Reexamination of Mononobe-Okabe Theory of Gravity Retaining Walls Using Centrifuge Model Tests

Gravity retaining walls are widely used in Japan because of their simplicity of structure and ease of construction. In design procedure, the seismic coefficient method is widely employed, in which the earth pressure and inertia force are calculated by converting the seismic force into a static load. Earth pressure is usually calculated by the Mononobe-Okabe formula, which applies Coulomb's earth pressure computed from the equilibrium of forces in the static state. However, the Hyogoken-Nambu Earthquake of 1995 prompted the need to reexamine seismic design methods for various civil engineering structures. Gravity retaining wall is one of such structures whose seismic design has to be reexamined and rationalized. At this moment there is no clear empirical basis for converting the seismic force into a static load. The design method has to take into account the behavior of gravity retaining walls during earthquakes. At the Public Works Research Institute, model tests were conducted on gravity retaining walls using a centrifuge. The acceleration and displacement of a retaining wall and its backfill as well as the earth pressure acting on the wall were measured simultaneously together with the deformation behavior of the wall and its backfill, using a high-precision high-speed camera. The data show that the hypothetical conditions of the Mononobe-Okabe formula do not appropriately express the real behavior of backfill and gravity retaining walls during earthquakes.     

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.     

Centrifuge Model Tests on a Cohesive Soil Slope Under Excavation Conditions

Problems induced by slope excavations are quite common. An in-flight excavation device was realized to simulate the live excavation of slopes at high g levels during centrifuge model tests. A series of centrifuge model tests was conducted to simulate the excavation of a slope at different inclinations and heights, and the effect of the excavation size was taken into consideration. The displacement histories of points over the slope were measured by an image capture and displacement measurement system. Measurement results showed that the excavation-induced deformation process could be divided into several phases with different displacement distribution features. The excavation was found to only affect a restricted zone of the slope whose boundary could be outlined by an A-surface. A strain analysis was conducted to determine the excavation-induced strain localization area of the slope. The degree of strain localization increased as the excavation time increased, but the width of the strain localization area was nearly invariable. Shear failure first occurred near the excavation surface and then extended upwardly to the slope surface under excavation conditions, while tension failure played a dominant role in the upper part of the slip surface. The strain localization area moved towards the slope surface with an increasing slope inclination. The lower part of the final slip surface was located in the strain localization area.     

大坝堆石体压缩模量计算方法的改进

为提高大坝堆石体压缩模量计算方法的适应性和可靠性,通过分析传统计算方法受拱效应影响较大的局限,提出了采用沉降仪的沉降值和土压力计的压应力值共同计算堆石压缩模量的方法.通过对比2座200 m级堆石坝实测成果,证明引入土压力计的压应力值后,能够取得比较明显的改进效果.     

垃圾填埋场防渗衬垫特性的离心模型试验与数值分析研究

垃圾填埋场中垃圾体和防渗土工膜的受力变形特性是普遍关心的问题,在现场试验尚无法获得满意的监测结果时,利用离心模型试验和有限元数值分析进行探讨性研究也是可以尝试的途径。通过离心模型试验研究和有限元计算,模拟了垃圾填埋场中填埋垃圾大变形条件下土工防渗膜的变形性状,在模型比率80的条件下,可以找到相应的模型材料来模拟实际工程中的土工膜;选用的防渗膜模拟材料和木屑土能较好地模拟垃圾填埋场中材料;邓肯-张模型和Goodman接触面模型能模拟垃圾土和土工膜以及界面的工作状态;土工膜中最大拉应力靠近坡肩,可达到极限抗拉强度的1/1.5倍;考虑到HDPE膜的抗拉安全,最大设计坡长应加以控制。     

