浅硬场地剪切波速测试标准差对地震动影响初探

采用Shake 2000程序,以Turkey Flat试验场地为模型,通过输入不同类型下多种强度的地震波,计算研究多工况下剪切波速测试标准差对地表加速度反应谱和峰值加速度的影响。结论为:(1)浅硬场地上剪切波速测试标准差对地震动的影响很大,影响程度与输入地震波的强度和频率以及场地剪切波速计算值有关;(2)如果将反应谱残差大于20%或加速度峰值差别大于20%定义为统计意义上的不可忽略,那么剪切波速测试标准差对计算结果的影响在大多数情况下均不可忽略;(3)当输入波的卓越周期与场地特征周期接近时,浅硬场地上剪切波速测试标准差引起的反应谱变化非常显著;(4)只有当输入波的卓越周期与场地特征周期相差较大且输入波强度偏小时,剪切波速测试标准差引起的反应谱变化才可略去;(5)当浅硬场地上剪切波速实测结果低于统计均值时,地震动计算结果的偏差一般明显大于剪切波速实测结果,高于统计均值时引起的偏差,且地震输入越强表现越明显。     

Stochastic analysis of ground response using non-recursive algorithm

During an earthquake, the amplitudes of seismic wave may amplify significantly as it propagates through the soil layers near the ground surface. Analysis of site amplification potential is strongly influenced by the uncertainty associated to the definition of soil thickness and its properties. In this paper, the non-recursive algorithm is used in linear and nonlinear Hybrid Frequency Time Domain (HFTD) approaches for stochastic analysis of site amplification. The non-recursive algorithm causes time reduction of analysis that is the essential base of stochastic analysis. The selected soil stochastic parameters are shear wave velocity, density, damping and thickness. The results of sensitivity analysis also show that the damping ratio is the most effective parameter in PGA at ground surface. The stochastic peak ground acceleration, response spectrum and amplification factor at the ground surface are determined by the two approaches for four sites with different average shear wave velocities. Comparison of the results shows that the nonlinear HFTD approach predicts closer response to real recorded data with respect to linear HFTD.     

P波入射下断层参数对地表地震动的影响

基于应用透射人工边界条件的显式有限元法计算断层破碎带宽度及力学参数变化、地震动入射角变化时二维断层场地模型P波入射下地表地震动场的分布。结果表明:(1)低速度破碎带的存在导致整个场地都有P波转换为SV波的分量,且在断层破碎带的区域出现断层陷波;(2)低速度破碎带的存在使输入场地恒定的能量向破碎带集聚放大,随着破碎带宽度增大或其介质波速降低集聚放大效应增大;(3)场地放大效应是频率相关的,宽度较宽或介质波速较低的断层破碎带对输入地震动中较低的频率成份放大显著;(4)竖向断层破碎带能阻隔斜入射地震P波,随着入射角增加隔震效应更显著。     

回填土不确定性对场地地震反应的影响

选取某核电场地控制性钻孔的厚度、剪切波速、密度等实际勘探数据,通过改变回填土剪切波速,分析了回填土不确定性对场地地震动参数的影响。研究结果表明:在回填土层厚度不变和模型总厚度不变的情况下,地表的水平向峰值加速度随着回填土剪切波速的增大而减小,但水平向峰值加速度增幅逐渐减小;回填土剪切波速到达一定的波速就不再影响地表水平峰值加速度;随着回填土剪切波速的增加,整个反应谱的谱值都普遍减小。     

