25th Meeting

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European Working Group on Internal Erosion

in Embankment Dams & their Foundations

Book of Abstracts

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endorsement of any product or firm by the organizing committee of the 25th Meeting of the

European Working Group. The organizing committee of the 25th Meeting of the European

Working Group does not accept any responsibility for the statements made or the opinions

expressed in this publication.

Copyright ©2017 Deltares

Book of Abstracts of the 25th meeting of the Working Group on Internal Erosion in

embankment dams and their foundations

Edited by: V.M. van Beek and A.R. Koelewijn

ISBN: 978-90-827468-0-8

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Delft , The Netherlands

25th Meeting European Working Group

on Internal Erosion in Embankment

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Dr.

André

Koelewijn

Deltares,

The

Netherlands

Mr. Jan Jaap Heerema

Rijkswaterstaat, The Netherlands

Prof. Dr. Adam Bezuijen

Ghent University, Belgium

Prof. Dr. Cristina Jommi

Delft University of Technology, The Netherlands

Prof. Dr. Stéphane Bonelli

IRSTEA, France

Prof. Dr. Adam Bezuijen

Ms. Maaike Blauw

Mr. Jan Blinde

Mr. Ulrich Förster

Mr. Jan Jaap Heerema

Prof. Dr. Cristina Jommi

Dr. André Koelewijn

Dr. Maria Konstantinou

Dr. Esther Rosenbrand

Ms. Maria Luisa Taccari

Dr. Vera van Beek

Dr. Ane Wiersma

Mr. Aron Noordam

Reviewers

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• Dams have usually been built to their present height in one stage, dikes have often been improved over time, with a present crest height considerably higher than the initial height. In case of a significant raise, the original design assumptions no longer apply and modifications to the existing structure to provide sufficient safety may be costly.

• Dams have a limited crest length, while dikes may extend for hundreds of kilometres. This poses quite different possibilities and challenges for inspection and monitoring.

• Dams are built across a stream to block it, while dikes are built along it and only guide the stream. For the design and construction of dams the flow must be sufficiently blocked, including through the foundation. Many dikes on untreated sandy foundations are vulnerable to backward erosion.

• Dams always require provisions to pass the flow, for dikes this is rare. Concentrated leak erosion along culverts and spillways often poses a threat to dams but rarely to dikes.

Each of the above points will be illustrated by practical cases, focusing on the various mechanisms of internal erosion which are dealt with first.

A.R. Koelewijn

Deltares, Delft, Netherlands

R. Bridle

Dam Safety Ltd, Great Missenden, United Kingdom

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Research needs for improved dam safety risk management of

internal erosion

R.J. Fannin

University of British Columbia, Vancouver, Canada

D.N.D. Hartford

BC Hydro, Burnaby, Canada

Keywords: Dam safety, risk management, internal erosion.

Dams play a central role in the stewardship of Canada’s hugely valuable water resources, for which storage, flood control and hydropower generation are the key national interests. There are more than 10,000 dams across the country, most of which are owned by the federal and provincial governments, electric utilities, industrial and mining companies, irrigation districts, and municipalities. British Columbia, like the provinces of Quebec, Manitoba, Newfoundland and Labrador, and also the territory of Yukon, generates almost 90 % of its energy from hydropower sources. Canada is now the world’s biggest producer of hydroelectric power, generating 350 TWh/year, or approximately 13 % of global output.

Three of the biggest embankment dams in the world, based on reservoir storage capacity, are located in Canada. They represent an enormous investment by Canadian society at-large and, like much of our public infrastructure, these northern-climate embankment dams are ageing. An effect of the ageing process, at susceptible locations, is for water seeping from the reservoir to erode, with time, fractions of soil in the embankment dam. Such internal erosion is probably the greatest, and least understood, risk of failure in embankment dams worldwide.

The industry need is to address the scientific issue of spatial and temporal influences on the occurrence of internal erosion in embankment dams, and more specifically:

• What is the relation between soil type, effective stress and seepage-flow that explains where internal erosion initiates within a zoned embankment dam?

• What explains the time-rate at which internal erosion continues, and hence determines when internal erosion progresses to constitute a significant risk to a dam?

The two research questions are defined by the classic need to develop a theoretical model, calibrate it, verify it, and then validate it against field observations. Drawing upon industry experience with dam safety risk management, and related guidelines for dam engineering practice in Canada, the USA and internationally, we outline a research framework to address the two questions.

R.J. Fannin

D.N.D. Hartford

BC Hydro, Burnaby, Canada

Research needs for improved dam safety risk management of internal

erosion

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3 Fine-tuning the evaluation of suffusion of silt-sand-gravel

soils – a comparative study of LTU and UNSW tests

H. Rönnqvist

RQV Teknik AB

Keywords: suffusion, internal erosion, glacial till, dams.

Swedish embankment dams are usually constructed with core soils of glacial till. A widely graded soil sourced from moraine deposits, till comprises many fractions, from silt and sand to gravel and stones, all crushed and mixed by the action of glaciation. Interestingly, this type of soil is remarkably similar to that in other parts of the world that were once glaciated: typically cohesionless and practically non-plastic. Statistics reveal that these core soils undergo internal erosion incidents more frequently than other soil types; however, they are less likely to fail. This indicates vulnerability to the initiation of internal erosion but resistance to its progression, suggesting a potential self-filtering ability that arrests the continuation. Almost simultaneously, Australia’s UNSW carried out GBE-suffusion tests on silt-sand-gravel soils that were similar in gradation to the glacial tills tested at Sweden’s LTU for suffusion. This paper makes a comparative assessment of these two studies, with the objective of improving and fine-tuning the existing evaluation tools for silt-sand-gravel soils in dams.

H. Rönnqvist

RQV Teknik AB

Fine-tuning the evaluation of suffusion of silt-sand-gravel soils –

a comparative study of LTU and UNSW tests

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Physical modelling of backward erosion piping in levee

foundation subjected to repeated flooding

A. Takahashi & K. Horikoshi

Tokyo Institute of Technology, Japan

T. Maruyama

East Japan Railway Company, Japan (Formerly, Tokyo Institute of Technology, Japan)

Keywords: centrifuge modelling, piping, levee foundation, repeated flooding

Backward erosion piping is one of the causes of levee damage. Breach of levee without overtopping occurred in the Yabe River during the 2012 Northern Kyushu Flood. Investigation Committee reported that this was caused by piping (Ministry of Land, Infrastructure, Transport and Tourism, Japan 2013). In the 2015 Kinugawa River Flood, it was also pointed that piping was one of factors that may have accelerated the levee breach (Ministry of Land, Infrastructure, Transport and Tourism, Japan 2016). Following the study by Koito et al. (2016), centrifuge model tests are conducted to examine the piping progression under the repeated seepage flow. An attempt is also made to estimate relationship between levee damage and number of repetitions using the linear cumulative damage hypothesis.

