土(尾矿)坝的三维有效应力动力反应分析

本文首先根据试验资料整理出可用于三维应力状态的振动孔隙水压力增长的公式,然后提出了一个计算饱和土体(尾矿)地震反应的三维动力分析方法。该法不仅考虑到动剪应变与动剪切模量之间的非线性关系,而且还考虑到随振动孔隙水压力的逐渐增长而变化的动力性质。这个方法能够求得地震过程中三维应力状态土坝的加速度、动应力,动应变以及振动孔隙水压力随着时间的发展以及液化的开始和发展的过程。 以安徽铜陵尾矿(副)坝为例,采用空间等参数法,用本文提出的分析方法和振动孔隙水压力公式,详细地分析了该坝在0.1g和0.2g地震情况下的各种反应量和稳定情况。     

软件ABAQUS在饱和土体动力响应分析中的应用

研究表明:地震作用下土体的动力特性及变形特性与超静孔隙水压力的发展变化密切相关,因此,在土体动力分析过程中考虑孔隙水压的影响是非常必要的。本文通过对基本方程的推导,借助于大型有限元软件ABAQUS,进行了饱和土体在动力作用下孔隙水压变化的数值模拟。计算结果表明,ABAQUS完全适用于此类问题的数值模拟,并且稳定性和收敛性较好。     

考虑土体固液二相性质的地下管线地震反应研究

本文将管线周围的土体视为固-液二相介质，采用考虑地震过程中土体内孔隙水压力的增长和消散的有效应力分析方法，并以非线性的土体本构模型模拟土体动力特性的改变以及在孔隙水压作用下的变形，以求接近土体的真实地震反应。研究表明随着土体软化程度的不同，受到土体约束的管线应力亦发生相应变化。文中详细研究了地震动峰值、管线直径、行波波速和入射角度等对管线应力的影响。     

Q_3黄土残余变形对孔隙压力控制作用试验研究

土体残余变形与孔隙压力的作用关系,是理解土体受荷力学响应行为的重要问题。利用室内静(动)三轴试验,获取天然黄土受载后的轴向残余变形、体积残余变形、孔隙气(水)压力变化的时程数据,研究残余变形和孔隙压力的对应关系,厘定残余变形对孔隙压力的控制作用。结果表明:轴向残余变形描述孔隙气(水)压力的响应过程及绝对量值时,具有局限性;孔隙气(水)压力的响应过程及成因具有一致性,与初始应力条件及加载形式无关;孔隙水压力的绝对量值对围压具有强依赖性,体积残余应变相同时水体对孔隙水压力绝对量值的贡献是气体对孔隙气压力的10倍。孔隙压力的响应过程及成因,取决于固相颗粒介质和气、水相介质之间的作用机理;其绝对量值,决定于气、水相介质的渗透性、压缩性等固有特性。     

关于岩土孔隙压力参数问题

孔隙压力参数是用来表述饱和岩土受力变形时所受力与超孔隙压力关系的参数,是地震地下水前兆机理研究中的重要问题。本文从室内试验、现场试验与野外观测的某些结果,分析了不同岩土、不同受力条件下孔隙压力参数的若干特征     

离心机振动台试验中侧向流动地基位移分析方法研究

通过分析倾斜可液化地基的离心机振动台试验,对离心机振动台试验中地基土体位移分析方法进行了研究,得到了能够有效测量和计算可液化地基地震过程中和震后侧向流动的位移的方法。这种分析方法能够通过对加速度数据的处理和计算,有效地克服位移传感器在地震过程中反应不够灵敏导致的位移过程测量的误差,得到合理的地震过程中地基位移变化;同时利用位移传感器对低频位移测量准确的优势,通过量测的震后侧向位移值对计算位移值进行修正,可以得到地震结束后,地基的最终变形。本研究为土工离心机振动台试验中侧向流动地基的位移测量和分析提供可靠有效的方法,也能够为进一步的可液化地基离心模型试验的土体位移相关研究分析提供必要基础。     

