Liquefaction and post-liquefaction properties of sand-silt mixtures and undisturbed silty sands

Severe damage to earth structures mainly consisting of fine sands containing non-plastic silt has often occurred due to the liquefaction of the fill materials. However, the changes in the liquefaction susceptibility and post-liquefaction undrained behaviour of fine sands associated with the increase in the amount of non-plastic silt have not been well understood under a constant degree of compaction which has been employed as the construction management index for these structures. To clarify this point, a series of undrained cyclic triaxial liquefaction tests, followed by monotonic loading, was conducted on seven sand-silt mixtures with fines contents ranging from 0 to 100% in the present study. Undrained triaxial compression tests without precedent cyclic loading were also performed on the same materials for comparison purposes. In these tests, cylindrical specimens with an initial degree of compaction of 95%, prepared by the wet tamping method, were employed to simulate the construction conditions of earth structures. The test results showed that: (1) the liquefaction resistance, and the static strengths of specimens with and without precedent cyclic loading history decreased with increasing fines content ranging from 0 to 50%; however, they increased afterward, and (2) sand-silt mixtures with fines contents of 30, 50, and 65% consistently exhibited extremely small post-liquefaction strengths without showing any significant changes in the excess pore water pressure generated in the precedent liquefaction tests, which may lead to the post-liquefaction flow failure of earth structures. From these results, the risk of employing a uniform degree of compaction irrespective of the fines content was presented. In the present study, similar triaxial liquefaction tests, with measurements of the shear wave velocities by means of bender elements, were also conducted on both undisturbed and reconstituted non-plastic medium-fine sands containing fines which were retrieved from road embankments damaged possibly due to liquefaction brought about by the 2011 off the Pacific coast of Tohoku earthquake. The soil ageing effects were briefly discussed from the test results.     

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.     

Liquefaction resistance of fibre-reinforced silty sands under cyclic loading

Whether the so-called double porosity in soils with a loose and natural packing state is a concept with real-world implications is a fundamental yet controversial question in the study of cyclic undrained shear behaviour of fibre-reinforced silty sands. An attempt is made here to clarify the question by means of particle-level modelling combined with 41 undrained cyclic triaxial shear tests. The study shows that the initial Random Loose Packing changes to Random Close Packing and then Close Packing with silt content increments. The transition from random to close packing occurs at a threshold silt content which is relatively lower in coarser sands. For sands with <40% silt content, the rate of pore pressure growth with loading-unloading cycles increase with silt content increment. Reverse trend applies to silty sands with >40% silt content. Irrespective of fine content, fibres tend to sit deep into the silt pellets and encrust the macro-pore spaces. Generally, increasing fibre content leads to an increase in the average number of contacts per particle, dilation and easier dissipation of excess pore water pressure, a decrease in contact forces and improved liquefaction resistance. For sands with >40% silt content, effectiveness of fibre reinforcement diminishes with increasing sand median size.     

MICP胶结钙质砂动力特性试验研究

以南海某岛的钙质砂为材料进行微生物(MICP)胶结加固,通过动三轴试验和SEM微观结构试验,研究了MICP胶结钙质砂在不同胶结程度和不同动应力水平下的动强度、动变形、动孔压、有效应力路径的发展规律和MICP胶结的微观机理。结果表明:通过MICP胶结的钙质砂动剪应力比和抵抗变形的能力得到明显提高,这表明MICP胶结作用能显著改善钙质砂抗液化能力。MICP胶结钙质砂的孔压发展可以分为4个阶段:初始阶段—稳定发展阶段—快速发展阶段—完全液化阶段,当孔压发展到快速阶段末期,孔压曲线出现凹槽,试样开始失稳,最后发生破坏。整个振动循环过程中土体的变形和强度变化与有效应力路径和孔压的发展密切相关,当试样发生破坏时,有效应力路径表现出循环活动性。MICP胶结作用生成了方解石结晶包裹在砂土颗粒表面或填充于砂颗粒之间,这改变了土体的性质,使得土体的黏聚力和内摩擦角均有所提高。     

Cyclic resistance of two unsaturated silty sands against soil liquefaction

The changes of the cyclic resistance of two silty sands under unsaturated, partially saturated and fully saturated conditions are examined based on a series of undrained cyclic tests conducted using triaxial test apparatus specially equipped for testing unsaturated soils as well as ordinary triaxial test apparatus for testing partially saturated and fully saturated soils. Based on the observations of volumetric strain, pore air and pore water pressures of unsaturated soil specimens, the possibility of soil liquefaction triggering under different degrees of saturation is examined and discussed. The changes in the cyclic resistance under different degrees of saturation are then examined. Those two unsaturated silty sands with different grain size compositions are found to give rise to different responses on the volumetric strain as well as pore air and pore water pressure developments during undrained cyclic loading, leading to different relations between cyclic resistance and degree of saturation, covering unsaturated, partially saturated and fully saturated conditions.     