Model levee in the centrifuge tests is shown in Fig. 1. In the tests, only the slope on the protected side is modelled and its slope is 1V:3H. The seepage length is 200 mm and the thickness of the permeable foundation ground is 50 mm in the model scale. Seepage test is conducted in a centrifugal acceleration field of 50G. Corresponding prototype seepage length is 10 m and the thickness of the foundation ground is 2.5 m. Silica No. 8 (relative density = 30%; void ratio = 1.14; hydraulic conductivity = 9.6×10-3 _{cm/s) is used for the model foundation ground and Kaolin clay (water content =} 55%; unit mass = 13.5 kN/m3_{; unconfined compression strength, q}

u= 14 kPa; secant modulus at qu/2, E50 = 0.13 MN/m2_{) is used for the model embankment. The ground water level in the protected side is} maintained at the ground surface level. Rise of water level on the flood side is modelled by supplying water to the reservoir in the upstream side as in the tests by Horikoshi and Takahashi (2015). In Case 1, the flood water level is monotonically raised. In Cases 2, 3 & 6, the small flooding is repeated at the beginning and the flood water level is gradually increased. In Case 4, an irregular repeated seepage history is given. In Case 5, relatively large flood water level is applied repeatedly. Figure 2 shows typical time histories of the water level in the flood side. The average hydraulic gradient here is calculated by dividing the water level difference between upstream and downstream by the seepage length. If this definition is adopted, according to the scaling laws for the seepage flow in the centrifuge test, apparent hydraulic conductivity of the soil is 9.6×10-3_{cm/s × 50 = 4.8×10}-3_m/s.

0 0.1 0.2 0.3 0.4 0 50 100 150 200 0 0.1 0.2 0.3 0.4 0 50 100 150 200 250 300 Time (min) Case 2 Case 4 A ver ag e hy dr au lic g rad ien t

A. Takahashi & K. Horikoshi

Tokyo Institute of Technology, Japan

T. Maruyama

East Japan Railway Company, Japan (Formerly, Tokyo Institute of Technology, Japan)

Physical modelling of backward erosion piping in levee foundation

subjected to repeated flooding

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5

Table 1. Average hydraulic gradient at which soil ejecta from Area A is observed near slope toe,icr

Case 1 2 3 4 5 6

Ave. hydraulic gradient 0.15 0.17 0.17 0.21 0.21 0.13

(a) For marked soil ejection (b) for excessive slope settlement Figure 3. Cumulative damage curves for (a) marked soil ejection and (b) excessive slope settlement

Table 1 summarizes the average hydraulic gradient at which ejecta of the coloured sand from Area A (see Fig. 1) is observed at the notch on the slope toe in the first seepage step. Hereafter, this average hydraulic gradient is called the critical average hydraulic gradient, icr. Since the seepage flow exceeding

icrcontributes to deterioration of the levee foundation according to the preliminary analysis, icris used as

a threshold value for counting the number of effective repetitions in the following analysis.

To obtain the relationship between levee damage and the number of repetitions, linear cumulative damage hypothesis is employed. This has been used in the field of metal fatigue and has been also applied to the assessment of liquefaction potential in the geotechnical engineering. Here, two damage levels are considered. One is the marked onset of soil ejection and the other is excessive settlement of the levee slope. For the former, ejection of the coloured sand from Area B (see Fig. 1) is considered, while the average slope subsidence of s/H=2% is considered for the latter. Here, s is the average settlement of the levee slope, H is the levee height, and s/H=2% corresponds to the volume of ejecta of 40cm3_{. The peak of the average hydraulic gradient of each cycle is denoted as i}

peak hereafter. Relationships between the number of effective repetitions, Nef, and ipeak are plotted and the cumulative damage curves are constructed with iteration. To count Nef, the threshold value is considered. i.e., the

number of floods whose ipeakexceeds icris counted.

Figure 3 shows estimated cumulative damage curves for two damage levels of the levee. Although data points used are rather scattered, the cumulative damage curves can be constructed. This fact indicates that the levee damage can be roughly estimated by the linear cumulative damage hypothesis. In the case with the monotonic water level rising (Case 1), the average hydraulic gradient required to cause marked soil ejection from Area B is 0.23, while that is 0.32 for the excessive slope settlement (s/H=2%). Since there is no data for the smaller Nef for the excessive settlement (see Fig. 3(b)), the estimated

damage curve overestimate the required average hydraulic gradient for the monotonic loading case (Nef=

1). The data points for the irregular seepage pattern (Case 4) are located well above the estimated damage curves, i.e., the estimated damage curves give us conservative assessment results. This suggests that consideration of the loading order may be needed for better assessment.

Horikoshi, K. & Takahashi, A., (2015). Suffusion-induced change in spatial distribution of fine fractions in embankment subjected to seepage flow, Soils and Foundations, 55(5): 1293-1304.

Koito, N, Horikoshi, K. & Takahashi, A., (2016). Physical modelling of backward erosion piping in foundation beneath levee. Proc. 8th International Conference on Scour and Erosion, Oxford, 445-451.

Ministry of Land, Infrastructure, Transport and Tourism, Japan, (2013). Report of Investigation Committee on Yabe River levee breach in 2012 (in Japanese).

Ministry of Land, Infrastructure, Transport and Tourism, Japan, (2016). Report of Investigation Committee on Kinugawa River levee breach in 2015 (in Japanese).

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Pipe depth measurement in small-scale backward erosion

piping experiments

K. Vandenboer

Ghent University

V.M. van Beek & A. Bezuijen

Deltares, Ghent University and Deltares

Keywords: Backward erosion piping, erosion, embankments, groundwater flow.

Backward erosion piping is an important failure mechanism for water-retaining structures, a phenomenon that results in the formation of shallow pipes at the interface of a sandy or silty foundation and a cohesive cover layer. Although the pipe depth reveals a lot of information on the backward erosion process, it has never been measured systematically. In this study we used a contactless laser triangulation sensor to measure the pipe depth during and after small-scale backward erosion experiments with a circular exit for three poorly graded sands with mean grain sizes varying from 0.155 mm to 0.544 mm. The pipes prove to be extremely shallow and the pipe depth close to the pipe tip is just large enough to let a particle through. As the pipe grows, the pipe depth increases due to scour and reallocation of grains, allowing for a higher flow rate and more grains to pass. Furthermore, the pipe often consists of a shallow part in the middle and deeper parts at the outside.

K. Vandenboer

Ghent University

V.M. van Beek & A. Bezuijen

Deltares, Ghent University and Deltares

Pipe depth measurement in small-scale backward erosion piping

experiments

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7 Physical measurements of the backward erosion piping

process

B.A. Robbins

U.S. Army Engineer Research and Development Center, Vicksburg, MS, USA

V.M. van Beek

Deltares, Delft, The Netherlands

Keywords: piping, backward erosion, seepage.

A novel laboratory device is presented, in which the process of backward erosion piping is observed in cylindrical sand samples oriented horizontally. The cylindrical shape of the testing device constrained the location of the erosion path to the top of the sample, thereby allowing pore pressure measurements to be made in both the eroded pipe and the surrounding soil. Additionally, the pipe depth and width were measured. From the measurements, the local hydraulic gradient upstream of the pipe tip and the critical shear stress in the bottom of the eroded pipe were calculated. Results indicate that the local critical hydraulic gradient measured over a distance of 10 cm upstream of the pipe is not influenced by experiment scale. Further, the measurements suggest that the sediment transport in the eroded pipe can be adequately modelled using classic sediment transport theory for open channel flow.