近岸水平场地液化侧向大变形机理及软化模量分析方法

本文依据震害现象和实验探讨近岸水平场地地面液化侧向大变形机理，改进现有软化模量分析技术，给出一套地面液化侧向大变形的分析方法。近岸水平场地侧向大变形机理因地基中孔隙水压力升高、土体模量衰减、土骨架变软使偏应变得到充分发展所致，其水平永久侧移可用从底部到顶部呈增加形式的整体变形描述。利用本文方法，对1995年阪神地震中近岸沉箱岸壁和土体液化侧向大变形进行了数值模拟，结果与震后实测结果和试验结果在主要特征上一致，说明改进的软化模量法可以用于地面液化侧向大变形的分析。     

振动能量下砂土的体变与孔隙水压力

本文建立了砂土在振动能量下的体积变化和孔隙水压力生长的新型本构关系。鉴于砂土的非线性振动反应和多因素的影响,文中采用了内缓变理论(Endochronic Theory)将本构关系表达为单一的内缓变量(软化参数)的函数。 试验证明在基于建议的本构关系计算的孔隙水压力值与相应的振动能量下,福建标准砂试样中产生的孔隙水压力很接近。同时又根据该本构关系预估了实际地震能量下饱水砂层中孔隙水压力的反应,回判地震液化势,获得了较满意的结果,具有一定的实用性。     

NEES系统中振动离心机最新进展及国内振动离心机发展设想

美国地震工程网络模拟系统(NEES)代表了国际地震工程试验技术的最新发展趋势,其中振动离心机的建设占有重要地位,而国内振动离心机的现状与我国辽阔的地域、复杂的工程地质条件、高速发展的经济建设以及严峻的地震形势十分不相称。文中概述了我国振动离心机的现状和客观需求,对NEES系统中振动离心机的发展进行了跟踪研究,通过分析UCDavis和RPI两台振动离心机的发展历程和最新进展,阐述了我国振动离心机发展的必要性及重点。强调应加强离心机振动台技术的研发,根据国内当前土动力学和岩土地震工程科学研究的需要,提出了以振动负载为主要指标的思想,指出了发展重点应放在振动负载、频宽、多向、低频位移、辅助试验功能和网络化功能等方面,对一些重要指标提出了建议。     

不同结构的岩石试件变形破坏与孔隙压力关系的实验研究

根据华北地区地震地下水位动态观测井的含水层岩性、埋深与承压水头大小等实际条件，对具有不同结构的四种岩性试件，在不同围压、初始孔隙压力、变形速率下，进行了变形破坏过程中孔隙压力变化特征的实验研究，探讨了岩性、结构、围压、初即孔隙压力，变形速率等因素对井水位映震特征的影响。     

Evolution of the shear wave velocity during shaking modeled in centrifuge shaking table tests

Two in-flight shear wave velocity measurement systems were developed to perform the subsurface exploration of shear wave velocity in a centrifuge model. The bender elements test and the pre-shaking test used in the study provided reliable and consistent shear wave velocity profiles along the model depth before and after shaking in the centrifuge shaking table tests. In addition, the use of the bender elements measurement system particularly developed here allowed continuous examination of the evolution of shear wave velocity not only during and after the shaking periods in the small shaking events but also during the dissipation period of excess pore water pressure after liquefaction in the large shaking events. The test results showed that the shear wave velocity at different values of excess pore water pressure ratio varied as the effective mean stress to the power of 0.27, to a first approximation. Consequently, a relationship between the shear wave velocity evolution ratio and the excess pore water pressure ratio is proposed to evaluate the changes in shear wave velocity due to excess pore water generation and dissipation during shaking events. This relation will assist engineers in determining the shear stiffness reduction ratio at various r_u levels when a sand deposit is subjected to different levels of earthquake shaking.     

Boundary effects of a laminar container in centrifuge shaking table tests

Two dynamic centrifuge model tests were performed to simulate dry or saturated sand deposits subjected to 1 Hz base shaking. This experimental study investigated the boundary effects of a laminar container on the seismic response acquired from accelerometers and from pore pressure transducers, both of which were embedded in the sand bed at various depths and distances from the end walls. Under the tested configurations and the employed input motion used in the study, the test results revealed minimal boundary effects on the seismic responses. The measured maximum amplitude, main frequencies, phase lags of acceleration, and the profiles of the calculated RMS acceleration amplification factor were not affected by the boundaries if the instruments were positioned at a distance of more than one-twentieth of the model length from the end walls and were not positioned on the ground surface. No obvious discrepancies were observed in the time histories of excess pore water pressure, measured at a distance of one-fourth of the model length from the end walls. These results infer that variations in the seismic response at the end walls were minimal; hence the laminar container used in the study may be used effectively to simulate 1D shear wave propagation in centrifuge shaking table tests. However, for other testing configurations, a similar study should be undertaken for evaluating the boundary effect of the laminar container on the seismic responses.     

