砂土液化大变形的弹塑性循环本构模型

循环剪切过程中饱和砂土的3个体积应变分量(有效球应力变化引起的体变、剪切引起的可逆性体变和不可逆性体变)的变化规律决定了液化后剪应变的发展。基于上述机理、对剪切引起的可逆性和不可逆性体变的数学描述、体积相容性条件以及边界面本构理论框架,建立了一个可描述饱和砂土液化后大变形的弹塑性循环本构模型。通过对饱和砂土排水和不排水循环扭剪试验结果的模拟表明,该模型不仅可以合理地模拟饱和砂土循环加载条件下从液化前到液化后、从小剪应变到大剪应变的变形发展过程,而且可以合理地模拟饱和砂土液化后再固结大体变的累积特性。本文的研究为定量描述砂土液化后大变形提供了一条合理而有效的途径。     

垂向荷载作用下饱和砂土的液化分析

 博士学位论文摘要　在对近40 a 来关于饱和砂土液化的研究工作调研的基础上, 就垂向荷载作用下饱和砂土液化问题, 进行了一维应变分析, 得到了垂向荷载作用下, 饱和砂土液化的发展过程以及砂土液化区域的扩展过程这两方面的特性。首先, 在固结仪上对往复荷载作用下有侧限的饱和砂土的本构关系进行了系列实验。实验材料包括松散细砂、密实细砂、松散粗砂(分别代表不同密实度和不同颗粒粒径构成的砂土) 。实验过程中, 加卸载路径有等幅的情况, 也有任意的情况。根据实验数据给出了用双曲线表述的应力2应变关系。这种形式的表述, 既避免了用切线模量形式表述的困难, 又解决了用对数形式表述的不能用于有效应力接近于零的问题; 而且形式简单, 又便于应用。接着, 考虑到砂土液化是孔隙水和骨架相互作用的结果, 将饱和砂土作为固液两相连续介质, 建立了垂向荷载作用时饱和砂土的一维应变动力学模型。该模型考虑了孔隙率的变化, 将骨架作为弹塑性材料, 且将渗透率与孔隙率的变化相联系, 使其与实际情况尽可能符合; 同时考虑在一般振动条件下的压力范围内, 可以忽略颗粒和孔隙水的压缩性, 使模型更简洁。然后, 在上述本构及动力学模型的基础上, 对饱和砂土的变形和液化发展进行了数值模拟。模拟结果表明, 垂向振动对饱和砂土的液化发展有显著的影响。在垂向振动作用下, 砂土首先在外载作用端产生局部区域的液化, 然后液化区逐渐向远处扩展, 且由于阻力的影响, 扩展速度随着液化区厚度的不断增长而逐渐减小。砂土骨架的可压缩性越大, 即砂土的初始切线模量越小、初始极限应变越大以及初始孔隙率越大, 则砂土的液化发展越快; 外载强度越高, 即外载幅值和频率越大, 砂土的液化发展越快; 砂土的渗流性越小, 排水越困难, 即初始渗透率越小, 砂土的液化发展越快。由此可认识到, 饱和砂土液化实际上是骨架和孔隙水相互作用的结果; 骨架为了抵抗外部扰动将产生压缩趋势, 从而挤压孔隙水, 力图使孔隙水排出,如果孔隙水难以排出, 两种介质的变形就出现不协调; 这就会导致骨架结构破坏、失去承载能力, 即饱和砂土发生液化。这些结果表明, 垂向荷载对饱和砂土液化的影响不可忽略。     

振动荷载下饱和砂土液化区的发展

本文以两相介质理论为基础 ,将饱和砂土简化为一维情况 ,对饱和砂土液化区的范围做了一些理论探讨。结果表明 ,液化区扩展随时间而逐渐变缓 ,且固相密度越大 ,发展越慢 ,而dp/dn越大 ,则液化区扩展越快。     

饱和粉土液化及液化后力学特性试验研究

对饱和粉土进行了一系列动三轴液化及液化后试验,即进行饱和粉土液化试验及在饱和粉土液化后孔隙水压力进行了不同程度消散后,再进行三轴剪切试验。探讨了饱和粉土的液化特性以及液化后孔压消散程度对液化后粉土力学特性的影响规律。试验结果表明:饱和粉土的抗液化强度随着所施加动应力幅值的增大而减小;液化后饱和粉土孔压消散程度越大(即再固结程度越大),则液化后粉土的不排水抗剪强度越大,其应力应变关系曲线表现为应变硬化型,土体的切线模量随着应变的增大而减小;液化后粉土的孔压进行了不同程度消散后,再进行的三轴剪切试验中应力应变关系呈对数曲线关系。     

