西北地区典型河谷城市岩土体剪切波速的特征分析

为获取西北地区典型河谷型城市的场地剪切波速分布特征,应用数理统计方法分析了天水市1294组不同岩土体的剪切波速数据,并比较分析了不同地貌单元的等效剪切波速。结果显示,不同岩土体的剪切波速均具有随深度变化或风化程度差异而引起的较大变化区间,其区间差值为467～982m/s不等;不同岩土体的剪切波速分布均呈现出正态分布特征,其峰值区间和优势区间的剪切波速值可以反映岩土体的物理性状;粉土、粉质粘土、卵石和圆砾等松散堆积体的剪切波速与深度具有很好的相关性和拟合函数,而泥岩、砂岩和砾岩等成岩体的剪切波速与深度相关性较差,其剪切波速大小主要受岩体的裂隙发育和风化程度影响;不同地貌单元结合部位较单一地貌单元的等效剪切波速为大;II类场地等效剪切波速变化范围为152~319m/s,III类场地等效剪切波速变化范围为122~228m/s。该场地不同岩土体的剪切波速分布特征具有普遍一致性,反映了典型河谷城市的地层分布及地层岩性。     

西北地区典型河谷城市岩土体剪切波速的特征分析

为获取西北地区典型河谷型城市的场地剪切波速分布特征,应用数理统计方法分析了天水市1 294组不同岩土体的剪切波速数据,比较分析了不同地貌单元的等效剪切波速。结果显示,不同岩土体的剪切波速均具有随深度变化或风化程度差异而引起的较大变化区间,其区间差值为467~982m/s不等;不同岩土体的剪切波速分布均呈现出正态分布特征,其峰值区间和优势区间的剪切波速值可以反映岩土体的物理性状;粉土、粉质粘土、卵石和圆砾等松散堆积体的剪切波速与深度具有很好的相关性和拟合函数,而泥岩、砂岩和砾岩等成岩体的剪切波速与深度相关性较差,其剪切波速大小主要受岩体的裂隙发育和风化程度影响;不同地貌单元结合部位较单一地貌单元的等效剪切波速为大;Ⅱ类场地等效剪切波速变化范围为152~319m/s,Ⅲ类场地等效剪切波速变化范围为122~228m/s。该场地不同岩土体的剪切波速分布特征具有普遍一致性,反映了典型河谷城市的地层分布及地层岩性。     

表层土剪切波速的不确定性对地表设计谱平台值的影响

土层剪切波速对地表反应谱有重要的影响。选取构建了不同场地类别的235个场地地震反应模型,人工合成了12条强弱及频谱特性不同的加速度时程,运用一维土层地震反应的等效线性化波动方法,研究了不同地震动输入下表层土剪切波速的不确定性对设计谱平台值的影响,统计给出了表层土剪切波速的变异率与地表设计反应谱平台值间的经验公式。结果表明:地表设计反应谱平台值与表层土剪切波速的变异率呈负相关关系,即随表层土剪切波速变异率的增大而减小;不同地震波输入条件下各类场地类别地表设计谱平台高度与表层土剪切波速变异率的数学模型为一阶指数函数。     

单孔法波速测试在宿州市区建筑场地类别划分的应用及该区特征研究

建筑场地的不同类别反映不同场地条件对基岩地震震动的综合放大效应,建筑场地类别是根据土层等效剪切波速和覆盖层厚度进行划分的。这里介绍了单孔法波速测试在计算土层等效剪切波速度,划分建筑场地类别上的应用,以及通过对宿州市区40个建筑场地的201个土层等效剪切波速数据Vse20和场地类别划分结果进行统计分析、研究,得出该区域类似地层条件的土层等效剪切波速近似呈正态分布特征,估算范围在195.89m/s～222.46 m/s,推测建筑场地类别为Ⅲ类。     

场地类别分类方案研究

基于性能的抗震设计,要求工程师设计出具有预期抗震性能的结构,一个关键因素是地震作用的确定,这在很大程度上取决于局部场地条件。通过收集和分析北京、苏州和唐山城区956个钻孔资料,建立地表20 m和30 m深土层走时平均剪切波速VS20和VS30的关系式;现场钻探获取北京城区深105 m的典型钻孔原状土样,试验给出各类土体动剪切模量和阻尼比曲线;建立北京城区170个钻孔的场地反应计算模型,采用Nakamura提出的HVSR法和陈国兴等提出的弱震法估算场地基本周期TS值,结合国内外现行抗震规范的场地分类及一些学者对场地分类的研究成果,提出两种新的场地分类建议方案：基于等效剪切波速VSE和覆盖土层厚度H（地表至剪切波速VS ≥ 500 m/s的基岩深度）的双指标场地分类方案及基于VSE、H和TS的三指标场地分类方案。提出的场地分类方案对我国现行抗震规范场地分类方法的改进有参考价值。     

