Seismic behavior of an inverted T-shape flexible retaining wall via dynamic centrifuge tests

In the design procedure for a retaining wall, the pseudo-static method has been widely used and dynamic earth pressure is calculated by the Mononobe–Okabe method, which is an extension of Coulomb’s earth pressure theory computed by force equilibrium. However, there is no clear empirical basis for treating the seismic force as a static force, and recent experimental research has shown that the Mononobe–Okabe method is quite conservative, and there exists a discrepancy between the assumed conditions and real seismic behavior during an earthquake. Two dynamic centrifuge tests were designed and conducted to reexamine the Mononobe–Okabe method and to evaluate the seismic lateral earth pressure on an inverted T-shape flexible retaining wall with a dry medium sand backfill. Results from two sets of dynamic centrifuge experiments show that inertial force has a significant impact on the seismic behavior on the flexible retaining wall. The dynamic earth pressure at the time of maximum moment during the earthquake was not synchronized and almost zero. The relationship between the back-calculated dynamic earth pressure coefficient at the time of maximum dynamic wall moment and the peak ground acceleration obtained from the wall base peak ground acceleration indicates that the seismic earth pressure on flexible cantilever retaining walls can be neglected at accelerations below 0.4 g. These results suggest that a wall designed with a static factor of safety should be able to resist seismic loads up to 0.3–0.4 g.     

Elastk-plastic dynamic analysis of structures using known elastic solutions

A numerical method is shown to analyse the dynamic elastic-plastic responses of those structures with known elastic solutions. The displacement at one point at time t caused by a unit load applied at another point at zero time, called dynamic influence coefficient, is calculated from the known elastic solutions. Incremental plastic strain is accounted for by a set of additional incremental loads, so the stiffness matrix and the eigenvectors do not vary with time. From the incremental load including that caused by the incremental plastic strain, the displacement vs. time of the structure is obtained. This method is applied to simply supported beams with bilinear stress-strain relations with different strain-hardening rates and to a simply supported elastic-ideally plastic rectangular plate. This procedure can be extended to structures with no available known analytical elastic solutions. For these structures, the elastic solutions can be obtained by the finite element method.     

烟囱模型动力试验研究

烟囱结构的地震反应及其破坏机理是地震工程界长期讨论的问题，对高耸烟囱结构的模型试验研究是研究此工程问题的重要方法，作者通过对45m和180m高大烟囱结构的动力模型试验，对烟囱结构的地震反应及其破坏机理进行了模型试验研究，得出了有意义的结论，为这一问题的理论研究提供了有力的依据。     

Dynamic centrifuge testing of a cantilever retaining wall

Two correctly-scaled model cantilever retaining walls of different stiffnesses were tested under dynamic loading conditions in a centrifuge. A medium-dense fine sand was retained with a range of backfill slopes. For the centrifuge model, an earthquake-generating mechanism was designed to produce seismic shaking equivalent to that generated at ground surface in the epicentral area of an earthquake of approximate magnitude 5–5. The response of the model retaining walls to the input dynamic motion was measured by strain gauges, pressure transducers and accelerometers. From the measurements plots were constructed of moment, shear, pressure and displacement over the height of the walls as a function of time. The results are compared with calculations based on the quasi-static Mononobe-Okabe theory. Although the calculated resultant force is in reasonable agreement with the experiments, the moments can be substantially different. Residual values of all parameters at the end of shaking are considerably greater than the initial static values. It is recommended that dynamic behaviour be incorporated in the earthquake design of retaining walls.     

