循环荷载下砂土变形的细观数值模拟Ⅰ：松砂试验结果

采用颗粒流方法模拟不同排水条件下砂土的双轴试验，研究了循环荷载作用下松砂渐进破坏过程中配位数、接触方向、粒间接触力的演化规律，应用表征上述量的组构参数研究了砂土的诱发各向异性，探讨了饱和砂土液化、状态转换面产生的微细观机理。研究表明：宏观的液化对应于细观组构上配位数的连续累积丧失和粒间接触力的不断减小，其根本原因在于循环荷载往返过程中，组构各向异性与应力各向异性发展的不匹配。研究成果对于揭示砂土变形的细观机理以及建立砂土的细观力学模型都具有意义。     

动态三轴试验确定砂土抗液化强度

结合压电陶瓷弯曲元动测技术 ,对杭州砂进行控制剪切波速的动三轴试验 ,验证了砂土抗液化强度与弹性剪切模量间的相关性。试验结果表明 ,土体抗液化强度与其剪切模量的平方成线性关系 ,这使得结合剪切波速测试的动三轴试验研究土体液化更为简单、有效。根据研究实例 ,提出了进行液化判别的一般步骤 ,与常规方法比较后说明该方法的合理性和优越性     

砂坡液化稳定问题的动三轴液化试验研究

堤坝砂壳边坡在强地震时经常出现破坏现象。动三轴试验是进行砂土液化研究的必要手段。本文除讨论排水条件和密实条件外,还提出了不同砂土液化时总强度降低规律,这对于大致地鉴定砂坡的稳定性是有益的。     

关于“砂土液化动稳态强度分析”的讨论

拜读了王星华、周海林先生的“砂土液化动稳态强度分析”(发表于《岩石力学与工程学报》2003年第1期,以下简称“文[1]”)一文,受益颇多。针对文[1]所提出的动稳态强度问题,谈谈自己的看法,供大家参考。文[1]有如下阐述:“中砂、中密砂或密砂在周期振动力作用下,会发生液化和应变软化,最终达到一个稳定的残余强度。在此,残余强度在不同的振动周期,砂土的孔隙水压幅值不再增加,应力路径相互重叠,但是应变幅值却以恒定的速率不断增加,借助单调荷载中的稳定状态概念,称这种振动力作用下的相对稳定状态为动稳态。”在我国工程界,所谓液化定义为:…     

循环荷载下砂土变形的细观数值模拟I:松砂试验结果

采用颗粒流方法模拟不同排水条件下砂土的双轴试验,研究了循环荷载作用下松砂渐进破坏过程中配位数、接触方向、粒间接触力的演化规律,应用表征上述量的组构参数研究了砂土的诱发各向异性,探讨了饱和砂土液化、状态转换面产生的微细观机理。研究表明:宏观的液化对应于细观组构上配位数的连续累积丧失和粒间接触力的不断减小,其根本原因在于循环荷载往返过程中,组构各向异性与应力各向异性发展的不匹配。研究成果对于揭示砂土变形的细观机理以及建立砂土的细观力学模型都具有意义。     

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。     

砂土动三轴应力应变滞回曲线试验及其动力特性研究

通过动三轴试验研究砂土轴向应变的发展过程,从应力状态变化和理论解析两个方面,分析了砂土应力应变滞回曲线及其动力学特性,利用试验测量的滞回曲线来确定土的剪切模量和阻尼比。分析了影响滞回曲线形状的主要因素,解释了砂土在动载荷作用下应力应变滞回曲线出现不规则变化情况的原因。研究结果可为地震或振动等作用下工程建筑的安全性评价提供参考和借鉴。     

Comparison of sand liquefaction in cyclic triaxial and simple shear tests

The liquefaction resistance and correction factors K_σ and K_α of Nakdong River sand obtained from cyclic triaxial (CTX) tests were compared with those determined by cyclic simple shear (CSS) tests to ascertain the importance of the reduction factor C_r and correction factors K_σ and K_α in liquefaction evaluations, especially in view of the lack of comparative liquefaction assessments based on different laboratory test apparatuses. All samples used for the comparisons were obtained from the same type of sand by using similar preparation methods and they were subjected to similar stress states to minimize the number of factors influencing the comparison results; moreover, the apparatuses used in the two tests were manufactured by the same company and all tests were conducted by a single operator. It was found that the liquefaction resistance in CTX tests was always greater than that in CSS tests. Furthermore, C_r varied from 0.63 to 0.36, and it depended on the relative density D_r and initial static shear ratio α. K_σ, which increased with the normal effective stress σ′_nc in CTX tests, was identical to K_σ observed in CSS tests when α was increased up to 0.1. By contrast, K_α in the CSS tests was 58%–97% of K_α measured in the CTX tests, and it depended on the combined effect of D_r, σ′_nc, and α. The relationship between K_α and α in both CTX and CSS tests was well represented by a parabolic function. Moreover, the differences in K_α values between the CTX and CSS tests were also found to be a parabolic function of α. This information can be used for converting CTX (or CSS) values into equivalent CSS (or CTX) values.     

