Effect of installation geometry on dynamic stability of small earth dams retrofitted with a geosynthetic clay liner

Geosynthetic clay liners (GCLs), used to repair small earth dams, are typically installed with the GCL panel placed parallel to the upstream slope of the dam or on the surface of benches cut into the upstream side of the earth dam fill. While the former requires less earthwork, leading to a more cost-effective and rapid construction, it can potentially introduce a plane of weakness if the interface shear strength between the GCL and the cover soil is less than the shear strength of the cover soil. The inclusion of benches in the upstream slope of an earth dam can potentially be an effective strategy for reducing the significance of this preferential failure plane, resulting in an increased seismic performance during earthquakes. However, the expected increase in seismic performance has not yet been quantified in large-scale shaking table tests. In this study, a full-scale shaking table test on an earth dam with a GCL installed parallel to the upstream slope of the dam is reported and compared to previously published results from tests on an identical earth dam with the GCL placed in the benched configuration under the same seismic boundary conditions. The results indicate that, for the configuration tested, the seismic deformation of the benched installation was half of that of the earth dam with the GCL installed parallel to the slope, providing significant motivation for adopting the benched installation method.     

Model Test and Consolidation Analysis of Failure of a Loose Sandy Embankment Dam During Seepage

Failures of earth dam embankments and river dikes, which are constructed of sandy soils with low dry densities, have been observed to occur during rising of water levels. In this paper, a large-scale physical model test was conducted in order to investigate the behavior of a small dam embankment as water levels rose. The test results were simulated by use of a consolidation analysis method coupled with an elastoplastic model for unsaturated soils. All parameters used in the simulation were obtained from element tests; oedometer, triaxial compression, soil water retention, and permeability tests. First, in order to verify the parameters identified, direct shear tests were simulated using the consolidation analysis method; results of the simulation and the tests were consistent. From the embankment model test it was found that the crest of the embankment moved upstream at the first stage of impounding and then moved back. After a seepage surface appeared on the downstream slope, tension cracks occurred on the downstream slope, and sliding occurred through the crack and the downstream toe. The results of the simulation were consistent with those of the model test. This consolidation analysis method could be used to simulate the complex deformations induced by saturation collapse and shear strains and even failure behavior. Old embankments constructed with loose densities might have histories in which cracks occurred on the downstream slopes when the reservoir water level rose, and their stabilities might have decreased.     

300m级超高面板堆石坝变形规律的研究

采用三维有限元数值分析方法研究了300m级超高混凝土面板堆石坝的变形规律。指出坝体分区、筑坝材料特性特别是流变特性是影响超高面板堆石坝变形性状的主要因素,要重视超高面板堆石坝上游部分坝体变形的鼓肚现象。建议了合理选择筑坝材料,适当提高下游堆石区填筑标准、全断面均衡填筑、上部1/3左右坝体采用变形模量较高的坝料,待坝体变形基本趋于稳定才浇筑面板以及考虑设置面板永久水平缝等工程措施来改善超高面板堆石坝的应力变形性状。     

不透水地基上堤坝渗流计算的理论解

提出堤坝流态分区的概念和在流量固定条件下,上下游流态互不影响的法则。这使堤坝的渗流求解趋于简化和方便。增添新的理论计算式计有:上游区浸润线反弯点位置的计算式,下游区浸润线方程的解析开拓式和式前的待定常数的计算式以及下游区的附加渗径长度计算式等。按努美罗夫理论解?L1=C1H1-C2q/k和柯钦娜理论解q/k=μh0,高精度计算出上游区附加渗径长度?L1的比例系数C1和C2以及流量和出逸点高度关系式的比例系数μ,并据以提出有足够精度的C1,C2和μ的拟合计算式,以便于应用。经数值比较,得出比例系数C1全同于边坡有压流附加渗径长度的比例系数C0的结果。对于有尾水位的堤坝,提出了计算出逸点高度和下游区附加渗径长度的两经验计算式,据以计算的结果和有限单元法相符。     