Shaking table tests and dynamic analyses of masonry wall buildings with frame-shear walls at lower stories

This paper describes shaking table tests of three eight-story building models： all are masonry structures in the upper stories, with or without frame-shear walls of one- or two- stories at the bottom. The test results of damage characteristics and seismic responses are provided and compared. Then, nonlinear response analyses are conducted to examine the reliability of the dynamic analysis. Finally, many nonlinear response analyses are performed and it is concluded that for relatively hard sites under a certain lateral stiffness ratio （i.e., the ratio of the stiffness of the lowest upper masonry story to that of the frame- shear wall story）, the masonry structure with one-story frame-shear wall at the bottom performs better than a structure built entirely of masonry, and a masonry structure with frame-shear wall of two stories performs better than with one-story frame- shear wall. In relatively soft soil conditions, all three structures have similar performane. In addition, some suggestions that could be helpful for design of masonry structures with ground story of frame-shear wall structure in seismic intensity region VII, such as the appropriate lateral stiffness ratio, shear force increase factor of the frame-shear wall story, and permissible maximum height of the building, are proposed.     

Shaking table tests and dynamic analyses of masonry wall buildings with flame-shear walls at lower stories

This paper describes shaking table tests of three eight-story building models: all are masonry structures in the upper stories, with or without frame-shear walls of one- or two- stories at the bottom. The test results of damage characteristics and seismic responses are provided and compared. Then, nonlinear response analyses are conducted to examine the reliability of the dynamic analysis. Finally, many nonlinear response analyses are performed and it is concluded that for relatively hard sites under a certain lateral stiffness ratio (I.e., the ratio of the stiffness of the lowest upper masonry story to that of the frame-shear wall story), the masonry structure with one-story frame-shear wall at the bottom performs better than a structure built entirely of masonry, and a masonry structure with frame-shear wall of two stories performs better than with one-story frame-shear wall. In relatively soft soil conditions, all three structures have similar performane. In addition, some suggestions that could be helpful for design ofmasomy structures with ground story of frame-shear wall structure in seismic intensity region VII, such as the appropriate lateral stiffness ratio, shear force increase factor of the frame-shear wall story, and permissible maximum height of the building, are proposed.     

基于多弹簧模型的钢框架结构三维弹塑性地震反应分析

文章提出了考虑剪切变形弹塑性刚度影响的多弹簧模型的空间梁柱单元,用于反复加载下钢构件的数值模拟。应用多轴应力状态下的塑性应力-应变关系理论,在单元模型中考虑了弹塑性区域剪切变形对单元的弹塑性刚度的影响,针对单元模型的塑性区长度和弹簧布置两个参数,文中给出了合理建议取值。数值模拟分析表明,所提出的单元模型能够很好地模拟钢构件的弹塑性性能。在此基础上,以多高层钢结构商业设计软件MTS为平台,进行三维钢框架结构弹塑性动力时程分析模块的开发。最后,文章对一纯钢框架结构足尺振动台试验进行数值模拟,模拟分析结果表明,本文所提出的多弹簧单元模型及开发的动力分析模块能够较好地模拟钢结构在地震作用下的弹塑性性能。     

挡土墙地震反应非线性波动模拟

本文运用解耦近场非线性波动数值模拟方法研究挡土墙地震反应，为反映墙土体系在地震作用下的位移机制，引入了Desai薄层单元模拟墙土间接触面，并采用双线型本构关系作为接触面单元和土体的非线性模型，在此基础上给出了解决P—SV问题的非线性显式有限元时域递推公式，为进一步发展非线性波动数值模拟技术提供了有益经验。为验证本文方法及适用性，将数值模拟结果与Zeng，X．和Madabhushi，X．P．G．等的离心机试验和弹塑性数值模拟结果进行对比。结果表明：墙土体系加速度、挡土墙顶底相对滑移、沉降和墙体倾角等同离心机试验模拟结果基本吻合，与弹塑性数值模拟结果相似。     

加筋土挡墙变形特性研究进展

从理论研究、模型试验、数值模拟和实地调查四方面评述了国内外加筋土挡墙变形特性的研究成果。理论研究方面分析并归纳了研究方法和挡墙变形计算公式;模型试验和数值模拟介绍了已有的静、动力模型试验成果和常用的数值模拟方法。实地调查介绍了现有调查结论及部分规范、标准规定的位移限值。通过4种方式的研究情况介绍,指出了当前加筋土挡墙变形特性研究中存在的一些不足,并进一步阐述了今后的研究方向。     

