Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited Talk-11) CONTROLLING INTERNAL EROSION IN EARTH DAMS AND THEIR FOUNDATIONS: CASE STUDIES A. Soroush, Associate Professor, Amirkabir University of Technology, President of Iranian Geotechnical Society (IGS) P. T. Shourijeh, Formerly PhD Student, Amirkabir University of Technology, Member of Iranian Geotechnical Society (IGS) A. Mohammadinia, Formerly MSc Student, Amirkabir University of Technology, Tehran, Iran ABSTRACT: This treatment reviews case histories of a handful of embankment dams recently completed or under construction in Iran. Special attention is devoted to characteristics of core materials, in relation to internal erosion, and the substantiation of filters through NEF (No Erosion Filter) tests. Filter design and proportioning for fine-grained low-plasticity soils (viz. CL, CL-ML, and ML) is critically elaborated and exemplary comparison with contemporary filter design criteria is provided. Guides concerning the NEF testing procedure are also recommended. INTRODUCTION Internal erosion and piping present serious risks to the stability of embankment dams. An excellent review of dam incidents up to 1986 by Foster et al. [1] revealed that 48% of earth and rockfill dam failures were caused by piping and internal erosion. Contemporary researches have indicated that even in modern zoned dams internal erosion and piping are still major threats of damage that may eventuate to dam failure [2]. The sequences of internal erosion through a zoned embankment are clearly described by Fell et al. [3]. The process of internal erosion can be broken into four phases; initiation of erosion, continuation of erosion, progression of erosion and formation of a breach. While initiation of erosion to some extent depends on characteristics of the core, filters act as barriers to stop continuation of erosion. If the filter fails, erosion will progress and may lead to breaching. The importance of filters in dam safety along with the usually high costs of filter production makes filter design and substantiation a contentious issue. Heretofore, numerous filter design criteria have been proposed, from which a few are more accepted and implemented. Filters in modern dams generally respect the criteria presented in Table 1 that were proposed by Sherard and Dunnigan [4]. Based on analysis of an extensive NEF test database, Shourijeh and Soroush [5] suggested minor modifications to Sherard and Dunnigan [4] criteria (cf. Table 1). Although criteria of Table 1 have lead to proven performances, filter testing still provides the most confident and reliable method for selection of filters [6 & 7]. The No Erosion Filter (NEF) test is recognized as a competent filter test especially for fine grained soils. Many researchers have repeated NEF testing to substantiate appropriate filters and to assess filter criteria credibility [8, 9, 10, 11 & 12]. Fine-grained low-plasticity silty and silty-sandy soils, such as CL, CL-ML, ML and SC, are considered as competent core materials given that they satisfy permeability requirements of central sealing (i.e. core) elements. The use of these materials has been reported in numerous cases [13, 14 & 15]. However, from the viewpoint of internal erosion these soils are disadvantageous, as they have feeble erosion resistance [16, 17 & 18]. In analysis of dam incidents, Foster et al. [1] noticed that 34% of cores which had experienced erosion damages or failures were consisted of CL soils and 18% were ML soils. For such soils, special attention should be devoted to internal erosion; propensity of core cracking should be minimized and proper critical filters should be executed. Besides, cohesionless soils impose practical difficulties during dam construction, especially in core compaction [19]. Many embankment dams have been completed or currently are under construction in Iran, and fortunately, aspects of modern dam engineering are considered in their design features. Specifically speaking, great emphasis is placed on designing appropriate critical filters to prevent internal erosion. Filters are strictly delimited according to criteria and in most cases confirmed/delineated via NEF tests. In the last decade extensive experimental investigations have focused on internal erosion and piping in Amirkabir University of Technology. These efforts have served as a platform for both state of the art researches in geotechnical engineering, and professional consultancy to numerous dam projects. This paper deals mainly with some of the authors’ experiences regarding filter design for low plasticity core soils. NEF TESTING Since NEF testing has played a pivotal role in the selection of filter materials for the dam case histories, a complete yet brief description of NEF testing practiced by the authors is presented. Experiences by Soroush et al. [20], and Soroush and Shourijeh [21] resulted in semi-standard procedures for NEF testing. For NEF tests herein, the main container of the apparatus, illustrated in Figure 1, is a Plexiglas cylinder with internal diameter of 11 cm and height of 30 cm. Hence, specimen fabrication and testing procedure is as follows: 64
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Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited Talk-11)
CONTROLLING INTERNAL EROSION IN EARTH DAMS AND THEIR FOUNDATIONS:
CASE STUDIES
A. Soroush, Associate Professor, Amirkabir University of Technology, President of Iranian Geotechnical Society (IGS)
P. T. Shourijeh, Formerly PhD Student, Amirkabir University of Technology, Member of Iranian Geotechnical Society (IGS)
A. Mohammadinia, Formerly MSc Student, Amirkabir University of Technology, Tehran, Iran
ABSTRACT: This treatment reviews case histories of a handful of embankment dams recently completed or under
construction in Iran. Special attention is devoted to characteristics of core materials, in relation to internal erosion, and the
substantiation of filters through NEF (No Erosion Filter) tests. Filter design and proportioning for fine-grained low-plasticity
soils (viz. CL, CL-ML, and ML) is critically elaborated and exemplary comparison with contemporary filter design criteria is
provided. Guides concerning the NEF testing procedure are also recommended.
INTRODUCTION
Internal erosion and piping present serious risks to the
stability of embankment dams. An excellent review of dam
incidents up to 1986 by Foster et al. [1] revealed that 48% of
earth and rockfill dam failures were caused by piping and
internal erosion. Contemporary researches have indicated that
even in modern zoned dams internal erosion and piping are
still major threats of damage that may eventuate to dam
failure [2].
The sequences of internal erosion through a zoned
embankment are clearly described by Fell et al. [3]. The
process of internal erosion can be broken into four phases;
initiation of erosion, continuation of erosion, progression of
erosion and formation of a breach. While initiation of erosion
to some extent depends on characteristics of the core, filters
act as barriers to stop continuation of erosion. If the filter
fails, erosion will progress and may lead to breaching. The
importance of filters in dam safety along with the usually
high costs of filter production makes filter design and
substantiation a contentious issue.
Heretofore, numerous filter design criteria have been
proposed, from which a few are more accepted and
implemented. Filters in modern dams generally respect the
criteria presented in Table 1 that were proposed by Sherard
and Dunnigan [4]. Based on analysis of an extensive NEF test
database, Shourijeh and Soroush [5] suggested minor
modifications to Sherard and Dunnigan [4] criteria (cf. Table
1). Although criteria of Table 1 have lead to proven
performances, filter testing still provides the most confident
and reliable method for selection of filters [6 & 7]. The No
Erosion Filter (NEF) test is recognized as a competent filter
test especially for fine grained soils. Many researchers have
repeated NEF testing to substantiate appropriate filters and to