Assessment of soil liquefaction incorporating earthquake characteristics

This paper investigates the effect of nature of the earthquake on the assessment of liquefaction potential of a soil deposit during earthquake loading. Here, the nature of the earthquake is included via the parameter V, the ‘pseudo-velocity’, that is the gross area under the acceleration record of the earthquake at any depth below the ground surface. By analysing a number of earthquake records from different parts of the world, a simple method has been outlined to assess the liquefaction potential of a soil deposit based on the pseudo-velocity. For many earthquakes occurred in the past, acceleration records are available or can be computed at the ground level or some other depth below the ground surface. Therefore, this method is a useful tool at the preliminary design stage to determine the liquefaction potential before going into a detailed analysis. Validation of the method is carried out using a database of case histories consisting of standard penetration test values, acceleration records at the ground surface and field observations of liquefaction/non-liquefaction. It can be seen that the proposed method has the ability to predict soil liquefaction potential accurately, despite its simplicity.     

The effects of liquefaction-induced lateral spreading on pile foundations

Observations of pile foundation performance during previous earthquakes have shown that pile failure has been caused by lateral ground movements resulting from soil liquefaction. The recognition that lateral ground movements may play a critical role in pile performance during an earthquake has important implications for design and risk assessment, and requires that analytical models be devised to evaluate these potential problems.In this paper, parametric studies were conducted to estimate the maximum bending moments induced in piles subjected to lateral ground displacement. The results are summarized in charts using dimensionless parameters.The analyses reveal that the existence of a nonliquefiable layer at the ground surface can affect significantly the maximum bending moment of the pile. When a relatively thick nonliquefiable layer exists above a liquefiable layer, neither the material nonlinearity of the soil nor loss of soil stiffness within the liquefiable layer significantly affect the maximum bending moment. When the thickness of the liquefiable soils is greater than about three times that of an overlying intact layer, soil stiffness in the liquefiable layer must be chosen carefully when evaluating the maximum bending moment.     

Liquefaction-induced damage to houses and its countermeasures at Minami-Kurihashi in Kuki City during the 2011 Tohoku Earthquake,Japan

Sand boiling and liquefaction-induced damage to houses and infrastructures occurred in Minami-Kurihashi, Kuki City, during the 2011 off the Pacific Coast of Tohoku Earthquake, Japan. After the earthquake, extensive site investigations were conducted in the affected areas, including 14 borehole surveys and 43 sounding tests, where Piezo Drive Cone penetrometer, a newly developed test method, was used which could be effectively employed in detecting local change of soil profiles. A filled sandy soil layer existed near the ground surface in the affected areas, which originated from reclamation works using dredged materials to construct housing lots. In addition, a Holocene sandy soil layer existed partly at a depth of about 10–13 m. Though these two layers were evaluated to be potentially liquefiable, the liquefaction-induced damage was observed to concentrate in the areas where the reclamation works had been executed, suggesting that the liquefaction of the reclaimed layer caused such damage. It was deduced that possible liquefaction of the Holocene layer did not contribute to the damage and to the occurrence of sand boiling at the ground surface. As countermeasure against future liquefaction, ground water lowering method has been selected, and in-situ tests and numerical analyses were executed to predict the long-term ground settlement. A subsequent study on detailed design of the selected countermeasure is underway as of June 1, 2015.     

Remediation of piled foundations against lateral spreading by passive site stabilization technique

Physical modeling tests were conducted on pile foundations to measure the seismic performance of a new ground improvement technology, called passive site stabilization, for use on sites susceptible to liquefaction and liquefaction-induced lateral spreading. The method involves the slow injection of a low-viscosity stabilizer in conjunction with the natural groundwater flow. The effectiveness of the treatment using dilute colloidal silica as the stabilizer was tested by two centrifuge models that simulated soil–pile interaction of a 2×2 end-bearing pile group embedded in a multilayer soil deposit of 10-m thickness. The models utilized a laminar box and involved gently inclined soil profiles with and without the applied soil improvement. Response of the pile groups and the lateral spreading behaviors of the treated and untreated soil under a simulated base shaking were investigated and compared. The results showed that treatment with dilute colloidal silica stabilizer minimized permanent lateral deformations and reduced the liquefaction potential of the soil. Significant reductions occurred in the measured pile bending moments and axial forces because the layer treated with dilute colloidal silica did not liquefy. Thus, the technique can be an alternative to traditional methods of ground improvement.     

