水利水电工程专业毕业设计外文翻译

附录一 外文翻译

英文原文

Assessment and Rehabilitation of Embankment Dams

Nasim Uddin, P.E., M.ASCE1

Abstract: A series of observations, studies, and analyses to be made in the

field and in the office are presented to gain a proper understanding of how an embankment dam fits into its geologic setting and how it interacts with the presence of the reservoir it impounds. It is intended to provide an introduction to the engineering challenges of assessment and rehabilitation of embankments, with particular reference to a Croton Dam embankment.

DOI: 10.1061/(ASCE)0887-3828(2002)16:4(176)

CE Database keywords: Rehabilitation; Dams, embankment; Assessment. Introduction

Many major facilities, hydraulic or otherwise, have become very old and badly deteriorated; more and more owners are coming to realize that the cost of restoring their facilities is taking up a significant fraction of their operating budgets. Rehabilitation is, therefore, becoming a major growth industry for the future. In embankment dam engineering, neither the foundation nor the fills are premanufactured to standards or codes, and their performance correspondingly is never 100% predictable. Dam engineering—in particular, that related to earth structures—has evolved on many fronts and continues to do so, particularly in the

context of the economical use of resources and the determination of acceptable levels of risk. Because of this, therefore, there remains a wide variety of opinion and practice among engineers working in the field. Many aspects of designing and constructing dams will probably always fall within that group of engineering problems for which there are no universally accepted or uniquely correct procedures.

In spite of advances in related technologies, however, it is likely that the building of embankments and therefore their maintenance, monitoring, and assessment will remain an empirical process. It is, therefore, difficult to conceive of a set of rigorous

assessment procedures for existing dams, if there are no design codes. Many agencies (the U.S. Army Corps of Engineers, USBR, Tennessee Valley Authority, FERC, etc.) have developed checklists for field inspections, for example, and suggested formats and topics for assessment reporting. However, these cannot be taken as procedures; they serve as guidelines, reminders, and examples of what to look for and report on, but they serve as no substitute for an experienced, interested, and observant engineering eye. Several key factors should be examined by the engineer in the context of the mandate agreed upon with the dam owner, and these together with relevant and appropriate computations of static and dynamic stability form the basis of the assessment. It is only sensible for an engineer to commit to the evaluation of the condition of, or the assessment of, an existing and operating dam if he/she is familiar and comfortable with the design and construction of such things and furthermore has demonstrated his/her understanding and experience.

Rehabilitation Measures

The main factors affecting the performance of an embankment dam are (1)seepage; (2)stability; and (3) freeboard. For an embankment dam, all of these

factors are interrelated. Seepage may cause erosion and piping, which may lead to instability. Instability may cause cracking, which, in turn, may cause piping and erosion failures. The measures taken to improve the stability of an existing dam against seepage and piping will depend on the location of the seepage (foundation or embankment), the seepage volume, and its criticality. Embankment slope stability is usually improved by ?attening the slopes or providing a toe berm. This slope stabilization is usually combined with drainage measures at the downstream toe. If the stability of the upstream slope under rapid drawdown conditions is of concern, then further analysis and/or monitoring of resulting pore pressures or modi?cations of reservoir operations

may eliminate or reduce these concerns. Finally, raising an earth ?ll dam is usually a relatively straightforward ?ll placement operation, especially if the extent of the raising is relatively small. The interface between the old and new ?lls must be given close attention both in design and construction to ensure the continuity of the impervious element and associated filters. Relatively new materials, such as the impervious geomembranes and reinforced earth, have been used with success in raising embankment dams. Rehabilitation of an embankment dam, however, is rarely

achieved by a single measure. Usually a combination of measures, such as the installation of a cutoff plus a pressure relief system, is used. In rehabilitation work, the effectiveness of the repairs is difficult to predict; often, a phased approach to the work is necessary, with monitoring and instrumentation evaluated as the work proceeds. In the rehabilitation of dams, the security of the existing dam must be an overriding concern. It is not uncommon for the dam to have suffered significant distress—often due to the deficiencies that the rehabilitation measures are to address.

The dam may be in poor condition at the outset and may possibly be in a

marginally stable condition. Therefore, how the rehabilitation work may change the present conditions, both during construction and in the long term, must be assessed, to ensure that it does not adversely affect the safety of the dam. In the following text, a case study is presented as an introduction to the engineering challenges of embankment rehabilitation, with particular reference to the Croton Dam Project.

