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.     

1976年唐山大地震CPT液化数据库检验

1976年唐山大地震引发了范围广、灾害重的液化震害。铁道部科学研究院等单位于1977、1978年对液化场地进行了单桥静力触探测试。但单桥静力触探在数据指标方面存在缺陷,与国际标准不接轨。中国地震局工程力学研究所与东南大学等单位于2007年对上述唐山地区部分测点再次进行了孔压静力触探（CPTU）测试。本文通过对比2次静力触探数据,利用Robertson土质分类图,进行新孔压静力触探数据土类分层检验,将土类检验结果与单桥静力触探测试时钻孔柱状图进行对比,发现大部分测点土层土类均能较好对应,现场测试力学指标沿深度变化趋势较相符,仅剔除了错误点T2、T3。对所有测点选定液化层,分别建立基于单桥静力触探测试比贯入阻力p_s和基于孔压静力触探测试锥尖阻力q_c的液化数据库。利用基于静力触探测试（CPT）的我国规范液化判别方法检验了2个数据库,发现对单桥静力触探测试的数据库液化判别效果好,而对孔压静力触探测试的数据库液化判别效果较差,说明经过30年的时间,土层液化可能已发生较大改变。因此,基于孔压静力触探测试的液化数据库可靠性较低,基于该数据库对液化判别方法进行改进意义较小。     

Assessment of various CPT based liquefaction severity index frameworks relative to the Ishihara (1985) H1–H2 boundary curves

The objective of the study presented herein is to develop an understanding of the predictive trends of four different liquefaction severity index frameworks, with emphasis on the utility of the frameworks for assessing liquefaction vulnerability in Christchurch, New Zealand. Liquefaction induced land damage was widespread following the four major earthquakes in Christchurch (M_w 5.9–7.1) between 4 September 2010 and 23 December 2011. As part of the rebuilding effort, a major focus, to date, has been on assessing/developing approaches for evaluating vulnerability to liquefaction induced damage in future events. The four liquefaction severity index frameworks that are evaluated herein are: the one-dimensional volumetric reconsolidation settlement (S_V1D), the Liquefaction Potential Index (LPI), and two new liquefaction severity indices developed following the major earthquakes in Christchurch, namely the Ishihara inspired LPI (LPI_ISH) and the Liquefaction Severity Number (LSN). To assess the predictive trends of the four severity index frameworks, the H₁–H₂ boundary curves developed by Ishihara (1985) are used as a reference of comparison. In large part, the severity index frameworks serve the same purpose as the Ishihara boundary curves, but they alleviate some of the difficulties in implementing the Ishihara boundary curves for assessing the highly stratified soil profiles that underlie much of Christchurch. A parametric study was performed wherein relatively simple soil profiles are evaluated using all the procedures and contour plots of calculated S_V1D, LPI, LPI_ISH, and LSN values were superimposed onto the Ishihara boundary curves. The results indicate that the LPI_ISH and LSN indices yield similar trends as the Ishihara boundary curves, whereas the S_V1D and LPI indices do not. Furthermore, little field data is available to assess the severity indices for the scenarios where the trends in the LPI_ISH and LSN indices differ.     

Assessment of liquefaction evaluation procedures and severity index frameworks at Christchurch strong motion stations

The objective of the study presented herein is to assess three commonly used CPT-based liquefaction evaluation procedures and three liquefaction severity index frameworks using data from the 2010–2011 Canterbury earthquake sequence. Specifically, post-event field observations, ground motion recordings, and results from a recently completed extensive geotechnical site investigation programme at selected strong motion stations (SMSs) in the city of Christchurch and surrounding towns are used herein. Unlike similar studies that used data from free-field sites, accelerogram characteristics at the SMS locations can be used to assess the performance of liquefaction evaluation procedures prior to their use in the computation of surficial manifestation severity indices. Results from this study indicate that for cases with evidence of liquefaction triggering in the accelerograms, the majority of liquefaction evaluation procedures yielded correct predictions, regardless of whether surficial manifestation of liquefaction was evident or not. For cases with no evidence of liquefaction in the accelerograms (and no observed surficial evidence of liquefaction triggering), the majority of liquefaction evaluation procedures predicted liquefaction was triggered. When all cases are used to assess the performance of liquefaction severity index frameworks, a poor correlation is shown between the observed severity of liquefaction surface manifestation and the calculated severity indices. However, only using those cases where the liquefaction evaluation procedures yielded correct predictions, there is an improvement in the correlation, with the Liquefaction Severity Number (LSN) being the best performing of the frameworks investigated herein. However scatter in the relationship between the observed and calculated surficial manifestation still remains for all liquefaction severity index frameworks.     

