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.     

Role of particle shape on the shear strength of sand-GCL interfaces under dry and wet conditions

Interface shear strength of geosynthetic clay liners (GCL) with the sand particles is predominantly influenced by the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. This study brings out the quantitative effects of particle shape on the interaction mechanisms and shear strength of GCL-sand interfaces. Interface direct shear tests are conducted on GCL in contact with a natural sand and a manufactured sand of identical gradation, eliminating the particle size effects. Results showed that manufactured sand provides effective particle-fiber interlocking compared to river sand, due to the favorable shape of its grains. Further, the role of particle shape on the hydration of GCL is investigated through interface shear tests on GCL-sand interfaces at different water contents. Bentonite hydration is found to be less in tests with manufactured sand, leading to better interface shear strength. Grain shape parameters of sands, surface changes related to hydration and particle entrapment in GCL are quantified through image analysis on sands and tested GCL surfaces. It is observed that the manufactured sand provides higher interface shear strength and causes lesser hydration related damages to GCL, owing to its angular particles and low permeability.     

地震荷载作用下饱和砂层孔隙水压力的增长与消散

本文用有限元法对地震荷载和不同排水条件下陡河水库土坝砂基中孔隙水压力的增长与扩散过程,以及振动结束后孔隙水压力的重分布与消散过程进行了计算分析。通过动力三轴试验,给出了饱和砂的残余孔隙水压力的全量表达式和不同液化破坏阶段的临界孔隙水压力比的表达式。本文定义了液化势评价准则,并对该坝在1976年唐山大地震时的稳定性进行了分析。得到的结果能较好地说明宏观破坏现象,同时也表明砂砾井和砂砾层有明显的排渗作用。     

Shear strength and failure mechanism of needle-punched geosynthetic clay liner

The internal shear strength of a geosynthetic clay liner (GCL) within composite liner systems is crucial for the stability of landfills and should be carefully considered in the design. To explore the shear strength and failure mechanism of the extensively used needle-punched GCL, a series of displacement-controlled direct shear tests with five normal stress levels (250–1000 kPa) and eight displacement rates (1–200 mm/min) were conducted. The shear stress to horizontal displacement relationships exhibit well-defined peak shear strengths and significant post-peak strength reductions. The monitoring results of the thickness change indicate that the degree of volumetric contraction is related to the reorientation of fibers and dissipation of pore water pressure. Furthermore, the peak and residual shear strengths both depend on the displacement rate because of the rate-dependent tensile stiffness of needle-punched fibers and shear strength of the soil/geosynthetic interface. Through additional tests and lateral comparison, it was discovered that the shear behavior of sodium bentonite, degree of hydration, and pore water pressures all affect the shear mechanisms of the NP GCL. In particular, the failure mode transfers from fiber pullout to fiber rupture with the increase in water content as the hydrated bentonite particles facilitate the stretching of needle-punched fibers.     

Seismic performance of small earth dams with sloping core zones and geosynthetic clay liners using full-scale shaking table tests

In Japan, a large number of old small earth dams are in critical need of repair due to leakage and poor earthquake resistance. In addition to cohesive soils, geosynthetic clay liners (GCLs) are used as impervious materials to repair such dams. This paper discusses the seismic performance of small earth dams, with reservoirs on their upstream side, repaired with a sloping core zone and a GCL on the basis of the results of full-scale shaking table tests performed at the E-Defense facility. The main focus is on the differences in mechanical behavior between the upstream and downstream sides of the dam. The results elucidate that the effective stress of the upstream embankment materials increased because of the undrained shear behavior of the compacted soils, although the deformation on the upstream side was larger than that on the downstream side. A large phase difference in the measured accelerations between the upstream slope and the downstream slope was also observed. Therefore, it is concluded that significant differences occurred in the dynamic behavior of the upstream side and the downstream side.     