Pile response in submerged lateral spreads: Common pitfalls of numerical and physical modeling techniques

A three dimensional dynamic numerical methodology is developed and used to back-analyze experimental data on the seismic response of single piles in laterally spreading slopes. The aim of the paper is not to seek successful a-priori (Type A) predictions, but to explore the potential of currently available numerical techniques, and also to get feedback on modeling issues and assumptions which are not yet resolved in the international literature. It is illustrated that accurate simulation of the physical pile–soil interaction mechanisms is not a routine task, as it requires the incorporation of advanced numerical features, such as an effective stress constitutive soil model that can capture cyclic response and shear-induced dilation, interface elements to simulate the flow of liquefied ground around the pile and proper calibration of soil permeability to model excess pore pressure dissipation during shaking. In addition, the “conventional tied node” formulation, commonly used to simulate lateral boundary conditions during shaking, has to be modified in order to take into account the effects of the hydrostatic pore pressure surplus that is created at the down slope free field boundary of submerged slopes. A comparative analysis with the two different lateral boundary formulations reveals that “conventional tied nodes”, which also reflect the kinematic conditions imposed by laminar box containers in centrifuge and shaking table experiments, may underestimate seismic demands along the upper part of the pile foundation.     

Uplift mechanism for a shallow-buried structure in liquefiable sand subjected to seismic load: centrifuge model test and DEM modeling

Based on a centrifuge model test and distinct element method(DEM), this study provides new insights into the uplift response of a shallow-buried structure and the liquefaction mechanism for saturated sand around the structure under seismic action. In the centrifuge test, a high-speed microscopic camera was installed in the structure model, by which the movements of particles around the structure were monitored. Then, a two-dimensional digital image processing technology was used to analyze the microstructure of saturated sand during the shaking event. Herein, a numerical simulation of the centrifuge experiment was conducted using a two-phase(solid and fl uid) fully coupled distinct element code. This code incorporates a particle-fl uid coupling model by means of a "fi xed coarse-grid" fl uid scheme in PFC3D(Particle Flow Code in Three Dimensions), with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate the uplifts of the shallow-buried structure and the sharp rise in excess pore pressure. The corresponding micro-scale responses and explanations are provided. Overall, the uplift response of an underground structure and the occurrence of liquefaction in saturated sand are predicted successfully by DEM modeling. However, the dynamic responses during the shaking cannot be modeled accurately due to the restricted computer power.     

Centrifuge modeling of the behavior of caisson-type quay walls during earthquakes

A series of 2-D centrifuge modeling tests with an in-flight shaker were carried out in order to model both the deformation characteristics of backfill and the seismic responses of caisson-type walls embedded in soils with various permeabilities. The rotational and translational modes were found to be in phase or various degrees out of phase with each other for quay walls embedded in soils with varying permeabilities. The alternative pumping and suction processes in excess pore water pressure that are caused by a wall's vibrations increase the level of damage because large amounts of backfill are forcedly leaked into the sea. The test results show that the rotational mode makes the dominant contribution to the changes in excess pore water pressure and in the earth pressure in the deep layers of soil, but the translational mode makes the dominant contribution to these pressures in the shallow layers. The average shear wave velocities were found to decrease rapidly to values as low as 1/8th of the velocity measured at the beginning of shaking.     

Soil–pile-bridge structure interaction in liquefying ground using shake table testing

The evaluation of seismic pile response is particularly useful for geotechnical engineers involved in the design of foundations in liquefying site. Shake table testing was performed to study the dynamic interactive behavior of soil–pile foundations in liquefying ground under different shaking frequency and amplitude. The soil profile consisted of a clayey layer over liquefiable sand over clay. The model was tested with a series of El Centro earthquake motions with peak accelerations ranging from 0.15g to 0.50g, and time step from 0.006 to 0.02 s. Representative data, including time histories of accelerations and excess pore pressure ratios that characterize the important aspects of soil–pile interaction in liquefying ground are presented. The shaking frequency has no significant effect on the magnitudes of excess pore pressure ratio, ground and pile accelerations and pile bending moments. Excess pore pressure ratio, ground acceleration and pile acceleration, and pile bending moment largely depend on the shaking amplitude.     