上海地区场地剪切波速的特征分析

通过对上海地区场地剪切波速资料的统计分析,较深入地探讨了上海地区场地剪切波速在100m深度内的分布范围及其变化规律,可在工程实践中参考使用。     

基于标贯试验的岩土剪切波速多因素公式分析

剪切波速是建筑场地类别判定的重要依据.剪切波速Vs在岩土工程设计中应用广泛,如划分建筑场地土类型和场地类别、液化判别、建(构)筑物地基地震反应分析等.《建筑抗震设计规范》(GB 50011—2010)中明确提出"建筑的场地类别,应根据土层等效剪切波速和场地覆盖层厚度划分为四类."准确地得到土体剪切波速才能够准确地进行建...     

基于互相关函数的单孔法波速测试优化算法

土的剪切波速是岩土工程领域的一个重要参数。建筑场地类别划分、地基饱和砂土液化判别、地震反应分析等都依赖于剪切波速,由于其重要性,对它的测定技术和精确程度应重点研究。针对工程中广泛采用的单孔法剪切波速测试,提出了一种基于互相关函数计算土层剪切波速的优化算法。克服了传统数据处理方法中经常遇到的负波速、波速数值明显过大或过小和同一土层各测点波速差别较大等4种不合理因素,提高了数据分析的精度和效率。     

常规土类剪切波速与埋深经验公式的可靠性评价

国内常规土类剪切波速与埋深的统计关系公式(刘红帅等《常规土类剪切波速与埋深的关系分析》一文中,表2—5中的幂函数和一元二次函数模型公式)作为全国性经验公式,已成为地区经验公式检验对比的重要参考,但缺乏较系统的可靠性评价。为此,选取国内典型地区(北京、鲁西、成都、天津、常州、武汉6个区域)的剪切波速与埋深的统计经验公式,检验全国性常规土类剪切波速与埋深经验公式的可靠性。结果表明:幂函数模型预测的剪切波速随深度增大而增大,符合定性认识,而一元二次模型有可能出现不合理的回弯反常现象,不应继续采用。全国性剪切波速经验公式在不同地区的预测精度差异显著,在大多数地区对于绝大数土类预测的相对误差总体在±20%以内,在少数地区对于大数土类预测的相对误差超过±20%,仅在近地面20 m内的相对误差较大,最大可达40%左右。建议优先选择适合当地的剪切波速经验公式;当缺乏本地公式,需选用全国性剪切波速幂函数型经验公式时,应先经过本地实测资料检验确认后方可使用;20 m范围的剪切波速最好以实测为准,这有助于降低全国性剪切波速经验公式带来的显著误差。     

波速测试技术在场地抗震评价中的应用研究

波速测试技术是地震勘探方法之一,也是一种简便、快速、准确的原位测试技术。它通过获取的地震波在经过不同深度的岩、土层的直达波旅行时,求得纵波、剪切波波速,进而通过弹性波理论,计算出岩、土层动弹性力学参数,同时也可以根据剪切波的波速,来划分场地土类型和建筑场地类别,以及计算卓越周期,这些参数可以为场地抗震设计及地震安全性评价提供依据。这里简述了波速测试技术的工作原理和野外实测技术,以及数据处理和资料解释全过程,并且结合某一建筑场地的实例,说明了波速测试技术在场地抗震评价中的应用及其效果。     

Multichannel analysis of surface waves (MASW) for seismic site characterization using 2D genetic algorithm at Bahrah area,Wadi Fatima,Saudi Arabia

The present study deals with the seismic site classification of Bahrah area, Wadi Fatima, to characterize the local site conditions. The dynamic behavior of sediments was studied by the application of surface wave inversion. The multichannel analysis of surface waves (MASW) shallow geophysical technique was utilized for site classification. MASW survey was carried out at 66 sites along with 13 seismic refraction profiles at suitable localities. MASW and seismic refraction profiles were processed and compared with the available borehole data. The integration of MASW techniques with seismic refraction and borehole data progressively enhanced the subsurface visualization and reliability of the shear wave velocity estimation in the subsurface in the study area. The subsurface shear-wave velocity model was achieved by the solution of an inverse problem-based dispersion of surface waves and propagation in a vertically heterogeneous medium. The 2D genetic algorithm was employed for the inversion of dispersion curves to obtain velocity and thickness of subsurface layers. The depth to engineering bedrock and velocity of shear waves in the first 30 m was deciphered and mapped. The depth of bedrock in study area varies from 4 to 30 m, and V _{S 30} ranges from 320 to 800 m/s. The most of study area falls in B and C class categories in addition to few sites of D class according to the NEHRP guidelines.     

