土体地震反应分析的简化有效应力法

本文提出了一个计算二维土体地震反应的简化有效应力法。该方法将整个地震特时分成若干时段，对每个时段用等效线性迭代进行次线性分析，等效剪应变幅值取该时剪应变均方根值的√２倍。     

A Discussion on Dynamic Characteristics of Undisturbed Loess by Dynamic Triaxial Test

Based on the dynamic triaxial test system and using the fitted wave of the Wenchuan earthquake and 1 Hz constant amplitude sinusoid,the paper compares the results of tests on undisturbed loess samples under different loads and vibration modes but under same saturated conditions.Results of the comparative experiment show:The stress-strain curves have a similar trend under random seismic loading and constant amplitude sinusoidal loading,but the random seismic loading is more sensitive to failure strength of the undisturbed loess samples under the same stress.     

相对密实度对随机波与定次数波作用下土体变形关系的影响

通过砂土的一系列动三轴实验,研究不规则地震荷载作用下与定次数等幅荷载作用下土体变形间的关系,给出砂土相对密实度对二者间关系的影响规律。结果表明:真实地震荷载下土的变形发展与等幅正弦荷载明显不同,应变发展时程的形态主要受地震动的形态控制;应变比C与砂土相对密度间关系具有规律性,随相对密度增大而降低,若采用以20周作为标准作用次数、0.65倍地震波峰值为等幅荷载代替不规则的地震荷载,修正真实地震应力下的残余变形,其应变比C随砂土密实度的增大而减小。同时,冲击型荷载的应变比C`要远大于振动型荷载。     

荷载大小对随机波与定次数波作用下土体变形关系的影响

通过砂土的一系列动三轴实验,研究不规则地震荷载作用下与定次数等幅荷载作用下土体变形间的关系,给出了地震荷载的大小对二者间关系的影响规律。结果表明:真实地震荷载下土的变形发展与等幅正弦荷载明显不同,应变发展时程的形态主要受地震动的形态控制;若采用以20周作为标准作用次数的等幅荷载代替不规则的地震荷载,修正真实地震应力下的残余变形,荷载大小对随机波与定次数波作用下土体变形间关系没有影响。     

Effect of overconsolidation ratio on dynamic properties of binary mixtures of silica particles

Overconsolidated soils are ubiquitous in nature due to multiple mechanisms; however, the stress-history-based studies of small strain stiffness on binary mixtures, such as silty sand, are limited even though natural sand deposits are commonly mixtures of sand particles with varying amounts of fines. Consequently, this study quantified the stress-history-based dynamic properties of binary mixtures, such as sand-sand mixtures with different size small particles, and silty sand mixtures with small amounts of non-plastic fines, up to the critical fines content. By performing bender element tests on those mixtures according to fines content, size ratio, and overconsolidation ratio, the stress-history-based G_max of binary mixtures was evaluated. For the relevant data analysis, the OCR (overconsolidation ratio) exponent in the G_max formulation was expressed in terms of stress exponents during loading and unloading. It was found that the effect of OCR on the estimation of G_max increased with a decrease in size ratio (or increase in size difference), since the stress exponents during loading increased more significantly with a decrease in size ratio due to the pronounced change in interparticle coordination between large grains. However, the variation of stress exponents during unloading of different mixtures was relatively small due to the prevalent elastic deformation. It was demonstrated that the maximum stress history effect of tested mixed soils was observed at a fines content of approximately 5%, which was smaller than critical fines content of silty sand. This behavior was attributable to the delay in critical fines content observed during unloading, when compared to that observed during loading.     