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。     

Evaluation of CPT-based Liquefaction Procedures at Regional Scale

The liquefaction database describing the response of the Christchurch area in the 2010–2011 Canterbury Earthquake Sequence (CES) provides a unique basis for evaluating the regional application of various liquefaction analysis procedures, from liquefaction triggering analyses through to liquefaction vulnerability parameters. This database was used to compare the Robertson and Wride [17], Moss et al. [15] and Idriss and Boulanger [7] liquefaction triggering procedures as well as evaluate the impact of the 2014 versus 2008 Cone Penetration Test (CPT)-based liquefaction triggering procedure by Idriss and Boulanger on four liquefaction vulnerability parameters (S_V1D, LPI, LPI_ISH and LSN), the correlation of those parameters with observed liquefaction-induced damage patterns in the CES, and the mapping of expected damage levels for 25, 100 and 500 year return period ground motions in Christchurch. The effects on S_V1D, LPI, LPI_ISH and LSN were small relative to other sources of variability for the majority of the affected areas, particularly where liquefaction was clearly severe or clearly not. Nonetheless, considering the separation of the land damage populations as well as consistency between the events, the the IB-2008 liquefaction triggering procedures appears to give a slightly better fit to the mapped liquefaction-induced land damage for the regional prediction of liquefaction vulnerability for the Christchurch soils. The Boulanger and Idriss [1] triggering procedure produces improved agreement between the liquefaction vulnerability parameters and observations of damage for: areas south of the Central Business District (CBD) where there tends to be higher soil Fines Content (FC), and localized areas that experienced liquefaction during the smaller Magnitude (M) earthquake events. Implementation of the 2014 liquefaction triggering procedure for mapping of expected liquefaction-induced damage at 25, 100 and 500 year return period ground motions is shown to require use of representative Peak Ground Acceleration (PGA)-M values consistent with the de-aggregation of the seismic hazard. Use of equivalent magnitude-scaled PGA-M7.5 pairs, where the equivalency relates to previously published MSF relationships, with the 2014 liquefaction triggering procedure is shown to be unconservative for certain situations.     

Semi-empirical procedures for evaluating liquefaction potential during earthquakes

Semi-empirical procedures for evaluating the liquefaction potential of saturated cohesionless soils during earthquakes are re-examined and revised relations for use in practice are recommended. The stress reduction factor (r_d), earthquake magnitude scaling factor for cyclic stress ratios (MSF), overburden correction factor for cyclic stress ratios (K_σ), and the overburden normalization factor for penetration resistances (C_N) are discussed and recently modified relations are presented. These modified relations are used in re-evaluations of the SPT and CPT case history databases. Based on these re-evaluations, revised SPT- and CPT-based liquefaction correlations are recommended for use in practice. In addition, shear wave velocity based procedures are briefly discussed.     

Earthquake damage to earth dams in Japan—maximum epicentral distance to cause damage as a function of magnitude

Small scale agricultural earth dams have been damaged by several past earthquakes. Damage to earth dams occurred at distances from the epicentre that increased in proportion to the scale of the earthquakes. On studying the damage a constant relation between the magnitude of the earthquake and maximum epicentral distance where the damage occurred was obtained. This relationship is similar to that obtained between the magnitude and the maximum epicentral distance at which liquefaction occurs. This, and the fact that earth dams which have been damaged several times due to liquefaction were near the critical epicentral distance, suggests that liquefaction of the foundation ground is the main cause of damage to earth dams.     

Liquefaction-induced deformation of earthen embankments on non-homogeneous soil deposits under sequential ground motions

Damage of embankments during earthquakes is widely attributed to the liquefaction of foundation soil. Previous studies have investigated the dynamic response of embankments by mainly considering uniform sand foundation and a single earthquake event. However, the foundation of an embankment consists of many sublayers of soil from liquefiable sand to relatively impermeable layer, and during earthquakes a mainshock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil structures. Accordingly, the investigation of liquefaction-induced deformation of earthen embankments on various liquefiable foundation conditions under mainshock–aftershock sequential ground motions is carried out by a series of dynamic centrifuge tests in this study. The liquefiable foundation includes uniform sand profile, continuous layered soil profile, and non-homogeneous soil profiles. Effects of various foundation conditions on embankment deformations are compared and analyzed. From the test results, it is found that the embankment resting on non-homogeneous soil deposits suffer more damage compared to the uniform sand foundation of same relative density. The test results also suggest that the sequential ground motions have a significant effect on the accumulated deformation of embankment.     

Spatial variation of magnitude scaling factors during the 2010 Darfield and 2011 Christchurch,New Zealand,earthquakes

The combination of well-documented liquefaction response during the Darfield and Christchurch, New Zealand, earthquakes, densely-recorded ground motions for the events, and detailed subsurface characterization provides an unprecedented opportunity to investigate the significance of the spatial variation of magnitude scaling factors (MSF) on liquefaction triggering. Towards this end, MSF were computed at 15 SMS sites across Christchurch and its surroundings using two established approaches. Trends in the spatial variation of the MSF computed using number of equivalent cycles (n_eq) from both approaches were similar, with the spatial variation being more significant for the Christchurch earthquake than the Darfield earthquake. However, there was no consistent trend for regions with lower computed MSF having experienced more severe or widespread liquefaction. Additionally, there is a general correlation between MSF and a_max, but because a_max ranges more widely than MSF it has a greater influence on the resulting seismic demand imposed on the soil than MSF does. Nevertheless, the spatial variation of the MSF is deemed significant enough that it warrants being considered for incorporation into future variants of simplified liquefaction evaluation procedures.     

