Laboratory Tests for Backwards Piping Erosion

Laboratory Tests for Backward Erosion Piping

U.S. Army Engineer Research and Development Center, Geotechnical and Structures Laboratory, U.S.A.

Abstract. The U.S. Army Engineer Research and Development Center (ERDC) is conducting research to investigate internal erosion, specifically backward erosion piping, by use of laboratory-scale model testing to understand and properly capture the physics of the problem. Preliminary results show that lower void ratio models, using poorly-graded soils with a coefficient uniformity of 1.5, required a higher critical gradient to initiate backward erosion.

Keywords. Internal erosion, seepage, piping, backwards erosion

1. Introduction

Internal erosion is an important safety issue for both large (>15 m in height) and small (<15 m in height) dams, dikes, and levees. The U.S. Army Corps of Engineers (USACE) has identified internal erosion as a major risk driver contributing to failure modes or reduced performance of dams and levees. Internal erosion can occur in the embankment prism, in the foundation, from the embankment prism to the foundation, and around obstructions in the embankment or foundation. Foster et al. (1998; 2000) reported on a study of dam failures for large dams constructed between 1800 and 1986, concluding that internal erosion was responsible for 46% of the embankment dam failures where the mode of failure was known. They also reported that this failure mode was slightly less frequent than failure by overtopping. Additionally, they reported that most failures occurred in the embankment. There are numerous reported studies of internal erosion incidents, such as ICOLD (1995), which identifies failures and events by dam type and age and by cause of failure; Brown and Gosden (2008), a study of dams in the United Kingdom; Engemoen and Redlinger (2009) and Engemoen (2011, 2012), a detailed analysis of internal erosion incidents for the U.S. Bureau of Reclamation; and Schaefer et al. (2011) report of internal erosion incidents for USACE dams in the United States.

A recent publication comprising a view of internal erosion around the world is Bulletin 164 (ICOLD 2013). In summary, this publication states that internal erosion is a significant cause of incidents and, to a lesser extent, failure for older dams. The cause of these incidents can include poor compaction and incorrectly designed filters. The study also showed that failure initiates in concentrated leaks, by backward erosion, by contact erosion, or by suffusion when the hydraulic forces imposed by seepage exceed the capacity of the soils in the dam and its foundations to resist them. The greatest hydraulic forces occur during periods of high water level as floods pass through the reservoir. Homogeneous (unzoned) dams are particularly vulnerable because if erosion initiates, no materials capable of arresting the erosion are present. Case histories, where sufficient details are available, are presented in this bulletin. This bulletin reveals that internal erosion is a major problem in dams and levees, potentially affecting every country.

As shown in the aforementioned publications, significant levee failures have been ascribed to seepage-driven internal erosion processes. The current state of knowledge regarding internal erosion, and more specifically backwards erosion piping, is largely empirical. As a result, the engineering profession is not yet capable of predicting onset or evolution of significant internal damage in a manner that leads to quantified risks, identification and monitoring, or reliable remediation. Much of our © 2015 The authors and IOS Press.

This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License.

knowledge rests on 40 years of multi-scale testing conducted in the Netherlands (Sellmeijer 1981, 1988; Sellmeijer et al. 2011); van Beek et al. 2008, 2010; 2012a; 2012b). This body of knowledge was supplemented by work conducted at the University of Florida (Schmertmann 2000). However, the tests were focused on soil types and seepage scenarios specific to the Netherlands and Florida. Therefore, the applicability of the resulting theories to different soil types is questionable, leaving engineers to rely on personal judgment rather than objective knowledge when performing risk assessments.

Efforts are currently underway by the U.S. Army Corps of Engineers (USACE) to perform risk assessments of all dams and levees within their portfolio. The majority of these dams and levees are earthen structures. Findings from the assessments show that the major risk drivers, which contribute to failure modes or reduced performance of these earthen structures, are related to internal erosion, surface erosion from overtopping, slope instability, poorly-designed and constructed intrusions (such as pipe crossings), and other factors to a lesser degree (such as burrowing animals).

2. Test Plan 2.1. Background

As detailed in ICOLD (2013), failures and incidents by internal erosion of embankments and their foundations are categorized into three general failure modes: internal erosion through the embankment, internal erosion through the foundation, and internal erosion of the embankment into the foundation. The process of internal erosion may be broadly broken into four phases: initiation, continuation, progression, and breach (ICOLD 2013). The four mechanisms of initiation of erosion are concentrated leaks, backward erosion, contact erosion, and suffusion. The reader is referred to ICOLD (2013) for detailed discussion.

