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。     

荷载类型对土体变形影响研究

以动三轴实验为基本手段,研究了定次数等幅荷载与不规则地震荷载作用下土体变形间的关系,探讨地震荷载类型对二者关系的影响规律。结果表明:等幅正弦荷载与地震荷载作用下土单元的变形时程曲线明显不同,其形态明显受地震波形的控制。当采用以0.65倍地震峰值折合最大幅值进行的正弦应力波实验代替真实地震波作用,以20周为标准的荷载转换方法,地震波与定次数等幅荷载作用下土体变形受地震荷载类型的影响,冲击型荷载作用下的应变比远大于振动型荷载作用下的应变比。同时,其应变比C随砂土密实度的增大而减小。     

土体永久变形中地震动有效荷载研究

通过黏土和砂土的一系列动三轴试验,研究土体永久变形中地震动的有效荷载问题,提出了黏土和砂土应变门槛值与有效荷载幅值下限值的关系,给出了有效荷载幅值的计算公式和有效荷载的推荐值.结果表明,真实地震荷载下土的变形发展与等幅正弦荷载有明显不同,应变发展时程的形态主要受地震动的形态控制.应变门槛值与有效幅值下限值呈非线性关系,应力幅值下限值比应变门槛值大,且在较小的应变门槛值情况下需要比率差别更大的有效幅值才能产生相应的应变.以使土体产生超过总应变10%增量的应力值作为有效荷载幅值,砂土情况下幅值应大于峰值的四分之一,黏土情况下幅值应大于峰值的三分之一.     

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。     

重复循环荷载作用下原状黄土动力特性试验研究

为探究重复循环荷载作用下含水率、固结压力对原状黄土动力特性、变形性状的影响规律,利用英国GDS双向动态三轴试验系统模拟交通荷载,对海东地区原状黄土进行动三轴试验研究。试验结果表明：单一重复循环动荷载作用下,在加载初期,海东地区原状黄土的轴向动应变随着循环次数的增大急剧增加,后期缓慢增加至趋于稳定,即发生应变硬化现象。在加载初期,含水率、固结压力对原状黄土的轴向动应变无明显影响,当循环次数N>400时,原状黄土的轴向动应变随含水率的升高而增加,随固结压力的增大而减小。动剪切模量随循环次数的增加出现先减小后增加再减小的变化,随含水率的升高有较大幅度的降低。动阻尼比随循环次数的增加先增加后减小,随含水率的升高而增大。表明海东地区原状黄土所具有的大孔隙架空结构使其在重复循环荷载作用下易发生振动变形。     

辽宁台安砂土液化特性动三轴试验研究

为探讨台安砂土液化相关特性及发展规律,针对辽宁台安砂土进行一系列不排水动三轴试验,研究台安砂土在动荷载作用下动孔压及动应变变化规律,分析影响台安砂土发生液化的主控因素,揭示动载作用次数下台安砂土动力响应特征。结果表明：动应力水平不同的各相对密度试样u_d曲线可分为后期陡增和匀速增长两种类型,动应力幅值较大时,u_d始终以较快速度增长,呈现匀速增长特征,动应力幅值较小时,u_d曲线则呈现初期缓增后期陡增的特点;相对密度D_r和动应力幅值CSR是决定砂土液化的主控因素,在同一里氏震级等效动载作用次数下,随着D_r持续发展,液化所需CSR增长逐渐加快,当D_r足够大时,砂土基本处于密实状态,此时难以发生液化;循环荷载作用下,台安砂土呈现出初期整体受压、中期拉压平衡、后期受拉凸显的动力响应特征,到后期试样拉应变迅速增大,试样易发生受拉破坏。研究成果可为辽宁台安地区建筑物抗震设计提供参考依据。     