堰塞湖坝体动力特性及加速度分布规律大型振动台模型试验研究

 2008年汶川八级地震形成了至少257个堰塞坝,主震后发生的大量余震可能会影响堰塞坝的动力安全状态。堰塞坝体的动力特性参数(包括自振频率和阻尼比等)和加速度分布规律是堰塞坝地震安全研究的基础内容。通过大型振动台模型试验,研究在余震作用下模型堰塞坝体的动力特性参数、加速度分布规律及二者的影响因素,并根据动力相似律,计算原型坝体的动力特性参数。共进行2组不同材料的振动台模型试验,分别模拟含黏粒较多且颗粒较小(坝体I)和基本不含黏粒且颗粒较大(坝体II)的2种坝体。在不同地震波形输入、不同加速度峰值和不同水位条件下进行振动台试验。研究成果表明：(1) 模型坝体具有较稳定的X向和Z向自振频率和阻尼比。(2) 先期振动使坝体自振频率降低,阻尼比有增大趋势;坝体I的自振频率小于坝体II。水位变化对2种坝体自振频率的影响规律不一致。(3) 加速度放大倍数随高程增大而增大,最大加速度发生在坝顶处;相同高程测点加速度放大倍数最大值出现在上游或下游靠近坝坡表面处,即“表面放大”效应明显,说明坝坡表面容易受地震作用破坏。(4) 所含频谱成分与坝体自振频率接近的地震波会引起最大的加速度反应。Z向振动使坝体测点X向加速度放大倍数增大。加速度放大倍数一般随输入加速度峰值的增大而减小。     

坝体碾压混凝土温度场监测与分析

通过分析某大坝坝体碾压混凝土温度监测原始资料,研究碾压混凝土温度分区变化过程和内部混凝土温度变化规律;绘制坝体碾压混凝土温度场,发现下游侧等温线与下游坝面平行,上游侧水下部分等温线与坝面相交。等温线在坝面附近密,在坝体内部稀,说明温度梯度在坝面附近大,在坝体内部小。坝体碾压混凝土夏季温度场有一个高温核与两个低温核,而冬季温度场只有一个高温核,表明夏季温度场比较复杂。     

水位骤降对边坡稳定性影响的模型试验研究

 自然界中存在着大量的临水边坡,比如河岸、海堤、土石坝、水库库岸以及湖岸等,边坡外水位的骤降极易诱发此类临水边坡的滑坡。通过大型模型试验研究水位骤降引致临水边坡滑坡的原因及失稳模式。模型边坡的尺寸为15 m×5 m×6 m(长×宽×高),边坡部分的高度为4 m。在试验中,通过水位控制系统实现坡外水位的骤降,利用数码摄像、高精度传感器、侧面示踪点等仪器设备详细记录水位骤降过程中边坡内的孔隙水压力、土水总压力,滑动面形态及坡面裂缝的形成和发展过程,揭示水位骤降引致边坡失稳的原因及失稳模式。试验结果表明,坡外水位骤降时,坡内水位的下降速度显著滞后于坡外,产生指向坡外的渗流,是滑坡产生的重要原因;松散填土边坡的失稳模式为有多重滑面的牵引破坏模式。该研究结果有助于深入认识水位骤降引致滑坡的机制,可为治理此类滑坡提供科学依据。     

Géologie et barrages en béton compacté au rouleau (BCR)

RCC dams are often presented as an alternative to earth or rockfill embankments. They seem to be able to accept slightly greater deformation than ordinary concrete. The five examples quoted, recently built in France, show that RCC has been used for vertical upstream walling in a very varied range of soils, despite their traditional profiles.     

Damage to agricultural facilities

The 2011 off the Pacific Coast of Tohoku Earthquake severely damaged agricultural facilities in Tohoku district, damaged coastal dikes and farmland behind the embankment, and disrupted lifelines, such as irrigation systems, in districts far from the coastline.This report introduces an outline of the damaged agricultural facilities on the basis of the investigations made during inspection visits to those facilities by National Institute for Rural Engineering (NIRE). In Fukushima Prefecture, many small earth dams, other dams, and pipelines were damaged. Among a total of 3730 dams and small earth dams in Fukushima Prefecture, 745 small earth dams were damaged by sliding failure or settlement of embankment. Small earth dams at 3 locations were breached, inflicting severe damage on the regions downstream.The embankments of two dams for agricultural use, the Nishigo dam and the Hatori dam, located in the Tohoku region, were damaged. In parts of Fukushima Prefecture where trunk pipelines for agricultural use have a total length of 17.8 km, at 7 places pipelines were either exposed or experienced leakages, and severe deformation which did not satisfy standard values was noted at 149 locations.This paper reports the state of damage typical to each type of facility and the restoration measures that were undertaken.     