The efforts reported in this paper are focused on backward erosion piping, which mainly occurs in the foundation but may also occur within the embankment. Generally, this type of erosion occurs in non-plastic soils. A simple

schematic of this phenomenon is shown in Figure 1. The erosion process begins at a free surface on the downstream side of the embankment and progresses beneath the embankment. This requires a material suitable to form a roof or maintain an open pipe, hydraulic gradients large enough to erode particles from the foundation, and enough flow velocity to transport the eroded particles downstream.

Figure 1. Schematic of backward erosion piping (ICOLD 2013).

2.2. Test Plan Overview

The overall objective of this study is to expand the scientific knowledge available regarding the evaluation of internal erosion, specifically the progression of backwards erosion, through integrated theoretical, laboratory, field, and computational investigations relevant to USACE sites. In particular, this research focuses on laboratory experiments that include soil types

commonly encountered at USACE sites, evaluation of the appropriateness of extrapolating laboratory results to full-scale conditions, and development of capabilities of both analytical and numerical analysis used to assess seepage regimes specific to internal erosion in USACE dams and levees. Field tests and numerical investigations will also be conducted to evaluate flood-fighting technologies in terms of effectiveness at reducing internal erosion potential.

2.3. Technical Approach

The objectives of this research program will be achieved through a three-pronged approach that can be broken down into laboratory testing, numerical and theoretical research, and field activities. Laboratory testing began in 2014 and is the focus of this paper. Laboratory testing activities consist of horizontal internal erosion flume testing. Synthetic mixtures of sands and gravels will be created to test soil types (both uniform and mixtures) such that the entire particle-size range from fine sand to gravel is tested. Additionally, tests will be conducted such that a range of coefficients of uniformity between 5 and 20 are represented. The flume tests consist of horizontal flow through a soil specimen placed in a flume under a minor confining stress. The horizontal gradient required to initiate piping will be recorded. Additionally, videos of the experiments will be used to document the piping behavior that occurs throughout each test. This information will allow for comparison of controlled piping experiments to numerical results of the tests. This small scale, controlled, laboratory test program will also be used as a test bed to evaluate the potential of geophysics for monitoring internal erosion. Techniques, such as resistivity, self-potential, and seismic tomography, will be evaluated on the sample for effectiveness at identifying seepage pathways.

Additional horizontal internal erosion flume tests will be conducted to extend the materials tested to include materials of glacial origin. Glacial materials typically contain a broad range of particle sizes (from silts to boulders). Therefore, this testing will be conducted at a much larger scale to accommodate the larger

particles present. Efforts will be coordinated such that dam specific materials can be tested to answer project related questions for ongoing risk assessments at individual dams. Additionally, this testing will include flume tests conducted on materials gathered from failure sites that are investigated during the field activity phase of this research. This will allow for a direct comparison of the laboratory test results to the field conditions at sites that progressed to failure.

Additionally, laboratory tests will be conducted in phases to test a hypothesized, engineering scale theory explaining the development and progression of sand boils in confined aquifers. Laboratory testing will be structured to specifically test the influence of confining layer defect size on aquifer internal erosion, the influence of confining layer erodibility on internal erosion progression, and the influence of aquifer permeability on confining layer void enlargement. The laboratory tests will be used to validate the macro-scale and pore-scale numerical models. The results of the laboratory tests will be used in conjunction with the numerical model results to adjust the engineering scale theory until it reasonably represents the behavior expected in the field.

3. Laboratory Testing 3.1. Equipment

Several testing devices were developed to accomplish the objectives of this study. One of the developed test flumes was designed to be a close representation of the device that Schmertmann (2000) used in his study of internal erosion. The flume is shown in Figure 2. Dimensions of the flume are 2.5 m long, 0.45 m wide and 0.3 m tall. There are numerous ports along the back wall to accommodate installation of piezometers to record pressures along the full horizontal length of the constructed model and at two elevations of the model. An inflatable bladder was placed on the bottom of the container and a Plexiglas cover on the top of the container. A constructed model is shown in Figure 3. Note that a fully permeable end wall constrains the model on the outflow side of the model. Also note that the model has a sloping exit face on the outflow side

Figure 2. Internal erosion test container.