双向动荷载下重塑红黏土动变形特性研究

利用SDT-20型三轴仪探究双向动荷载下红黏土动变形特性。结果表明:相位差为0其他条件相同时红黏土动剪切模量随径向动荷载幅值增加而减小;双向动荷载下红黏土动剪应变与振动次数近似呈指数型关系增长,并存在一个临界循环次数;随含水率升高和固结应力增大,重塑红黏土破坏模式由受拉破坏变为受压破坏;径向动荷载幅值的增加使重塑红黏土更容易发生受拉破坏;双向动荷载下阻尼比随动剪应变增加无明显规律,动剪应变小于1%时阻尼比的变化无规律,动剪应变大于1%时随动剪应变增加阻尼比处于稳定平衡阶段。含水率对阻尼比变化规律有明显影响,含水率小于20%时阻尼比随含水率增大而增大,含水率大于20%时其对阻尼比影响可以忽略。     

相对密实度对含黏粒砂土动剪切模量与阻尼比影响的试验研究

利用GCTS共振柱系统开展一系列含黏粒砂土的动力特性试验,研究相对密实度对含黏粒砂土动剪切模量和阻尼比的影响,并探讨其影响规律。试验结果表明:相对密实度对动剪切模量、动剪切模量比、阻尼比、最大动剪切模量均有影响。动剪切模量随着相对密实度的增加而变大,但动剪切模量增大的幅度更大;当黏粒含量较小时,动剪切模量比随相对密实度的增加而变大,当黏粒含量较高(≥16%)时,相对密实度对动剪切模量比基本没有影响。阻尼比随相对密实度的增加逐渐变小;最大动剪切模量随相对密实度的增加而增大,但黏粒含量不同,其最大动剪切模量增大幅度不同,同时建立最大动剪切模量随相对密实度变化的关系式,并给出相关参数。     

波浪荷载作用下砂土变形特性的模拟试验研究

利用新研制的“土工静力-动力液压三轴-扭转多功能剪切仪”模拟波浪荷载作用，对福建标准砂进行了4组三轴一扭转耦合振动剪切试验，以研究海床砂土在波浪荷载作用下的变形特性及其对砂土初始密度和固结压力的依赖性。试验结果表明，应力-应变呈现出良好的双曲线关系，与等效线性模型理论相吻合；阻尼比和模量随应变幅值的变化规律均依赖于土体的初始密度和固结压力；归一化后的E／Emax-ε及G／Gmax-γ曲线对相对密度和固结压力仍具有明显的依赖性；但完全归一化后的E／Emax-ε/εr及G／Gmax-γ/γr的关系曲线则与固结压力无关。     

砂土震陷分析中多维地震荷载等效循环周数计算

借助有限元数值方法模拟不同砂土试样在多向和单向地震荷载输入条件下的动三轴试验,选取覆盖大震、中震、近场、远场及不同土质条件的155组多向地震输入时程对不同砂土试样分别进行单向和多向加载,建立考虑多向地震荷载作用的等效循环周数计算方法,并研究震级、震中距和砂土特性对等效循环周数比的影响。研究表明,震级和震中距对等效循环周数比影响不明显,砂土特性对等效循环周数比的影响具有主导作用,砂土相对密度为45%、60%、80%和100%时,其对应的等效循环周数比均值分别为1.58、1.75、1.93和2.08,相对密度越大,比值越大,结合该比值,可以较好地应用文中提出方法进行多维地震荷载等效循环周数计算。     

Impact and cyclic shaking on loose sand properties in laminar box using gap sensors

This paper focuses on using high-frequency GAP-SENSORs (GSs), accelerometers, and load cells in a laminar shear box (LSB) filled with loose Toyoura sand to understand the effects of impact loads and cyclic shaking at 1-G on soil properties. The shear wave velocity at small strain (V_s) was calculated directly from first arrival reference using displacement time-history of two GSs under impact loading. Moreover, from first peak using the reduced deformation amplitude technique, damping ratio was calculated. In addition, shaking table tests were performed under harmonic loading with amplitude of acceleration inside the model ground varying from 0.02 g to 1 g. The frequencies of excitation varied from 1 Hz to 10 Hz. GSs and inside accelerometers were used to directly measure the outside lateral deformation and shear stress at different elevations of LSB, respectively. Results show that the shear modulus (G) and the damping ratio (D) behavior of model sand are generally consistent with the behavior presented by similar tests using only accelerometers. In addition, damping ratio increases as frequency loading increases. Characteristic changes in two shear stress components in shaking loading conditions were also investigated using high precision inside load cells.     

