A simple method for evaluating liquefaction potential from shear wave velocity

The simplified procedure using shear wave velocity measurements is increasingly used to evaluate the seismic liquefaction potential of soils. This procedure is based on finding the boundary separating the liquefaction and non-liquefaction cases through the analysis of liquefaction case histories, following the general format of the Seed-Idriss simplified procedure based on standard penetration test (SPT) data. It is noted that many assumptions have been made in the simplified procedure. This paper develops a simple method for evaluating the liquefaction potential of soils from shear wave velocity by using the optimum seeking method to directly analyze the liquefaction history data and quantify the influence of major factors affecting the liquefactions potential of soils. The factors considered are the earthquake magnitude, the vertical effective overburden stress, the shear wave velocity, the peak acceleration at the ground surface of the site, and the fines content of the soil. The most important factor has been identified as the shear wave velocity. The developed method uses the measured data directly and in a very simple way. Neither stress-correction of shear wave velocity nor calculation of cyclic shear stress as in the simplified procedure is required. Comparisons indicate that the developed simple method has a higher success rate for evaluating liquefaction potential of soils than the simplified procedure. A case study is presented to illustrate the application of the developed simple method and further confirms its accuracy.     

剪切波速在评估地基饱和砂层地震液化可能性中的应用

对利用“门槛剪切波速”作为初判手段的起源 ,及用现场剪切波速校正室内试验“初始液化”动剪应力幅比的功能 ,进行了论述。对是否有“临界剪切波速”和“临界动剪应变幅”等问题 ,进行了讨论。最后提出了笔者的看法。     

饱和砂土的剪切波速与抗液化强度相关性研究

依据扭剪振动原理,开发出剪切波速与振动三轴联合试验装置。进而针对取自20 多个工程场地、埋深在20 m 以内的12种大量原状砂土,分析了同一类土、在同一固结压力作用下,原状与重塑土样抗液化强度与其初始剪切波速之间的相关性。结果表明,当判别液化的应变标准不超过6%时,土的抗液化强度与其初始剪切波速之间存在良好的对应关系。当应变破坏标准超过6%,土的初始剪切波速与抗液化强度之间不具备唯一对应关系。     

波速测试在东营地区工程勘察中的应用

介绍了波速测试的基本原理、测试方法、波速计算公式和成果应用，并结合工程实例加以阐述，总结出该测试方法具有操作简单、波形清晰、快速有效的优点。     

相同剪切波速的原状与重塑饱和松散粉土的抗液化强度

采用剪切波速与振动三轴联合试验装置,研究了具有松散结构的原状和重塑饱和粉土抗液化强度之间的相关性。结果表明,具有松散结构的原状与重塑饱和土样的振动应变随振动次数的变化趋势是一致的。如果把重塑饱和土样的初始剪切波速恢复到原状饱和土样的初始剪切波速,重塑土样的抗液化强度也被恢复到原状土样的抗液化强度。此时,原状与重塑土样的抗液化强度与抗液化强度曲线之间的相关系数大于0.89,而且这种良好的相关性与确定破坏振次的液化判别标准无关。这里的研究结果支持饱和松散试验土样的初始剪切波速可以作为控制其抗液化强度的一个关键参数。     

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.     

土层等效剪切波速与平均剪切波速对比分析

通过简化的理论推导与实例分析，比较了太原市土层平均剪切波速与等效剪切波速计算结果的差异性，提出了一般情况下Vse值总是小于Vsm值，且两者的差异性与土层的剪切波速值间的变异系数有关。     

深孔剪切波速测试孔下探头贴壁方法的改进

着重介绍深孔剪切波速测试时,探头在孔下贴壁的新方法、原理及其应用。同时对沿用的旧方法的缺点及新方法的优点做了介绍,并进行了对比分析,结果表明新方法测试精度大大提高,准确地判定了建筑场地土类型及场地类别,为建筑设计提供了详实的抗震设计参数。     

离散元模拟中颗粒材料剪切波速的剪切振动确定方法

砂土等颗粒材料的剪切波速是反映其工程性质的重要参数,离散元模拟中实现剪切波速实时测试对研究砂土材料力学性质变化的细观机理具有重要意义。本文基于软件 PFC^3D ,在试样模型中设置激发源和接收源,通过对激发源施加产生剪切振动的速度脉冲、在接收源监测扰动信号,实现了剪切波在颗粒材料中的传播模拟和信号监测,提出了利用激发和接收信号的互相关系数确定剪切波传播时间和评价剪切波传播质量的方法。系统分析了激发频率、激发幅值、激发源和接收源尺寸、阻尼等因素对剪切波速测定的影响,给出了合理的参数取值范围。研究结果得到了均匀颗粒试样理论解的验证。     

弯曲元试验高精度测试土样剪切波速方法

在HX-100型多功能三轴仪上开发了压电陶瓷弯曲元剪切波速测试系统,研究了高精度测试不同种类和刚度土样剪切波速的方法以及剪切波速弥散性问题,并将弯曲元测试结果与DrnevichLong-Tor型共振柱试验结果进行了对比。研究结果表明,对不同种类和刚度的土样,通过选用合适的激发波形和频率就可以消除接收波形的近场效应和过冲现象,高精度地确定土样剪切波速和极小应变剪切模量;在所使用的激发频率范围内,剪切波速不具弥散性。     