动态土工真三轴仪在砂土液化研究中的应用

为了模拟实际场地中土体变形和强度特性,研究开发了揭示侧限条件下饱和砂土震动液化机理的试验方法。文中采用了动态真三轴试验系统,该设备采用刚性板加柔性面的混合边界加载装置,并应用先进的CATS试验控制系统。试验结果表明应用动态土工真三轴仪进行砂土液化实验是可行的,并初步分析了侧限条件下的液化机理,即土体将在轴向压力、侧压和孔隙水压力相等的条件下发生液化,表明振动荷载过程中的应力重分布对砂土液化强度和孔隙水压力发展等具有显著的影响。     

Image based local deformation measurement of saturated sand specimen in undrained cyclic triaxial tests

To evaluate the local deformation properties of sand specimen directly in the undrained triaxial test, a procedure based on Particle Image Velocimetry was developed through using a transparent membrane. Cylindrical specimens using a mixture of silica sand particles in black and white colors were prepared by the moist tamping method with five sand layers and the air pluviation method. Comparisons of local deformations between the direct evaluation by sand particles patterns and indirect evaluation by dots pasted on the membrane were made for a single specimen. The results obtained in the undrained cyclic triaxial tests revealed that the local deformation properties of medium dense and dense silica sand specimens evaluated by the indirect observation were consistent with those evaluated by the direct observation before the onset of initial liquefaction, except for some regions near the bottom ends of the specimen. Unlike the air pluviation method, the moist tamping method induced undesirable stratification within the specimen, and this resulted in the occurrence of specimen necking. Meanwhile, a non-uniform local strain distribution was observed along the specimen prepared by moist tamping method even under an axial strain less than 2%. The relative movement between the membrane and sand specimen, or slippage, occurred at the liquefied status. The extent of vertical slippage was also impacted by the relative density of the specimen.     

地震P波和S波耦合的变围压动三轴试验模拟

地震发生时,尤其是直下型地震发生时,P波对场地土体的作用不可忽略。通过变围压动三轴设备施加的特殊应力路径, 模拟了P波和S波耦合对饱和软黏土动力特性的影响,即循环剪应力和循环正应力耦合对饱和软黏土动力特性的影响。试验结果表明：变围压动三轴试验得到的包括循环剪应力和循环正应力的应力路径可以较好地模拟P波和S波的耦合作用;循环剪应力和循环正应力的相位差对饱和软黏土的动强度有较大的影响,当相位差为180°时,土体的动强度最低,当相位差为0°时,动强度最高;随着循环正应力幅值的增大,土体的动应变发展速度与常规恒定围压动三轴试验下的动应变发展速度相比,差距逐渐增大。这些发现一方面表明强震过程中P波的作用不可忽略,另一方面表明,在某些情况下,常规恒定围压动三轴试验高估了饱和软黏土的动强度,对抗震设计不利。     

循环荷载下饱和南沙珊瑚砂的液化特性试验研究

针对饱和南沙岛礁珊瑚砂,开展了一系列不排水循环加载试验,研究了相对密度D_r和初始围压■对饱和珊瑚砂的超孔隙水压力、应变发展、有效应力路径及动强度特性的影响,并比较了珊瑚砂与福建砂的液化特性差异。试验表明,珊瑚砂的超静孔压Δu的发展模式与石英砂的有较大区别,可用修正后的Seed模型进行表征。珊瑚砂液化时的累积能量耗散远比福建砂的大。珊瑚砂的轴向应变ε_(DA)随着循环振次增加而逐渐变大,不会发生急剧增大的现象。在有效应力路径接触相转换线后,珊瑚砂会发生剪胀和剪缩交替出现的现象,仍然会存在有效应力。较之福建砂Δu的波动特征,珊瑚砂Δu的波动更大,且当Δu接近■时波动明显增大,产生"瞬时液化"现象。珊瑚砂的动强度随着D_r以及■的增大而增大。珊瑚砂的动强度大于石英砂的动强度。     

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。     

Static liquefaction behavior of saturated fiber-reinforced sand in undrained ring-shear tests

The problem of static liquefaction of sand is nowadays a classical soil mechanics subject. Using a ring-shear apparatus, we explore the possibility of fiber reinforcement as a new method to improve the liquefaction resistance of sand. In order to understand the effect of the fiber content and sand density on the static liquefaction behavior of fiber-reinforced sand, a series of undrained ring-shear tests were carried out on saturated samples with different fiber content and sand density, and the test results and mechanisms of fiber reinforcement were then analyzed. The results indicate that the undrained shear behavior of fiber-reinforced loose samples is not greatly influenced by the presence of fiber, but for medium dense and dense samples, the presence of fiber clearly affects their undrained behavior. Untreated specimens showed a continuous decrease in shear resistance after failure, while the specimens treated with fiber showed fluctuations even after shear failure, and these fluctuations become stronger with increasing fiber content. The peak shear strength increases with the fiber content, especially in dense specimens. After shearing, all the fiber-reinforced and untreated dense samples maintained structural stability, while the unreinforced loose samples showed a completely collapse of structure. The presence of fibers may thus limit or even prevent the occurrence of lateral spreading that is often observed in unreinforced sand.     

不排水循环荷载条件下胶结砂土宏微观力学性质离散元模拟研究

天然或人工胶结的存在能够提高砂土的抗液化能力,从宏微观尺度对其动力学性质进行研究具有重大意义.将已有的三维完整胶结接触模型引入到三维离散元程序中,对胶结砂土不排水循环三轴剪切试验进行三维离散元模拟,研究颗粒间胶结、循环应力比对离散元试样宏微观力学性质的影响.研究结果表明,胶结的存在能够抑制轴应变和孔压的发展,提高砂土的...