波浪荷载下含细粒砂土动力特性试验研究

采用空心圆柱扭剪仪对含细粒饱和砂土进行动力特性试验。试验结果表明:波浪荷载作用下,砂土的液化周次明显小于动三轴和动扭剪试验的振动周次;随着细粒含量的增大,含细粒砂土的液化周次先减小后增加,细粒含量为10%左右时最小;细粒含量较小时,含细粒砂土超静孔隙水压发展规律类似纯砂,当细粒含量达到15%以上时,逐渐呈现出类似黏土的规律;细粒含量、有效围压对模量归一化曲线无影响;掺加细粒后砂土的阻尼比归一化曲线受围压影响比较明显,有效围压越大,阻尼比越小。     

沉积成层饱和粉土质砂振动孔压发展规律的试验研究

具有层状结构砂土的力学特性有别于纯净的砂土或含有一定细粒含量均匀混合砂土,利用自行制作的土样制备箱采用水中沉砂法制备沉积成层粉土质砂,通过动三轴液化试验研究这种沉积成层饱和粉土质砂的振动孔压发展规律。试验结果发现:对于沉积成层饱和粉土质砂,孔压一般达不到有效固结压力,仅为有效固结压力的70%~80%,而相应的轴向双幅应变可达到0.05左右;当围压为50 kPa时,其振动孔压发展模式可用反正弦函数表示;当围压为100,200 kPa时,其振动孔压发展模式都可用双曲线函数表示。     

冲击载荷下饱和砂土密实与排水过程的初步分析

 考虑液化后饱和砂土在重新固结时砂土骨架的压缩变形, 对冲击造成液化的饱和砂土在排水与密实过程中砂面沉降速度、沉降量、超孔隙水压力等参数随时间的变化进行了初步的分析。在对液化后饱和砂土的渗透系数进行必要的修正后, 分析计算结果与实验结果吻合得较好。     

饱和粉砂土液化和应变特性试验研究

对取自齐泰高速公路路基的粉砂土,在实验室内按照不同的干密度进行了重塑,完成了三组不同干密度的振动三轴液化试验。分析了饱和粉砂土的抗液化强度和液化过程以及应变发展规律,根据动三轴试验结果,通过曲线拟合方法建议了粉砂土振动液化过程中的永久应变势模型,给出了不同干密度下土样的回归曲线。     

饱和砂土振后再固结变形规律的试验研究

本文在考虑振液化的各种影响因素的基础上，对饱和砂土的振后再固结变形进行了深入系统的研究，得出了一系列重要特性和规律。     

砂土液化后大变形的机理

基于试验观察和机理分析,揭示了不排水往返加载条件下饱和砂土初始液化后的剪切大应变和三个体积应变分量(有效球应力变化引起的体变、可逆性剪切体变和不可逆性剪切体变)之间的内在联系,阐明了三个体积应变分量的组合变化规律控制了饱和砂土液化后大变形的产生和发展,界定了液化后循环剪切大变形过程中交替出现的三种物态(摩擦接触状态、临界接触状态和悬浮状态)及其产生条件,解释了液化后流滑和再固结体变形成的机理,给出了一个物理概念明确并具有普遍性的建立液化后大变形弹塑性本构模型的合理途径。     

Evaluation of post-liquefaction volumetric strain of reconstituted samples based on soil compressibility

In this study, we performed undrained cyclic–torsional–shear tests on clean and silty sands to examine post-liquefaction volumetric strain. Accordingly, we proposed a novel density index—ultimate volumetric strain—to evaluate the density of clean sand and silty soils; the proposed density index is based on soil compressibility. We discussed the test results with respect to relative density, skeleton void ratio, and ultimate volumetric strain. Consequently, we confirmed that ultimate volumetric strain showed good correlation with post-liquefaction volumetric strain irrespective of soil type, whereas values for relative density and skeleton void ratio were discordant. Further, we observed that, compared to maximum shear strain, accumulated shear strain was a better index for representing the effect of loading history. From these results therefore, we proposed a chart to evaluate post-liquefaction volumetric strain based on ultimate volumetric strain and accumulated shear strain.     

饱和南京细砂初始液化后特大流动变形特性试验研究

针对地震中倾斜场地砂土液化流动大变形特征与机理,以具有明显片状颗粒结构特征的南京细砂为研究对象,采用了英国GDS公司的动态空心圆柱扭剪仪,开展了饱和南京细砂液化后特大流动变形特性的循环扭剪试验研究,主要分析了橡皮膜效应、有效围压、循环加载幅值和初始静剪应力等因素对南京细砂液化流动大变形特性的影响规律及其机理。试验结果表明:在其它条件不变时,随着有效围压增加,饱和南京细砂抗液化强度和抗单向液化流动累积变形强度都有所增加。当保持围压不变时,饱和南京细砂抗单向液化流动累积变形的强度有增加的趋势。随着初始静剪应力比的变大,南京细砂的抗液化和抗单向累积变形强度特征可以分为三个不同阶段。在无初始静剪应力条件下饱和南京细砂液化后仍以循环液化流动变形为主和单向流动累积变形为辅。但是,随着初始静剪应力比的增大,饱和南京细砂抗液化强度和抗单向液化流动累积变形强度明显降低,即主要发生单向液化流动累积变形破坏。当初始静剪应力比接近或超过循环动剪应力比时,饱和南京细砂的抗液化和抗单向流动累积变形的强度又明显增大,但仍以液化单向流动累积破坏为主。当初始静剪应力比继续增大到一定程度时,饱和南京细砂已经很难液化,其抵抗单向累积变形的强度也明显降低,主要原因应为此时试样发生的是塑性累积大变形破坏。同时,试验结果表明饱和南京细砂的液化后流动大变形特性也明显区别于含圆形颗粒为主的日本丰浦砂。     