B.A. Robbins

U.S. Army Engineer Research and Development Center, Vicksburg, MS, USA

V.M. van Beek

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Contribution for assessing filter efficiency in zoned dams

A. Benamar

Normandie Univ, UNIHAVRE, CNRS, LOMC, 76600 Le Havre, France

S. Azirou & A. Tahakourt

LGCA, Faculté de Technologie, Université de Bejaia, Algeria Keywords: erosion, filter, flow, plasticity, porosity.

Dam filters are mainly designed using filter criteria based on the grain size distribution. The main design criteria against which performance is assessed are a criterion for retention of fine particles. The characteristic size of finer fraction influences the size distribution of the filter pore and hence the retention capacity of flowing particles, and the permeability of the filter itself. This paper reports experimental results obtained on the soil-filter system behaviour subject to different hydraulic and geometrical conditions. The device used for erosion test in vertical flow conditions is quite similar to that described by Sherard et al. (1985) for No Erosion Filter (NEF) test. It is devoted to investigate the filtration of cohesive soils by granular filters with the presence of a crack. Many core soils and filters were used. The objective of this study was to determine the effectiveness of the filter to protect the silt submitted to erosion under controlled water flow. The hole erosion test (without filter) performed on lean clays is devoted to investigate the erodibility of the soil and also to provide the boundary condition at the filter inlet for further test including a downstream filter. The test was carried out using three successive pressures (25, 50 and 75 kPa) and the eroded mass was measured after each pressure step. The results show that overall applied pressures the eroded mass increases. A method based on porosity reduction and particle size distribution of base soil, for estimating the filter efficiency, is presented. The model should help in the design and the quality control during filter construction. Moreover, important results are developed for predicting the filter performance and capacity retention of fine particles. Particles transport and filtration through each granular filter were analysed as regards to filter retention capacity and particles size selection. The plasticity of base soil influences greatly the filtration since slightly plastic soils are more erodible than plastic soils. The analysis of hydraulic conductivity in the filter is of a great concern and leads to understand the filtration process. A comparison of the efficiency of the filters is assessed toward the usual required criteria and the most appropriate for the dam filters. Matching experimental results with filter design criteria reveals that many of them are conservative. The retention capacity is computed for many combinations of filter-base soil at each applied hydraulic load. The results indicate that the retention capacity for different combinations increases with applied pressure. As assessed previously, the eroded mass from base soil broadly increases with increasing pressure. The filter porosity variation was assessed and correlated with clogging particles volume. The evolution of such parameter may be an indicator of likely filter clogging. A new approach of filter clogging was proposed by evaluating a damage index which is affected by various parameters such as the ratio D15/d85 and the size of eroded particles. An approach linking the geometrical parameters (damage index) to the hydraulic conductivity leads to an estimation of the filter performance which provides an interesting and realistic criterion. The analysis of the base soils erodibility (without downstream filter) indicates that plasticity is an important parameter in the consideration of erodibility rate of the base soil. The grading analysis of retained particles provides more quantitative data about the particle size selection. The results show that slightly plastic base soil (CL2) produced the deposition of a large number of particles compared to the very plastic soil (CL1) which is less erodible but dispersive.

A. Benamar

Normandie Univ, UNIHAVRE, CNRS, LOMC, 76600 Le Havre, France

S. Azirou & A. Tahakourt

LGCA, Faculté de Technologie, Université de Bejaia, Algeria

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9 Development of a coaxial cell for porosity measurements

during contact erosion experiments

T. Bittner

School of Civil Engineering, The University of Queensland, Australia

T. Bore

1

_{, A. Scheuermann}

1

_{, M. Bajodek}

1

_{& K.J. Witt}

2

1School of Civil Engineering, The University of Queensland, Australia 2_{Bauhaus Universität Weimar, Germany}

Keywords: Contact erosion, porosity measurements, granular soils, Time Domain Reflectometry

The rearrangement of soil particles during erosion, which is basically a transient mixing of base and filter particles, is accompanied by changes in porosity, leading to overall settlements which frequently can be severe for geotechnical structures such as levees. Therefore, not only the geometric and hydraulic boundary conditions, but also porosity changes are an important parameter that has to be observed during experiments. Classical approaches, like layer-wise analysis after the test, are usually not sufficient to allow any upscaling to technical dimensions. Furthermore, numerical approaches are under development allowing the computational modelling of hydro-mechanical problems in general. A decisive parameter governing both, hydraulic processes as well as the mechanical reaction, is the porosity. Former experiments have shown that Spatial TDR (Time Domain Reflectometry) is a promising technology for real time and spatial monitoring of porosity distributions. In order to use this measuring principle, an erosion experiment has to be designed and built to meet the requirements of this technique. The erosion cell itself serves additionally as the TDR-probe. Glass beads are used as an idealisation of a granular soil in order to minimise the effects of the grain angularity and different compositions of natural soils of varying sources, and thus allowing a high rate of repeatability. The data obtained in these experiments will help to get a better understanding about the progress of the erosion process and can be used for the calibration of numerical simulations.

T. Bittner

School of Civil Engineering, The University of Queensland, Australia

T. Bore

1

_{, A. Scheuermann}

1

_{, M. Bajodek}

1

_{& K.J. Witt}

2

1_{School of Civil Engineering, The University of Queensland, Australia} 2_{Bauhaus Universität Weimar, Germany}

Development of a coaxial cell for porosity measurements during

contact erosion experiments

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Erosion behaviour of gap-graded soils due to upward flow

R. Correia dos Santos & L. Caldeira

Laboratório Nacional de Engenharia Civil

E. Maranha das Neves

Instituto Superior Técnico

Keywords: Internal erosion, suffusion, gap-graded soils, upward seepage tests, internal stability

A laboratory study aiming at the evaluation of the suffusion behaviour of coarse gap-graded soils is presented. Six granular gap-graded soils missing the medium-to-coarse sand fraction have been examined. Four soils have no fines, one has 5% of non-plastic fines, and one has 5% of clayey fines (with plasticity index of about 14%). The use of available methods to assess internal stability of soils suggests that the majority of the selected soils are susceptible to suffusion. Testing has been carried out in the Upward Flow (UF) seepage test. A cylindrical seepage cell is used to impose vertical flow, from the bottom to the top, along a soil specimen with 200 mm-diameter and 150 mm-thick. During an UF test, the hydraulic gradient in the specimen is slowly increased in steps. The observation of the erosion behaviour at the top surface of specimen, together with the evolution of the discharge flow rate, allows determining the hydraulic gradients causing initiation of erosion on top of the specimen and development of suffusion in the soil. Some tests have been conducted with a low friction sheet placed in the inner surface of the test cell, to evaluate the influence of the cell wall roughness in the soil erosion behaviour. A ‘sand boiling’ phenomenon has been observed in soils exhibiting suffusion, resulting in the deposition of the finer particles at the specimen surface. Laboratory testing on soils with no fines clearly shows that the higher the fine sand content the higher the amount of material deposited on the specimen top, but the gradients associated to initiation of suffusion and development of 'sand boiling' also increase. Whenever high hydraulic gradients are not likely to occur, the gap-graded soil with 5% of plastic fines should be more resistant to initiation and development of suffusion than the gap-graded soil with 5% of clayey fines.