泉州市规划区剪切波速与地基土类型关系

通过参与泉州市规划区抗震防灾规划勘察积累的546个钻孔的岩土层剪切波速数据,分析和总结了岩土类型与剪切波速的关系,为估算确定建筑场地类别提供依据。     

饱和土剪切波速在基桩缺陷定量分析中的应用研究

在基桩缺陷定量分析中准确定量地描述桩土之间的相互作用是非常重要的。以黏滞阻尼器模拟桩土之间的相互作用,对6种饱和土进行了桩土相互作用模型试验和剪切波速试验。试验结果表明,可以对这些饱和土在土剪切波速与阻尼系数之间建立一致的相关关系。同样依据试验结果分析了试验阻尼系数和理论值的差别,得到了以剪切波速的函数表示的理论值修正系数。由桩身纵向振动响应解析解利用正交试验方法分析了阻尼系数、瞬态冲击力的脉冲宽度和幅值、桩弹性模量、桩密度、桩长以及桩截面积对速度波衰减过程的影响,确定了阻尼系数是影响速度波衰减的唯一显著因素,得到了速度波沿桩身的衰减规律。进而结合土剪切波速与阻尼系数之间的相关关系和桩身缺陷处的能量分配特性建立了利用土剪切波速和桩顶速度响应进行基桩缺陷定量分析的关系式。根据建立的关系式分析了利用剪切波速确定阻尼系数所带来的误差,结果证明由剪切波速最终得到的缺陷程度相对利用试验阻尼系数得到的结果误差一般在10%以内,所以利用剪切波速确定阻尼系数是可行的。     

关于估算各土层剪切波速引发的新问题

结合工程实例,通过估算两类场地的等效剪切波速,对其引发的持力层问题进行了探讨,从而为场地类别的判定奠定了理论基础,以使所提抗震参数与持力层实际情况相适应。     

最优化法在砂土液化势评价中的应用

探讨了应用Ｆｉｂｏｎａｃｃｉ 最优化法评价砂土液化势的理论和原理,给出了相应算法和流程,并建立了评价模型,实例应用表明基于Ｆｉｂｏｎａｃｃｉ 最优化法来判别及评估砂土液化势是有效可行的,且取得了理想的结果。     

砂土地震液化的不确定性判别法综述

通过对不同自由场地液化势估计的方法和公式进行总结和比较,发现剪切波速比其它波的波速更能反映地基岩土层的工作性能,剪切波速法近年来也逐渐受到人们的重视,这是因为基测定简便易行,且和场地分类可以共用,同时它既不影响钻孔周围土的性态,又较少操作条件和方法的影响,判别精度同基它经验方法也同阶,是人很好的经验判别公式。     

适于不同深度土层液化的剪切波速判别公式

剪切波速测试是工程上常用的现场技术,正逐步成为液化判别方法的基本指标之一。利用早期的Andrus数据库对中国《岩土工程勘察规范》方法和Andrus方法进行检验,发现了二者存在的问题。提出了双曲线形式的剪切波速判别模型和公式。采用新的Kayen数据库对三者进行了对比检验,并讨论了提高判别精度的可能性和方式。结果表明:中国现行的《岩土工程勘察规范》中剪切波速判别液化方法,无论对浅层还是深层土,判别结果均严重保守,甚至会把十分密实的砂土判成液化,十分不合理;国际上应用广泛的Andrus方法对浅层土判别结果过于保守,对深层土判别成功率可以接受,但其临界剪切波速曲线在深处存在回弯的不合理现象;所提出的双曲线液化判别模型和公式,能够深浅兼顾,无论对浅层还是深层土都能给出较好的判别结果,克服了中国规范方法和Andrus方法的弊端,且形式简单便于工程应用;采用剪切波速进行液化判别时,应采取多次测试,以降低数据离散性,提高判别的准确性。     

花岗岩残积土剪切波速经验取值问题的探讨

通过福州东站附近30余个钻孔的实测波速资料,利用回归分析方法,研究花岗岩残积土标贯击数与剪切波速的定量关系及其应用。     

Effects of specimen density and initial cyclic loading history on correlation between shear wave velocity and liquefaction resistance of Toyoura sand

Some previous studies have shown a good correlation between the shear wave velocity, V_s, and the cyclic resistance ratio, CRR. Recently, however, a V_s-based liquefaction assessment method has become an alternative and supplementary method to the conventional N_SPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric may generate different relations between V_s and the CRR even if the tested specimens are of the same soil material. In the current study, a series of V_s measurements and undrained cyclic triaxial tests is performed on Toyoura sand specimens with different soil fabrics for three different specimen densities. The fabric of the specimens is varied by applying initial cyclic loading. The results of the V_s measurements indicate that the V_s of the specimen is affected by the initial cyclic loading histories, and the results of the undrained cyclic triaxial tests show that there is a good correlation between V_s and the CRR. However, the correlation varies depending on the specimen density even when the tested material is Toyoura sand only. In other words, the soil-type specific correlation between V_s and the CRR depends on the specimen density. Therefore, the results indicate that both V_s and the specimen density are necessary parameters for an accurate assessment of the CRR.     

利用弯曲元测量土体表层剪切波速的初步试验

土体剪切波速是土力学研究与岩土工程设计的关键参数之一。根据压电弯曲元波速测量的方向性,提出了侧向激发与接收的测量模式,使得弯曲元技术可用于室内和现场土体表层剪切波速的测量。在砂土模型内的初步试验表明,以激发元和接收元内侧间距为剪切波传播距离,并采用时域初达波法判别剪切波到达时间,则侧向激发与接收方式弯曲元的剪切波速测量结果准确可靠,精度与常规"对向激发与接收"方式相同。该弯曲元测试方法为当前大量涉及的取样扰动评价及地基处理质量控制等重要问题提供了便捷可靠的无损检测手段。