液化规范判别与实际震害现象的差异对比——以唐山地震震害为例

砂土液化判别是地震地质灾害判定和预测的一项重要工作,判别方法以规范为主,但在实际工作中发现规范的判别结果与实际情况存在差异。本文在查阅大量唐山地震震害资料的基础上,选择3个典型工程场地,根据实际钻孔现场标贯原位测试数据,采用《建筑抗震设计规范》中的方法进行液化判别,发现规范判别结果与实际震害现象之间存在着一些差异。分析认为,抗震设防烈度与实际地震动不同、地下水位变化、上覆非液化土层的厚度、局部场地效应、地震动持时以及实验造成的人为误差等,均是造成差异的原因。分析结果也表明,规范中的判别方法具有较普遍的适用性和较强的实用性,但由于基础数据的局限性及判别公式本身存在的定性异常,其判别结果的合理性还有待于进一步研究论证,这也是造成判别结果与实际震害现象存在差异的原因之一。本文的研究结果对地震液化的机理认识、判别方法的完善,均具有一定的意义。     

GEOLOGICAL HAZARD CHARACTERISTICS AND MACROSCOPIC EPICENTER OF NOVEMBER 25, 2016, ARKETAO,XINJIANG, MW6.6 EARTHQUAKE

The M_W6.6 Arketao earthquake occurred on November 25, 2016 in Muji Basin of the Kongur extensional system in the eastern Pamir. The region is the Pamir tectonic knot, one of the two structural knots where the India plate collides with the Eurasian plate. This region is one of the most active areas in mainland China. The seismogenic structure of the earthquake is preliminarily determined as the Muji dextral-slip fault which locates in the north of Kongur extensional system. Based on field surveys of seismic geological hazard, and combined with the characteristics of high altitude area and the focal mechanism solution, this paper summarizes the associated distribution and development characteristics of sandy soil liquefaction, ground fissures, collapse, and landslide. There are 2 macroscopic epicenters of the earthquake, that is, Weirima village and Bulake village. There are a lot of geological hazards distributed in the macroscopic epicenters. Sand liquefaction is mainly distributed in the south of Kalaarte River, and area of sand liquefaction is 1 000m². The liquefaction material gushed along the mouth of springs and ground fissures, because of the frozen soil below the surface. More than 60% of soil liquefactions are formed in the mouth of springs. According to the trenching, these liquefactions occurred in 1.8 meters underground in the gray green silty clay and silty sand layers. The ground fissures are mainly caused by brittle failure, and the deformation of upper frozen soil layer is caused by the deformation of lower soil layer. The ground fissures at Weirima village are distributed in a chessboard-like pattern in the flood plain of Kalaarte River. In the Bulake village, the main movement features of the ground fissure are tension and sinistral slip, and the directions of ground fissures are 90°~135°. The collapse and landslide are one of the important geological disasters in the disaster area. The rolling stones falling in landslide blocked the roads and smashed the wire rods, and the biggest rolling stone is 4 meters in length. We only found a small landslide in the earthquake area, but there are a large number of unstable slopes and potential landslides in the surroundings. The ground fissures associated with sand liquefaction are an important cause of serious damage to the buildings.     

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.     