珊瑚礁钙质沉积物液化特性及其机理研究

针对南沙群岛永暑礁小澙湖珊瑚礁沉积物-钙质砂土的多孔隙及颗粒破碎特点,利用现场原位扁铲侧胀试验和室内动静三轴-扭剪试验手段,开展不同条件下钙质砂土的液化特性分析。试验结果表明,在7度地震烈度下浅海钙质砂土会发生液化;在波浪荷载作用下,饱和钙质砂土峰值孔压随振动次数的增加可分为抛物线型增长、线性增长、指数型增长和稳定状态等四个阶段,而每一阶段均对应有钙质砂颗粒间孔隙和内孔隙的不断调整和破碎过程;在均压固结条件下其液化机理归结为流滑破坏。     

往返荷载下的饱和砂

饱和砂土的强度、液化和破坏问题是水工建筑物地基和土石坝抗震安全中考虑的重要问题。本文建议以极限平衡作为饱和砂土在往返荷载下开始破坏的准则,并且提出了相应的试验和分析方法。阐明了液化与开始破坏的区别。提供了两种估计饱和砂土在往返荷载下孔隙水压力变化的途径,其中之一考虑了它们的扩散和消散作用。     

基于流动性的饱和砂砾土液化机理

砂砾土的地震液化至今仍存较大的争议,相应的液化机理解释主要沿用传统的砂土液化分析思路和方法。利用动态圆柱扭剪仪开展了100 mm直径、3组典型级配（含砾量分别为37%,45%和60%）的饱和砂砾土试样循环动三轴实验。基于实验得到的应力-应变率关系曲线,定义了反应饱和砂砾土流动性的平均流动系数和流动性水平。实验发现,初始动应力比对不同含砾量下的平均流动系数-孔压比关系曲线无影响;相对密度越大、含砾量越大,饱和砂砾土的流动性水平越低;有效固结压力对饱和砂砾土平均流动系数-孔压比关系曲线的影响与含砾量相关。推测饱和砂砾土在循环荷载下的流动性由其粗粒接触状态和数量决定;粗粒间的接触在高孔压状态下不能顺利解除是饱和砂砾土与饱和细粒土抗液化性能的本质区别。提出的基于流动性的饱和砂砾土液化机理较好地解释了以上现象。     

水泥固化漳州滨海风积砂液化特征试验研究

利用GDS动三轴仪对水泥固化漳州滨海风积砂进行液化强度试验,分析水泥固化漳州滨海风积砂在不同水泥剂量、不同龄期、不同基准应变和不同等幅循环应变幅值下的液化特性。试验结果表明:砂样的抗液化能力随水泥掺量的增加近似呈线性提高;随龄期增长而增强,但基准应变越高,增强越弱;随循环应变幅值的减小而显著增强;基准应变对砂样抗液化能力的影响受水泥掺量的制约。孔压半对数模型可以较好的反应水泥固化漳州滨海风积砂土的孔压发展规律;存在一个临界基准应变,使得抗液化能力随水泥剂量的增加得到最大化提高。     

Prediction of pore water pressure generation leading to liquefaction under torsional cyclic loading

Undrained cyclic loading tests were performed under torsional shear in a hollow cylindrical apparatus on four sands of various densities, initial stress levels, gradings and origins to establish the pattern of excess pore water pressure generation with cycles leading to initial liquefaction. Two equations were derived to predict this pattern. The first is based on the method introduced by Ishibashi et al. (1977) and incorporates density and the effective stress level into the original equation. The second involves a unique relationship between the excess pore water pressure and the shear work imparted to the sand and it was obtained independent of the shear stress amplitude when the dissipated shear work was normalized with respect to density and the effective stress level. The excess pore water pressure required to induce liquefaction was found to necessitate lower normalized shear work from finer sands. These equations can be used to assess the liquefaction potential and/or can be directly related to the amount of seismic energy dissipated in the field.     