区域土壤地震液化预测简化方法

利用可大范围获取的空间参数给出区域土壤液化的评估方法,可预估震前各地区土壤液化可能性,可快速评估震后震区土壤液化情况,对预防减轻地震灾害以及地震快速响应救援都具有重要意义的.本文以我国1976唐山大地震液化调查资料为背景,提出了区域土壤液化预测的四参数简化评估方法,并用2011年新西兰基督城地震进行了检验.选取场地平均剪切波速V s30、复合地形指数CTI、场地到河流的距离DR以及地面峰值加速度PGA,分别代表土壤密实程度、饱水状态、地质年代和遭遇的地震作用大小,并基于我国唐山地震液化区域调查资料生成的样本,运用经典二元Logistic回归方法,建立了区域土壤液化预测模型和评估公式,回归检验结果良好,整体回判成功率为77%,非液化和液化区域成功率分别为73%和78%.将公式应用于评估2011年新西兰基督城地震液化情况,区域液化和非液化成功率分别为82%和88%,总体成功率为84%.以上结果表明本文模型可靠,方法正确,可用于区域土壤液化震前预估以及土壤液化震后快速评估.     

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。     

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。     

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.     

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。     

Assessing CPT-based methods for liquefaction evaluation with emphasis on the cases from the Chi-Chi, Taiwan, earthquake

In the early morning (1:47 Taiwan time) of September 21, 1999, the largest earthquake of the century in Taiwan (M_w=7.6, M_L=7.3) struck this island country. The earthquake killed more than 2400 people and caused great destruction to buildings, bridges, dams, highways, and railways. One of the causes for heavy damages to the structures is soil liquefaction and ground settlement during the earthquake. In this paper, investigation of soil liquefaction and case histories of liquefaction are presented. Three CPT-based simplified methods, the Robertson method, the Olsen method, and the Juang method, are examined using the case histories derived from the Chi-Chi earthquake. The results of the comparison show that the Juang method is more accurate than the two methods in predicting liquefaction potential of soils based on the cases derived from the Chi-Chi earthquake, although all three methods are quite comparable in accuracy.     

Liquefaction potential evaluations by energy-based method and stress-based method for various ground motions

An energy-based liquefaction potential evaluation method (EBM) previously developed was applied to a uniform sand model shaken by seismic motions recorded at different sites during different magnitude earthquakes. It was also applied to actual liquefaction case histories in Urayasu city during the 2011 M9.0 Tohoku earthquake and in Tanno-cho during the 2003 M8.0 Tokachi-oki earthquake. In all these evaluations, the results were compared with those by the currently used stress-based method (SBM) under exactly the same seismic and geotechnical conditions. It was found that EBM yields similar results with SBM for several ground motions of recent earthquakes but has easier applicability without considering associated parameters. In Urayasu city, the two methods yielded nearly consistent results by using an appropriate coefficient in SBM for the M9.0 earthquake, though both overestimated the actual liquefaction performance, probably because effects of plasticity and aging on in situ liquefaction strength were not taken into account. In Tanno-cho, EBM could evaluate actual liquefaction performance due to a small-acceleration motion during a far-field large magnitude earthquake while SBM could not.     

Lateral spreading: Evidence and interpretation from the 2010–2011 Christchurch earthquakes

In the 2010–2011 Canterbury earthquakes widespread liquefaction occurred over nearly half of the urban area of Christchurch. The most severe damage to buildings and infrastructure was often associated with lateral spreading and consequent large ground distortion and permanent ground displacements. This paper presents analysis, results and interpretation of lateral spreads using measurements from detailed ground surveying at a large number of locations along the Avon River. Classification of lateral spreads based on the magnitude and spatial distribution of permanent ground displacements is first presented, and then key characteristics of soil layers and ground conditions associated with different classes of lateral spreads are identified and discussed. Evidence of both global effects from topographic features and local effects related to density, thickness and continuity of critical layers is presented highlighting the need for a systematic approach in the engineering evaluation of lateral spreading in which particular attention is given to key factors governing lateral spreading.     

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.     

地表刚性对地震液化作用的影响

一种新的假说阐述了在大地震中饱和沙土的液化现象。这是建立在多反射理论和FEM动力反应计算的基础上的。计算中应用了在1994年三陆外海M7.5地震时对八户港土层的假设,其结果与观测到的地震现象十分符合。这种方法也被应用于其他曾经发生液化的地区并被证实是在液化机理可接受限度内。富井地震(1948年6月,M=7.1)发生在福井盆地中央,深度为15 km,引起了大范围的破坏,伤亡人数达到5 300人。这次地震在各处引起大面积的液化,导致大范围的稻田被砂石所覆盖。但是,尽管加速度很大,盆地的几部分没有被液化。应用这种假说,我们分析了在相同情况下地表层的差异。结果发现液化的产生取决于地表层刚性引起的应变大小,假设与实际情况一致。