Modification of soil properties induced by suffusion: mechanical and X-ray tomography characterization

(1)

September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.

31

Modification of soil properties induced by suffusion:

mechanical and X-ray tomography characterization

C.D. Nguyen

1

_{, N. Benahmed}

1

_{, E. Andò}

2

_{, L. Sibille}

2

_{, P. Philippe}

1

_{, R. Aboul Hosn}

2

_{, L.}

Li-Hua

1

_{& A. Wautier}

1

1_{IRSTEA, Research Unit RECOVER, Aix-en-Provence, France} 2_{University Grenoble Alpes, CNRS, Grenoble, France}

Keywords: Internal erosion, suffusion, mechanical behavior, microstructure, x-ray CT.

Internal erosion is a complex phenomenon which represents one of the main sources of risk to the safety of earthen hydraulic structures such as embankment dams, dikes and levees. Its occurrence may cause instability and failure of these structures with consequences that can be dramatic. The specific situation of erosion by suffusion is the one characterized by the migration of the finest soil particles through the surrounding soil matrix formed mainly of large grains. Such phenomenon can lead to change/degradation of the initial microstructure and, hence, a change in the physical, hydraulic and mechanical characteristics of the soil.

The direct comparison of the mechanical behavior of original soil before erosion and eroded soil is often used to investigate the effect of internal erosion on soil strength (shear strength at peak/critical state). However, the experimental results presented in the literature on the mechanical consequences of internal erosion on non-cohesive soils are somehow contradictory (Chang et al. 2011, Xiao and Shwiyhat, 2012, Ke and Takahashi, 2014a, 2014b). The potential reasons might be attributed to the relatively heterogeneous erosion process and the way in which the coarse grain are rearranged afterwards, which is not taken into account, nor mentioned, in the analysis.

To further analyze the mechanical response of soils subjected to internal erosion, suffusion tests are performed on granular soil samples under different hydraulic gradients using a newly developed erosion permeameter; while high resolution x-ray computed tomography technics (x-ray CT) is used to capture the micro-structural evolution induced by the erosion process. Thereafter, triaxial compression tests are carried out on the eroded samples and “intact” ones (not eroded) having the same initial conditions in terms of density and fines content. The results obtained from triaxial tests show that the suffusion affects the mechanical response of eroded soil, mainly in terms of volumetric strains. Besides, the images from x-ray CT show that the erosion process is highly heterogeneous, with some preferential paths, leading to a heterogeneous sample in terms of distribution of fine particles, hence void ratio (Figure 1). Due to the observed microstructure heterogeneities, the influence of suffusion on the soil mechanical behavior cannot be assessed only by a direct comparison of the mechanical response of eroded and not eroded soils. A special attempt to investigate the microstructure modifications, especially the void ratio distribution (overall and intergranular) and the fine contents should be made.

C.D. Nguyen

1

_{, N. Benahmed}

1

_{, E. Andò}

2

_{, L. Sibille}

2

_{, P. Philippe}

1

_{, R. Aboul Hosn}

2

_,

L. Li-Hua

1

_{& A. Wautier}

1

1_{IRSTEA, Research Unit RECOVER, Aix-en-Provence, France} 2 _{University Grenoble Alpes, CNRS, Grenoble, France}

Modification of soil properties induced by suffusion: mechanical and

X-ray tomography characterization

(2)

September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.

32

Figure 1. Evidence of erosion process heterogeneity with preferential paths around the sample.

Chang, D.S., & Zhang, L.M. (2011). A stress-controlled erosion apparatus for studying internal erosion in soils. Geotechnical Testing Journal, 34(6), 579-589.

Ke, L., & Takahashi, A. (2014a). Experimental investigations on suffusion characteristics and its mechanical consequences on saturated cohesionless soil. Soils and Foundations, 54(4), 713-730.

Ke, L., & Takahashi, A. (2014b). Triaxial erosion test for evaluation of mechanical consequences of internal erosion. Geotechnical Testing Journal, 37(2).

Xiao, M., & Shwiyhat, N. (2012). Experimental investigation of the effects of suffusion on physical and geomechanic characteristics of sandy soils. Geotech Testing J, 35(6), 890-900.