Surface soil assessment in the Ubhur area,north of Jeddah,western Saudi Arabia,using a multichannel analysis of surface waves method

Ten boreholes drilled in Ubhur area up to the depth of bedrock indicted the shallow depth of bedrock where the average depth ranges between 10 and 15 m. The standard penetration test N-values of these boreholes were measured and averaged. Based on N-values to the depth of bedrock, Ubhur area can be classified as site class C and D. Multichannel analysis of surface waves technique has been applied along seventy six profiles using 24-channel geophone array and 4.5Hz vertical geophones with 1m geophone spacing and sledgehammer and/or weight drop as seismic energy sources. Values of shear wave velocity to 30 m are calculated and then averaged (Vs30) where it ranges between 310.08 m/s and 1139.8 m/s. Therefore, Ubhur area can be classified into site class B, C and D based on site classification of the national earthquake hazards reduction program (NEHRP) recommendations. Accordingly, the greatest part of the study area falls in site class C while class B and D covered limited areas in the western and the eastern parts respectively. Depending on the shallow depth of bedrock in the study area, the Vs30 parameter is not applicable in the study area so the average values of Vs for the soil thickness, excluding the bedrock, have been calculated and mapped for site class C and D only. So Vs30 approach is not applicable for areas with shallow depth of bedrock which gives higher classification.     

Research of reservoir watershed fine zoning and flood forecasting method

In the present study, a set of correlation relations between standard penetrations tests (SPT-N values) and shear wave velocity (Vs) for different categories of soils is developed for Dholera region, Gujarat state, Western India. Shear wave velocities were measured using multichannel analysis of surface wave (MASW at 42 sites) and by PS logging (at 16 sites). SPT-N values were taken in total 87 geotechnical boreholes at depth interval of 3 m. Seismic site classification is done as per National Earthquake Hazards Reduction Program of the USA. Total 336 pairs of SPT-N values and Vs data at different depths are used to develop the regression correlation between uncorrected SPT-N value and Vs and compared with the results of other workers from India and worldwide for all soils type. It is found that regression correlations developed for clay and all soils are almost similar to each other, for sand, the coefficient value is less and for silt, it is higher. The new regression equation gives good prediction performance. The present correlation can be used for the seismic hazard study for the study area and also for the other areas having similar soil strata using a process of validation.     

Determination of shear wave velocity and depth to basement using multichannel analysis of surface wave technique

The dispersive characteristics of Rayleigh type surface waves were utilized to estimate shear wave velocity (Vs) profile followed by imaging the shallow subsurface granitic layers in the heart of Hyderabad. The reliability of Multichannel Analysis of Surface Waves (MASW) depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Multichannel recording leads to effective identification and isolation of various factors of noise. Calculating the 1-D shear wave velocity (Vs) field from surface waves ensures high degree of accuracy irrespective of cultural noise. The main advantage of mapping the bed rock surface with shear wave velocity (Vs) is the insensitivity of MASW to velocity inversion besides being free from many constraints such as contrast in physical properties etc. Modeling of surface waves data results a shear wave velocity (Vs) of 250?C750 m/s covering the top soil to weathering and up to bedrock corresponding to a depth range of 10?C30 m. Further, the computed N values (which is an indicator of site characteristic) based on the harmonic shear wave velocity up to a depth of 5 m is found to be quite high (> 25?C30) well above 5 indicating the site to be safe and strong enough and not prone to liquefaction. A pair of selected set of results over granites are presented here as a case study highlighting the salient features of MASW.     

Soil void ratio correlation with shear wave velocities and SPT N values for Indo-Gangetic basin

In this study attempt has been made to understand in-situ void ratio in Indo-Gangetic basin (IGB) and to form empirical relations between void ratio and shear wave velocity (V_s), N values considering subsoil investigation data. Multichannel analysis of surface wave (MASW) test and standard penetration test was carried out along with soil property measured at 25 locations. The general soil profile varied from silty sand to clay of low compressibility, ground water level fluctuated between 1-27 m, depth of borehole varied from 20-40 m. Regression analysis was conducted on 202 data sets of void ratio and shear wave velocity, 293 data sets of void ratio and SPT- N value, which resulted in inverse correlations between void ratio and V_s, SPT N value. The datas were segregated into fine, coarse grained data based on engineering classification and relations were developed separately. Until now, no studies have related in-situ void ratio to V_s and SPT N. These correlations will be useful to predict void ratio for sites having measured values of V_s and N value. These void ratios can be further used to assess liquefaction susceptibility.     