Case Study

The Croton Dam Project is located on the Muskegon River in Michigan. The project is owned and operated by the Consumer Power Company. The project structures include two earth embankments, a gated spillway, and a concrete and masonry powerhouse. The earth embankments of this project were constructed of sand with concrete core walls. The embankments were built using a modified hydraulic fill method. This method consisted of dumping the sand and then sluicing the sand into the desired location. Croton Dam is classified as a ??high-hazard‘‘ dam and is in earthquake zone 1. As part of the FERC Part 12 Inspection (FERC 1993), an evaluation of the seismic stability was performed for the downstream slope of the left embankment at Croton Dam. The Croton Dam embankment was analyzed in the following manner. Soil parameters were chosen based on standard penetration (N) values and laboratory tests, and a seismic study was carried out to obtain the design earthquake. Using the chosen soil properties, a static finite-element study was conducted to evaluate the existing state of stress in the embankment. Then a one-dimensional dynamic analysis was conducted to determine the stress induced by

the design earthquake shaking. The available strength was compared with expected maximum earthquake conditions so that the stability of the embankment during and immediately after an earthquake could be evaluated. The evaluation showed that the

embankment had a strong potential to liquefy and fail during the design earthquake. The minimum soil strength required to eliminate the liquefaction potential was then determined, and a recommendation was made to strengthen the embankment soils by insitu densification.

Seismic Evaluation

Two modes of failure were considered in the analyses—namely, loss of stability and excessive deformations of the embankment. The following analyses were carried out in succession: (1) Determination of pore water pressure buildup immediately following the design earthquake; (2) estimation of strength for the loose foundation layer during and immediately following the earthquake; (3) analysis of the loss of stability for postearthquake loading where the loose sand layer in the embankment is completely liquefied; and (4) liquefaction impact analysis for the loose sand layer for which the factor of safety against liquefaction is unsatisfactory.

Liquefaction Impact Assessment

Based on the average of the corrected SPT value and cyclic stress ratio (Tokimatsu and Seed 1987), a total settlement of the 4.6 m(15 ft) thick loose embankment layer due to complete liquefaction was found to be 0.23 m (0.75 ft).

Permanent Deformation Analysis

Based on a procedure by Makdisi and Seed (1977), permanent deformation can be calculated using the yield acceleration, and the time history of the averaged induced acceleration. Since the factor of safety against flow failure immediately following the

earthquake falls well short of that required by FERC, the Newmark type

deformation analysis is unnecessary. Therefore, it can be concluded that the embankment will undergo significant permanent deformation following the earthquake, due to slope failure in excess of the liquefaction-induced settlement of 0.23 m (0.75ft).

Embankment Remediation

Based on the foregoing results, it was recommended to strengthen the embankment by in situ densification. An analysis was carried out to determine the minimum soil strength required to eliminate the liquefaction potential. The analysis was divided into three parts, as follows. First, a slope stability analysis @using the computer program PCSTABL (Purdue 1988)# of the downstream slope of the left embankment was conducted. Strength and geometric parameters were varied in order to determine the minimum residual shear strength and minimum zone of soil strengthening required for a postearthquake stability factor of safety, (FS)>1.Second, SPT corrections were made. The minimum residual shear strength correlates to a corrected/normalized penetration

resistance value (N1) of 60. From this value, a backcalculation was performed to determine the minimum field measure standard penetration resistance N values (blows per foot). Third, liquefaction potential was reevaluated based on the minimum zone of strengthening and minimum strength in order to show that if the embankment is strengthened to the minimum value, then the liquefaction potential in the downstream slope of the left embankment will, for all practical purposes, be eliminated.

Conclusion

Key factors to be considered in dam assessment and rehabilitation are the completeness of design, construction, maintenance and monitoring records, and the

experience, background, and competence of the assessing engineer. The paper presents a recently completed project to show that the economic realization of this

type of rehabilitation inevitably rests to a significant degree upon the expertise of the civil engineers.

References

Duncan, J. M., Seed, R. B., Wong, K. S., and Ozawa, U. (1984). ??FEADAM: A

computer program for finite element analysis of dams.‘‘ Geotechnical Engineering Research Rep. No. SU/GT/84-03,Dept. of Civil Engineering, Stanford Univ., Stanford, Calif.