渤海海域黏性土剪切波速与抗剪强度统计关系的初步研究

剪切波速与地基土的抗剪强度、剪切模量和卓越周期等参数密切相关,是地震安全性评价中判定场地类别的一个主要指标和参数。鉴于海域工程中剪切波速往往难以直接由原位测得,而室内实验结果又常常与野外现场物探测试值存在较大差异,因此,如何通过其他途径有效获取满足工程需要的剪切波速参数在海域工程的地震安全性评价等方面具有迫切的实用需求。为此本文通过对渤海海域数十个石油平台项目中一系列饱和黏性土样品的剪切波速与抗剪强度实验数据的统计分析,尝试采用多种可能的函数来拟合确定二者之间的经验关系。结果表明:对于渤海海域黏性土剪切波速V_s与抗剪强度S_u之间的最佳统计经验关系为幂函数V_s=53.751S_u~(0.376)。此关系可为渤海海域工程中通过不排水抗剪强度估算剪切波速提供一种简便可行的实用性方法。     

Surface and downhole shear wave seismic methods for thick soil site investigations

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.     

Shear wave velocity-based liquefaction evaluation in the great Wenchuan earthquake： a preliminary case study

The great Wenchuan earthquake (Ms = 8.0) in 2008 caused severe damage in the western part of the Chengdu Plain. Soil liquefaction was one of the major causes of damage in the plain areas, and proper evaluation of liquefaction potential is important in the definition of the seismic hazard facing a given region and post-earthquake reconstruction. In this paper, a simplified procedure is proposed for liquefaction assessment of sandy deposits using shear wave velocity (Vs), and soil liquefaction from the Banqiao School site was preliminarily investigated after the earthquake. Boreholes were made at the site and shear wave velocities were measured both by SASW and down-hole methods. Based on the in-situ soil information and Vs profiles, the liquefaction potential of this site was evaluated. The results are reasonably consistent with the actual field behavior observed after the earthquake, indicating that the proposed procedure is effective. The possible effects of gravel and fines contents on liquefaction of sandy soils were also briefly discussed.     

Micro-zoning of the natural radioactivity levels and seismic velocities of potential residential areas in volcanic fields: The case of Isparta (Turkey)

This study involved the analysis of the locations with the lowest earthquake risk in the Isparta province of Turkey and the radiological properties of these locations. The most convenient residential areas in the region in terms of radiology and earthquake risk were identified. The radiological characteristics of soil are an important factor in the evaluation of residential areas. Gamma ray spectrometry and multi-source seismic refraction methods were used to conduct radioactive and seismic measurements. The measured ²³⁸U, ²³²Th and ⁴⁰K activity concentrations were used to calculate the absorbed gamma dose rate, the annual effective dose rate, the radium equivalent and the external hazard index of the environment to estimate the radiological risk of natural radioactivity. The compressional and shear wave velocities were calculated based on the results of multi-source seismic refraction. The calculated parameters were used to generate micro-zone maps. The shear wave velocity was used to analyze the stiffness of looseness of the soil. The radiological risks of the area were determined. Both the seismic and radiological hazards were considered in determining the appropriate areas for residential development. In the results of this study, the best location for residential development was demonstrated to be on limestone, and the worst location was determined to be on alluvium.     

饱和黄土液化判别方法的两点发现

黄土液化实例多与现有可液化土地质年代规定不符。采用动三轴弯曲元试验设备对原状黄土饱和过程进行剪切波速跟踪测试,发现黄土浸水、结构先破坏再固结形成新的稳定结构的特点,证明经历过饱和的黄土已不能再视其为饱和前的地质年代。兰州马兰黄土中的黏粒有些是以黏土团块的形式存在,而黏土团块并不影响其他部分粉质土的液化,因此对于此类土应用黏粒含量进行液化判别时,应考虑将黏土团块不计入黏粒含量。     

Soil characterization of Tınaztepe region (İzmir/Turkey) using surface wave methods and nakamura (HVSR) technique