基于TOPSIS法的堆石坝施工效果综合评价

堆石坝种类较多,堆石体填筑质量的优劣是这类大坝安全运行关注的核心问题之一。在施工过程中,每一个填筑单元质量效果评价因素包括单位面积碾压时间T(i)、碾压超速次数N(i)、振动状态不达标次数L(i)、碾压遍数合格率R(i)和平均压实厚度H(i)。基于此,通过决策矩阵模型计算出各个指标的权重,采用了TOPSIS法计算每个填筑单元各指标的最优隶属度进而排序,形成矩阵模型。从施工进度和质量两方面系统地对堆石坝施工质量效果进行评价。TOPSIS法能够较为客观、全面地反映堆石坝填筑质量的优劣,便于对每一个填筑单元进行及时监控和评价。该方法简单,计算方便,可用于土石坝填筑质量的综合评价。     

深厚覆盖层坝基施工期模型试验及数值分析

本文论述了一座水电站河谷右深槽覆盖层坝基施工期地基模型试验。模型试验实测沉降量与用分层总和法、数值分析法计算结果接近。通过模型试验得出了坝基的最大承载力,还进行了施工方法的模型试验,验证了设计方案。大量的试验资料还表明,用电测百分表和液压千斤顶稳压器效果良好。     

鲁布革心墙堆石坝变形的反馈分析

鲁布革坝于１９８９年建成后已经受了高水位的考验。本文利用该坝在填筑及蓄水期积累的大量观测资料及施工期直接从坝上取样的试验成果进行了计算模型的对比验证和模型参数的反馈分析。分析结果表明，按Ｄｕｎｃａｎ模型计算的水平位移明显偏大，按双屈服面弹塑性模型计算的沉降和位移比较理想，但为了更好符合实际变形过程，必须补充流变变形。文中提出了一个流变变形的计算模型，并通过反馈分析确定了模型参数。     

混凝土面板堆石坝接缝止水的实验和分析

混凝土面板堆石坝周边缝止水的损坏是其漏水的主要原因。针对工程实用形式的周边缝铜止水和ＰＶＣ止水，首次进行了承受错位变形的破坏实验，揭示了止水极限变形能力和开裂规律，提出了接缝止水的计算模型、设计准则及提高铜止水极限抗裂能力的工程措施。     

桩间土拱效应离心模型试验研究

抗滑桩的合理桩间距是设计中的关键参数之一。本文通过桩间净距分别为4 d、5d和6 d的粘性土抗滑桩离心模型试验,分析了桩间土拱形态变化及破坏状态。试验结果表明,桩间距较大时,土拱曲线形状趋于平缓,土拱矢高较小,土拱的受力趋于梁的受力,难以形成稳定的土拱;桩间距较小时,土拱矢高较大,土拱的受力呈现受压的特征,容易形成稳定的土拱,即随着桩间距的增大,土拱效应减弱。同时从理论上分析了不同桩间距土拱的破坏图式,提出粘土中合理桩间净距为桩径的4倍左右,该结论对抗滑桩的设计具有一定的参考意义。     

软黏土地层盾构隧道开挖面稳定性离心试验研究

针对软黏土地层中隧道开挖面稳定性开展离心模型试验,由远程控制模型推进模拟隧道开挖面被动渐进破坏。通过数字图像处理分析技术对开挖面被动渐进破坏模式进行了探究,结合相应数值模型对比,分析了开挖面极限支护压力以及地表变形规律。研究结果表明:在开挖面被动破坏状态下,隧道仰拱处首先出现与水平方向近似成45°+φ/2向上发展的滑动破裂带,当其发展接近隧道拱顶时,拱顶滑动破裂带形成,两滑动破裂带呈漏斗状向上逐步发展至地表;随着模型向前推进,开挖面压力前期呈现线性快速增长趋势,随后土压力增速逐步减小,开挖面压力值最终趋于稳定,分析确定软黏土地层开挖面被动破坏状态下的支护压力可控制在1~1.9倍的静止土压力;开挖面被动破坏会引起前方地表产生隆起。     