FERC. (1993). ??Engineering guidelines for the evaluation of hydropower

projects.‘‘ 0119-2.

Makdisi, F. I., and Seed, H. B. (1977). ??A simplified procedure forestimating earthquake induced deformations in dams and embankments.‘‘ Rep. No. EERC 77-19,

Univ. of California, Berkeley, Calif.

Purdue Univ. (1988). ??PCSTABL: A computer program for slope stability analysis.‘‘

Rep., West Lafayette, Ind.

Schnabel, P. B., Lysmer, J, and Seed, H. B. (1972). ??SHAKE: A computer program

for earthquake response analysis of horizontally layered site.‘‘ Rep. No. EERC 72-12, Univ. of California, Berkeley, Calif.

Seed and Harder. (1990). ??An SPT-based analysis of cyclic pore pressure generation

and undrained residual strength.‘‘ Proc., H. Bolton Seed Memorial Symp., 2, 351–376.

Tokimatsu, K., and Seed, H. B. (1987). ??Evaluation of settlements of sands due to

earthquake shaking.‘‘ J. Geotech. Eng., 113(8), 861–878.

中文翻译

土石坝的评估和修复

摘要：在野外实地、办公室里已进行的一系列的观察，研究，分析，使本文获得了对石坝如何适应其地质环境，以及如何与水库相互影响的正确的认识。本文旨在通过对克罗顿堤坝进行的的案例分析，介绍大坝评估和修复过程中会遇到的技术难题。

引言

水利或其他工程上的许多大型设备，已经非常陈旧且磨损严重；更多的业主逐渐意识到维护设施的费用在运营成本里所占的比重越来越大。因此，未来修复产业将会蓬勃发展。在土石坝建设工程上，无论是地基还是填土质量都不能在生产前达到标准或规范，并且也不能100%预测出他们的性能表现。大坝建造工程，尤其是土质结构工程，在许多方面已经取得进步并将继续改进，特别是在节约资源和可接受风险水平的测定方面更是需要改进。因此在该领域，仍存在多种改进意见和实践方法。因为该领域没有公认的标准或唯一的施工程序，设计和建造大坝过程中可能会遇到一些工程建设上的问题。尽管相关技术有所进步，但是这些技术很大一部分是关于大坝建造的，而对其维护，监测和评估方面的技术都处在实验阶段。因此，如果没有统一的设计规范，很难制定出一套严格的对建成大坝的评估制度。许多机构（美国陆军工程兵团，田纳西流域管理局，联邦能源监管委员会等）已经开发出用于实地检测的核对表，例

如，可行的评估报告和主题。但是这些不能被当做固定程序，只能充当指导，参考，或作为需要观察，记录之处的范例。这种核对表决不能代替一个有经验的，观察力极强的工程师。在业主同意施工后，工程师应该检测几个关键因素，这些因素相关的，结合适当的静态和动态稳定性的计算结果，就形成了评估报告的基础。如果工程师熟悉并习惯于设计建造大坝，并且对该领域有足够的了解且有丰富的工程实践经验，这种评估报告则是工程师们所能提交的唯一合理的报告。

修复措施

影响堤坝性能的主要因素有：(1)渗流( 2)稳定性 （3）超高。 对于一个堤坝来说，所有这些因素都是相关联的，渗流会导致腐蚀和管道渗漏，使大坝失稳。失稳则会导致坝体开裂，反过来会导致渗漏和腐蚀。为提高大坝的稳定性，防止渗漏管涌所采取的措施取决于溢出点位置（地基还是坝体），渗流量及其临界值。加高路堤边坡稳定性通常要通过填平斜坡或是加重压脚。这种斜坡加固工程通常会结合下游坡脚的排水措施。如果担心快速水位下降情况下的上流坡面的稳定性会下降，那么深入分析或监测产生的孔隙水的压力或微调水库的操作方式会消除（对于失稳）的顾虑。最后加高土坝通常是相对简单的填充操作，尤其是加高程度相对较小的填充操作更为简单。新旧填充物的接触面必须在设计和建造时被给予足够的关注以确保防水层和相关过滤器是一个连贯的整体。相对较新的材料，如防水的土工膜和加固土已被成功运用于大坝的加高工程。然而，单靠这一解决措施，大坝修复程度收效甚微。通常，需结合多种解决措施，如安装一个带减压系统的截流器。在修复工程中，维护的效果是很难预测的。通常，在修复过程中进行阶段性的监测和仪器的评估是很必要的。在大坝修复过程中，必须高度重视建成大坝的安全问题。大坝因维护措施不完备而遭受重大损失的例子是很常见的。

