September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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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, JapanT. 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 9.6×10-3cm/s × 50 = 4.8×10-3m/s.
Figure 1. Model setup Figure 2. Typical time histories of water level in flood side
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
September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
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).