前期动载对低塑性粉土静态和动态强度的影响

 低塑性粉土广泛存在于世界范围内,在地震中容易产生液化现象,然而一些基础设施破坏不仅见于地震中也发生在地震后,这就决定了研究低塑性粉土震后行为的必要性。以美国中部密西西比河沿岸低塑性粉土为试验材料,研究动载对低塑性粉土静态和动态强度的影响。在动三轴仪上对试样施加动载引起超孔隙水压力,排水重固结后,分别对2组震动后试样进行静态和动态三轴强度试验。试验结果表明,当液化水平小于0.70时,前期动载对粉土的不排水剪切强度影响不大;相反地,只有当液化水平大于0.70,密西西比河沿岸粉土的震后重固结体积应变和不排水剪切强度才伴随着液化水平的提高显著增加,但相对于砂土而言,重固结体积应变在较低的液化水平时即有明显增加。与前期动载对不排水剪切强度的影响不同,当动载所引起的液化水平为0.35或轴向应变为0.2%时,抗液化强度达到最大值,若液化水平大于0.35,抗液化强度伴随液化水平提高而降低。如果前期荷载引起较大的压应变,在重固结后第二次动载循环中,轴向压力相比轴向拉力引起较小的超孔隙水压力。     

随机地震作用下砂土液化概率分析

为了评价随机地震作用下饱和砂土孔隙水压力发展的概率,本文介绍了一种新的概率模型。模型中考虑了砂土动力特性的非线性、液化抗力的不确定性以及孔压发展的非线性,计算了每一荷载循环终了时的孔压累积概率分布函数,当孔压比等于1时,直接从累积概率分布函数中可得出液化概率。应用本方法,对日本新泻地震时的三个典型场地进行了分析,计算结果和现场震害良好一致,证实了本文方法的适用性。     

循环加载频率对砂土液化模式的影响试验研究

为了研究循环加载频率对饱和砂土的液化特性的影响,针对密实度为35%、50%、70%的福建标准砂进行了振动频率为0.05Hz、0.1Hz、0.5Hz、1Hz、2Hz的循环扭剪试验,并对密实度为50%的珊瑚砂和细砂进行了振动频率为0.1Hz和1Hz的循环扭剪试验。结果表明：无论是松砂还是密砂,其剪胀剪缩特性与加载频率密切相关,在低频荷载作用下表现出显著的剪胀特性,达到初始液化后孔隙水压力波动,土体仍具有抵抗液化能力,呈现“硬化型”液化模式;在高频荷载作用下表现出显著的剪缩特性,达到初始液化后孔隙水压力保持稳定,循环液化模式呈现“软化”特征,珊瑚砂和细砂的孔隙水压力特征和液化模式也同样受加载频率的影响,说明循环加载频率显著影响饱和砂土的剪胀剪缩特性,进而影响液化模式;液化阶段产生的流滑变形大小与加载频率密切相关,低频荷载作用下所产生的流滑变形显著大于高频荷载作用下的流滑变形。     

Experimental investigation on liquefaction and post-liquefaction deformation of stratified saturated sand under cyclic loading

The soils are often distributed in a stratified structure. When an earthquake occurs, there are always risks of liquefaction for stratified soils which can cause serious consequences. The research aims to investigate the liquefaction and post-liquefaction deformation of saturated sand with stratified structure. The influences of liquefaction and post-liquefaction deformation were analyzed by varying the thickness, position, and layers of powdery sands. The findings showed that the correlations between the times taken to reach liquefaction for cyclic loading and the thicknesses of the powdery sandy interlayer are non-linear. With the optimal thickness, the powdery sand interlayer can effectively prevent the transfer of power water pressure. And two-layer powdery interlayer in the sample was more favorable to resist pore water pressure than that with single layer. The strength of post-liquefaction deformation and failure patterns are closely related to the distribution of powdery layer. The work provides new research evidence on the liquefaction and failure mechanism of saturated sands with stratified structure under cyclic loading which often happen during earthquakes.

     

Characterizing the effects of fines on the liquefaction resistance of silty sands

Recent earthquakes in New Zealand and Japan indicate that evaluating the liquefaction potential of silty sands remains an area of difficulty and uncertainty in geotechnical engineering. This paper presents a comprehensive experimental study along with analysis and interpretation in the framework of critical state soil mechanics, with the aim to address the complicated effects of fines. Two series of sand-silt mixtures, formed by mixing two different base sands with the same type of non-plastic silt, were tested under a range of packing density, confining pressure and silt content, and a unified correlation was established between the cyclic resistance and the state parameter that collectively accounts for the effect of packing density and confining pressure. The proposed correlation is independent of packing density, confining pressure, fines content and base sand, and allows prediction of the cyclic resistance of silty sands under different states. Furthermore, the mechanism of the fines-content induced reduction of cyclic resistance and the mechanism of the base-sand effect observed from the tests are elaborated in the sound theoretical context. The present study suggests that the critical state soil mechanics is a rational and appealing framework for liquefaction analysis of both clean and silty sands.     