Figure 3. Internal erosion test container with model ready for testing.

A second container was also constructed to be a close representation of the device that was used by van Beek et al. (2012a; 2012b). This container is shown in Figure 4 having dimensions of 0.8 m long, 0.3 m wide, and 0.1 m tall. The device also has ports for piezometers along the length of the model and at two elevations (as can be seen in Figure 4 on the front side of the model). The container also has an end wall to constrain the model at the outflow side. Note that the model is also constructed with a small sloping face.

The containers shown in Figure 2 through Figure 4 are useful for studying horizontal flow through a soil layer. A third container has been constructed to study the three-dimensional effects of vertical flow through a confining layer. This container is shown in Figure 5. Because testing has not started in this container, this paper will not report on any results, but it is important to note that such a device is necessary to explore the three-dimensional aspects of piping that most often result in boils at the ground surface.

Figure 4. Internal erosion test container.

Figure 5. Circular erosion test container for modeling three-dimensional exit conditions.

3.2. Material

As previously mentioned, it is the intent of this study to look at the effects of internal erosion on soils that range in size from fine sand to cobble. To that end, soils have been acquired that span a broad range of grain sizes. Grain-size distributions for a selection of the tested soils are shown in Figure 6. The percent finer is recorded as the weight of each sieve used in the sieve analysis plus the weight of the soil retained in each sieve. Note that these soils are poorly graded, subrounded to rounded with a coefficient of uniformity of approximately 1.5. This is an example of one soil type used in the laboratory tests. Soils of broader gradation and varying coefficients of uniformity will also be studied. Initially, all the constructed models consist of homogeneous soil deposits. The test plan also includes studying mixtures of soils that are well graded, including the inclusion of fines into some of the models.

Figure 6. Grain-size distribution for selected soils tested in this study.

4. Preliminary Results

The testing program described above is in progress and results are beginning to emerge. Preliminary results from two test series are briefly highlighted in this paper. The first series consisted of testing on the soils shown in Figure 6 using the smaller apparatus (Figure 4). The models were all constructed by rotating the container such that the longest dimension was in the vertical direction. The soil was then dry rained into the container at two different void ratios (relatively dense and relatively loose void ratios) and saturated. The container was then rotated back into the horizontal orientation shown in Figure 4. Flow was introduced into one side of the container and initiation, continuation, and progression of the backward erosion pipe was recorded. Preliminary results are shown in Figure 7 as a plot of void ratio versus critical, horizontal gradient. It is apparent from the plot that the two groups of results centered on the relative dense void ratio and the relatively loose void ratio. It is also apparent, and expected, that the lower void ratio models required a higher critical gradient for backward erosion to occur. These results are consistent with those found by other researchers, such as Sellmeijer et al. (2012) and van Beek et al. (2012a; 2012b) in their studies. However, the tested soils discussed in this paper are different than soils tested in previous research. The final results from the ERDC tests will serve to expand the database of material that has been tested for backward erosion piping from internal seepage.

Figure 7. Results of backward erosion piping for one series of soils tested.

The second series of preliminary test results that are shown in Figure 8 consist of tests on gravel. These tests were conducted in the larger container shown in Figure 2. The models were constructed by dry raining the material into the container followed by saturation. The gravel in this series of tests had a Cu = 1.67 and a D60 =

7.79 mm and was constructed at two relative densities as with the previously discussed soil. A typical, fully developed backward erosion pipe (post-test) is shown in Figure 9.

Figure 8. Preliminary results of backward erosion piping for one series of gravel tested.

Figure 9. Backward erosion pipe in gravel.

0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1 10 Percent Finer Grain Size (mm) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.40 0.50 0.60 0.70 0.80 0.90 Critical Gradient Void Ratio

Small Flume - 40/70 Sand Small Flume - 30/50 Sand Small Flume - 20/40 Sand Small Flume - 16/30 Sand Small Flume - 12/20 Sand Small Flume - 8/12 Sand

0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,55 0,60 0,65 0,70 0,75 C rit ic al G rad ie n t

5. Conclusions

This paper highlighted a testing plan for evaluation of the internal erosion failure mode in embankments, specifically focused on backward erosion piping. A selected sample of test results from two types of soil was presented to highlight the work that is being performed. Testing results have shown that the soils tested are consistent with results reported by previous researchers (e.g. Schmertmann 2000; Sellmeijer 1981, 1988) while expanding the available information on backward erosion piping of soils not previously studied. This research will continue to study a broader range of soil types, which will be cross-checked with results from field testing of sites that have experienced internal erosion, leading to the development of new or improved methods for more accurately quantifying the risks associated with backwards erosion piping.