高混凝土面板堆石坝地震损伤机理研究

以紫坪铺面板堆石坝为例,基于堆石料的黏弹性模型和地震残余应变模型计算分析了高混凝土面板堆石坝的地震响应,并结合震害调查结果分析了高混凝土面板堆石坝的地震损伤机理。研究表明,输入地震加速度在坝顶附近和坝坡表面显著放大,呈现出显著的鞭梢效应,导致坝顶和下游坝坡上部堆石体松动滚落。地震导致大坝堆石体产生显著剪缩,坝体断面整体向内收缩,刚性混凝土面板与垫层料之间脱空,脱空后面板与垫层料之间的摩擦力大幅减小甚至消失,面板在自重和地震惯性力联合作用下向下滑动,致使面板水平施工缝发生错台,面板表面产生裂缝。地震还导致岸坡附近左右坝段堆石体向河谷中央位移,致使岸坡附近面板垂直接缝发生拉伸破坏,河床中部垂直接缝及附近混凝土面板发生挤压破坏。数值计算和震害调查结果均表明,高混凝土面板堆石坝的地震损伤现象主要与其堆石体地震残余变过大,以及堆石体与防渗系统之间变形不协调密切相关,故强震区修建高面板坝应尽可能提高堆石体压实密度,以减小坝体的地震残余变形。     

Microcomputer-Aided Evaluation of Earthquake-Induced Permanent Slope Displacements

Ageotechnical engineering microcomputer program has been developed for the determination of permanent slope displacements resulting from earthquake shaking. The Newmark procedure, in which accelerations in excess of yield accelerations are double integrated to obtain displacements, has been incorporated into the program DISPLMT. Several user options are available for describing the yield acceleration function, including variation with time and variation with displacement. Screen graphics are available which allow the user to observe the down slope movements of the Newmark sliding block as they increase with time during the simulated earthquake. DISPLMT has been used to calculate the permanent movements of an earth dam using acceleration-time histories and soil shear stresses determined from a separate analysis. Modification to the conventional Newmark procedure, by introduction of a "slip layer," has been made in the analysis of a dam. Permanent slope displacements can be analyzed using the recommended procedure, which involves fairly simple computations that can be performed in a reasonable period of time with the exclusive use of microcomputers.     

水化状态对含针刺GCL复合衬里抗剪强度的影响分析

针刺GCL和HDPE土工膜(GM)在防渗工程中应用广泛,含多层界面的复合衬里整体抗剪强度是边坡稳定性分析的关键。介绍了含针刺GCL复合衬里的大单剪试验方法,并且对比分析了针刺GCL初始状态分别为干燥和完全水化两种情况下的复合衬里抗剪强度。结果表明,复合衬里的剪切破坏不会发生在干燥针刺GCL内部界面,而GCL干燥状态下的复合衬里单剪强度未必高于GCL完全水化状态下的复合衬里单剪强度。结合含GCL复合衬里的剪切破坏机理,阐述了针刺GCL的水化状态对复合衬里抗剪强度的影响。含GCL复合衬里在不同水化状态下的界面滑移稳定性都应引起工程人员的重视。     

Earthquake induced deformation of earth dams

The paper summarizes the different modes of failure affecting earth dams as a consequence of earthquake shaking. A case study is presented to illustrate seismic deformation analysis using the Alibey Earth Dam, Istanbul, Turkey as an example. The dynamic behaviour, failure modes and mechanisms of failure of the dam under possible ground motions were investigated. The results from finite element analysis are compared with the experimental study.      