微生物处理砂土不排水循环三轴剪切CFD-DEM模拟

微生物诱导碳酸钙沉积是一种新型的地基处理技术,处理后的土体可以看成一种结构性土。首先,在已有三维含颗粒抗转动和抗扭转模型及三维胶结破坏准则的基础上,通过考虑颗粒碰撞接触过程中颗粒本身的塑性变形及率相关性的接触黏滞阻尼,建立考虑循环荷载作用下的三维胶结模型;然后,参考已有研究,建立了反硝化反应在加固砂土中的时效性关系。并引入CFD-DEM耦合程序,用以模拟分析不同胶结含量以及不同气泡含量下,微生物处理砂土在固结不排水循环剪切试验中的力学特性;最后,从宏微观角度分析生物胶结与生物气泡对砂土抗液化性能的影响及其作用机理。研究表明,胶结和气泡共同存在对抗液化能力的提升并没有起到“1+1=2”的效果;胶结的存在提高了非饱和砂土的抗液化能力,明显抑制孔压比和轴向应变的发展,力学配位数得到了提升;而气泡的存在却降低了胶结砂土的抗液化能力,使得胶结砂土达到初始液化的振次减少,轴向应变向受拉方向大幅增长,力学配位数下降明显。     

循环荷载下滨海风积砂孔压发展规律试验研究

利用GDS动三轴试验系统,在不同的基准应变下对福建漳浦滨海风积砂施加不同幅值的等幅循环应变荷载,分析了滨海风积砂在循环应变作用下的孔压发展规律。试验结果表明:滨海风积砂的孔压在初始段迅速增长,之后循环应变幅值不同,孔压有不同的动力响应。小幅值作用下,过渡段大致为“S”发展趋势,后期试样结构崩溃,孔压值迅速达到围压值。密实砂抗液化能力随着基准应变的增大和循环应变幅值的减小而增大,但是中密砂受基准应变的影响很小。根据实验数据建立孔压增长对数模型,模拟结果与试验结果一致性较好。     

双向耦合剪切条件下饱和松砂的液化特性试验研究

为了模拟海床及海洋建筑物遭受波浪荷载时所引起的循环应力,进行了一系列均等固结条件下的应力控制式轴向–扭转双向耦合循环剪切试验。加载路径在σd/2-τ应力空间内为椭圆。试验在保证椭圆面积不变的情况下,分别变化竖向和扭转向的荷载分量幅值,以此来探讨双向耦合剪切试验中各个分量的变化对饱和松砂的循环强度特性的影响。试验结果表明砂土在双向耦合荷载作用下,其液化强度与加载椭圆路径的面积和两个荷载分量比值密切相关。当轴向应力与剪应力幅值的比值保持不变时,砂土液化强度随着椭圆面积的增大而降低。而在椭圆面积保持不变时,当竖向与扭转向荷载分量的比值小于某一临界值0.6～0.75时,砂土液化强度随着比值的增加而增大,当竖向与扭转向荷载分量的比值大于某一临界值0.6～0.75时,砂土的液化强度随着比值的增加而减小,在临界值0.6～0.75之间表现出最高的强度。另外,在一个周期内孔隙水压力的循环变化与轴向应力相位一致,与循环剪应力相位相差90。     

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.     

Centrifuge model tests on large-diameter monopiles in dense sand subjected to two-way lateral cyclic loading in short-term

To evaluate the lateral resistance of rigid monopiles for wind turbines in dense sand under lateral cyclic loading, centrifuge model tests are performed, focusing on the base resistance and degradation of the soil resistance under two-way lateral cyclic loading in the short-term. The slenderness ratio (embedded pile length to diameter) is varied from 3.75 to 8 and the loading frequency is in the range of 0.002 to 0.4 Hz in the prototype scale. Under cyclic loading with a maximum horizontal displacement of 5% of the pile diameter, the build-up of excess pore water pressure is observed, but the maximum value of the average excess pore water pressure ratio is around 50% in the steady-state for dense sand whose relative density is 80%. A simple analytical model for the rigid piles, considering the base resistance, is derived and then used to quantify the significance of the resistance at the pile base and the degradation of the soil resistance under cyclic loading. When the slenderness ratio is less than 5, a significant contribution of the moment resistance at the base is confirmed. The estimation of the degradation of the horizontal subgrade reaction coefficient using the simple analytical model suggests that, through cyclic shear tests for the determination of the deformation properties of the soil in a laboratory, it is possible to estimate the degradation of the soil stiffness and the parameters for the reduced sway-rocking type of foundation model.