R. Correia dos Santos & L. Caldeira

Laboratório Nacional de Engenharia Civil

E. Maranha das Neves

Instituto Superior Técnico

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11 Historical information and advanced tools for flood

protection and structures management

S. Aielli, S. Parodi, S. Pavan & A. Rosso

AIPo – Interregional Agency for the river Po

Keywords: levees, sand-boils, filtration, flood management, data management

The river Po is the main river of Italy, and crosses the entire North Italy from the Western Alps to the Adriatic sea. Its middle-lower course is characterized by an imposing levees’ system, which origins go back to the XVI Century, and which size has been continuously increased until nowadays. As it often occurs, when dealing with earth dykes, this system is subject to seepage phenomena, that can lead to embankments piping if not properly managed. In particular, during last flood events (years 1994 and 2000), about 130 sand boils have been detected, surveyed and controlled. The first organized list of filtration phenomena has been built in 2004 by the River Po Basin Authority (AdBPo), and published in a document named "Catasto delle arginature maestre del fiume Po" (i.e. "Real estate registry of main embankments of the river Po") After the last large flood event of November 2014, AIPo (the Interregional Agency for the River Po, i.e. the public body responsible for levee construction and maintenance), together with AdBPo, felt the necessity to update and integrate this registry, adding specific information about the hydraulic condition (flood height and duration) associated to new sand boils activation or old sand boils reactivation. This paper describes how the collection of historical data, together with most recent information, even from the 2016 flood event, allowed the creation of an information database (DB), with a simple but efficient structure ready to be updated with data from future events. Monographic data sheets containing basic information for understanding every filtration phenomena can be obtained by querying this DB. Main contents are: identification number of the filtration phenomena and/or of the surveyed sand boil; exact location of the phenomena given by geographical coordinates and support maps; time of activation (when known); critical flood height, estimated accounting for maximum flood height without activation and minimum recorded flood height with activation; any other available information (e.g. size curve of the transported material, soil characteristics, stratigraphy of the subsurface, etc.). At the conclusion of this work, the aim is to demonstrate that the proper organization of all these information, together with the interpretation of the interactions among hydraulic factors and local soil characteristics, can help the deep phenomena’s understanding, and make the DB a useful tool for levees’ hazard analysis, allowing a modern and efficient management of earth embankments and other flood protection structures.

S. Aielli, S. Parodi, S. Pavan & A. Rosso

AIPo – Interregional Agency for the river Po

Historical information and advanced tools for flood protection and

structures management

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The Coursier Dam sinkholes: a case study of backward

erosion as a consequence of filter incompatibility

R.J. Fannin

D. Roos

Aurecon, Tshwane, South Africa

Keywords: Coursier Dam, filter incompatibility, backward erosion, sinkhole.

Modern filter criteria are routinely used in engineering practice for the design of filters in embankment dams. Although somewhat well-developed, these criteria are based on results from a variety of non-standardised test devices and methods, and are rarely validated by means of field data or full-scale testing. Furthermore, very little work has been done towards understanding how filters built before the advent of modern filter design may be assessed.

To address this knowledge gap, the Continuing Erosion Filter (CEF) test and empirical criteria for the assessment of existing filters had been developed. Decommissioned in 2003, following a long history of sinkholes, piping and seepage-related incidents, Coursier Dam presents an excellent opportunity for study. CEF tests have been conducted on soils sampled at the dam site, to determine the material susceptibility to filter incompatibility.

It is concluded that the lower core from Coursier Dam is susceptible to filter incompatibility where it is in contact with a stratum of the foundation, and that this filter incompatibility may explain the occurrence of sinkholes. The finding is supported by the results of a parametric study on soil from another dam site. Furthermore, it is found that CEF testing, in conjunction with the empirical criteria for filter assessment, provides useful insights into the phenomenon of base-filter compatibility.

R.J. Fannin

D. Roos

Aurecon, Tshwane, South Africa

The Coursier Dam sinkholes: a case study of backward erosion as a

consequence of filter incompatibility

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13 Numerical simulation of the groundwater flow leading to

sand boil reactivation in the Po River

M.F. García Martínez, M. Marchi, L. Tonni & G. Gottardi

DICAM, University of Bologna, Bologna, Italy

A. Bezuijen

Department of Civil Engineering, Ghent University, Gent, Belgium; Deltares, Delft, Netherlands

A. Rosso

Agenzia Interregionale per il fiume Po, Parma, Italy

Keywords: groundwater flow, backward erosion piping, sand boil , Po River

The reliability of river embankments is essential for flood risk management. The Po River, which flows through the North of Italy, is safeguarded over half of its length by major river embankments. Performance assessment of such water-retaining structures has become a major concern following some significant flood events in the past. Among the possible initiating causes of failure, backward erosion piping turns out to be particularly threating in the middle-lower stretch of the river. In particular, the November 2014 high-water event triggered the formation or reactivation of a few important sand boils. The paperpresents a preliminary 3D finite element model of the groundwater flow through a selected cross section of the Po river, located in the Province of Reggio Emilia, which experienced a reactivation of piping phenomena after the 2014 event. The numerical model, based on a detailed geotechnical characterizationobtained from in situ tests, was calibrated on the basis of the 2014 high-water event measurements andverified for a subsequent event that took place in November 2016, though without any relevant sand boil reactivation. Results are discussed with the aim of providing some insight into the mechanism under study.

M.F. García Martínez, M. Marchi, L. Tonni & G. Gottardi

DICAM, University of Bologna, Bologna, Italy

A. Bezuijen

Department of Civil Engineering, Ghent University, Gent, Belgium; Deltares, Delft, Netherlands

A. Rosso

Agenzia Interregionale per il fiume Po, Parma, Italy

Numerical simulation of the groundwater flow leading to sand boil

reactivation in the Po River

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Experience in 3-D modeling tricks and fitting techniques in

seepage and piping prediction in levee subsoil

A. Cavagni

1

_{, S. Cremonesi}

2

_{, M.G. Tanda}

3

_{, M.D. Giliberti}

4

_{, L. La Torre}

4

_{, G. Zanichelli}

4

_&

S. Pavan

5

1_{Graduate in Civil Engineering from the University of Parma, Italy}

2_{Graduate in Environmental and Land Engineering from the University of Parma, Italy} 3_{Department of Engineering and Architecture, University of Parma, Italy}

4_{Department of Flood Risk Prevention and Mitigation-Interregional Agency of the Po River, Parma, Italy} 5_{Interregional Agency of the Po River, Subsidiary office of Ferrara, Italy.}

Keywords: 3-D numerical simulation, fitting techniques, piping, decision making, flood risk management. Levee failure due to flood-induced sand boils is one of the main causes that generate flood occurrence (Vergnani M. and Zanichelli G., 2002; Toth, 2004; TACFD 2002). It represents serious hazard for population and structures due to the difficulty in forecasting where erosion initiates.