渗流作用下的坝坡稳定有限单元分析

本文从渗透力的基本概念出发。在指出作为常规坝坡稳定分析的条分法存在的问题后,提出结合渗流分析,直接用有限单元法进行坝坡稳定分析,开给出了单一圆弧和复会圆弧滑动面的计算公式。为了验证计算方法和编制的程序的正确性,我们对六个发生滑坡的土坝进行调查分析并在实验室内进行水力模型试验,试验和分析计算都得出了将要发生滑坡事故的结论,而且出现的最危险滑弧位置和水力条件也和实际情况甚为一致。然而按照常规条分法计算分析却不能得出各土坝滑坡的肯定结论,这就说明了本文论证的有限单元分析方法的准确性和可靠性。     

花凉亭水库砂壳坝的抗震性能分析与加固研究

经过对花凉亭水库黏土心墙砂壳坝进行地质勘察取样试验和液化分析,由于其上游坝坡砂土密实度不够,在7度地震时,水下部分可能发生液化并产生滑坡,需要进行抗震加固处理。我国目前已进行过抗震加固的土石坝不多,工程经验较少,还没有成熟的加固设计方法和技术规范。本工程应用砂土动强度试验成果,采用动力有限元分析对大坝抗震加固方案进行技术论证,并提出了帮坡压重的抗震加固方案,即在上游坝坡的水下部分抛块石帮坡压重和水上部分填筑石渣料帮坡压重,有效解决了水库蓄水条件下的大坝上游坝坡的抗震加固问题,其经验可供类似工程借鉴。     

强度参数的不确定性对土石坝坝坡失稳概率的影响

目前对土石坝坝坡的稳定性,主要是在坝料特性的确定参数基础上,采用极限平衡方法求出的坝坡抗滑稳定安全系数来评价。但实际的坝料特性参数往往是具有一定变化范围的变量,因此,安全系数大的坝坡不一定就比安全系数小的坝坡的失稳概率小。在分析研究12座均质土坝坝料强度参数统计特性的基础上,采用蒙特卡罗法对坝坡最危险滑裂面进行失稳概率分析,研究土石坝坝料强度参数的均值、变异性和相关性对坝坡失稳概率的影响。结果表明:强度参数的不确定性对坝坡的失稳概率有较大的影响,失稳概率随强度参数的均值的增大而减小,随变异系数的增大而增大。两个强度参数的相关性对坝坡的失稳概率也有显著的影响。由此可知,土石坝坝坡稳定性评价中,在确定性分析的同时,若能够考虑强度参数的不确定性,给出坝坡失稳概率,有利于更科学合理地评价土石坝的安全。     

堤坝非稳定渗流试验研究与计算

本文以砂模型试验为基础,探讨了不透水地基均质土坝非稳定渗流的物理力学过程。分析了影响给水度μ值的若干因素,从给水度在非稳定渗流过程中的作用,提出了在计算中必须考虑μ值随时间的变化过程。 经有限单元法大量计算,从自由水面变化过程、流场分析及上游边坡稳定综合分析可知,式（4）按不同时段选取相对应的μ值进行计算,基本符合堤坝非稳定渗流的物理状态。     

Numerical analysis of nonlinear dynamic behavior of earth dams

A numerical study is conducted to investigate the dynamic behavior of earth dams. The numerical investigation employs a fully nonlinear dynamic finite difference analysis incorporating a simple elastic perfectly plastic constitutive model to describe the stress-strain response of the soil and the Rayleigh damping to increase the level of hysteretic damping. The extended Masing rules are implemented into the constitutive model to explain more accurately the soil response under general cyclic loading. The soil stiffness and hysteretic damping change with loading history. The procedures for calibrating the constructed numerical model with centrifuge test data and also a real case history are explained. For the latter, the Long Valley (LV) earth dam subjected to the 1980 Mammoth Lake earthquake as a real case-history is analyzed and the obtained numerical results are compared with the real measurements at the site in both the time and frequency domains. Relatively good agreement is observed between computed and measured quantities. It seems that the Masing rules combined with a simple elasto-plastic model gives reasonable numerical predictions. Afterwards, a comprehensive parametric study is carried out to identify the effects of dam height, input motion characteristics, soil behavior, strength of the shell materials and dam reservoir condition on the dynamic response of earth dams. Three real earthquake records with different levels and peak acceleration values (PGAs) are used as input motions. The results show that the crest acceleration decreases when the dam height increases and no amplification is observed. Further, more inelastic behavior and more earthquake energy absorption are observed in higher dams.     

