reported by other authors in similar soils.     

Coupled Creep and Seepage Model for Hybrid Media

The permeability coefficient of a rock mass depends mainly on the aperture of the joint and the porosity of the block, which may alter with time when creep of the rock mass is taken into account. Therefore, a coupled creep and seepage model for hybrid media is proposed in this paper. Large-scale and strongly permeable joints are simulated according to their spatial distributions, while other discontinuities are treated as equivalent continuum. Based on the fundamental mechanism of creep effects on the permeability of the rock mass, together with empirical equations for hydraulic conductivity, coupled creep and seepage equations for filled joints, rough joints, and equivalent continuum are proposed. By application of these equations, governing equations for the coupled creep and seepage model are deduced. A simplified numerical solution is proposed to solve the coupled creep and seepage model. The coupled model is shown to simulate the evolvement of seepage, deformation, and stress field in a gravity dam. By comparing the results derived by coupled and uncoupled models, it is concluded that the coupling between creep and seepage should be taken into account when performing engineering design of large dams.     

Physically Based Model for Evaluation of Rock Scour due to High-Velocity Jet Impact

Scour of rock may occur downstream of dam spillways, as a result of the impact of high-velocity jets. The phenomenon is traditionally assessed by means of (semi-) empirical methods. These partially neglect basic physical processes responsible for rock mass breakup. Therefore, a model to evaluate the ultimate depth and time evolution of scour in jointed rock is presented. The model is based on near-prototype scaled experimental investigations of transient water pressures in artificially created rock joints and on a numerical modeling of the measured pressures. It describes two different ways of rock mass destruction, i.e., failure by instantaneous or progressive breakup of closed-end rock joints, and failure by dynamic ejection of single rock blocks. The corresponding computational methods are easily applicable to practice, without neglecting relevant physics. The basic principles are outlined and applied to the well-known scour hole at Cabora-Bassa Dam.     

基于三维模型的矿床品位分布规律研究——以陕西龙头沟金矿为例

以陕西龙头沟金矿为例,通过收集矿山已有的勘查和生产数据,选用Micromine软件平台,并采用普通克里格方法进行插值,建立了矿床的品位矿块模型。在总结矿床地质特征的基础上,从矿化域变异函数拟合参数、矿块模型三维展布及品位分布特征等方面,分析了龙头沟金矿床品位空间分布规律。结果表明：龙头沟金矿床低品位矿基本连续,而高品位矿连续性差;高品位工业矿体在深部倾向延伸方向的连续性好于走向方向。矿化带在55线以东走向延伸和深部倾向延伸均未封闭,且临近EW向与NW向构造交会部位,具有较好的找矿前景,可作为下一步工作靶区。     

Three-Dimensional Joint/Interface Element for Rough Undulating Major Discontinuities in Rock Masses

Major civil engineering structures are being constructed now a days in complex geological environment with faults, shear zones, and other major discontinuities. These major discontinuities can cause a variety of problems in both surface and underground constructions. Unfavorably dipping major discontinuities may create unstable conditions in underground openings and contribute to the deformations of a rock mass under external static loading. Hence, rock–structure interaction analysis should simulate arbitrarily oriented rough and undulating major discontinuities within the rock mass, as well as the undulating interface along the structure and the rock mass such as dam foundations and underground excavations intersected by fault/shear zones. Realistic simulation of the mechanical behavior of rock joints is a prerequisite for successful numerical modeling of discontinuous rocks. When joint modeling is designed to include different degrees of joint roughness, dilation, and aperture, then realistic response depends upon the appropriate constitutive models and the way these parameters interact with stress change. Due to low values of the normal and tangential module, a unique characteristic of a rock discontinuity is that dilation may occur as soon as relative slip takes place and this may significantly alter the stress distribution, particularly around an underground excavation. In view of these practical requirements, a generalized formulation of a three-dimensional joint/interface element has been proposed here to account for dilatancy, roughness, and undulating surface of discontinuities.     

K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona

At least 13 times during the Pleistocene Epoch lava flowed into the inner gorge of the Grand Canyon and formed lava dams, as high as 600 m, that temporarily blocked the flow of the Colorado River. K-Ar ages on these lava dams indicate that the seven youngest formed within a short period of time between about 0.6 and 0.4 mega-annum (Ma). The physiography of the lava dam remnants within the canyon shows that each dam was destroyed by erosion, the Colorado River rapidly reaching its pre-existing grade level, before the next dam was emplaced by new eruptions. The total time for emplacement and destruction for an individual lava dam was probably as little as 0. 01-0.02 million years. The K-Ar ages of the two oldest dams, the Lava Butte dam (0.577 +/- 0.054 Ma) and the Prospect dam (0.684 +/- 0.051 Ma) are somewhat younger than the physiography of their remnants suggest.     

Structural Stability of Concrete Gravity Dams Strengthened by Rockfill Buttressing: Hydrostatic Load

Rockfill buttressing is often considered to strengthen existing gravity dams that have inadequate stability to resist the estimated hydrostatic and seismic loads. Various simplified methods for static stability analyses of composite concrete–rockfill dams, which represent the rockfill as equivalent forces, are discussed. Numerical analyses of composite dams using nonlinear rockfill and interface constitutive models are then considered. Hydrostatic stability analyses of a 35?m composite dam are carried out to compare the results obtained from simplified methods and numerical analyses. Parametric analyses are performed to investigate the effects of various modeling parameters such as the friction angle of the concrete–fill interface, the friction angle of the concrete–foundation interface, and the reservoir elevation during the fill placement. Numerical analyses results show that lowering the reservoir prior to construction of the rockfill does not have a significant effect on the stress response of the strengthened dam in the case analyzed. For design purpose, it is shown that the simplified minimum/maximum earth pressure method is always on the safe side irrespective of the concrete–rockfill friction angle.     

Seismic Response of Liquid Storage Tanks Incorporating Soil Structure Interaction

A frequency domain method is presented to compute the impulsive seismic response of circular surface mounted steel and concrete liquid storage tanks incorporating soil-structure interaction (SSI) for layered sites. The method introduces the concept of a near field region in close proximity to the mat foundation and a far field at distance. The near field is modeled as a region of nonlinear soil response with strain compatible shear stiffness and viscous material damping. The shear strain in a representative soil element is used as the basis for strain compatibility in the near field. In the far field, radiation damping using low strain soil response is used. Frequency dependent complex dynamic impedance functions are used in a model that incorporates horizontal displacement and rotation of the foundation. The focus of the paper is on the computation of the horizontal shear force and moment on the tank foundation to enable foundation design. Significant SSI effects are shown to occur for tanks sited on soft soil, especially tanks of a tall slender nature. SSI effects take the form of period elongation and energy loss by radiation damping and foundation soil damping. The effects of SSI for tanks are shown to reverse the trend of force and moment reduction under earthquake loading as is usually assumed by designers. The reasons for this important effect in tank design are given in the paper and relate to the very short period of most tanks, hence, period lengthening may result in load increase. A comparison is made with SSI effects evaluated using the code SEI/ASCE 7-02. Period elongation is found to be similar for relatively stiff soils when assessed by the code compared with the results of the dynamic analysis. For soft soils, the agreement is not as good. Code values of system damping are found to agree reasonably well with an assessment based on the dynamic analyses for the range of periods covered by the code. Energy loss by material damping and radiation damping is discussed. It is shown that energy loss may be computed using the complex dynamic impedance function associated with the viscous dashpot in the analytical model. The proportion of energy loss in the translation mode compared to that dissipated in the rotational mode is addressed as a function of the slenderness of the tank. Energy loss increases substantially with the volume of liquid being stored.