随机地震荷载作用下黄土动本构关系的试验研究

本文利用动三轴试验系统,试验研究了随机地震荷载作用下,非饱和原状黄土的动本构关系。结果表明,其本构关系依然服从双曲线模型,但模型参数随所施加的随机地震荷载时程的不同而不同。在试验中还对不同随机地震荷载和正弦波等效荷载作用下的模型参数、剪切模量和阻尼比进行了对比研究,初步建立了动本构模型参数与所施加的随机地震荷载的卓越周期之间的定量关系。通过试验和计算,证实了曼辛准则对黄土的适用性。     

车辆荷载对软土地区海底沉管隧道的影响分析

假定软土地区海底沉管隧道地基土为Kelvin模型,车辆荷载是随时间变化的波动荷载形式,引入黏弹性地基梁模型,利用模态叠加法给出三种情况下沉管隧道的竖向位移、弯矩和地基反力的解答。结合天津海河沉管隧道工程实例,分析车辆速度、地基土模量对沉管隧道竖向位移及弯矩的影响。研究结果表明:车辆荷载引起的管段中点振动振幅达5mm左右,振动周期为0.25s;引起的管段中点弯矩为15 500kN·m左右,且车速越大,管段振动一个周期所需时间越短,振动越剧烈,但对振动幅度及弯矩影响不大;地基土模量越大,振动幅度和弯矩越小,但对周期影响不大。     

地震波类型对砂土震陷影响的数值模拟分析

地震波是一种随机的、不规则作用的动荷载脉冲,可分为振动型和冲击型。不同类型的地震波会对砂土液化和变形等产生重要影响,而传统的砂土震陷计算方法往往忽视这种因素,只考虑最大加速度幅值。通过编写UMAT子程序,在非线性有限元软件ABAQUS中开发亚塑性砂土边界面模型,对不同地震波类型下不同相对密度的砂土进行动单剪试验模拟,得到一系列砂土剪应变及竖向应变的时程曲线,并与室内试验结果进行对比分析。研究表明:在同一工况下,同类型的地震波引起的砂土竖向应变相近,不同类型引起的竖向应变差异明显;振动型地震波比冲击型引起的竖向应变更大。     

季节变化对列车行驶路基动应力的影响及简化计算方法

我国众多铁路干线分布于深季节冻土地区。铁路路基土层的冻融状态随着季节的交替变化而改变,相应的列车行驶时引起的路基动应力分布也有所不同。考虑路基土体的参振效应,通过改进车辆-轨道-路基垂向耦合动力学模型获取不同季节列车行驶振动荷载时程,进而通过动力有限元数值模拟方法,研究季节变化对列车行驶引起的路基动应力分布规律的影响。研究表明:路基土中的动应力幅值及其沿路基深度的分布规律与该时期路基土的冻融状态密切相关,基于此结论,提出深季节冻土地区不同季节铁路冻土下限范围内路基动应力的简化计算方法。该研究对于优化季节性冻土地区铁路路基设计方法,完善路基长期动力稳定性能评价方法等具有重要意义。     

地震荷载作用下黄土地基震陷研究

在某大型企业的地震安全性评价工作中，采用随机地震荷载输入动三轴进行了黄土震陷试验，探讨了该场地黄土在不同动荷载作用下的震陷特性。并对该场地地基未来可能发生的震陷灾害进行了评价，为工程地基抗震处理提供了依据。     

Temporal fluctuations of water repellency in forest soils of Galicia,NW Spain. Do soil samples dried at laboratory reflect the potential soil water repellency?

Water repellency is a widespread property of Pinus pinaster and Eucalyptus globulus forest soils in NW Spain and is particularly severe during the summer dry conditions. The aim of this work was to compare actual water repellency at field‐moist samples with potential water repellency after drying at 25 and 105 °C in samples collected at different times of year under four forest soils. Also, we investigated whether drying at 25 or 105 °C led to repellency values comparable to the highest levels reached under field conditions in the summer with a view to developing an appropriate sampling protocol towards estimating the maximum possible water repellency of a given soil as a key to establishing its environmental effects. The actual and potential water repellency was determined by using the water drop penetration time (WDPT) and molarity of an ethanol drop (MED) tests. Clear seasonal patterns of water repellency were observed from the results for the four forest soils, peaking in the dry period and disappearing after prolonged wet periods. Water repellency lasts longer in sandy loam soils than in more finely textured soils, and also under eucalyptus than under pine forests. Drying soil samples at 25 or 105 °C increased water repellency, as measured with the WDPT method, in the four soils, but especially in the non‐repellent samples collected during the wet period. The increase was more marked in the sandy loam soils than in the more finely textured soils, and also after drying at 105 °C than at 25 °C. MED measurements exposed a common trait in the four soils; thus, the water repellency values obtained under field conditions in summer invariably exceeded those obtained after drying at 25 or 105 °C. In addition, the repellency values for dried samples collected in the wet period were never comparable to the maximum levels observed under field conditions in the summer. Copyright © 2011 John Wiley & Sons, Ltd.     