高填路基盖板涵外界面受力状态形成机制研究

依据原位试验和数值模拟研究高填路基盖板涵外界面受力状态形成机制。结果表明:涵洞顶面压力大于上覆填土自重,呈非均匀分布形式,涵洞侧墙顶相对于顶板中部承受更大的压力,侧墙主要承担了来自两侧填土的附加压力;涵洞侧墙外水平压力远小于按土柱高度换算的静止土压力,涵洞地基的不均一地质条件导致涵洞单侧承受相对较大的水平力;涵土差异沉降导致涵洞体承担了大于上覆填土自重的压力,基底的不均匀沉降引起涵顶压力向一侧集中,侧墙外压力和基底压力非对称分布。实测涵顶压力约为填土自重应力的1.56~3.02倍,使用现行公路桥涵规范设计高填方盖板涵偏于危险。     

Total stress rapid drawdown analysis of the Pilarcitos Dam failure using the finite element method

Rapid drawdown is a critical design condition for the upstream or riverside slope of earth dams and levees. A new total stress rapid drawdown method based on finite element analysis is used to analyze the rapid drawdown failure that occurred at Pilarcitos Dam in 1969. Effective consolidation stresses in the slope prior to drawdown are determined using linear elastic finite element analysis. Undrained strengths from isotropically consolidated undrained (ICU) triaxial compression tests are related directly to the calculated consolidation stresses and assigned to the elements in the model by interpolation. Two different interpretations of the undrained strength envelope are examined. Strength reduction finite element analyses are used to evaluate stability of the dam. Back analysis suggests that undrained strengths from ICU tests must be reduced by 30% for use with this rapid drawdown method. The failure mechanism predicted for Pilarcitos Dam is sensitive to the relationship between undrained strength and consolidation stress.     

Lower San Fernando土坝破坏及加固的完全耦合分析

介绍一组关于LowerSanFernando土坝在 1971年SanFernando地震中响应的完全耦合有限元分析 ,利用一新研制的临界状态砂土模型模拟土在各种条件下的响应。分析结果表明 ,在堤坝的上游边坡会发生流动破坏。如果没有下游反压平台 ,堤坝有可能发生滑向下游的流动破坏。分析结果还表明加建上游反压平台会有效地防止上游的滑坡     

堆石料的亚塑性边界面模型及其验证

 基于高莲士教授针对天生桥面板堆石坝灰岩所进行的堆石料在不同应力路径下的大型三轴试验结果, 提出亚塑性理论边界面模型参数的简捷确定方法。通过编制调参程序, 对堆石料典型三轴试验进行数值模拟, 与试验成果的对比表明, 模型能较好地反映各种复杂加载路径下堆石料变形性状。进而对恒应力比转折、恒应力比加卸载、平面应变试验等土体单元可能出现的复杂应力路径进行了预测, 对深入了解高围压下摩擦型材料的屈服及应力路径相关等特性具有一定意义。同时合理的模型参数为进行面板堆石坝的变形预测和数值仿真模拟奠定了基础。     

非饱和土性状及其与工程问题的联系

首先讨论非饱和土研究的现状,指出目前存在的几种观点及其特点,然后针对不同类型的工程问题分别讨论了非饱和土性状与这些工程问题的联系,最后对非饱和土理论实用化发展方向进行了思考和讨论。     

土坝渗透稳定可靠性分析方法及应用

采用确定性分析方法分析土坝渗透稳定的理论和方法已经较为完善,但它由于没有考虑坝体材料参数的变异性,因此得出的安全分析成果并不能完全可靠地表征其安全性。利用概率与统计的方法对土坝渗透稳定进行可靠性分析则可考虑坝体材料等一些不确定因素对土坝的渗透稳定影响。首先利用统计方法对坝体材料物理参数进行分析,得出了参数的概率分布类型,然后运用可靠性分析原理,构造土坝渗透破坏功能函数,分析土体有效重度、内摩擦角,坝体、坝基材料渗透系数对功能函数的影响,并采用几何法对坝体渗透破坏可靠指标进行计算,得出坝体发生渗透破坏的概率。利用上述方法对一土坝渗透稳定进行了分析。结果分析表明,坝体渗透破坏的概率随着土体有效重度和内摩擦角变异性的增加而增加,并且土体内摩擦角的变异性对坝体渗透破坏的影响较大,坝体渗透变异性对坝体渗透破坏的影响较大;另外,随着库水位的升高,坝体发生渗透破坏的概率也显著增加。