唐山东矿区地震易损性分析

地震易损性分析是小区域震害预测的一种新方法,是以震害经验为基础,综合分析地基土、第四系厚度、地下水埋深、砂土液化、边坡、断层和地形等地质因素对震害的影响,并将这些影响以数字形式表示。以唐山东矿区为例,介绍地震易损性分析在我国的应用     

Estimation of Nonlinear Time-dependent Soil Behavior in Strong Ground Motion Based on Vertical Array Data

To improve our understanding of nonlinear elastic properties of soils, a method is proposed of estimation of stress-strain relations of soils in situ in strong ground motion based on vertical array data. Strong motion records provided by seismic vertical arrays allow estimation of nonlinear stress-strain relations in soil layers at different depths, from the surface down to the location of the deepest device. As an example, records obtained during the main shock of the 1995 Kobe earthquake at Port-Island, SGK, and TKS sites were used to estimate the stress-strain relations in the soil profiles. For different layers, different types of nonlinear stress-strain relations were selected, according to the profiling data. To account for temporal changes in the soil behavior, consecutive parts of records were examined, and for successive time intervals, the relations were found showing the best-fit approximation to the observed data. At Port Island and SGK sites, where the strongest accelerations were recorded, the obtained stress-strain relations showed systematic changes in the upper layers (0–14 m), such as, a progressive reduction of the slopes of the stress-strain curves due to liquefaction at Port Island and reduction and recovery of the slopes at SGK and TKS sites. At the three sites, the stress-strain relations remained stable in layers below 11–14 m. Thus, the proposed approach gives us a representation of the soil behavior in layers at different depths in strong ground motion; it allows calculation of the propagation of arbitrary seismic signals in the studied profiles and estimation of nonlinear components in the ground response by the nonlinear system identification technique. The method can also be applied to evaluate the ground response at sites where profiling data are available and an imposed motion can be estimated.     

基于支持向量机的砂土液化预测分析

将支持向量机方法应用于砂土地震液化预测问题.考虑影响砂土液化的因素,选用震级、标贯击数、相对密实度、土层埋深、地震历时、地面运动峰值加速度和震中距7个影响因子作为液化判别指标,建立了砂土液化预测的支持向量机模型.以砂土液化实测数据作为学习样本进行训练,建立相应函数对待判样本进行分类.研究结果表明:支持向量机模型分类性能良好,是砂土地震液化预测的一种有效方法,可以在实际工程中进行推广.     

Seismotectonic and liquefaction studies of an industrial site in Northern India

Foundation soil of the proposed fertiliser complex in Northern India is examined for its susceptibility to liquefaction during an earthquake. Information on geotectonic set up and earthquake occurrences in the region around the site is used for defining the earthquake parameters of the ground motion. The effective peak ground acceleration for the site is estimated to be of the order of 0.15 g. Laboratory tests were carried out on soil samples obtained from the site on a horizontal vibration table. The test results were used in determining the possibility of liquefaction employing the methodology developed at the Department of Earthquake Engineering, University of Roorkee. About 10 m of a thick soil layer below the top 1.5 m stiff clay is likely to liquefy. Remedial measures used at the site to counter the possibility of liquefaction are mentioned.     

循环荷载下液化对土层水平往返变形的影响

采用多工况振动台实验研究液化对土层水平往返变形的影响.以干砂实验为参照,分析孔压增长与土层加速度和土层往返变形之间的关系.结果表明:液化将引起土表加速度显著降低,减小惯性力传递,但同时会引起土层往返剪应变明显增大.对往返变形而言,液化土层往返剪应变就可达到1%～5%的大变形状态,且液化土层往返剪应变沿深度呈下大上小分布.土层中孔压比0.4～0.8是往返变形出现放大的敏感段,在孔压比0.8左右而不是在1.0达到最大.作为其结果,土层液化将对刚性上部结构振动起减震作用,但同时增大的往返剪应变也易导致基础和地下结构破坏,特别是对液化层与下部非液化层交界处的构件更敏感.     