To determine the shear wave velocity structure and predominant period features of T?naztepe in ?zmir, Turkey, where new building sites have been planned, active–passive surface wave methods and single-station microtremor measurements are used, as well as surface acquisition techniques, including the multichannel analysis of surface waves (MASW), refraction microtremor (ReMi), and the spatial autocorrelation method (SPAC), to pinpoint shallow and deep shear wave velocity. For engineering bedrock (V _s > 760 m/s) conditions at a depth of 30 m, an average seismic shear wave velocity in the upper 30 m of soil (AV_s30) is not only accepted as an important parameter for defining ground behavior during earthquakes, but a primary parameter in the geotechnical analysis for areas to be classified by V _s30 according to the National Earthquake Hazards Reduction Program (NEHRP). It is also determined that Z1.0, which represents a depth to V _s = 1000 m/s, is used for ground motion prediction and changed from 0 to 54 m. The sediment–engineering bedrock structure for T?naztepe that was obtained shows engineering bedrock no deeper than 30 m. When compared, the depth of engineering bedrock and dominant period map and geology are generally compatible.     

Experimental and conceptual evidence about the limitations of shear wave velocity to predict liquefaction

Based on the liquefaction performance of sites with seismic activity, the normalized shear wave velocity, V_s1, has been proposed as a field parameter for liquefaction prediction. Because shear wave velocity, V_s, can be measured in the field with less effort and difficulty than other field tests, its use by practitioners is highly attractive. However, considering that its measurement is associated with small strain levels, of the order of 10⁻⁴–10⁻³%, V_s reflects the elastic stiffness of a granular material, hence, it is mainly affected by soil type, confining pressure and soil density, but it is insensitive to factors such as overconsolidation and pre-shaking, which have a strong influence on the liquefaction resistance. Therefore, without taking account of the important factors mentioned above, the correlation between shear wave velocity and liquefaction resistance is weak.In this paper, laboratory test results are presented in order to demonstrate the significant way in which OCR (overconsolidation ratio) affects both shear wave velocity and liquefaction resistance. While V_s is insensitive to OCR, the liquefaction resistance increases significantly with OCR. In addition, the experimental results also confirm that V_s correlates linearly with void ratio, regardless of the maximum and minimum void ratios, which means that V_s is unable to give information about the relative density. Therefore, if shear wave velocity is used to predict liquefaction potential, it is recommended that the limitations presented in this paper be taken into account.     

3-D surface wave group velocity distribution in Central-Southern Africa

Group velocities estimated from fundamental mode Love and Rayleigh waves are used in a tomography process in central-southern Africa. The waves were generated by eighteen earthquakes, which occurred along the East African Rift and recorded at BOSA, LBTB and SLR seismic stations in southern Africa. The group velocities from Love and Rayleigh waves were isolated using the Multiple Filter Technique (MFT) at the period range of 10 to 50 seconds. The tomography method developed by Ditmar and Yanovskaya (1987) and Yanovskaya and Ditmar (1990), was applied to calculate the lateral distribution of surface wave group velocities in central-southern Africa. The results of the tomographic inversion were plotted as distribution maps. In addition to the maps, I also produced two velocity cross-sections across the area of study. The velocity distribution maps show the regional tectonic units, though with poor resolution. The azimuthal bias of the surface wave paths is reflected in the distribution of the group velocities. The Moho depth appears to correlate with velocities at a period of about 30 s. A low velocity feature observed beneath the Zimbabwe craton implies a thickening upper asthenosphere and lithospheric thinning beneath the Zimbabwe craton. Also estimated was a shear wave velocity model beneath the Zimbabwe craton.     

Threshold seismic energy and liquefaction distance limit during the 2008 Wenchuan earthquake