The objective of this paper is to show how the 3-dimensional modeling can improve, in comparison with a 2-dimensional modeling, the evaluation of the failure probability (Lanubile and Tanda, 2016). That because the subsoil heterogeneity, the field measurements and the visual field observation of sand boils can be taken into account in the 3D approach.

We present two piping – sensitive subsoil case studies in Italy, along the Po river: Sacca di Colorno (Cavagni, 2017) and Caselle Landi (Cremonesi, 2017).

The analysis has been carried out using FEMWATER (Hsin-Chi, 1997) a 3-D Finite Element model, which is able to solve the saturated-unsaturated flow field both in steady and unsteady conditions. The embankments considered have been described by means of a detailed Digital Terrain Model with 1m grid. The heterogeneity of the foundation soil has been recovered on the basis of the existing investigations consisting of several boreholes and geo-electrical tomographies. These data have been processed by the graphical interface of the Groundwater Modeling System (GMS, 2017) software. The hydraulic parameters, when available, have been set at the values determined by field tests (Morreale, 2014) or at literature values (De Marsily, 1986; Van der Zee, R.A, 2011), on the basis of the geological description. The soil properties of the unsaturated zones have been represented with the Van Genuchten curves (1980) with parameter set in agreement with Carsel and Parrish (1988).

Both cases have been analyzed in unsteady conditions using the historical flood waves occurred in the Po river in 2000 and 2014; for these events records of the occurrence or not occurrence of the sand boils are available and they have been used as a check of the reliability of the soil heterogeneity identification and the hydraulic parameters (Ozkan, S., 2003).

In the Sacca di Colorno (Cavagni, 2017) case (Figure 1-a), the 3-D modeling gave new information on the plausible direction of the seepage flux that caused the formation of a big sand-boil, suggesting a new perspective for the remediation measures, since the existing hydraulic barriers, intended to prevent the sand boil formation, have been revealed as partially inefficient. In particular, the simulations pointed out that an unexpected, preferential flow direction origins from a gravel lens located downstream in the floodplain, that is saturated only for remarkable river levels.

In the Sacca di Colorno case, 2-D simulations carried out in previous studies did not notice the effect of the above mentioned gravel lens and suggested the design of hydraulic defense structures in the upstream and front side of the river. The 3-D modeling, with the overview consideration of the entire subsoil, can give information free of assumed flow direction as the ones unavoidable with the 2-D modeling.

In the Caselle Landi (Cremonesi, 2017) case (Figure 1-b), the 3-D modeling allowed a better

A. Cavagni

1

_{, S. Cremonesi}

2

_{, M.G. Tanda}

3

_{, M.D. Giliberti}

4

_{, L. La Torre}

4

_{, G. Zanichelli}

4

_&

S. Pavan

5

1_{Graduate in Civil Engineering from the University of Parma, Italy}

2_{Graduate in Environmental and Land Engineering from the University of Parma, Italy} 3_{Department of Engineering and Architecture, University of Parma, Italy}

4_{Department of Flood Risk Prevention and Mitigation-Interregional Agency of the Po River, Parma, Italy} 5_{Interregional Agency of the Po River, Subsidiary office of Ferrara, Italy.}

Experience in 3-D modeling tricks and fitting techniques in seepage and

piping prediction in levee subsoil

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15

several, dangerous sand-boils. The 2-D simulations carried out in previous studies suggested the realization of an embankment with an extension involving the entire site while the 3-D simulation allowed a location and extent of the barrier optimized with regards to economic budget and hydraulic efficiency.

(a) (b)

Figure 1. Computation grids for the (a) Sacca di Colorno and (b) Caselle Landi sites.

Finally, 3-D numerical models offer more reliable tools in flood risk management but a key role of the process is played by the detailed knowledge of the ground sediment structure and of the accurate geologic conceptual model.

Carsel R., Parrish R. (1988). Developing joint probability distributions of soil water retention characteristics, Water Resour Res.

Cavagni A. (2017). 3D modeling of the seepage and piping phenomena in a Po River embankment in the Parma province, MSc Thesis in Civil Engineering, Parma University, Italy (in Italian).

Cremonesi S. (2017). 3D modeling of the seepage and piping phenomena in a Po River embankment in the Lodi province, MSc Thesis in Environmental and Land Engineering, Parma University, Italy (in Italian).

De Marsily (1986). Quantitative Hydrogeology, Academic Press Inc., London.

GMS (2017).http://www.aquaveo.com/software/gms-groundwater-modeling-system-introduction

Hsin-Chi J.Lin, Richards D.R. Talbot C.A., Yeh G.T., Cheng J. R., Cheng H. P., Jones N. L. (1997). FEMWATER A Three-Dimensional Finite Element Computer Model for Simulating Density-Dependent Flow and Transport in Variably Saturated Media, U.S. Army Engineer Waterways Experiment Station, Vicksburg.

Lanubile R. and Tanda M. G. (2016). Analysis of the Stresses Induced by Preferential Flow Paths in River Embankments, In RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA - vol. 39, 2016. Morreale L. and Rosso A., (2014). Analisi numerica del comportamento di alcuni tratti arginali del Fiume Po nei

territori di Caselle Landi (LO) e Villaregia di Porto Viro (RO) per la risoluzione delle problematiche legate al fenomeno del sifonamento in rapporto ai livelli idrometrici del Fiume Po durante l’evento di piena dell’ottobre 2000. Interregional Agency of the Po River, Parma (in Italian).

Ozkan, S. (2003). Analytical study on flood induced seepage under river levees. Doctoral Dissertation. Department of Civil and Environmental Engineering/ Faculty of Louisiana State University.

TACFD - Technical Advisory Committee on Flood Defenses (2002). Technical report on Sand Boils (Piping). The Netherlands.

Tóth, S. (2004). Case Study on Failure Mechanism of Flood Embankments Due to Rapid Sand Boiling on Alluvial Flood Plains and the Identification of Vulnerable Levee Sections. International Conference on Case Histories in Geotechnical Engineering. 8

Van Der Zee, R.A. (2011). Influence of sand characteristics on the piping process. Research to the influence of the grain size and other characteristics on the critical head of piping, MSc thesis. Delft University of Technology. Van Genuchten, M.T. (1980). A closed-form equation for predicting the hydraulic conductivity of unsaturated

soils. Soil Sci. Soc. Am. J. 44:892-898.

Vergnani M. and Zanichelli G. (2002). I Ponti del Po e le sue Piene, Interregional Agency of the Po River, Parma (in Italian).

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Keywords: flood-control works, piping, environmental consequences, model boundaries, escalation. This paper aims to show what happens when engineering decisions are taken based on an artificial model that, though perfectly coherent within its boundaries, is constrained into an area which does not match the actual natural limits. The case study considers the escalation of flood-control works, such as levees, levee elevations and cut-off walls, built over the years, to protect the riparian town of Boretto (RE) along the Po river. It illustrates how the exposition to piping, along the Po river (Italy) instead of being solved, has been transferred constantly downstream in the past 70 years.

