ICOLD EUROPEAN CLUB Working Group on Uplift Pressures under Concrete Dams

The report illustrates the results of the activities carried out by the European Working Group. Four subjects were selected by the Group: "Regulatory Rules and/or Normal Practice" adopted in different Countries; "Analysis of Measurement Data", referring to recent relevant studies; "Numerical Modelling" to compute uplift pressures; "Clearing Of Drainage Systems". As far as regulatory rules are concerned, it should be noted first that their degree of detail is variable from Country to Country, but in most cases they are not very definite and rigid. In some cases (Spain) there is only an indication to take uplift pressures into consideration, without any further directive or constraint. In general the more detailed guidance relates to gravity, hollow-gravity and buttress dams. For other dam types, uplift pressures are not among the load factors to be considered or are left completely as the designer’s responsibility. The most specific rules can be found in the Italian Regulations (Technical Rules, 1982), where a linear or bi-linear distribution of uplift pressure is prescribed (for dams without or with drainage system), with headwater and tailwater pressures at the dam heel and toe, and with a maximum allowed reduction of uplift at the drainage line (this maximum allowed reduction corresponds to the reduction commonly adopted in normal practice). The uplift reduction can only be adopted if the diameter and the spacing of the drains comply with regulatory limit values. In other Regulations (Germany, Portugal) analyses by means of numerical hydraulic models are envisaged, at least for the foundations of major dams. In some cases (Spain, Portugal) an abnormal increase of uplift pressures should be evaluated, in addition to normal operating conditions. In the most recent Regulations (Spain, 1996; Portugal, 1993) the word “uplift” is replaced by the more general term “pore pressures”. In Portuguese Regulations a study of the mechanical effects of the water in terms of effective stresses is explicitly required. Referring to normal practice, flow-net analyses and the use of conventional linear/bi-linear distribution are the most commonly applied approaches. Reduction factors ranging between 0.25 and 0.6 are normally adopted to take into account drain effectiveness. 3 ANALYSIS OF UPLIFT MEASUREMENT DATA The Group recognised the value in preparing a summary of results (information, indications and, if possible, conclusions) derived from the analysis of measured uplift pressures at existing dams taking into account the effects of the main factors influencing the uplift pressure distribution. Among the influencing factors, the following were included: foundation characteristics; effects of grout curtains; drains and other methods to control and limit uplift pressures; response of uplift to headwater variations; uplift in exceptional loading conditions (flood, earthquake). It was also interesting to investigate the correlation between estimated uplift pressures using currently accepted methods and actual measured uplift pressures. The Group considered it appropriate to concentrate the analysis of this subject on gravity dams. Through information available to the members of the Group, a literature survey and personal contacts, important recent studies on this subject were identified. These studies were carried out by EDF, the Swiss National Committee of Large Dams, and EPRI (USA) who promoted two studies. Two studies (EDF, EPRI) were specifically addressed to gravity dams. The other two (Swiss National Committee, EPRI) also included other types of dams, but most of the results were still relevant to gravity dams. The Group reviewed such studies highlighting the findings considered of main interest. The common motivation behind all these studies is the acknowledgement that design assumptions about the effect of drains, grout curtains, cut-offs, and other methods of controlling and limiting uplift pressures, have never been fully validated. This becomes of particular interest in the safety reassessment of existing dams, where many questions and differences of opinion arise as to uplift assumptions. Many dams would require modification to meet updated safety standards. A better understanding of the interaction of structural features and uplift pressure distribution can contribute to avoid unnecessary modifications. 3.1 EXAMINED STUDIES 3.1.1 Study carried out by EDF (France) This study (Ref. 1), concluded in 1995, examined the uplift pressures measured in the foundation of 31 EDF gravity dams (260 measurement points, in total). The effects of the drainage systems, grout curtains, stress levels and headwater variations on uplift pressures in the foundation and at the damfoundation interface were investigated. The chronological records of the measured uplift pressures were processed by means of statistical techniques (multiple linear regressive analyses) to distinguish the components associated with different external factors (hydrostatic load, ambient temperature, time), and dimensionless/normalised coefficients were used to analyse and compare the measurement data. 3.1.2 Study carried out by the Swiss Committee of Large Dams This study (Ref. 2) was carried out by a working group of the Swiss National Committee set up in 1986, and was concluded in 1992. Approximately 70 dams (38 arch dams, 25 gravity dams, 3 arch-gravity dams, 4 buttress dams) were investigated with respect to geology and foundation treatment, examining the measurement data that were available for about 70% of the investigated dams. The study was expanded to include theoretical principles and details about measurement techniques used in Switzerland. 3.1.3 Study carried out by EPRI (USA) The Project “Uplift Pressures Under Concrete Dams” (Ref. 3) was promoted by EPRI to determine if existing records of uplift pressure readings could provide a reasonable basis for evaluating current analytical methods of estimating uplift pressure distribution, and to examine the influence of several factors on uplift pressures. Among the factors of interest were: influence of dam foundation; effectiveness and reliability of grout curtains and drainage systems; uplift pressures within the dam; effects of rapid changes in head water or tailwater levels; uplift pressures in exceptional loading conditions. The study examined a large amount of records of uplift pressure data at existing concrete dams. To this aim, a comprehensive questionnaire was prepared and sent to more than 100 organisations, in USA and foreign countries, obtaining responses from 63 of them. Foreign contacts were primarily made through the various ICOLD National Committees. Foreign responses therefore represented a number of organisations within those countries. Many agencies, companies, states, municipalities and other organisations contributed to the project. The study also had access to data and findings produced by similar research studies undertaken by the US Army Corps of Engineers and Edison Electric Institute. Altogether, data were collected for 225 dams. The Project also reviewed a considerable amount of published data regarding uplift at existing dams. Adequate information for analysis and interpretation was obtained for 148 dams (89 US dams, 59 foreign dams), of which 130 were gravity dams. Consequently, the conclusions and recommendations were limited to concrete gravity dams on rock foundations. Following the preliminary review of data for all dams, the project was split into the following phases: Classification of dams which provided good data for the aim of the study; Detailed study of the data for each dam and each parameter; Study of the interrelationships of multiple parameters; Development of trends, conclusions, recommendations. Much of the site data provided were incomplete because construction and foundation information was inadequate. As a result, the study was unable to validate assumptions about uplift with a high degree of confidence. However, it was possible to arrive at some interesting conclusions to direct the path of subsequent research. 3.1.4 Study carried out by EPRI(USA) The Project “Uplift Pressures, Shear Strengths and Tensile Strengths for Stability Analysis of Concrete Gravity Dams” (Ref. 4) was developed in 19891992, after the conclusion of the previous EPRI Project, to examine some aspects of the subject in more detail. In addition to uplift pressures, the Project also aimed to establish ranges of shear and tensile strengths and cohesion values for concrete-to-rock interfaces. As far as uplift pressures are concerned the objectives of the three-year study were the following: Evaluate geological conditions, foundation treatment, and foundation drainage with respect to their influence on uplift; Evaluate drain clearing methods; Develop a rational approach for extrapolating measured uplift to design flood levels. A comprehensive study of uplift pressures at existing gravity dams was undertaken to meet these objectives. Data from over 150 gravity dams was reviewed and 17 well-instrumented host dams were selected. The selected dams were built between 1912 and 1974 and ranged from 30 to 170 m in height. A variety of sedimentary, metamorphic, and igneous rock foundations were represented.