Estimating the possible region of liquefaction occurrence during a strong earthquake is highly valuable for economy loss estimation, reconnaissance efforts and site investigations after the event. This study identified and compiled a large amount of liquefaction case histories from the 2008 Wenchuan earthquake, China, to investigate the relationship between the attenuation of seismic wave energy and liquefaction distance limit during this earthquake. Firstly, we introduced the concept of energy absorption ratio, which is defined as the absorbed energy of soil divided by the imparted energy of seismic waves at a given site, and the relationship between the energy absorption ratio and the material damping ratio was established based on shear stress–strain loop of soil element and the seismic wave propagation process from the source to the site. Secondly, the threshold imparted seismic energy of liquefaction was obtained based on existing researches of absorbed energy required to trigger liquefaction of sandy soils and the ground motion attenuation characteristics of the 2008 Wenchuan earthquake, and the liquefaction distance limit of this earthquake was estimated according to the proposed magnitude–energy–distance relationship. Finally, the field liquefaction database of 209 sites of the 2008 Wenchuan earthquake was used to validate such an estimation, and the field observed threshold imparted seismic energy to cause liquefaction in recent major earthquakes worldwide was back-analyzed to check the predictability of the present method, and several possible mechanisms were discussed to explain the discrepancy between the field observations and the theoretical predictions. This study indicates that seismic energy attenuation and liquefaction distance limit are regional specific and earthquake dependent, and 382 J/m³ is the average level of threshold imparted seismic energy to cause liquefaction for loose saturated sandy soils, and the corresponding liquefaction distance limit is approximately 87.4 km in fault distance for a M_w?=?7.9 event in the Chengdu Plain. The proposed regional energy attenuation model and threshold imparted seismic energy may be considered as an approximate tool in evaluating the liquefaction hazard during potential earthquakes in this area.     

The porosity of saturated shallow sediments from seismic compressional and shear wave velocities

This study devises a new analytical relationship to determine the porosity of water-saturated soils at shallow depth using seismic compressional and shear wave velocities. Seismic refraction surveys together with soil sample collection were performed in selected areas containing water-saturated clay–silt, sand and gravely soils. Classification of clay–silt, sand and gravel dense soils provided the coefficient of experimental equation between the data sets, namely, Poisson's ratio, shear modulus and porosity values. This study presents a new analytical relationship between Poisson's ratio and shear modulus values, which are obtained from seismic velocities and porosity values of water-saturated material computed from water content and grain densities, which are determined by laboratory analysis of disturbed samples. The analytical relationship between data sets indicates that when the shear modulus of water-saturated loose soil increases, porosity decreases logarithmically. If shear modulus increases in dense or solid saturated soils, porosity decreases linearly.     

土层剪切波速测试中的不确定性对场地地震动参数的影响分析——以Ⅲ类场地为例

本文选取山东地区2个Ⅲ类场地的工程地质勘探及土层剪切波速等资料,将土层厚度按5个深度段,每个分段给出了4个土层剪切波速的改变量,通过改变不同深度段土层剪切波速,建立了19种土层地震反应分析模型,分析了不同深度段,不同概率水平下土层剪切波速的变化对场地地震动参数的影响。研究表明,不同深度段土层剪切波速的变化对场地地震动参数的影响有差异。具体表现为,土层剪切波速的改变在1—10m、11—40m和地震输入界面处三个深度段对地震动加速度峰值影响较大;其中,41—70m和71—100m两个深度段剪切波速的改变对地震动加速度峰值影响小;在土层深度1—10m时,剪切波速降低,峰值变大,剪切波速的改变与峰值的改变呈负相关;在其它深度段,剪切波速降低,峰值变小,剪切波速的改变与峰值的改变呈正相关。剪切波速的改变在1—10m和11—40m两个深度段对地震加速度反应谱影响较大;在41—70m、71—100m和地震输入界面三个深度段对地震加速度反应谱影响很小。     

不同深度土层剪切波速的变化对场地地震动参数的影响分析——以Ⅱ类场地为例

以Ⅱ类场地为例,选取了山东地区2个场地的工程地质勘探及剪切波速等资料,通过改变不同深度段波速,分别建立土层模型,计算分析了不同深度段、不同概率水平条件下剪切波速的变化对场地地震动参数的影响。研究结果表明,剪切波速的变化对场地地震动加速度峰值影响在浅层影响最大,基岩输入面处次之,深层最小;对特征周期的影响,在浅层影响最大,深层次之、基岩输入面处最小。研究结果为进一步研究土层剪切波速测试中的不确定性对场地地震动参数的影响及合理确定场地地震动参数提供一定的参考。     