可液化倾斜场地中桩基动力响应振动台试验研究

为研究倾斜场地中桩基的动力响应,以2011年新西兰地震中受损的Dallington桥为原型,设计并完成可液化倾斜场地桥梁桩-土相互作用的振动台模型试验。试验再现了喷砂、冒水、地裂缝、场地流滑等宏观现象。试验结果表明,土层足够的液化势及惯性是造成倾斜场地侧向流滑的必要条件;浅层土相比深层土更易液化,液化层中的加速度由下至上呈现逐渐衰减的趋势,而未液化砂土层却表现为逐渐增大的特征;深部测点的桩侧土压力明显大于浅部测点,且土体的液化会弱化土对结构的压力;结构应变最大值位于上部桥台,而结构弯矩在桩身中部及土层分界面附近出现两个较大值,桩端嵌固及倾斜场地流滑是造成出现两个弯矩较大值的主要原因。     

Site classification using equivalent soil profiles for building-liquefaction interaction

The seismic behaviour of a building on a liquefiable deposit is a complex interaction which involves quantifying both shaking induced damage and permanent ground deformation-related damage. In this paper the key parameters that influence both surface shaking and foundation settlements have been identified as the depth, thickness and liquefaction resistance of an equivalent liquefiable layer. These parameters can be used to develop an ‘equivalent soil profile’ that is analogous to the equivalent single degree-of-freedom that reduces the complexity of the dynamic response of a building into comparable and easily understood quantities. The equivalent soil profile is quantified independent of the seismic hazard, making it compatible with performance based design and assessment frameworks such that the building and soil profile can be directly assessed at different levels of seismic hazard. Several numerical studies are presented that demonstrate the influence of these key parameters on the ground surface shaking and foundation settlement. A set of criteria are proposed for classifying soil profiles into 22 different soil classes for regional loss assessment. An algorithm was developed for automatically fitting the equivalent soil profile to a cone penetration test trace and issues with the fitting are discussed. Field reconnaissance was undertaken to collect additional data to support existing datasets on the performance of buildings in Adapazari, during the 1999 Kocaeli, Turkey, earthquake (Mw = 7.4). The field case history data was used to investigate the correlation between the depth, thickness and liquefaction resistance of an equivalent liquefiable layer, on the extent of foundation permanent deformation. The case history data showed that in general a shallow, thick and weak liquefiable layer near the surface results in significant settlement but a lack of data for buildings on non-liquefiable deposits and the additional complexities involved with real buildings and soil deposits, meant that the trends observed in the idealised numerical models could not identified in the field case history data set.

     

Cyclic undrained behavior of silty sand

Laboratory cyclic triaxial tests were performed to investigate the effect of fine content on the pore pressure generation in sand. Strain-controlled, consolidated undrained tests have been performed with a cyclic shear strain range of 0·015-1·5%. These tests were carried to 1000 cycles or to initial liquefaction, which ever occurred first. Triaxial tests were performed on pure sand silt specimens and specimens with silt additions of 10, 20, 30, and 60% by weight. Two types of silt, a non-plastic silt and a low plasticity silt (PI 10) were used as control materials. The main parameters varied in this study were the amount of silt, the plasticity index of silt, and the void ratio where the observed parameter was the pore pressure generation. For all silt contents, silt plasticity and the number of loading cycles have no significant effect at strain levels below 0·01%. Therefore, threshold strain for silty sands have approximately the same value as sands. For both non-plastic and low plasticity silts, there is a significant increase in the generated pore pressure at high strain levels.     

Peak surface strains during strong earthquake motion

Peak amplitudes of surface strains during strong earthquake ground motion can be approximated by ε = Aν_max/β₁, where ν_max is the corresponding peak particle velocity, β₁ is the velocity of shear waves in the surface layer, and A is a site specific scaling function. In a 50 m thick layer with shear wave velocity β₁ 300 m/s, A 0·4 for the radial strain ε_rr, A 0·2 for the tangential strain ε_rθ, and A 1·0 for the vertical strain, ε_z. These results are site specific and representative of strike slip faulting and of soil in Westmoreland, in Imperial Valley, California. Similar equations can be derived for other sites with known shear wave velocity profile versus depth.     