Static and dynamic elastic moduli of organic-rich chalk

The elastic moduli and anisotropy of organic-rich rocks are of great importance to geoengineering and geoprospecting of oil and gas reservoirs. In this paper, we probe into the static and dynamic moduli of the Ghareb–Mishash chalk through laboratory measurements and new analytical approaches. We define a new anisotropy parameter, ‘hydrostatic strain ratio’ (Ω), which describes the differential contraction of anisotropic rocks consequent to hydrostatic compression. Ω depends on the C₁₁, C₁₂, C₁₃ and C₃₃ stiffness constants of a transversely isotropic material, and therefore enables a unique insight into the anisotropic behaviour of TI rocks. Ω proves more sensitive to anisotropy within the weak anisotropy range, when compared with Thomsen's ε and γ parameters. We use Ω to derive static moduli from triaxial compression tests performed on a single specimen. This is done by novel employment of a hydrostatic-deviatoric combination for transversely isotropic elastic stiffnesses. Dynamic moduli are obtained from acoustic velocities measurements. We find that the bedding-normal velocities are described well by defining kerogen as the load-supporting matrix in a Hashin–Shtrikman model (‘Hashin–Shtrikman (HS) kerogen’). The dynamic moduli of the Ghareb–Mishash chalk in dry conditions are significantly higher than the static moduli. The dynamic/static moduli ratio decreases from ∼4 to ∼2 with increasing kerogen content. Both the static and dynamic moduli decrease significantly with increasing porosity and kerogen content. The effect of porosity on them is two times stronger than the effect of kerogen.     

油水两相渗流多孔介质弹性波传播动力学模型研究

弹性波在储层渗流场中的传播与衰减规律是研究波场强化采油动力学机理的重要基础.基于等效流体理论和饱和静态流体弹性波传播Biot理论,建立油水两非混相流体渗流条件下储层多孔介质中弹性波传播的动力学模型,通过算例求解与分析,发现含油水两相渗流储层多孔介质中同时存在着3种纵波P1、P2、P3和1种横波S;受频率和含水饱和度的影响,各波波速和品质因子呈现出不同变化规律,4种体波波速与频率、饱和度正相关,P1、P2波品质因子与饱和度正相关,P3和S波品质因子与饱和度负相关;最后,通过与传统静态弹性波模型结果对比,进一步分析了宏观渗流场对弹性波传播特征的影响规律,为揭示低频人工地震波辅助强化采油技术的动力学机理和工艺参数优化提供了重要理论依据.

     

Pseudo-dynamic active force and pressure behind battered retaining wall supporting inclined backfill

Knowledge of seismic active earth pressure behind rigid retaining wall is very important. Commonly used Mononobe–Okabe method considers pseudo-static approach, which gives the linear distribution of seismic earth force. In this paper, the pseudo-dynamic approach, which considers the effect of primary and shear wave propagations, is adopted to calculate the seismic active force. Considering the planar rupture surface, the effect of wide range of parameters like inclination of retaining wall, inclination of backfill surface, wall friction and soil friction angle, shear wave and primary wave velocity, horizontal and vertical seismic coefficients are taken into account to evaluate the seismic active force. Results are presented in terms of seismic coefficients in tabular form and variation of pressure along the depth.     

Centrifuge modelling of flexible retaining walls subjected to dynamic loading

This paper outlines the results of an experimental program carried out on centrifuge models of cantilevered and propped retaining walls embedded in saturated sand. The main aim of the paper is to investigate the dynamic response of these structures when the foundation soil is saturated by measuring the accelerations and pore pressures in the soil, displacements and bending moment of the walls. A comparison among tests with different geometrical configurations and relative density of the soil is presented. The centrifuge models were subjected to dynamic loading in the form of sinusoidal accelerations applied at the base of the models. This paper also presents data from pressure sensors used to measure total earth pressure on the walls. Furthermore, these results are compared with previous dynamic centrifuge tests on flexible retaining walls in dry sand.     