Experimental study on seismic response of soilbags-built retaining wall

The seismic performance of soilbags-built retaining wall model was studied experimentally. A series of small-scale shaking table tests with the input of different amplitude sinusoidal waves and a large-scale shaking table test in a designed laminar shear box with the input of the Wenchuan earthquake wave were carried out on soilbags' retaining wall models. For comparison, the small-scale shaking table tests were also conducted on horizontally reinforced retaining wall models. The horizontal acceleration responses, the Fourier spectra, the dynamic earth pressure and the lateral displacements of soilbags' retaining wall models were investigated in shaking table tests. The results show that the seismic response of the soilbags' retaining wall is equivalent to or even slightly better than that of the horizontally reinforced retaining wall. The fundamental frequency and the Fourier spectral characteristics of the soilbags’ retaining wall are similar to those of backfill sands. The dynamic earth pressure of the wall model fluctuates almost synchronously with the input Wenchuan wave and no residual earth pressure is induced by the seismic loading. The permanent lateral displacements are small when subjected to multiple shakings, providing a proof that the retaining wall of soilbags has a good seismic performance.     

Investigation of mechanisms of bentonite extrusion from GCL and related effects on the shear strength of GCL/GM interfaces

Two groups of laboratory tests were carried out to investigate the effect of bentonite extrusion from a hydrated GCL on the shear strength of GCL/GM interfaces. All tests were performed with the woven geotextile side of GCL against the GM. The first group of tests were one-dimensional loading tests in which the GCL/GM specimens were subjected to hydration and vertical loading involving different sequences and loading rates. The second group of tests were large direct shear tests that studied the effect of shearing on bentonite extrusion and hence on shear strength reduction. It was found that bentonite extrusion occurs more readily from GCL/GM interfaces subjected to a swelling-loading sequence than those subjected to a loading-swelling sequence. The quantity of extruded bentonite during the normal loading showed an increasing trend with an increase in loading rate. The total mass/area of extruded bentonite during the normal loading ranged from 0 to 21.9 g/m², which was less than the quantity of bentonite extruded during the subsequent shearing (i.e., 10.7 – 81.1 g/m²). It was found that the volume of bentonite extruded at the large shear displacement caused a significant strength loss equivalent with 8° in terms of interface friction angle. The influence of bentonite extrusion on the peak shear strength showed a magnitude of 3.5° in terms of interface friction angle. The relatively insignificant bentonite extrusion during hydration and normal loading was observed to have a minor effect on the strength loss. Observations from the experimental results provide further insight into the mechanisms of bentonite extrusion.     

大地震震害与震中距或断层距的相关性

震中距作为地震动衰减关系的参数,是地震现场快速评估的重要指标。根据汶川Ms 8.0级地震中378座严重震损土石坝的坐标数据,通过数理统计,分析不同梯段断层距和震中距下土石坝震损密度分布规律。对土石坝震损比例与震中距和断层距的相关性进行分析比较,并采用最小二乘法回归得出震损数量与震中距、断层距的关系式。结果显示汶川地震中土石坝的震损密度与断层距的相关性大于震中距。认为大震中宜采用断层距为评估现场震害的指标,中小震则宜选用震中距指标。     

Features of seismic hazard in large dam projects and strong motion monitoring of large dams

Earthquakes can affect large dam projects in many different ways. Usually, design engineers are focussing on ground shaking and neglect the other aspects. The May 12, 2008 Wenchuan earthquake has damaged 1803 dams and reservoirs, and 403 hydropower plants with an installed capacity of 3.3GW. Among these dams were the 132-m-high Shapai RCC arch dam and the 156-m-high Zipingpu concrete face rockfill dam. These recently completed dams are dam types which, up to now, have not experienced strong ground shaking. The widespread mass movements have caused substantial damage to dams and surface powerhouses in Sichuan. The different features of earthquake hazard are presented, i.e., ground shaking, faulting and mass movements. It is proposed to prepare project-specific safety plans for all dams, which consist of a matrix where the possible hazards and the corresponding countermeasures are listed. The earthquake behaviors of the Sefid Rud, Zipingpu and Shapai dams, which, in the past, have experienced strong ground shaking from nearby earthquakes, are discussed. Finally, the need for strong motion instrumentation of large dams is discussed. It is proposed that major dams with large damage potential, dams located in areas of high seismicity, and dams showing signs of abnormal behavior be equipped with strong motion instruments.     