Excess pore pressure dissipation and solidification after liquefaction of saturated sand deposits

To understand the post-liquefaction behavior of liquefied ground, it is important to get a better understanding and a more suitable characterization of the variation of excess pore pressure after liquefaction. In this paper, the soil permeability is considered as one of the key soil parameters for clarifying the mechanism of post-liquefaction behavior of liquefied ground. For this reason, a series of shaking table tests were conducted for a Toyoura sand deposit with different soil permeability values. Polymer fluid was used in model tests to vary the permeability of sand deposits. Excess pore pressures and settlements were measured in each test. A basic mechanism in post-liquefaction behavior and the solidification phenomenon after liquefaction were discussed based on these test results. Also, a new method for predicting the dissipation of excess pore pressure was developed. This study provides evidence of the important effect of soil permeability on the velocity with which the solidification front moves upward in liquefied ground. It is suggested that the value of coefficient of permeability of liquefied sand can increase to about 4.0 times the initial value. This variation of permeability after liquefaction should be taken into account in post-liquefaction analysis.     

Centrifuge modeling of loose fill embankment subjected to uni-axial and bi-axial earthquakes

The seismic risk is fairly high in Hong Kong even though it is located in an intreplate area with low to moderate seismicity. This is because of its high seismic vulnerability due to the presence of many steep loose fill slopes with a marginal static factor of safety, and a high consequence ‘value’ as a result of the dense population and intense economic activity in Hong Kong. In order to investigate the seismic stability and potential flow liquefaction of loose fill slopes, dynamic centrifuge tests in uni-axial and bi-axial directions were performed on saturated model embankments made of loose completely decomposed granite (CDG). Three windowed sinusoidal waves with peak shaking amplitudes ranging from 0.08 g to 0.3 g (prototype scale) were adopted. During the strong uni-axial shaking of 0.3 g, the measured maximum excess pore pressure ratios ranged from 0.70 to 0.85 and a relatively small crest settlement of 5.8 mm (0.22 m prototype) was measured. No soil liquefaction or flow slides were observed. Comparing the results between the strong uni-axial and bi-axial shaking, the maximum pore pressure ratios measured from the bi-axial test varied from 0.75 to 0.87, which were marginally larger than those obtained from the uni-axial test. Although the measured crest settlement during the bi-axial shaking was about 27% larger than that of the uni-axial test, soil liquefaction and flow slide did not occur. These test results suggest that loose CDG fill slopes are likely to be stable under the proposed design PGA ranging from 0.08 to 0.11 g in Hong Kong.     

地下RC结构物地震响应特征土工离心试验的模拟

地下结构物抗震稳定性的研究具有重要实际意义。基于动态土工离心试验,应用有效应力法分析了不同水平地震作用下土层和地下RC结构物的最大水平位移、不同埋深的土层加速度和超孔隙水压力变化规律,以及结构物的破坏发展特征,并进一步与试验结果进行了相互验证和对比,取得了较好效果,为重要地下结构物的抗震设计提供了依据和参考。     

饱和软土自由场地地震反应特性振动台试验

为了解软土自由场地地震反应特性,开展饱和软土自由场地地震反应特性大型振动台模型试验。分别从模型体系自振特性、震陷位移及不同土层深度测点的加速度、动孔压比等动力响应指标方面,较为深入和全面地分析饱和软土自由场地地震反应规律、破坏机理。同时还分析模型箱的"边界效应"以验证试验土箱的合理性、有效性和测试仪器性能,并由此进一步确定模型地基有效工作区域。研究表明:(1)地震动作用下,饱和软土自由场地特征频率降低,阻尼增大;(2)饱和软土自由场地对水平输入地震波具有短周期滤波、长周期放大效应,且强震作用下地基失效并表现为减隔震作用;(3)饱和软土自由场地动孔压比优势区域位于浅埋土层,并随着输入地震动强度的增大,该区域动孔压比优势逐渐减弱。该研究可为非自由软土场地试验研究提供必要的技术经验。