Rock-physics-based carbonate pore type characterization and reservoir permeability heterogeneity evaluation, Upper San Andres reservoir, Permian Basin, west Texas

The use of the shear wave velocity data as a field index for evaluating the liquefaction potential of sands is receiving increased attention because both shear wave velocity and liquefaction resistance are similarly influenced by many of the same factors such as void ratio, state of stress, stress history and geologic age. In this paper, the potential of support vector machine (SVM) based classification approach has been used to assess the liquefaction potential from actual shear wave velocity data. In this approach, an approximate implementation of a structural risk minimization (SRM) induction principle is done, which aims at minimizing a bound on the generalization error of a model rather than minimizing only the mean square error over the data set. Here SVM has been used as a classification tool to predict liquefaction potential of a soil based on shear wave velocity. The dataset consists the information of soil characteristics such as effective vertical stress (σ′_v0), soil type, shear wave velocity (V_s) and earthquake parameters such as peak horizontal acceleration (a_max) and earthquake magnitude (M). Out of the available 186 datasets, 130 are considered for training and remaining 56 are used for testing the model. The study indicated that SVM can successfully model the complex relationship between seismic parameters, soil parameters and the liquefaction potential. In the model based on soil characteristics, the input parameters used are σ′_v0, soil type, V_s, a_max and M. In the other model based on shear wave velocity alone uses V_s, a_max and M as input parameters. In this paper, it has been demonstrated that Vs alone can be used to predict the liquefaction potential of a soil using a support vector machine model.     

甘肃地区波速测试中的问题分析

土的弹性波速度是非常重要的力学参数,可用于场地类别划分、场地地震反应分析以及判别砂土、粉土、黄土液化、黄土震陷等。准确测得场地波速资料是进行土工设计和预测岩土地震灾害的关键技术环节。本文收集了甘肃地区地震安全性评价和小区化实测原位波速资料,分类指出了存在的问题,以反面案例的形式给出了波速数据处理时应注意的问题和分析方法。     

The use of MASW method in the assessment of soil liquefaction potential

The multi-channel analysis of surface wave (MASW) method is a non-invasive method recently developed to estimate shear wave velocity profile from surface wave energy. Unlike conventional SASW method, multi-station recording permits a single survey of a broad depth range and high levels of redundancy with a single field configuration. An efficient and unified wavefield transform technique is introduced for dispersion analysis and on site data quality control. The technique was demonstrated in the assessment of soil liquefaction potential at a site in Yuan Lin, Taiwan. The shear wave velocity and liquefaction potential assessments based on MASW method compares favorable to that based on SCPT shear wave measurements. Two-dimensional shear wave velocity profiles were estimated by occupying successive geophone spreads at several sites in central western Taiwan, at some of which sand boils or ground cracks occurred during 1999 Chi Chi earthquake. Liquefaction potential analysis based on MASW imaging was shown to be effective for estimating the extent of potential liquefaction hazard.     

Analysis of Influence Characteristics of Site Ground Motion in Ya'an Area,Sichuan

为分析雅安地区场地结构引起的震动特性,选取该地区重大安评项目中的实际测试钻孔进行场地地震反应分析,采用等效线性化方法,并以NGA强震记录为输入,开展场地反应计算,在全面分析该地区场地地震特征的基础上,以峰值速度为参数指标,研究场地条件引起的峰值速度放大效应。研究结果表明,雅安地区场地卓越周期约为0.2 s,自振周期为0.2 s的建筑结构破坏相对严重;随着基岩峰值加速度的增大,峰值速度呈增大趋势,峰值速度与峰值加速度变化趋势并非完全一致,分析场地效应时应考虑多种地震动参数的变化规律;不同地震输入下峰值速度放大系数为1～1.8;峰值速度放大系数k_v在不同强度基岩输入下,大致呈随等效剪切波速的增大而减小的趋势,软土场地更易引起场地峰值速度的放大,这种现象在芦山地震中较为明显。