Reconstruction of the CPT-based Liquefaction Database from 1976 Tangshan Earthquake

1976年唐山大地震导致了范围广、灾害严重的液化震害。地震发生后,已有学者对液化场地进行了2次静力触探测试。本文首先给出基于2次测试数据的唐山大地震CPT液化数据库,利用中国规范方法和NCEER推荐方法对数据库进行液化判别,发现针对第1次测试数据的判别成功率较高,而针对第2次测试数据的判别成功率较低。经30年的时间,绝大多数测试点的液化层强度与埋深均发生了较大变化,第2次测试时土层液化可能性已发生了较大变化,其测试数据对液化判别方法的改进意义较小。利用第2次测试的摩阻比R_f将第1次测试数据p_s分解为q_c和f_s,相比第2次测试数据,分解指标具有更高的可靠性。为此基于第1次测试分解数据构建唐山大地震CPT液化数据库,为液化判别方法改进提供数据支持。     

多功能参数静力触探在地震液化判别方法中的应用进展研究

地震液化是引起地基失稳和上部结构损害的直接原因之一,而液化震害预防的第一步就是对工程场地进行液化预测和判别。静力触探(CPT)作为最主要的原位测试技术,因其具有快速、低廉、高效等优点,被广泛用于液化判别。根据国内外近年来CPT技术的发展,对不同形式的CPT[孔压静力触探技术(CPTU)技术,电阻率孔压静力触探(RCPTU)技术、地震波孔压静力触探技术(SCPTU)技术等]在地震液化评价中的应用进行系统的论述。特别介绍一种基于状态参数进行液化判别的方法,另外还简要介绍基于概率统计分析法。比较和梳理各种液化判别方法的差异性,最后,在已有研究成果的基础上分析现有CPT液化判别法存在的问题与不足。分析结果表明:状态参数法能够同时考虑围压应力和孔隙比的影响,有效地将室内试验与现场试验联系起来;SCPTU、RCPTU液化判别框架还需进一步拓展与完善;概率统计分析法需结合相应软件使其更具适用性。     

地震液化判别及危害性评价

饱和砂土地震液化有可能诱发极为严重的破坏,已成为土动力学领域的重要研究课题之一。对液化的判别分为初判和复判。初判指根据已有的勘测资料或简单的测试手段,初步判别土层的液化可能。对于初判可能发生地震液化的土层,则再进行复判。鉴于土层液化的影响因素较多,我国规范建议采取经验方法,即标准贯入法,静力触探法,剪切波速法。单一判别方法都有局限性和适用范围,宜用各种方法综合判别。液化危害性评价使用危害性指标,分析液化对建筑物的危害程度。评价方法主要有液化指数法,震陷值法,谱强度比法和综合法。以评价指标为依据,划分液化影响的综合等级,全面反映液化危害程度。     

地震液化判别及危害性评价

饱和砂土地震液化有可能诱发极为严重的破坏,已成为土动力学领域的重要研究课题之一。对液化的判别分为初判和复判。初判指根据已有的勘测资料或简单的测试手段,初步判别土层的液化可能。对于初判可能发生地震液化的土层,则再进行复判。鉴于土层液化的影响因素较多,我国规范建议采取经验方法,即标准贯入法,静力触探法,剪切波速法。单一判别方法都有局限性和适用范围,宜用各种方法综合判别。液化危害性评价使用危害性指标,分析液化对建筑物的危害程度。评价方法主要有液化指数法,震陷值法,谱强度比法和综合法。以评价指标为依据,划分液化影响的综合等级,全面反映液化危害程度。     

基于CPTU原位测试状态参数的液化判别方法研究

砂土原位状态的性能一直以来都难以有效获取,而其体积变化特征的评价指标（状态参数）就是其中重要的一项。利用状态参数（Ψ）来代替相对密实度表征砂土的状态特性是一种新兴的趋势。考虑到砂土状态参数是关于体积变化的评价指标,在砂土地层中,随着围压的增加,砂土剪胀的趋势增加,液化阻力也增加。因此通过状态参数进行砂土液化的判别是可行的。以宿迁—新沂高速公路工程为背景,建立基于孔压静力触探（CPTU）的状态参数、周期阻力比（CRR）和标准贯入击数（N）之间的相关性模型。提出通过砂土的状态参数判别液化的两种新方法。     