Extraordinary flood events, in the river Po, occurred in November 1951 and 1994 and in October 2000. This latter flood caused 23 deaths, 11 missing and 40,000 homeless people. Between 1995 and 2000 cut-off walls were driven into the river embankments to reduce seepage forces, working from the crest downwards. Between 1998 and 2001 the same stretch of levee was raised to meet the system’s standard of protecting against the 200 year flood. As shown in Figure 1, three sand boils formed after the events in 1951, 1994 and 2000 (reactivated in 2014), each one appearing downstream from the previous one, on the landside of the main earthen levee without a significant change in distance from the levee toe.

Figure 1. Sand boils appeared in 1951 (nr. 35),in 1994 (nr .36) and a third one in 2000 which reoccurred in 2014. On the right, the freeway fly-over. Photo of 2017.

Every time a levee section was lifted and a cut-off wall was embedded in the embankment, its design and effectiveness was proved by the results of 2-D numerical calculation using Flow Nets and, more recently, SEEP/W (GEO –SLOPE 2004) limited to the area of the intervention. Here we see how engineering solutions that have been chosen and implemented in such an artificially designed area, though perfectly coherent and locally effective, have just transferred the problem to the areas beyond its boundaries, leading to an escalation in man-made interventions that may not be easily nor safely governed.

Until recently, as can be seen in the Po river Levee Atlas in Figure 2, flood control measures have been merely of structural kind, aiming at improving the hydraulic capacity of rivers. The atlas shows existing levees as well as relevant information concerning these levees. It provides a baseline description of the flood protection structures along the channel banks, in addition to locating sand boils and leaks in

M.D. Giliberti

1

_{, M. Cirincione}

2

_{, L. La Torre}

1

_{& G. Zanichelli}

1

1_{Department of Flood Risk Prevention and Mitigation - Interregional Agency of the Po River, Parma, Italy} 2_{IHO Cat. A Hydrographer}

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17

Figure 2. Po river Levee Atlas showing sand boil locations: nr. 35 and nr. 36. Blue and red lines indicate cut-off walls and their geometry. The green line indicates the raised levee stretch. The orange line indicates spur levees.

In order to evaluate levee performance and fixes, the approach was collecting: 1. Historical data: past levee performance, investigations, and repairs, supplemented with interviews to personnel from the Agency. 2. Topographic data: LIDAR survey along urban levees provides some significant advantage over the only use of aerial photographic imagery, like the smooth transition to GIS-based digital terrain models (DTM.2 m grid), 3. Subsurface data: intrusive and nonintrusive investigation methods. Sources were: a. Topographic maps; b. Aerial photography, c. Geological maps; d. Soil survey reports; e. Field reconnaissance observations.

Because structural works are extremely site dependent the choice of model boundaries must be given top priority. However levee geometry and fixes, to safeguard flood-prone areas, answer always to economic, social, environmental and political demands, thus influencing an optimal choice of the above mentioned boundaries. As a result of a not always optimal approach, we see an escalation of flood protection works unable to solve the situation. Following this escalation, we can notice a “maximum adding up” constraint that is not evident when defining boundary conditions in seepage and stability analysis of the designed measures. Also, protection from detrimental effects due to flooding and piping in soil foundation cannot lie in mere adding up of structural works, where design standards are based upon probability of event occurrence, or return period. Taking upstream, downstream and environmental consequences into consideration, should be common design practice.

The emerging, though still incomplete, research suggests the key concept of “engineering threshold”. In this case, building a new cut-off wall to avoid the reoccurrence of sand boils, seems to find its threshold in another civil and environmental engineering work, with an assessed higher vulnerability or exposed value, i.e. the freeway fly-over. But the fly-over as well, answers to a new social and economic demand. The new cut-off wall, though fit for purpose, could shift further downstream the sand boil under the fly-over, adding another step to the escalation.

Autorità di Bacino del Fiume Po. (2004) Aggiornamento Catasto arginature maestre del fiume Po da foce Tanaro

all’incile del Po di Goro.

Bocci, S. (2014). Studio delle sollecitazioni indotte da un percorso di filtrazione preferenziale in un argine

fluviale. MSc Thesis in Civil Engineer, Parma University, Italy.

Ozkan, S. (2003). Analytical study on flood induced seepage under river levees. Doctoral Dissertation. Department of Civil and Environmental Engineering/ Faculty of Louisiana State University.

Rockström J., Sachs J.D. (2013). Sustainable Development and Planetary Boundaries, Background paper for the High-Level Panel of Eminent Persons on the Post-2015 Development Agenda United Nations.

Tagliavini S. (2004). Lavori rialzo e ringrosso argine maestro dx fiume Po da Brescello a Guastalla. AIPO Modena

Technical Advisory Committee on Flood Defenses (2002), Technical report on Sand Boils (Piping). The Netherlands.

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Tóth, S. (2004). Case Study on Failure Mechanism of Flood Embankments Due to Rapid Sand Boiling on Alluvial Flood Plains and the Identification of Vulnerable Levee Sections. International Conference on Case Histories in Geotechnical Engineering. 8.

Van Der Zee, R.A. (2011). Influence of sand characteristics on the piping process - research to the influence of the grain size and other characteristics on the critical head of piping. MSc thesis. Delft University of Technology.

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19 The influence of the leakage length on the initiation of

backward erosion piping

A. Bezuijen

Ghent University, Ghent, Belgium / Deltares, Delft, the Netherlands

Keywords: Backward erosion piping, leakage length, example calculation

The earliest relation describing the risk of backward erosion piping, from Bligh in 1915, calculates an overall gradient over the dike. This gradient should be lower than a certain value depending on the material of the dike foundation for the dike to be safe. Later developments rationalize this number, showing the influence of grain size, permeability, density and added the influence of the thickness of the aquifer. However, the principle remains the same. The actual overall gradient is compared to a critical gradient.

This contribution will show, using analytical groundwater flow calculations that for the beginning of backward erosion piping, not only the overall gradient but also the leakage length of the semi-confined aquifer, usually present at the landward side of the dike, is of importance. This leakage length determines the piezometric head on the landward side of the dike. This is not very new, since this is also used in the ‘blanket theory’ developed and used in the USA, but a slightly different approach results in some new conclusions.

It will also be shown that the influence of the leakage length on the pipe progression and in ultimo on the breach of the dike is less. This means that with a long leakage length the difference in overall head between pipe initiation and breakthrough is larger than for the situation with a shorter leakage length.

A. Bezuijen

Ghent University, Ghent, Belgium / Deltares, Delft, the Netherlands

The influence of the leakage length on the initiation of backward

erosion piping

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erosion

F. Froiio

1

1_{École Centrale de Lyon; 36 Avenue Guy de Collongue, Lyon, France}

C. Callari

2

_{, A.F. Rotunno}

1,3

_{& A. Guidobaldi}

3 2_{University of Molise; Via De Sanctis, Campobasso, Italy}

3_{University of Rome “Tor Vergata”; Via del Politecnico 1, Rome, Italy}

Keywords: levees, backward erosion piping, Discrete Element Method, Lattice Boltzmann Method. As a major threaten for the safety of embankment dams and dykes, piping erosion is receiving increasing attention by the geomechanics community and different modeling approaches have been proposed in the last decade (Rotunno et al., 2017, Bonelli et al., 2008).