在开始修复的时候，大坝或许处于非常糟糕的状况或极不稳定的条件。因此，修复工作进展的如何会改变现有的大坝情况，无论是从大坝建设期或是长远来看，得一直进行对其评估和修复。接下来的文章里，将对克罗顿大坝工程维护案例进行分析，以此来介绍大坝修复过程中可能遇到的问题。

案例研究

克罗顿大坝工程坐落于密歇根州境内的马斯基根河上。工程的经营权和管理权归消费者电力公司所有。工程结构包括两座土石坝，一座有闸溢洪道，一座以混凝土和浆砌石修建的电站。工程中的土石坝属于砂石混凝土心墙坝。土石坝的填筑采用改进的水力冲填方法。这种方法包括倾倒沙子，然后泄水将沙子冲到所需的位置。克罗顿大坝被列为一个―高度危险‖的大坝，大坝所在地震区为1区。对克尔顿坝左侧下游斜坡进行的震后稳定性评估是联邦能源监管委员会的1993年的监测项目第12部分中的一部分。按以下方式对克罗顿堤坝进行分析。土壤参数选择基于标准贯入值（N）和实验室试验数据，并对大坝进行了抗震研究以获得设计地震烈度。采用所选择的土壤特性,以静态有限元方法进行研究，来评估堤坝现有的应力状态。然后进行一维动态分析，以确定设计地震烈度引起的应力。将堤坝的现有强度与预期最大地震影响进行比较，这样就可以对堤坝在地震期间以及震后瞬时的稳定性进行评估。评估结果表明，在设计地震影响下，堤坝很有可能会发生液化和溃坝。土体的最低强度要求消除土体中潜在的液化影响，并且建议通过现场压实来提高堤坝土体的强度。

抗震评价

在分析中考虑了两种失败模式，即大坝失稳和大坝过度变形，紧接着又进行了如下分析：（1）震后瞬时的孔隙水压力测定；（2)震后松散地基表面评估；（3）震后对大坝填土中的疏松砂岩层的液化程度分析；（4）震后砂岩层液化产

生的影响。

液化影响评价

根据修正的后的标准贯入试验值的平均值和循环应力比，在总共沉降的4.6m（15英尺）松散图层中，由于液化产生的沉降为0.23m（0.75英尺）。

永久变形分析

基于Makdisi和Seed（1977）的程序，永久变形可以使用屈服加速度计算，还可以用平均感应加速度的时间历程来计算。由于针对流量损失的安全系数随地震影响而变化，且联邦能源管制委员会在这方面的规定较缺乏，因此纽马克型变形分析并不是必要的。因此，可以得出结论：在地震发生后由于液化引起的沉降超过0.23m（0.75英尺），将引起边坡的失稳，最终将导致堤坝发生显著的永久变形。

堤防整治

基于上述分析结果，建议通过现场压实的方法加固大坝。通过分析，已经测定了能消除砂砾液化可能性的最小砂砾表面张力。这项分析如下所述分为三部分。第一，进行对大坝下游左侧斜坡的稳定性测试。使用不同的强度和几何参数以确定最小剪力强度和最小的土壤加强带。第二，对标准贯入试验进行了修正。最小的残余剪切强度对应于一个规范化的贯入阻力值（N1)。根据这个值，进行反算来确定最小惯入标准值。第三，基于最小土壤加强带和最大土壤加强带的数值重新评估沙砾的液化潜能，以显示假设大坝加固到最低值，那时在坝体左侧下游坡面的潜在液化危险是否被消除。

结论

大坝评估和修复的关键在于大坝设计，建造，维护和监测记录的完整性和评估者自身的工程建设经验，教育背景和工作能力。本文通过展示一个完整的工程项目来举例说明大坝的修复评估工作很大程度上取决于工程师的专业能力。

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