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.     

Influence of a cyclic and dynamic loading history on dynamic properties of dry sand, part I: cyclic and dynamic torsional prestraining

Initial fabric of a soil induced by its cyclic strain history is an important parameter together with the void ratio, state of stress and amplitude in respect to further accumulation of deformations under drained cyclic loading. It is of importance for the further deformation prediction to determine the initial fabric of the grain skeleton or the cyclic loading history of the soil. An attempt is made within this paper to correlate small strain stiffness of non-cohesive soil with its cyclic loading history. The results of performed cyclic and dynamic torsional tests show that small strain shear modulus is only moderately affected by cyclic prestraining even if high amplitudes are applied. A signature of prestraining history is observed in the tests since the sand memorizes its prestraining amplitude and the number of applied cycles.     

Cyclic shearing and liquefaction of soil under irregular loading: an incremental model for the dynamic earthquake-induced deformation

The paper presents a mathematical model for the deformation of soil under irregular cyclic loading in the simple-shear conditions. The model includes the possible change in the effective pressure in saturated soil due to the cyclic shearing, the reciprocal influence of the effective pressure on the response of the soil to the shear loading, and the pore pressure dissipation due to the seepage of the pore fluid. The hysteresis curves for the strain–stress relationship are constructed in such a way that they produce both the required backbone curve and the required damping ratio as functions of the strain amplitude. At the same time, the approach enables the constitutive functions involved in the model to be specified in various ways depending on the soil under study. The constitutive functions can be calibrated independently of each other from the conventional cyclic shear tests. The constitutive model is incorporated in the boundary value problem for the dynamic site response analysis of level ground. A numerical solution is presented for the dynamic deformation and liquefaction of soil at the Port Island site during the 1995 Hyogoken-Nambu earthquake.     

Experimental research on the development of residual strain in seasonal frozen soil under freezing-thawing and impact type traffic loads

Vehicle load is among the main factors affecting the deformation of subgrade soil.In this research study,the concept of impact type traffic load is introduced to investigate the effects of vehicle load based on the dynamic stress and displacement time histories acquired from seasonal frozen subgrade soils.Using freezing-thawing and dynamic triaxial tests and considering the amplitude and loading sequence of impact type traffic load,the residual deformation characteristics of subgrade soil under impact type traffic loads and freezing-thawing cycles is studied.It was found that under impact type traffic load,the residual deformation of soils increased sharply as the amplitude of impact type traffic load increased.It was also found that the increase in the amplitude of impact type traffic load led to the increase of residual deformation in a scale of power and exponential function.The amplitudes of impact type traffic load affect the development stress-strain path of the residual strain.After the soil experienced the proper amount of pre-vibration of the light load,residual deformation decreased by 15%.After freezing-thawing,the residual strain of soil increased as the amplitude of the impact type traffic loads increased.Also,when the amplification effect of freezing-thawing on the residual strain was basically stable,the residual deformation increased by about 10%.The peak impact type traffic load had a large effect on soil deformation after the freezing-thawing process,leading to the observation that of the earlier the peaks,the stronger the effect of freezing-thawing.After the soil was subjected to preloading with a small load,the influence of the freezing-thawing cycles gradually stabilized.The results may be useful in preventing and controlling the risk of subgrade soil failure when construction takes place spring thaw periods.     

地震波对干砂场地土层加速度放大系数影响分析

为研究土体加速度在向上传递时,不同地震动对其峰值的影响,设计制作了干砂场地离心振动台试验,通过对比得出加速度放大系数沿深度变化的规律.结果表明:地震动加速度幅值越大, 其沿深度变化的放大系数越小;在波的幅值增大时,其放大系数存在一定的衰减率,且衰减率随着幅值的增大而减小;在相同幅值不同频谱特性地震动作用下,等幅正弦波的加速度放大系数最大, LEAP波其次,El-Centro波最小.土体加速度放大系数及其衰减率的分析有助于研究场地的动力响应变化规律,对抗震设计具有一定参考价值.     

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.