Seismic performance of bar-mat reinforced-soil retaining wall: Shaking table testing versus numerical analysis with modified kinematic hardening constitutive model

Reinforced-soil retaining structures possess inherent flexibility, and are believed to be insensitive to earthquake shaking. In fact, several such structures have successfully survived destructive earthquakes (Northridge 1994, Kobe 1995, Kocaeli 1999, and Chi-Chi 1999). This paper investigates experimentally and theoretically the seismic performance of a typical bar-mat retaining wall. First, a series of reduced-scale shaking table tests are conducted, using a variety of seismic excitations (real records and artificial multi-cycle motions). Then, the problem is analyzed numerically employing the finite element method. A modified kinematic hardening constitutive model is developed and encoded in ABAQUS through a user-defined subroutine. After calibrating the model parameters through laboratory element testing, the retaining walls are analyzed at model scale, assuming model parameters appropriate for very small confining pressures. After validating the numerical analysis through comparisons with shaking table test results, the problem is re-analyzed at prototype scale assuming model parameters for standard confining pressures. The results of shaking table testing are thus indirectly “converted” (extrapolated) to real scale. It is shown that: (a) for medium intensity motions (typical of M_s≈6 earthquakes) the response is “quasi-elastic”, and the permanent lateral displacement in reality could not exceed a few centimeters; (b) for larger intensity motions (typical of M_s≈6.5–7 earthquakes) bearing the effects of forward rupture directivity or having a large number of strong motion cycles, plastic deformation accumulates and the permanent displacement is of the order of 10–15 cm (at prototype scale); and (c) a large number of strong motion cycles (N>30) of unrealistically large amplitude (A=1.0 g) is required to activate a failure wedge behind the region of reinforced soil. Overall, the performance of the bar-mat reinforced-soil walls investigated in this paper is totally acceptable for realistic levels of seismic excitation.     

Analytical evaluation of the dynamic distress of rigid fixed-base retaining systems

The current study proposes an analytical closed-form solution for the dynamic distress of rigid fixed-base retaining systems aiming at evaluating the main assumptions and limitations of the pertinent available elasticity-based methods. The new solution is actually an extension of the well-known model of Wood and is capable of evaluating the dynamic distress of either a single or a pair of rigid fixed-base walls interacting with each other, in the case of harmonic base loading. Wall distress is mainly evaluated in terms of dynamic earth pressures, shear forces and bending moments, while the original concept of a “distress spectrum” is introduced as a potential new tool for the seismic design of retaining structures. Distress and wall deformation are interrelated in a number of three-dimensional graphs, where dynamic interaction phenomena are evident. Finally, given the rigorous nature of the new solution, its results verify qualitatively and quantitatively the negligible amplitude of the computational errors of the approximate elasticity-based solutions proposed in the literature.     

Analysis of repeated-load laboratory tests on buried plastic pipes in sand

This paper describes a laboratory model test carried out on high-density polyethylene (HDPE), small diameter pipes buried in trenches, which subjected to repeated loadings to simulate the vehicle loads. Deformation of the pipe was recorded at eight points on the circumference of the tested pipes to measure the radial deformations and detect cross-sectional pipe profiles. Also settlement of the soil surface during the test up to 1000 cycles of loadings was recorded, until its value become stable or the excessive settlement was happened. The parameters varied in the testing program include height of buried depth, relative density of the sand and intensity of stress on the soil surface. The influence of various repeated loads (with magnitude of 250, 400 and 550 kPa) at relative densities of 42%, 57% and 72% in different embedded depth of 1.5–3 times of pipe diameter were investigated. Based on the results, in medium and dense sand relative density, the pipe embedded in depth of 3.0D and 2.0D, respectively, mostly remained undamaged (the maximum value of VDS is less than 5%) and increased the safety of buried pipes under different magnitude of repeated loads. The records of the pipe deformation and settlement of the soil surface due to the repeated loads have been compared in different conditions. These values increase rapidly during the initial loading cycles by a rate decreasing significantly as the number of cycles increase. The influence of the first cycle was also found to be one of the main behavioral characteristics of buried pipes under repeated loads. The ratio of deformation of pipe at first cycle to last cycle changes from 0.60 to 0.85 in different of tests. Finally for the obtained results, a non-linear power model has been developed to estimate the vertical diametral strain of buried pipe and settlement of the soil surface based on the model test data. It should be noted that only one type of pipe and one type of sand are used in laboratory tests.     