Soil and structure damping from single station measurements

In seismology and seismic engineering soils and structures are modeled as oscillators characterized by modal (resonance) frequencies, shapes and damping. In 1973 Cole proposed the RandomDec technique to estimate both the damping and the fundamental mode of structures from the recorded time series at a single point, with no need for spectral analyses. Here we propose a number of modifications to the original RandomDec approach, that we group under the name DECÓ, which allow to determine the damping as a function of the frequency and therefore the damping of all the vibration modes. However, the motion of structures is so amplified at the resonance frequencies that detecting the characteristic parameters by recording ambient vibrations is relatively easy. More interesting is to apply the DECÓ approach to the soil in the attempt to estimate the mode damping from single station measurements. On soils, the resonance frequencies are normally identified as peaks in the horizontal to vertical spectral ratios of microtremors. However, at these frequencies what is observed is a local minimum in the vertical spectral component, sometimes associated to local maxima in the horizontal components, whose visibility depend on the specific amount of SH and Love waves at the site. The determination of soil damping is therefore a much less trivial task on soils than on structures. By using microtremor and earthquake recordings we estimate the soil damping as a function of shear strain and observe that this is one order of magnitude larger than what is measured in the laboratory on small scale samples, at least at low-intermediate strain levels. This has severe consequences on the numerical seismic site response analyses and on soil dynamic modeling.     

Identification of soil degradation during earthquake excitations by Bayesian inference

A Bayesian inference approach is introduced to identify soil degradation behaviours at four downhole array sites. The approach of inference is based on a parametric time‐varying infinite impulse response filter model. The approach is shown to be adaptive to the changes of filter parameters and noise amplitudes. Four sites, including the Lotung (Taiwan), Chiba (Japan), Garner Valley (California), and Treasure Island (California) sites with downhole seismic arrays are analysed. Our results show two major types of soil degradation behaviour: the well‐known strain‐dependent softening, and reduction in stiffness that is not instantaneously recoverable. It is also found that both types of soil degradation are more pronounced in sandy soils than in clayey soils. The mechanism for the second type of soil degradation is not yet clear to the authors and suggested to be further studied. Copyright © 2003 John Wiley & Sons, Ltd.     

Model tests on XCC-piled embankment under dynamic train load of high-speed railways

Piled embankments, which offer many advantages, are increasingly popular in construction of high-speed railways in China. Although the performance of piled embankment under static loading is well-known, the behavior under the dynamic train load of a high-speed railway is not yet understood. In light of this, a heavily instrumented piled embankment model was set up, and a model test was carried out, in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train. Earth pressure, settlement, strain in the geogrid and pile and excess pore water pressure were measured. The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading. The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase. Accumulated geogrid strain has an increasing tendency after long-term vibration. The closer the embankment edge, the greater the geogrid strain over the subsoil. Strains in the pile were smaller under dynamic train loads, and their distribution was different from that under static loading. At the same elevation, excess pore water pressure under the track slab was greater than that under the embankment shoulder.     