Micro-scale modeling of saturated sandy soil behavior subjected to cyclic loading

Liquefaction induced damage to the built environment is one of the major causes of damage in an earthquake. Since Niigata earthquake in 1964, it has been popularly recognized that the liquefaction induced ground failures caused severe damage in various forms such as sand boiling, ground settlement, lateral spreading, landslide, etc. Since then, understanding the mechanism of liquefaction phenomena became very important to take measures against the liquefaction induced ground failures. To understand the mechanism of liquefaction, it is important to consider the soil as an assemblage of particles. A continuum approach may fail to explain some of the phenomena associated with liquefaction. Discrete approach, such as distinct/discrete element method (DEM), is an effective method that can simulate the mechanism of liquefaction and associated phenomena well at the microscopic level.     

基于重塑饱和砂土模型的现场液化试验方法

基于现场开展土体液化问题研究势必成为今后土动力学中的一个重要发展方向。目前人工激振下的现场液化试验方法还不够成熟,尚需进一步探索和发展。本文从试验设备组成、场地地震动激励、试坑布置、饱和砂土模型制备、数据测量与采集等5个方面论述该方法中的主要技术问题。研究表明:动力加载系统激励产生的地震动在0~7m/s2;系统工作频率13~15Hz,饱和砂土模型与基础边缘的距离在0.5~2.5m范围内,更适合进行液化试验;应用水沉法现场制备饱和砂土模型,要重点注意试坑防水和尺寸定位的问题;数据测量与采集中要充分考虑对现场液化问题认识不够这一因素的影响,需对数据测量与采集提出附加要求;试验实例初步表明,该方法可行,适合开展液化问题研究。     

河流沉积相土层结构对砂土液化的影响研究

目前相关规范主要依据工程场地单点的测试数据进行砂土液化判别,而实际的三维土层结构可能非常复杂。研究土层结构对砂土液化的影响机制,有利于提高砂土液化判别结果准确度。分析2008 年5 月28 日发生的松原M_S地震和2010—2011 年新西兰坎特伯雷地震序列中砂土液化点的分布,结果显示:砂土液化点主要位于高弯度河流的沉积相地层,凹岸侧蚀、凸岸沉积形成的边滩具有典型的二元结构,其顶部分布的黏土类不透水层有利于下伏饱和粉细砂等易液化土层的超孔隙水压累积;而辫状河流沉积相中,上覆黏土类不透水层间断分布特征明显。针对河流不同沉积相的土层结构建立简化场地模型,使用FLAC3D 进行砂土液化数值模拟,揭示出不同土层结构中超孔隙水压力的累积、消散和渗流过程机制,结果表明,河流沉积相土层结构对砂土液化场点的分布和地表变形具有显著影响。在合理的工程地质分区基础上,现有的液化判别方法有必要考虑场地的土层结构的影响。      

某长江大桥深度超过15米的砂土层液化势的综合评价

不同抗震设计规范的砂土液化判别方法或国内外其他有代表性的液化判别方法所采用的地震动参数和土性指标及其埋藏条件是不同的,因而采用这些方法对同一工程场地进行液化势预测时其评价结果通常有一些差异,甚至会得到相反的结论。为了给重大工程建设提供较为合理、可信的地基液化势预测结果,采用多种液化判别方法进行场地液化势的综合评价是比较客观的,也是必要的。本文结合某长江大桥桥基工程,采用建筑抗震设计规范的砂土液化判别方法、国内外有代表性的液化判别方法、有限元数值分析法等多种方法逐一对该工程场地砂性土层进行液化判别,并结合室内动三轴液化试验结果,对主桥墩不考虑冲刷条件和考虑一般冲刷深度5m条件时的砂性土层进行了液化势的综合评价,并将各土层的液化势分为液化、可能液化和不液化3个等级,得到了较为合理可靠的判别结果。