地基条件对仰斜式挡墙地震响应影响及墙高限值研究

地基条件和墙高是影响挡土墙地震响应特征的重要因素。建立不同地基条件的仰斜式挡土墙有限元时程分析模型,以墙身外倾最大危险状态为最不利时刻,研究地基条件和墙高对挡墙动力响应及墙-土相互作用的影响特征,并以满足力学检算和墙身位移限值为出发点,提出同时考虑地基条件和地震峰值加速度PGA的仰斜式挡墙墙高控制建议。结果表明：岩质地基挡墙墙背动土压力沿墙高呈中部大、上下小的凸形分布,大震下土压力较中震时有小幅减小;基底反力呈墙踵为0、墙趾集中的三角形图式,且随PGA和墙高的增加踵部脱空趋势更为明显;土质地基挡墙因墙底地基土变形对墙后填土的牵连作用,填土跟随墙身运动的趋势加剧,墙背动土压力与PGA呈正相关并沿墙高近似呈线性分布,于墙底处最大;墙身往复摆动使踵趾端地基土体塑性变形较基底中部明显,基底反力峰值向中部转移;根据最不利时刻稳定性、承载力检算,考虑对墙身位移合理限制,提出地震区仰斜式挡墙的允许墙高在设防PGA不超过0.2g时为8 m, 0.4g大震下硬质岩地基挡墙可达8 m,软质岩地基挡墙不宜超过6 m,碎石土、砂质黏土地基挡墙不宜超过4 m。     

Interpretation of transmissivity estimates from single-well pumping aquifer tests

Interpretation of single-well tests with the Cooper-Jacob method remains more reasonable than most alternatives. Drawdowns from 628 simulated single-well tests where transmissivity was specified were interpreted with the Cooper-Jacob straight-line method to estimate transmissivity. Error and bias as a function of vertical anisotropy, partial penetration, specific yield, and interpretive technique were investigated for transmissivities that ranged from 10 to 10,000 m(2)/d. Cooper-Jacob transmissivity estimates in confined aquifers were affected minimally by partial penetration, vertical anisotropy, or analyst. Cooper-Jacob transmissivity estimates of simulated unconfined aquifers averaged twice the known values. Transmissivity estimates of unconfined aquifers were not improved by interpreting results with an unconfined aquifer solution. Judicious interpretation of late-time data consistently improved estimates where transmissivity exceeded 250 m(2)/d in unconfined aquifers.     

Optimal design and dynamic impact tests of removable bollards

Anti-ram bollard systems, which are installed around buildings and infrastructure, can prevent unauthorized vehicles from entering, maintain distance from vehicle-borne improvised explosive devices (VBIED) and reduce the corresponding damage. Compared with a fixed bollard system, a removable bollard system provides more flexibility as it can be removed when needed. This paper first proposes a new type of K4-rated removable anti-ram bollard system. To simulate the collision of a vehicle hitting the bollard system, a finite element model was then built and verified through comparison of numerical simulation results and existing experimental results. Based on the orthogonal design method, the factors influencing the safety and economy of this proposed system were examined and sorted according to their importance. An optimal design scheme was then produced. Finally, to validate the effectiveness of the proposed design scheme, four dynamic impact tests, including two front impact tests and two side impact tests, have been conducted according to BSI Specifications. The residual rotation angles of the specimen are smaller than 30º and satisfy the requirements of the BSI Specification.