September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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Experimental assessment of hydraulic load effects on
suffusion of glacial till cores
I. Silva, J. Lindblom, P. Viklander and J. Laue
Luleå University of Technology, Sweden
Keywords: internal erosion, suffusion, glacial till, hydraulic gradients.
Internal erosion by suffusion occurs in the core of an embankment dam when the ability of the soil to resist seepage forces is exceeded and voids are large enough to allow the transport of fine particles through the pores. Soils susceptible to suffusion are described as internally unstable (ICOLD, 2015). As recognized by Sherard (1979), dams with core of broadly graded glacial moraines (tills) exhibit signs of internal erosion to a larger extent than dams constructed with other types of materials.
This contribution presents a description of the laboratory program and set-up defined as part of a research on internal erosion in embankment dams currently in progress at Luleå University of Technology (LTU). The aim of the research is to determine the hydraulic gradient to initiate internal erosion by suffusion in a given moraine used as core fill.
The testing program includes three categories of till soil: i) internally stable, ii) internally unstable; and iii) soils in the transition zone between the two first categories. The categories are defined based on the soil grain size distribution and according to the methods developed by Kenney & Lau (1985, 1986), the modified Burenkova (1993) method proposed by Wan & Fell (2008) and the unified plot approach proposed by Rönnqvist & Viklander (2015).
Samples of each category are prepared at four degree of compaction defined respect to the modified Proctor test. The degrees of compaction considered are: a) 95%, representing a well compacted material based on the recommendation of the current Swedish dam safety guidelines (Svensk Energy, 2012); b) 90% representing a material on the borderline of acceptance; c) 85% representing low compacted material; and d) 80% representing poorly compacted material.
Each sample of 200 mm thickness is compacted in four layers of 50 mm in a steel permeameter with a diameter of 300 mm in diameter and a maximum height of 450 mm. Tests are performed with a downward flow after upward saturation of the samples. A layer of coarse material of 150 mm thickness is placed over the sample in order to allow a uniform distribution of the downward flow. To obtain a quicker saturation of the samples, the air content in the gaseous phase is replaced by upward incorporation of CO2(carbon dioxide) before water saturation. A filter of 50 mm thickness is located at the bottom of the sample; the grain size distribution of this layer is defined according to the criteria given in the Swedish guidelines (Svensk Energi, 2012).
The hydraulic gradient will be increased stepwise until the onset of internal instability, which can be established on the basis of three conditions: end of visual observation of outflow turbidity, the hydraulic pressure head at various depths of the sample, and the rate of water flow through the sample attained steady values (Wan & Fell, 2008). Piezometer measurements are carried out at the interface between different materials and between the layers of 50 mm that compound the specimen.
Post-test examination and diagnosis of the samples will be performed following similar criteria to those applied by Rönnqvist et al. (2017), including vertical displacement measurements and post-test grain size distribution analysis by layer. Layers are identified with suffusion if the post-test gradation curve exhibit changes in distribution compared to the initial condition, without appreciable volume change.
Results will show the effect of grain size distribution and relative degree of compaction on the internal erosion susceptibility of glacial till soils at different hydraulic gradients.
I. Silva, J. Lindblom, P. Viklander & J. Laue
Luleå University of Technology, Sweden
Experimental assessment of hydraulic load effects on suffusion of
glacial till cores
September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
27
Burenkova, V.V. (1993). Assessment of suffusion in non-cohesive and graded soils. Proceedings, the first International Conference “Geo-Filters”, Karlsruhe, Germany, 20–22 October 1992, Filters in Geotechnical and
Hydraulic Engineering, Brauns, Heibaum & Schuler (eds), Balkema, Rotterdam, 357-360.
ICOLD (2015). Bulletin 164 Internal erosion of existing dams, levees and dikes, and their foundations. Volume 1:
Internal erosion processes and engineering assessment. International Commission on Large Dams, Paris.
Preprint 19 February 2015 from: http://www.icold-cigb.org.
Kenney, T.C. and Lau, D. (1985). Internal instability of granular filters, Canadian Geotechnical Journal, National Research Council Canada, 22(2), 215-225.
Kenney, T.C. and Lau, D. (1986). Internal instability of granular filters: Reply, Canadian Geotechnical Journal, National Research Council Canada, 23(3), 420-423.
Rönnqvist, H. and Viklander P. (2015). A unified-plot approach for the assessment of internal erosion in embankment dams. International Journal of Geotechnical Engineering, 10(1):66-80.
Rönnqvist, H., Viklander P. and Knutsson S. (2017). Experimental investigation of suffusion in dam core soils of glacial till. Geotechnical Testing Journal, 40(3):426-439.
Sherard, J. L. (1979). Sinkholes in dams of coarse, broadly graded soils. Trans., 13th ICOLD Congress, International Commission on Large Dams, New Delhi, India, Vol. 2, 25-35.
Svensk Energi – Swedenergy AB (2012). Kraftföretagens riktlinjer för dammsäkerhet (RIDAS), Avsnitt 7.2 Fyllningsdammar tillämpningsvägledning, in Swedish (Hydropower companies guidelines for dam safety, Section 7.2 Embankment dams implementation guidance), Stockholm, Sweden.
Wan, C.F. and Fell, R. (2008). Assessing the potential of internal instability and suffusion in embankment dams and their foundations. Journal of Geotechnical and Environmental Engineering, ASCE, 134(3), 401-407.