基于2011年新西兰地震的CPT液化特征研究

2011年2月22日,新西兰第二大城市克赖斯特彻市附近发生M_s6.3级地震,震中位于克赖斯特彻市西南10km处,震源深度为5km。地震造成了大量的人员伤亡和经济损失,重建费用估计高达40亿新西兰元,得到共识是砂土液化为震害主因。此次地震,液化震害极为严重,喷砂冒水范围广泛分布于城市内外,引起大量次生灾害,地震也为砂土液化特征研究,特别是检验和发展液化判别方法提供了大量宝贵的液化数据。本文收集整理震后132个勘察点的勘察资料,包括静力触探试验（以下简称"CPT"）数据、地表峰值加速度和地下水位等。通过对数据的研究,得到了地震液化场地加速度分布、地下水位分布和砂层埋深分布等特征。检验了我国现有的静力触探试验液化判别方法,结果表明:液化场地主要分布在PGA为0.5~0.65g附近,液化层5m以内居多,我国CPT液化判别方法对于10m以下判别存在明显的错误,同时此次地震还发现多个液化层位于20m以下,对于这个深度,国内CPT液化判别方法还未涉猎。     

砾性土抗液化能力与其液化判别方法研究进展

从砾性土的定义出发,回顾了砾性土从被认为不会液化到其液化现象引起工程师关注的研究历程及国内外研究进展。总结了含砾量、相对密度等因素对砾性土抗液化能力影响关系的研究现状。介绍了国际上对砾性土场地液化判别的研究成果及各种主流方法的优势与不足;分析了剪切波速方法应用于砾性土场地液化判别的可行性。研究认为:砾性土的液化现象正逐渐被科学界及工程界接受;含砾量对砾性土抗液化能力的影响研究仍然存在较大矛盾,相对密度等影响砂土抗液化能力的因素同样影响砾性土抗液化能力;砂土场地应用的标准贯入、静力触探等液化判别方法不适用于砾性土场地,国际上发展了基于动力触探和贝克尔贯入试验的液化判别方法;剪切波速判别方法在砾性土场地的液化判别中具有优势和潜力,是今后研究的方向之一。     

利用qc,D50评价砂土地震液化势研究

本文根据国内外地震液化调查资料，建立了利用双桥静力触探锥头阻力ｑｃ和平均粒径Ｄ５０判别地震液化势的半经验性方法。这种方法对评价地震液化势是颇为有效的。     

饱和黄土场地原位试验及液化势评价

目前,主要依靠室内动力试验对黄土液化势进行评价。由于黄土特殊的结构性,室内试验对其饱和的过程较为复杂,且与实际场地饱和黄土差异明显,导致室内黄土液化试验结果并不能代表现场饱和黄土的抗液化强度。本文选取兰州市西固区寺儿沟村某饱和黄土场地进行钻孔测试,现场实施了标准贯入试验、静力触探试验以及剪切波速测试。应用Robertson的土类指数分类图对该场地不同含水率黄土的土类进行了界定,确定了饱和黄土属于类砂土,有液化势。应用NCEER推荐方法,计算了3组原位试验数据的饱和黄土循环抗力比（CRR）,通过与1976年唐山地震和1999年集集地震液化土CRR对比,得出了饱和黄土抗液化强度很低的结论。     

江油火车站典型液化震害分析

2008年5月12日我国四川省发生的8.0级汶川大地震中,江油火车站外表看似完好,但地基沉降、墙体开裂严重,最终不得不废弃,而周围其他建筑物破坏较轻,具有典型研究价值。通过对江油火车站进行钻孔以及现场剪切波速测试,查明了火车站破坏主因、液化主要土类,并对现有液化判别方法进行了检验。结果表明:(1)江油火车站结构振动破坏相对较轻,主要为液化导致地基沉降而最终废弃;(2)江油火车站候车室地表喷出物为粉砂,但实际液化土与喷出物差别显著,主要为砂砾土液化;(3)现有液化判别方法主要基于砂土液化资料,不适用于砂砾土的液化评价,需要研究和发展新的砂砾土液化判别方法。     

Spatial variability of liquefaction potential in regional mapping using CPT and SPT data

Cone penetration test (CPT) and standard penetration test (SPT) are widely used for the site specific evaluation of liquefaction potential and are getting increased use in the regional mapping of liquefaction hazard. This paper compares CPT and SPT-based liquefaction potential characterizations of regional geologic units using the liquefaction potential index (LPI) across the East Bay of the San Francisco Bay, California, USA and examines the statistical and spatial variability of LPI across and within geologic units. Overall, CPT-based LPI characterizations result in higher hazard than those derived from the SPT. This bias may result from either mis-classifications of soil type in the CPT or a bias in the CPT simplified procedure for liquefaction potential. Regional mapping based on cumulative distribution of LPI values show different results depending on which dataset is used. For both SPT and CPT-based characterizations, the geologic units in the area have broad LPI distributions that overlap between units and are not distinct from the population as a whole. Regional liquefaction classifications should therefore give a distribution, rather than a single hazard rating that does not provide for variability within the area. The CPT-based LPI values have a higher degree of spatial correlation and a lower variance over a greater distance than those estimated from SPTs. As a result, geostatistical interpolation can provide a detailed map of LPI when densely sampled CPT data are available. The statistical distribution of LPI within specific geologic units and interpolated maps of LPI can be used to understand the spatial variability of liquefaction potential.     