The process is driven by two main erosion mechanisms: the upstream-oriented propagation of the erosion pipe and its radial enlargement (van Beek, 2015). We have recently proposed a discrete numerical model of the soil-pipe interface at the front region (Tran et al., 2016). The model was developed with an in-house 2D code based on the Discrete Element Method (DEM) coupled with the Lattice Boltzmann Method (LBM), for the description of the granular- and fluid phase, respectively (cf Lominé et al., 2013). At a larger scale of observation, we present herein a model of the whole front region, and report on the results of an extensive parametric study. Conclusions are drawn with regards to the kinetics of the backward erosion process and on the main resistance and degradation mechanisms driving the upstream propagation of the front (arching and damage). The numerical method is also discussed, along with the technical solutions enabling an effective implementation of the hydro-mechanical coupling at the grain scale.

Acknowledgements

Research supported by GIS VOR 2012, LTDS 2012 and PRIN 2010 - 2011 (2010BFXRHS 004) projects as well as by one PhD fellowship (MIUR). The research also benefited from several invitations of C. Callari at the École Centrale de Lyon - LTDS and of a VINCI mobility program (Italian-French University).

Bonelli, S. and Brivois, O. (2008). The scaling law in the hole erosion test with a constant pressure drop.

International Journal of Numerical and Analitycal Methods in Geomechanics 32:1573-1595.

Lominé F., Scholtès L., Sibille L., and Poullain P. (2013). Modeling of fluid-solid interaction in granular media with coupled lattice Boltzmann/discrete element methods: Application to piping erosion. International Journal

of Numerical and Analitycal Methods in Geomechanics 37:577-596.

Rotunno, A.F., Callari, C. and Froiio, F. (2017). A computational method for backward erosion piping. Models,

Simulations and Experimental Issues in Structural Mechanics 225-234.

Tran D.K., Prime N, Froiio F., Callari C., and Vincens E.. (2016). Numerical modeling of backward front propagation in piping erosion by DEM-LBM coupling. European Journal of Environmental and Civil

Engineering 1-28.

van Beek V.M., (2015). Backward erosion piping, initiation and progression. Phd-Thesis. Delft University of technology.

F. Froiio

1

1_{École Centrale de Lyon; 36 Avenue Guy de Collongue, Lyon, France}

C. Callari

2

_{, A.F. Rotunno}

1,3

_{& A. Guidobaldi}

3

2_{University of Molise; Via De Sanctis, Campobasso, Italy}

3_{University of Rome “Tor Vergata”; Via del Politecnico}1_{, Rome, Italy}

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21 Numerical modeling of small-scale experiments for a coarse

sand barrier as a measure against backward erosion piping

E. Rosenbrand

Deltares

V.M. van Beek

1

_{, J.M. van Esch}

1

_{, A. Noordam}

1

_{, F. Pederzani}

3

_{, K. Vandenboer}

2

_{& A.}

Bezuijen

1,2 1_Deltares

2_{Universiteit Gent,} 3_{Politecnico di Milano}

Keywords: backward erosion piping, measure, modelling, experiments.

Backward erosion piping can occur when an unfiltered exit is present on the downstream side of a levee and seepage forces are sufficient to transport sand grains. A pipe forms that progresses upstream, backwards, below the levee. When the pipe makes contact with the water on the upstream side of the levee, the flow rate in the pipe increases and progressive erosion of the cavity below the levee can cause failure of the embankment. Backward erosion piping has been studied extensively both numerically and by means of laboratory experiments (e.g. Sellmeijer (1988), Weijers and Sellmeijer (1993), van Beek et al (2014), Vandenboer et al. (2013), Robbins and van Beek (2015), Robbins et al. (2016), van Beek (2015).

One method to prevent the pipe from growing upstream is to place a barrier of coarse sand below the levee in the course of the path. The larger grains have a higher resistance to erosion, and furthermore the hydraulic gradient within the coarse barrier will be lower due to its higher permeability. Both factors contribute to the resistance against backward erosion piping. Van Beek (2016) observed that even in a relatively homogeneous sand, the presence of a slightly coarser layer band can already cause the pipe to deflect and develop parallel to the coarser layer. This was further studied in small-scale laboratory experiments by Negrinelli et al. (2016) who added a coarse sand layer to a uniform sand body. They found that a significant increase in the head drop is required for the pipe to progress across the barrier.

For application as a piping measure, the resistance of the coarse sand barrier against piping progression must be known. In practice, we wish to determine the maximum head drop over the levee which can be retained by the barrier. Currently Deltares is conducting a research programme to determine this maximum resistance of a coarse sand barrier as a piping measure for a pilot location in the Netherlands. The local gradient at the interface between the barrier and the pipe is considered as the critical issue that determines the strength of the barrier.

Small-scale laboratory experiments were conducted to determine the overall head drop beyond which the pipe progresses through the barrier, and measure the head distribution at 15 points in the model during the experiments. In order to relate the head drop across the setup to the local head gradient in the barrier, or conversely to relate the critical gradient in the barrier to the maximum head drop in a field situation, numerical modelling of groundwater flow distribution is used. This contribution concerns the analysis of the small scale experiments, and the modelling of these experiments by using a finite element groundwater simulation programme.

In the tests it was observed that for a given barrier sand, the local horizontal gradient in the barrier at the point that the pipe progresses into the barrier is similar, regardless of the downstream sand, or the depth of the barrier in the model. However with a lower relative density of the barrier, the pipe progresses into the barrier at a lower local gradient. This suggests that a strength criterion of the barrier may indeed be derived based on this local gradient, and that this criterion can be used to predict the

E. Rosenbrand

Deltares

V.M. van Beek

1

_{, J.M. van Esch}

1

_{, A. Noordam}

1

_{, F. Pederzani}

3

_{, K. Vandenboer}

2

_&

A. Bezuijen

1,2

1_Deltares

2_{Universiteit Gent,} 3_{Politecnico di Milano}

Numerical modeling of small-scale experiments for a coarse sand

barrier as a measure against backward erosion piping

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overall critical gradient for different geometries and background sands by using numerical groundwater modelling.

To model the results, it was necessary to introduce a low permeability layer on the upstream interface of the barrier with the fine sand. This suggests that possibly some fine material formed a filter cake during the experiments, as a high flow rate was applied. The upstream fine sand did not appear to be transported through the barrier however. The effect on the permeability was strongest for the experiment with a low relative density, possibly because there was more space for the background sand grains to penetrate. Although this provides an extra strength in the laboratory tests, the question is whether this may be counted on in field situations.

Sellmeijer, J.B. (1988) On the mechanism of piping under impervious structures.TU Delft, Delft. Weijers, J.B.A. and Sellmeijer, J. B. (1993) A new model to deal with the piping mechanism.

Beek, V.M. van, Bezuijen, A., Sellmeijer, J.B. and Barends, F.B.J. (2014) Initiation of backward erosion piping in uniform sands. Géotechnique 927–941.