地震作用下土坝抗裂稳定性的判别

调查表明,土石坝常见的震害形式是平行坝轴线的裂缝。文中通过一系列模型坝的实验,研究了裂缝形成的条件。利用迈勒累积损伤模型和随机振动理论提出了估价土坝在地震作用下抗裂性能的计算公式,并据此研究了一座实际土坝出现地震裂缝的可能性。     

Features of seismic hazard in large dam projects and strong motion monitoring of large dams

Earthquakes can affect large dam projects in many different ways. Usually, design engineers are focussing on ground shaking and neglect the other aspects. The May 12, 2008 Wenchuan earthquake has damaged 1803 dams and reservoirs, and 403 hydropower plants with an installed capacity of 3.3GW. Among these dams were the 132-m-high Shapai RCC arch dam and the 156-m-high Zipingpu concrete face rockfill dam. These recently completed dams are dam types which, up to now, have not experienced strong ground shaking. The widespread mass movements have caused substantial damage to dams and surface powerhouses in Sichuan. The different features of earthquake hazard are presented, i.e., ground shaking, faulting and mass movements. It is proposed to prepare project-specific safety plans for all dams, which consist of a matrix where the possible hazards and the corresponding countermeasures are listed. The earthquake behaviors of the Sefid Rud, Zipingpu and Shapai dams, which, in the past, have experienced strong ground shaking from nearby earthquakes, are discussed. Finally, the need for strong motion instrumentation of large dams is discussed. It is proposed that major dams with large damage potential, dams located in areas of high seismicity, and dams showing signs of abnormal behavior be equipped with strong motion instruments.     

A comparative study between gravel and rubber drainage columns for mitigation of liquefaction hazards

Liquefaction is one of the most destructive natural hazards that cause damage to engineering structures during an earthquake. This study aims to examine the effect of rubber and gravel drainage columns on the reduction of liquefaction potential of saturated sandy soils using a shaking table. Experiments were carried out in various conditions such as construction materials, different arrangements and diameters of drainage columns. Effects of the relative density and the input motion on the base test were investigated as well. The results demonstrate that rubber drainage columns have slightly better performance compared to gravel drainage columns at high relative density and high input acceleration. Soil improvement using gravel drainage columns, which leads to reduction in liquefaction effects at moderate input acceleration and low relative density, is a more effective method than that using rubber drainage columns. By increasing the number and diameter of gravel and rubber drainage columns, deformations due to liquefaction are reduced. The drainage rate of gravel drains is higher than that of rubber drains after shaking. Totally, the outcomes indicate that densification is the most important factor controlling liquefaction.     

砾性土层液化的触发条件

天然沉积砾性土场地液化是一个超出现有认识与现有规范的新问题,其触发条件至关重要,从震害现场调查提炼出相关认识最为可靠,是后续研究的基础和导引。鉴于2008年汶川地震砾性土液化规模远超以往,以其调查结果为主,综合历史砾性土液化全部资料,提出砾性土层液化的触发条件。现有资料分析表明:0.15g应为触发天然沉积砾性土层液化的地表最低地震强度,大规模砾性土层液化发生则需要0.2g~0.4g(Ⅷ度区)的地震强度;松散和接近松散状态是天然砾性土层液化的基本条件,液化砾性土密实度可随地震强度增大而增高但仍以稍密状态为上限;液化砾性土含砾量可达85%及更大,并且不随地震强度减弱而降低;高剪切波速天然砾性土层会发生液化,砾性土与砂土密实程度的剪切波速分界线相差悬殊,砂土液化判别公式不适于砾性土层;上覆渗透性差非液化土层(帽子)的存在是地下砾性土层可发生液化的必要条件,可称为帽子效应,此厚度至少应为0.5 m;地下水位与帽子间不能有过厚的可排水层间隙也是下卧砾性土层可发生液化的必要条件,可称为间隙效应,此间隙上限可取为2.0 m;区别于砂土液化判别方法,砾性土液化判别需要埋藏条件方面的特殊要求,否则容易出现误判。     