瀑布沟土石坝体及防渗墙的离心模型试验和数值分析

本文论述瀑布沟土石坝及坝基防渗墙在施工完建、蓄水至正常高水位、水位骤降及７．５度，８度地震烈度等不同设计条件下的离心模型试验。得出不同坝高的土石坝及防渗墙各部位的应力、变形值。采用有限元计算，并和有关工程的原型观测资料进行了对比分析。     

压实粘性填土挡土墙土压力离心模型试验

笔者利用自制的一套位移控制液压装置做了11组位移可控式压实粘性填土档土墙土压力的离心模型试验。本文对粘性填土的裂缝深度及其影响因素、土压力的大小、分布及其与墙体位移的关系等问题进行了试验分析。     

面板堆石坝砂砾石料填筑碾压试验研究

利用天然砂砾石进行大坝填筑,其碾压参数直接影响到大坝施工进度和沉降变形,也直接影响到大坝施工成本和安全稳定,碾压参数的优化一直是大坝填筑施工技术研究重点。为有效地指导现场施工,以某一工程为案例,通过面板堆石坝砂砾石料填筑碾压试验研究,分析数据成果,提出大坝填筑碾压参数。     

土坝坝体劈裂灌浆浆液固结分析

通过对若干土坝劈裂灌浆现场试验资料的分析，论述了浆液固结的机理及其影响因素。在此基础上，依据土坝坝体劈裂灌浆浆液固结的实际情况，提出了以下观点：浆液在灌浆期是渗透力作用下的固结过程，在回弹期是坝体侧向压力作用下的固结过程。并分别建立了灌浆期和回弹期浆体固结数学模型，用有限差分法对影响浆体固结的几种因素进行了分析。     

Analysis of Results of Consolidation Tests of Saturated Clayey Soils

It is shown that the dependence of soil deformation on the distance to the drainage boundary, which is noted by a number of experimenters for the stage of secondary consolidation, is explained by incorrect interpretation of test data. Laboratory results demonstrating an apparent lack of creep in the stage of primary consolidation are explained by characteristic features of the generalized consolidation equations and relationships between their parameters.__________Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 7–10, May–June, 2005.     

综合控制硐室爆破的原理及其在坝料开采中的应用

本文论述了集中药包及条形药包对岩石爆破破碎机理，提出了钢筋混凝土面板堆石坝坝料开采中综合控制硐室爆破的设计方法，并经工程实践证明效果显著。     

Nonlinear Earthquake Response Characteristics of a Central Clay Core Rockfill Dam

The effect of the 2008 Iwate-Miyagi Nairiku earthquake (M_J 7.2), Japan on the Aratozawa dam, which is a 74.4 m high rockfill dam with a central clay core located in the area, was studied with a main focus on the change in the vibration period, shear wave velocity, shear modulus, and pore-water pressure. In a vertical section of the central part of the dam, three sets of 3-component accelerometers and 15 pore water pressure meters were installed. During the main shock, the acceleration exceeded 10 m/s² at the gallery, inducing large shear strains in excess of 10^-3 and a sudden build-up of the excess pore water pressure in the core. Due to the large strains, the shear wave velocity and shear modulus showed a significant decrease from their initial values and the vibration period was elongated. The decreased shear wave velocity gradually recovered toward the end of the main shock, and continued to recover with the passage of time. The full recovery of the wave velocity was found to take at least one year, while the dissipation of the excess pore water pressure proceeded more quickly than the recovery of the wave velocity.     

Long-term behavior and safety assessment of Sance Rockfill Dam

Safety assessment of the 62 m high Sance Rockfill Dam based on long-term monitoring results and their simulation by numerical models is presented in the paper. Unexpectedly large settlements and horizontal movements of the crest together with the steep downstream slope questioned the safety of the dam. Calibrated 2D and 3D models fitting the monitoring results were applied for the safety assessment of the dam including slope stability analysis and estimation of the clay core cracking and hydraulic fracturing hazard. The modeling results contributed to the rehabilitation concept and project of the dam.