Vandenboer, K., Beek, V.M. van, Bezuijen, A., (2013) 3D FEM Simulation of Groundwater Flow During Backward Erosion Piping. Fifth Int. Young Geotech. Eng. Conf., 5–8.

Robbins B.A., and Beek, V.M. van, (2015) Backward Erosion Piping : A Historical Review and Discussion of Influential Factors ASDSO Dam Saf., 1–20.

Robbins, B.A. , Montalvo-Bartolomei, A., López-Soto, J., Stephens, I.J., (2016) Laboratory measurements of critical gradients of cohesionless soils,” USSD 2016 Annu. Conf., 1–11.

Beek, V.M. van, (2015) Backward Erosion Piping Initiation and Progression. TU Delft, Delft.

Negrinelli, G., Beek, V.M. van, Ranzi, R., (2016). Experimental and numerical investigation of backward erosion piping in heterogeneous sands,” in International conference on scour and erosion, 2016.

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23 Evaluation of Dutch backward erosion piping models and a

future perspective

V.M. van Beek & G.J.C.M. Hoffmans

Keywords: Backward erosion piping, Shields-Darcy model, Sellmeijer model, groundwater flow, incipient motion.

The prediction of backward erosion piping is important for safety assessment of dikes in the Netherlands, where subsurface conditions are prone to this erosion mechanism. In the current assessment methodology, the adapted Sellmeijer rule is in use. In combination with the national safety philosophy and uncertainty in input parameters, this model results in high failure probabilities. This paper evaluates the Sellmeijer model and the recently developed Shields-Darcy model alongside recent developments in research on modelling of backward erosion piping, leading to a future perspective.

V.M. van Beek & G.J.C.M. Hoffmans

Evaluation of Dutch backward erosion piping models and a future

perspective

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Understanding Piping using Pore Pressure Observations

C. Bocovich

1

1_{Colorado School of Mines}

W. Kanning

2

_{, M. Parekh}

3

_{& M. Mooney}

1

2_Deltares

3_{United States Army Corps of Engineers}

Keywords: Internal Erosion, Piping, Inversion, Data Driven Modeling

This presentation will focus on the use of densely spaced pore pressure observations to better understand the spatial and temporal progression of backwards erosion piping (piping). Pore pressure observations were collected during the IJkdijk 2009 experiments, conducted in the Netherlands to better understand piping as a failure mechanism and the monitoring of piping progression (Flood Control IJkdijk 2015). Patterns in pore pressure response during piping progression will first be presented by comparing the observed pore pressures to the upstream water level and to expected pore pressures. The changes in pore pressure distribution caused by spatial changes in hydraulic conductivity will then be presented followed by the introduction of inversion to back calculate changes in hydraulic conductivity from spatial pore pressure distributions. The inversion will be used on the IJkdijk 2009 spatial pore pressure observations at continuous snapshots in time. This will estimate the spatial and temporal progression of piping.

During the IJkdijk 2009 experiments, full scale embankments were designed and built to better understand backwards erosion piping progression. In the experiment, a sandy aquifer was built under a clay embankment. While the downstream water level was maintained constant, the upstream water level was incrementally increased. Please refer to the following sources for more information on the IJkdijk 2009 experiments (van Beek et al. 2010, Sellmeijer et al. 2011, van Beek 2015, Flood Control IJkdijk 2015, Parekh et al. 2016). Parekh et al. 2016 describes the temporal changes in pore pressure as piping progresses, noting that there is a point at which the pore pressure rapidly decreases followed by point at which the pore pressure stabilizes or starts to increase. Temporal changes in pore pressure are compared to expected pore pressures along a cross section in Figure 1 below. These changes in pore pressure are also observed spatially during snapshots in time. It is noted that a large decrease in pore pressure is surrounded by smaller decreases, implying that the pore pressure is affected locally and remotely of the pipe. Finite element modeling is used to confirm that pore pressure changes occur both local and remote of the pipe (Bocovich et al., 2017).

Figure 1. Observed and expected temporal changes in pore pressure from IJkdijk 2009 experiment

C. Bocovich

1

1_{Colorado School of Mines}

W. Kanning

2

_{, M. Parekh}

3

_{& M. Mooney}

1

2_Deltares

3_{United States Army Corps of Engineers}

25th Meeting

European Working Group on Internal Erosion

in Embankment Dams & their Foundations

Book of Abstracts

endorsement of any product or firm by the organizing committee of the 25th Meeting of the

European Working Group. The organizing committee of the 25th Meeting of the European

Working Group does not accept any responsibility for the statements made or the opinions

expressed in this publication.

Copyright ©2017 Deltares

Book of Abstracts of the 25th meeting of the Working Group on Internal Erosion in

embankment dams and their foundations

Edited by: V.M. van Beek and A.R. Koelewijn

ISBN: 978-90-827468-0-8

Delft , The Netherlands

25th Meeting European Working Group

on Internal Erosion in Embankment

Dr.

André

Koelewijn

Deltares,

The

Netherlands

Mr. Jan Jaap Heerema

Rijkswaterstaat, The Netherlands

Prof. Dr. Adam Bezuijen

Ghent University, Belgium

Prof. Dr. Cristina Jommi

Delft University of Technology, The Netherlands

Prof. Dr. Stéphane Bonelli

IRSTEA, France

Prof. Dr. Adam Bezuijen

Ms. Maaike Blauw

Mr. Jan Blinde

Mr. Ulrich Förster

Mr. Jan Jaap Heerema

Prof. Dr. Cristina Jommi

Dr. André Koelewijn

Dr. Maria Konstantinou

Dr. Esther Rosenbrand

Ms. Maria Luisa Taccari

Dr. Vera van Beek

Dr. Ane Wiersma

Mr. Aron Noordam

Reviewers

Contents

1

Internal erosion in dams and dikes: a comparison

A.R. Koelewijn

R. Bridle

A.R. Koelewijn

R. Bridle

Research needs for improved dam safety risk management of

internal erosion

R.J. Fannin

D.N.D. Hartford

R.J. Fannin

D.N.D. Hartford

Research needs for improved dam safety risk management of internal

erosion

3

Fine-tuning the evaluation of suffusion of silt-sand-gravel

soils – a comparative study of LTU and UNSW tests

H. Rönnqvist

H. Rönnqvist

Fine-tuning the evaluation of suffusion of silt-sand-gravel soils –

a comparative study of LTU and UNSW tests

Physical modelling of backward erosion piping in levee

foundation subjected to repeated flooding

A. Takahashi & K. Horikoshi

T. Maruyama

A. Takahashi & K. Horikoshi

T. Maruyama

Physical modelling of backward erosion piping in levee foundation

subjected to repeated flooding

5

Pipe depth measurement in small-scale backward erosion

piping experiments

K. Vandenboer

V.M. van Beek & A. Bezuijen

K. Vandenboer

_{, A. Scheuermann}

_{, M. Bajodek}

_{& K.J. Witt}

_{, A. Scheuermann}

_{, M. Bajodek}

_{& K.J. Witt}

_{, S. Cremonesi}

_{, M.G. Tanda}

_{, M.D. Giliberti}

_{, L. La Torre}

_{, G. Zanichelli}

_&