Coquitlam大坝抗震能力提高设计(英文)

Coquitlam 大坝建于 1913 年,坝高 30 m,为吹填土石坝。大坝位于加拿大 BC 省高地震危险带。风险人口数以万计。根据现行标准,大坝的抗震能力不符合要求,坝体冲填土和某些部位地基土松散,容易液化。在中度至重度的地震情况下,大坝将会遭受严重的损坏,并有可能溃决。提高大坝的抗震能力很有必要。选择最高设计地震为里氏7.5 级地震,地震地面水平峰值加速度 0.66g。在设计中,还需要对以下一些提高抗震能力的设计方案进行评估,包括结构修复、大坝重建、限制水库运行及退役。在综合考虑技术、社会、环境和经济因素的基础上,选择了兴建一座新的下游堤坝以提高其抗震能力,减少 Coquitlam 大坝的地震风险。新建坝包括一座土质心墙堆石堤坝和一个位于左坝肩的混凝土过渡带。新建堤坝大部分是建在具有承载力的淤泥土上,而混凝土过渡带则完全建在基岩上。由于现场和施工条件的限制,座落在现有大坝下游坝壳部分的新建堤坝上游坡的下部一小部分为沿原河道的液化砂砾石冲积层。先进的地震稳定性和变形分析结果表明,由于有足够的超高、厚厚的反滤体以及过渡带,大坝所产生的变形是可以接受的。为了控制基础渗流和出逸坡降,设计采用了塑性混凝土防渗墙、帷幕灌浆以及在下游增设一套减压井相结合的方案。为了监测新建坝的性态,在施工期和运行期的仪器监测设计中采用了基于破坏模式的方法。主要介绍了 Coquitlam新建堤坝的设计,包括大坝安全监测系统的设计。     

Seismic stability of embankments subjected to pre-deformation due to foundation consolidation

It has been reported that the major cause of earthquake damage to embankments on level ground surfaces is liquefaction of foundation soil. A few case histories, however, suggest that river levees resting on non-liquefiable foundation soil have been severely damaged if the foundation soil is highly compressible, such as thick soft clay and peat deposits. A large number of such river levees were severely damaged by the 2011 off the Pacific coast of Tohoku earthquake. A detailed inspection of the dissected damaged levees revealed that the base of the levees subsided in a bowl shape due to foundation consolidation. The liquefaction of a saturated zone, formed at the embankment base, is considered the prime cause of the damage. The deformation of the levees, due to the foundation consolidation which may have resulted in a reduction in stress and the degradation of soil density, is surmised to have contributed as an underlying mechanism. In this study, a series of centrifuge tests is conducted to experimentally verify the effects of the thickness of the saturated zone in embankments and of the foundation consolidation on the seismic damage to embankments. It is found that the thickness of the saturated zone in embankments and the drainage boundary conditions of the zone have a significant effect on the deformation of the embankments during shaking. For an embankment on a soft clay deposit, horizontal tensile strain as high as 6% was observed at the zone above the embankment base and horizontal stress was approximately half that of the embankment on stiff foundation soil. Crest settlement and the deformation of the embankment during shaking were larger for the embankment subjected to deformation due to foundation consolidation.     

Effect of gravel subgrade on hydraulic performance of geosynthetic clay liner

The effect of a gravel subgrade on the hydraulic performance of GCLs is investigated. Laboratory test results show that the GCL specimens exhibit significant variation in thickness when compressed against gravel. The maximum and minimum thicknesses of the specimen were about 20 and 3 mm, respectively, after consolidation by an effective stress up to 138 kPa. However, the permittivity of GCLs remained very low. The permittivity of both needle-punched and adhesive-bonded geotextile-supported GCLs decreased with increasing confining stress, regardless of the type of subgrade materials. In general, larger particles led to more significant migration of bentonite. Nevertheless, there was no significant difference in the degree of bentonite migration between the two GCLs investigated.