Influence of colloidal silica grout on liquefaction potential and cyclic undrained behavior of loose sand

Cyclic triaxial tests were performed to investigate the influence of colloidal silica grout on the deformation properties of saturated loose sand. Distinctly different deformation properties were observed between grouted and ungrouted samples. Untreated samples developed very little axial strain prior to the onset of liquefaction. However, once liquefaction was triggered, large strains occurred rapidly and the samples collapsed within a few additional loading cycles. In contrast, grouted sand samples experienced very little strain during cyclic loading. Additionally, the strain accumulated uniformly throughout loading rather than rapidly prior to collapse and the samples never collapsed. Cyclic triaxial tests were done on samples stabilized with colloidal silica at concentrations of 5, 10, 15, and 20%. In general, samples stabilized with higher concentrations of colloidal silica experienced very little strain during cyclic loading. Sands stabilized with lower concentrations tolerated cyclic loading well, but experienced slightly more strain. Thus, treatment with colloidal silica grout significantly increased the deformation resistance of loose sand to cyclic loading.     

On engineering equivalent linear analysis of soils in the frequency domain

This study investigated the performance, in the frequency domain, of an engineering equivalent linear method that is widely used in soil dynamics. A stochastic linear method using an extended differential equation by Wen and Baber was employed as the base line for evaluation. Stationary responses of single-degree-of-freedom non-linear systems under filtered white noise from both approaches were compared. For both a smooth nearly elasto-plastic material and soils, the study found that the engineering equivalent linear analysis underestimated rms displacements, but provided good estimates of rms acceleration responses. Furthermore, the frequency response functions of acceleration from both approaches were close at all levels of response.     

Damping and response measurement on A small-scale model of A cable-stayed bridge

A small-scale model of an existing cable-stayed bridge has been designed and built. Hammer, sinusoidal and random signal tests have been performed to determine the dynamic characteristics of the model. An attempt has been made to evaluate the real effect of the vibration of stays upon system damping. Initial measurements have been performed for a model where there was no vibration of the stays because of the absence of appropriately scaled additional mass of these stays. In this way the effect of the cables was eliminated in damping measurement. During the second series of measurements appropriate mass has been added to the stays, causing them to take part in the vibration. The comparison of the measured damping of the model for each case makes possible the estimation of the effect of the stays upon the system damping. By using the shaking table, two types of seismic excitation were applied, with corresponding response quantities in the critical sections being measured. Good agreement between the analytical and the experimental results was achieved.     

考虑应变软化的边坡稳定分析

在密砂、坚硬黏土及原状土的强度试验中,土试样常表现为应变软化,随着应变的增加其抗剪强度降低,并且发生渐进破坏。应用传统的极限平衡法,采用峰值强度作为计算参数进行稳定分析验算时,通常忽略土材料的渐进破坏过程,这可能会对边坡稳定评价带来不利影响。基于有限元极限平衡法,提出考虑应变软化的边坡稳定分析方法,并通过算例分析证明该方法的合理性。     

A semi-empirical formula for evaluating residual strain of soils under earthquake loading

A new semi-empirical formula for evaluating the residual strain of soils under earthquake loading is presented in this paper based on the incremental method and the increment model proposed by the authors. When the incident loading is uniform, the results calculated by the new formula are nearly the same as those by the existing formula. For excitation of the random earthquake loading, the results calculated by the new formula are compared to the results obtained by dynamic triaxial tests. The dynamic triaxial tests had been performed considering different seismic waves, confining stresses,consolidation ratios, and types of cohesive soils. The comparison between the calculated and tested results indicate that the presented formula can efficiently and practically describe the time-dependent process of the soil residual strains under actual seismic loads.     

Observations on non-linear dynamic characteristics of suspension bridges

For most forms of loading static and dynamic response of a suspension bridge is modelled adequately by linear analysis. By examining the two principal non-linearities of inclined and vertical hanger suspension bridges the limitations of linear analyses are shown. The inclined hanger configuration is shown to be the more strongly non-linear. Some examples are given for simplified planar analyses of this type of bridge to large amplitude vehicular, random, sinusoidal and propagating earthquake excitation, showing that the effect only becomes significant when deflections approach the extent of cable sag due to cable elasticity.