Upper Mississippi embayment shallow seismic velocities measured in situ

Vertical seismic compressional- and shear-wave (P-and S-wave) profiles were collected from three shallow boreholes in sediment of the upper Mississippi embayment. The site of the 60-m hole at Shelby Forest, Tennessee, is on bluffs forming the eastern edge of the Mississippi alluvial plain. The bluffs are composed of Pleistocene loess, Pliocene-Pleistocene alluvial clay and sand deposits, and Tertiary deltaic-marine sediment. The 36-m hole at Marked Tree, Arkansas, and the 27-m hole at Risco, Missouri, are in Holocene Mississippi river floodplain sand, silt, and gravel deposits. At each site, impulsive P- and S-waves were generated by man-made sources at the surface while a three-component geophone was locked downhole at 0.91-m intervals.

Consistent with their very similar geology, the two floodplain locations have nearly identical S-wave velocity (V_S) profiles. The lowest V_S values are about 130 m s⁻¹, and the highest values are about 300 m s⁻¹ at these sites. The shear-wave velocity profile at Shelby Forest is very similar within the Pleistocene loess (12 m thick); in deeper, older material, V_S exceeds 400 m s⁻¹.

At Marked Tree, and at Risco, the compressional-wave velocity (V_P) values above the water table are as low as about 230 m s⁻¹, and rise to about 1.9 km s⁻¹ below the water table. At Shelby Forest, V_P values in the unsaturated loess are as low as 302 m s⁻¹. V_P values below the water table are about 1.8 km s⁻¹. For the two floodplain sites, the V_P/V_S ratio increases rapidly across the water table depth. For the Shelby Forest site, the largest increase in the V_P/V_S ratio occurs at 20-m depth, the boundary between the Pliocene-Pleistocene clay and sand deposits and the Eocene shallow-marine clay and silt deposits.

Until recently, seismic velocity data for the embayment basin came from eartquake studies, crustal-scale seismic refraction and reflection profiles, sonic logs, and from analysis of dispersed earthquake surface waves. Since 1991, seismic data for shallow sediment obtained from reflection, refraction, crosshole and downhole techniques have been obtained for sites at the northern end of the embayment basin. The present borehole data, however, are measured from sites representative of large areas in the Mississippi embayment. Therefore, they fill a gap in information needed for modeling the response of the embayment to destructive seismic shaking.     

Estimation of the compound hazard severity of tropical cyclones over coastal China during 1949–2011 with copula function

The site effect plays very important role for planning design of structures and estimating seismic damage of existing structures, especially in seismicity active regions. Alia?a, located in the northern region of the ?zmir City, is high seismically active region in western Turkey. We identified the dynamic properties of shallow soil for Alia?a district (western Turkey) in terms of the average shear-wave velocity for the top 30 m of soil (Vs₃₀), predominant period (T ₀) and vulnerability index (Kg). We created the microzonation map that classes the shallow soil according to National Earthquake Hazards Reduction Program (NEHRP). Shear-wave velocity (Vs) was evaluated using the 1-D multichannel analysis of surface waves at 34 sites. The microtremor measurements were taken at 58 stations with a grid spacing of 1000 m in Alia?a district. The acquired HVSR results show that the T ₀ values change from 0.1 to 1.9 s, while the amplification factor (A ₀) values change between 1.5 and 12. Soil classification results illustrate that northern part of the study area, located in alluvial unit, has lower Vs₃₀ values. These values are observed within the range of 100–300 m/s. This part was classified as the D and E types according to NEHRP. In addition, the weak zones of the study area due to the local site effects were indicated by the Kg values which vary from 5 to 55. The northern part of the study area is high vulnerability index zone (Kg > 20). Last of all, we can mention that the northern part of our study area is the weakest zone according to the results of geophysical and geological data.     

Spatial distribution of soil shear-wave velocity and the fundamental period of vibration – a case study of the Saguenay region,Canada

The spatial distribution of soil shear-wave velocity and the fundamental period of vibration were selected as input parameters for the determination of potential seismic site effects in the Saguenay region, Canada. The methodology used in this study involved three clear steps. First, a 3D geological model of the surficial deposits was built taking into consideration the type, spatial distribution and thickness of the deposits. Second, representative average V_s values were determined for each of the major soil units. Finally, the average shear-wave velocity from the ground surface to bedrock (V_sav), the shear-wave velocity of the upper 30 m (V_s30) and the fundamental site resonance period (T₀) were calculated over a regular grid for the study area. The results include the spatial distribution of the fundamental site resonance period, the average shear-wave velocity in the first 30 m of the ground and the spatial distribution of National Building Code of Canada seismic soil classes for the Saguenay region.     

RSM仪与场地剪切波波速测试技术

介绍了采用检层法进行场地剪切波波速测试的技术和应用以及这种方法的原理、发展历史、测试技术和计算方法。本文还简要介绍了ＲＳＭ的设备和软件以及作者的现场实测经验