**2. Previous studies**

Many irrigation dams were not designed for earthquake resistance, and the cracks at the crest in the direction of the dam axis are remarkable. **Figure 1** shows the example of the crack at the crest of the Earth-fill dam. The relatively big open crack occurred. This type of open crack can be seen in the damaged Earth-fill dam so often. Not only the old Earth-fill dam but also the recent rock-fill dam has a crack in the dam-axis direction when the big earthquake hit. **Figure 2** shows the crack situation at the crest of the rock-fill dam constructed in 1988. This dam was designed for earthquake resistance. Although the acceleration was not measured, it was inferred that relatively big acceleration occurred because the earthquake inducing the crack was in 2011 off the Pacific coast of Tohoku. The open crack was propagated at a 3 m depth from the crest. The cracks shown in the figures cannot be seen to be induced by shear failure. The earthquake resistance is examined for shear failure with the slip-circle method according to the standard. It is difficult to

**Figure 2.** Crack at the rock-fill dam.

**Figure 1.** Crack at the Earth-fill dam.

24 Dam Engineering

The 1-G shaking table tests were carried out to investigate the dam and embankment behavior, and it was found that the acceleration response at the upper part of the embankment has a large effect on the behavior of the slope [3]. The crack in the dam-axis direction was also examined and the crack was considered to be caused by tensile stress. The tensile stress was affected by the vertical vibration as well as horizontal one [4]. Like others, the relation between the natural period and failure feature was investigated [5]. The effect of the aspect ratio of the dam on the vibration mode was also examined [6].

For the centrifugal loading tests, the acceleration response and residual deformation were investigated for the Earth-core rock-fill dam and concrete-faced rock-fill dam [7]. The effect of the liquefaction of foundation on the deformation of the dam was examined [8]. Moreover, the seismic response and liquefaction of loose embankments were also investigated [9].

As mentioned earlier, the previous studies focused on the seismic response, slope sliding and deformation and so the situation of the surface of the dam was focused. On the other hand, the authors investigated the behavior of the cross section by 1-G shaking table test [2] as mentioned earlier. **Figure 3** is the strain distribution of the model used for 1-G shaking table test. By using

**Figure 3.** Strain distribution by 1-G shaking test [2]: (a) shear strain and (b) volumetric strain.

image analysis, the behavior of cross section was observed. It was found that the share stain became large at the slope, and the large volumetric strain was observed at the upper part of the dam. In this study, a similar image analysis is used for the centrifugal loading test.

No. 7 silica sand and kaolin clay with the mixture ratio of 8:2 by dry weight. **Figure 5** shows the particle distribution of the mixture. The water content of the mixture was 13%. In order to evaluate the seismic behavior of the model, the gauge points were placed on the surface of the embankment model and the reference points were set at the foundation. The total number of the points is 82. To reduce the friction between embankment model and wall of the soil box, the silicone grease was painted on the wall surface. The accelerometers were installed on the foundation. The foundation was made of rigid material. By assuming that the distance of the reference points on the rigid foundation is not changed, the coordinate of

Seismic Crack Investigation in an Earth Dam by Centrifugal Loading Test

http://dx.doi.org/10.5772/intechopen.78788

27

The model was excited with a horizontal sine wave of 50 Hz whose amplitude is 1.5 mm. The input seismic wave corresponds to the earthquake ground motion with a peak acceleration of

In order to evaluate the displacement of the gauge points, the digital image analysis method

Firstly, the static image is taken before the model is tested. While the model is excited, continuous images are taken by the high-speed camera of which the shooting speed is 1000 fps. The images are transformed into black and white binary images. The noise reduction is, then, carried out as shown in **Figure 6**. The number of the points is confirmed to be 82 at this stage. Then, the coordinates of the gauge points are measured in the unit of pixels by calculating the center position of each white element representing the gauge point. Finally, the distance between two reference points at the foundation, which was precisely 150 mm, is measured in the unit of pixels. The scale calibration is carried out by the distance of reference points and the coordinate of the gauge points is calculated as the relative location of the reference point in the unit of mm. By repeating this procedure for all dynamic images, the displacement

the gauge points is calibrated.

**3.2. Digital image analysis**

about 300 gal, and the frequency is 1 Hz.

used in the 1-G shaking table test [2] is applied.

variation of each gauge point can be obtained.

**Figure 5.** Particle size distribution used for the embankment model.