Evaluation of soil liquefaction in the Chi-Chi, Taiwan earthquake using CPT

During the 1999 Chi-Chi, Taiwan earthquake, many sand boiling phenomena were observed in central Taiwan, which caused severe ground settlement and structure damages. According to the installed accelerograms, the peak ground surface horizontal accelerations in the liquefaction-affected areas range from 774.42 to 121.3 gal. The writers carried out an extensive investigation of soil liquefaction in this earthquake. In this paper, we present results of the CPT exploration and post-earthquake liquefaction analysis. Two hundred and seventy five (275) cone penetration test data were collected from the liquefaction-affected areas, and 46 liquefaction case histories and 88 non-liquefaction case histories were derived that can be used to evaluate the accuracy of existing liquefaction evaluation models. In addition, the strength of the liquefied soils after earthquake and the implication of its liquefaction potential in the future event are discussed.     

Estimation of residual shear strength ratios of liquefied soil deposits from shear wave velocity

For sites susceptible to liquefaction induced lateral spreading during a probable earthquake, geotechnical engineers often need to know the undrained residual shear strength of the liquefied soil deposit to estimate lateral spreading displacements, and the forces acting on the piles from the liquefied soils in order to perform post liquefaction stability analyses. The most commonly used methods to estimate the undrained residual shear strength (Sur) of liquefied sand deposits are based on the correlations determined from liquefaction induced flow failures with SPT and CPT data. In this study, 44 lateral spread case histories are analyzed and a new relationship based on only lateral spread case histories is recommended, which estimates the residual shear strength ratio of the liquefiable soil layer from normalized shear wave velocity. The new proposed method is also utilized to estimate the residual lateral displacement of an example bridge problem in an area susceptible to lateral spreading in order to provide insight into how the proposed relationship can be used in geotechnical engineering practice.     

CPT-based analysis of liquefaction and re-liquefaction following the Canterbury earthquake sequence

The Canterbury region experienced widespread damage due to liquefaction induced by seismic shaking during the 4 September 2010 earthquake and the large aftershocks that followed, notably those that occurred on 22 February, 13 June and 23 December 2011. Following the 2010 earthquake, the Earthquake Commission directed a thorough investigation of the ground profile in Christchurch, and to date, more than 7500 cone penetration tests (CPT) have been performed in the region. This paper presents the results of analyses which use a subset of the geotechnical database to evaluate the liquefaction process as well as the re-liquefaction that occurred following some of the major events in Christchurch. First, the applicability of existing CPT-based methods for evaluating liquefaction potential of Christchurch soils was investigated using three methods currently available. Next, the results of liquefaction potential evaluation were compared with the severity of observed damage, categorised in terms of the land damage grade developed from Tonkin & Taylor property inspections as well as from observed severity of liquefaction from aerial photography. For this purpose, the Liquefaction Potential Index (LPI) was used to represent the damage potential at each site. In addition, a comparison of the CPT-based strength profiles obtained before each of the major aftershocks was performed. The results suggest that the analysis of spatial and temporal variations of strength profiles gives a clear indication of the resulting liquefaction and re-liquefaction observed in Christchurch. The comparison of a limited number of CPT strength profiles before and after the earthquakes seems to indicate that no noticeable strengthening has occurred in Christchurch, making the area vulnerable to liquefaction induced land damage in future large-scale earthquakes.     

Magnitude scaling factors in liquefaction triggering procedures

A revised magnitude scaling factor (MSF) relationship for CPT-based and SPT-based liquefaction triggering analyses is presented in this paper. The revised MSF relationship incorporates functional dependency on the soil characteristics [represented by clean sand equivalent penetration resistances in the present form] as well as on earthquake magnitude. The revisions in MSF are based on the examination of cyclic testing results for a broad range of soil types and densities, analyses of strong ground motion records to develop relationships for the equivalent number of loading cycles for different soil properties, and the synthesis of those results into an MSF relationship suitable for implementation in practice. A separate study [2] showed that use of the revised MSF relationship in CPT-based and SPT-based liquefaction triggering procedures is well-supported by the case history databases. Other factors known to fundamentally influence the MSF are discussed.     

Seismic liquefaction potential assessment by using relevance vector machine

Determining the liquefaction potential of soil is important in earthquake engineering. This study proposes the use of the Relevance Vector Machine (RVM) to determine the liquefaction potential of soil by using actual cone penetration test (CPT) data. RVM is based on a Bayesian formulation of a linear model with an appropriate prior that results in a sparse representation. The results are compared with a widely used artifi cial neural network (ANN) model. Overall, the RVM shows good performance and is proven to be more accurate than the ANN model. It also provides probabilistic output. The model provides a viable tool for earthquake engineers to assess seismic conditions for sites that are susceptible to liquefaction.