References

Brown, A.J. and Gosden, J.D. (2008). Defra research into internal erosion, 15th Conference British Dam Society, University of Warwick, Thomas Telford London, 39-50. Engemoen, W.O. and Redlinger, C.G. (2009). Internal

erosion incidents, Bureau of Reclamation Dams, Proc. USSD Annual Conference, Nashville, Tennessee. Engemoen, W.O. (2011). Bureau of Reclamation experiences

with internal erosion incidents, Proc. of Workshop on Internal Erosion in embankment dams and their foundations, Brno, Czech Republic, April 2011, Institute of Water Structures, FCE, BUT, Brno, 11-18. Engemoen, W.O. (2012). Determining when to remediate an

embankment for potential internal erosion concerns, Proc ICSE-6 (6th International Conference on Scour and Erosion) Paris, August 27-31, Societe Hydrotechnique de France, Paris, Paper 78, 1481-1488. Foster, M., Spannagle, M. and Fell, R. (1998). Report on the

analysis of embankment dam incidents, UNICIV Report No.R374, School of Civil and Environmental Engineering, University of New South Wales ISBN: 85841 349 3; ISSN 0077-880X.

Foster, M., Fell, R. and Spannagle, M. (2000). The statistics of embankment dam failures and accidents, Canadian Geotechnical Journal, 37(5), 1000-1024.

International Commission on Large Dams (ICOLD). (1995). Dam failures statistical analysis, Bulletin 99, Paris. International Commission on Large Dams (ICOLD). (2013).

Internal erosion of existing dams, levees and dikes, and their foundations, Bulletin 164, Paris.

Schaefer, J.A., Redlinger, C.G. and O’Leary, T.M. (2012). Recent U.S. Army Corps of Engineers dam case histories of internal erosion, Internal Erosion in Embankment Dams and their Foundations, J-J Fry, J Riha and T Julinek, editors, Proceedings of the Institute of Water Structures, FCE, BUT, Brno.

Schmertmann, J.H. (2000). The non-filter factor of safety against piping through sand, ASCE Geotechnical Special Publication No. 111, Judgment and Innovation, Edited by F. Silva and E. Kavazanjian, ASCE, Reston, VA, 65-132.

Sellmeijer, J.B. (1981). Piping due to flow towards ditches and holes, Proc. Euromech 143, Delft, 69-72.

Sellmeijer, J. B. (1988). On the mechanism of piping under impervious structures, Doctoral dissertation, Technische Universiteit Delft, the Netherlands.

Sellmeijer, J.B., Lopez De La Cruz, J., van Beek, V.M. and Knoeff, J.G. (2011). Fine tuning of the backward erosion piping model through small scale, medium scale and IJKdijk experiments, European Journal of Environmental and Civil Engineering, 15(8), 1139-1154.

van Beek, V., Koelewijn, A., Kruse, G., Sellmeijer H., and Barends, F. (2008). Piping phenomena in heterogeneous sands - experiments and simulations, International Conference on Scour and Erosion, 453–459. Tokyo, Japan.

van Beek, V.M., Knoeff, J.G., Rietdijk, J., Sellmeijer, J.B. and De La Cruz, J.L. (2010). Influence of sand and scale on the piping process — experiments and multivariate analysis, Physical Modeling in Geotechnics, 1221–1226. Delft, Netherlands.

van Beek, V.M., Knoeff J.G. and Sellmeijer, J.B. (2012a). Observations on the process of piping by under-seepage in cohesionless soils in small, medium and full scale experiments, European Journal of Environmental and Civil Engineering, 15(8), 1115-1137.

van Beek, V.M., Bezuijen A., and Schenkeveld, F.M. (2012b). Piping in loose sands – the importance of geometrical fixity of grains, Eurofuge 2012, 1–10. Delft, Netherlands.