Comparison of Liquefaction-Induced Ground Deformation Between Results from Undrained Cyclic Torsional Shear Tests and Observations From Previous Model Tests and Case Studies

In order to investigate liquefaction-induced ground displacement, we conducted a series of undrained cyclic torsional shear tests on saturated Toyoura sand using a modified torsional apparatus capable of applying and measuring double amplitude shear strain up to about 100%. The limiting value of double amplitude shear strain, at which strain localization appears during undrained cyclic loading tests, was evaluated from the test results with reference to the change in the deviator stress during liquefaction. The limiting strain values, which increase with a decrease in the relative density of the specimen, were found to be consistent with the maximum amounts of liquefaction-induced ground displacement observed in the previous shaking table model tests and most of the relevant case studies. This feature is reasonable considering the reduction in the mobilized cyclic shear stress in liquefied soil due to the degradation of the shear resistance. As long as the liquefied soil layer remains in uniform deformation, these limiting strain values may be used in estimating the maximum amount of liquefaction-induced ground displacement.     

Influence of strain histories on liquefaction resistance of sand

The liquefaction resistance of sand increases with cyclic pre-shearing and pre-shaking as a result of earthquakes if the strain level in the pre-shearing is small. When larger shear strains are imposed, liquefaction resistance decreases. These complicated effects of pre-shearing histories on the liquefaction resistance are investigated in this study through a series of cyclic triaxial tests. Various combinations of cyclic stress amplitude and number of cycles of pre-shearing are examined. The tested sand is Toyoura Sand at 45% relative density, under a confining pressure of 50 kPa. Test results indicate that for the range of shear strain amplitude in pre-shearing smaller than 0.35%, the liquefaction resistance increases with pre-shearing. The increase in the liquefaction resistance depends strongly on the volumetric strain in the pre-shearing, and several effects of the shear stress amplitude and number of cycles can be negligible. Small volumetric strain of the order of 1% doubled the liquefaction resistance. Meanwhile, in the range of shear strain amplitude larger than 0.6%, the liquefaction resistance decreases. The liquefaction resistance decreases as the shear strain amplitude increases. Shear strain amplitude is one of the factors dominating this degrading effect, and the volumetric strain exerts beneficial effects to a certain extent. In this study, another series of tests are conducted to investigate the combined effects of small and large strain amplitude pre-shearing. It is observed that small shear strain pre-shearing cycles subsequent to large shear strain cycles erased the degrading effect of the latter. However, a large shear strain pre-shearing after small strain cycles degrades the beneficial effect of the small shear strain pre-shearing cycles previously applied to the specimens; however, the effects of the former small strain pre-shearing remains.     

评价饱和砂土液化过程中小应变到大应变的本构模型(英文)

在RambergOsgood模型的基础上,建立了一个实用的能描述饱和砂土从液化前小应变到初始液化后大应变范围的非线性本构模型。该模型主要有三个特点:①采用作者曾提出的移动相态转换线(MPTL)和移动临界状态线(MCSL)概念来较为合理地描述变形过程中的有效应力路径和临界状态;②采用一个应力软化模型来描述剪胀和定义超静孔压变化引起的骨架曲线的软化与硬化;③基于笔者等提出的液化后大变形理论来描述初始液化后产生的大剪切变形。文中通过对室内循环剪切试验结果的模拟,验证了模型的预测能力。该模型适用于水平饱和砂土地基的有效应力地震反应分析。     

论土体应力应变关系曲线类型和临界状态

为探讨应力历史对土体本构关系的影响,进行了一系列正常固结土和超固结土的排水剪切常规三轴压缩试验,对比具有不同应力历史的土体应力应变关系曲线发现,对于体应变,超固结比是决定性因素,而体应变对固结压力不太敏感。对于抗剪能力,固结压力是决定性因素,超固结比的影响也不可忽略,超固结比决定了是应变强化还是应变软化,且决定了应变软化的程度,但试样最终会达到一个统一的临界状态,具有大体相同的残余强度。根据塑性体应变与塑性剪应变之间的相互作用原理阐明,土体变形过程中,塑性体应变的变化控制了剪切抗力的升降,从而决定了土体应力应变关系曲线的类型;临界状态实际上是一个纯粹剪切变形过程,其中不发生塑性体应变与塑性剪应变之间的相互作用,压硬性、剪胀性、应力路径相关性统统消失,它与以前所经历的应力历史无关。     

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.     

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.