Grasp and evaluation of the unsaturated seepage behavior of soil by using MRI
© The Author(s). 2016
Received: 8 October 2016
Accepted: 25 November 2016
Published: 1 December 2016
It is essential for residents to understand ground disaster and improve disaster prevention awareness, because natural disasters have occurred frequently in recent years. In this study, nondestructive unsaturated seepage behavior was continuously visualized by using MRI (magnetic resonance imaging) with test equipment constructed of non-metal parts. Additionally, the water content of the soil from the MRI image was evaluated, the results of previous experiments were considered and compared.
The study revealed from the imaging with MRI that seepage will flow faster in the lower part of the cylinder than in the upper part. A discontinuous water content ratio change of the seepage line part was confirmed from image with the MRI. There was a tendency for air to be trapped on the border of different soil materials in an acrylic cylinder. It was possible for the water content of the sand in the cylinder to be evaluated from the MRI image.
The study suggests that complicated unsaturated seepage behavior can be easily visualized. It was suggested that air may be trapped in the different layer border of soil. And these finding will lead to the elucidation of relevance of slope collapse and the piping phenomenon with the rainfall.
KeywordsMRI Unsaturated soil Visualization Seepage Water retentivity Moisture profile
There is a very high uncertainty in predicting future ground disasters with rainfall and elucidation of the relationship between the slope collapse and piping phenomenon has been required (Araki et al. 2005; Iwami et al. 2013; Okumura et al. 2013; Araki et al. 2014; Araki et al. 2015; Yasufuku et al. 2015). In order to increase the interest of residents in ground engineering, there is a need for a visually-intuitive evaluation method of the ground. Furthermore, not only the numerical evaluation of the state of the ground by visualization is possible, but visual confirmation can also be performed at the same time. In this way new knowledge will be obtained about complicated seepage behavior of unsaturated ground. Visualization of the internal structure of the soil was carried out mainly using X-ray micro CT (Watanabe et al. 2015). The measurement method of the water contents in the ground using gamma rays has been studied by Kono et al. (1981). An MRI (magnetic resonance imaging) is used for the discovery the lesions in the medical field, and images of the inside of the human body can be directly obtained in a non-destructive manner. Moreover, the device can continue photographing for a long time. In this study, non-destructive unsaturated seepage behavior was continuously visualized by using MRI with test equipment constructed of non-metal parts. Additionally, the water content of the soil from the MRI image was evaluated, and the results of previous experiments were considered and compared.
Results and Discussion
Observation of unsaturated seepage behavior
The seepage proceeds approximately perpendicular for a short time after the start of the water supply, but over time, becomes faster in the lower part of two cylinder than in the upper part. These can be confirmed from Fig. 4. It is revealed that the water movement is affected by gravity even with 30 mm differences in height. There is a clearer white and black boundary in the area of the seepage front. Hereafter, this boundary will be called the seepage line. It can be confirmed that a seepage line is a disturbance (Fig. 4c). It is suggested that discontinuous water content change occurs on the seepage line. There is a black circle in the cylinder around 210 mm position (Fig. 4d). This is considered an air trap.
It is confirmed that seepage advances from the bottom as in the case of the cylinder filled with only Toyoura sand (Fig. 5a). The seepage line is almost orthogonal to the penetration direction when the seepage line reaches the Kaolin clay, and, it can be confirmed that the seepage front line of the Toyoura sand and the Kaolin clay are in a similar position (Fig. 5b). In the wetting process, it is because the seepage speed through the Toyoura sand layer is equal to that through the Kaolin clay layer. In addition, a black part can be identified on a border of the Kaolin clay and the Toyoura standard sand. As seen from Fig. 5c) it can be confirmed that the light and shade are not a uniform border of the left side and the bottom of the Kaolin clay and the Toyoura sand. The seepage line moves to the bottom before it reaches the Kaolin clay. It is shown that the moisture holding ability of the Kaolin clay is very much greater than that of the Toyoura sand.
As can be seen from Fig. 6, a black part towards lower left direction on a border of the Kaolin clay and the Toyoura sand. This seems to be an air trap, and the possibility that such an air trap occurs between materials of different quality of soil.
Water content estimation of the sample by MRI
The study suggests that complicated unsaturated seepage behavior can be easily visualized. A grasp and evaluation of the unsaturated seepage behavior was examined using an MRI device used for medical care. In this study, Toyoura sand was packed in an acrylic cylinder and the unsaturated horizontal seepage test was examined and imaged using an MRI device. A discontinuous water content change of the part of seepage line was confirmed. It was confirmed that there was a tendency for air to be trapped on the border of different soil materials in an acrylic cylinder. This study examined a technique to evaluate water content from an MRI image. These findings will lead to the elucidation of the relevance of slope collapse and the piping phenomenon with the rainfall.
This work was carried out by 2015 Strategy and Public projects (Research Activity Start-up) (in Japanese) from University of Yamanashi. The study was partly supported by 2016 Exploratory Fusion Research project (in Japanese) from University of Yamanashi and River Erosion Control Technology Research and Development Public Offering Regional Problem Division (Sabo) (in Japanese) from the Ministry of Land, Infrastructure, Transport and Tourism (MLIT).
HF collected and assembled data. KA designed drafting and conception of the study. KM participated in its design and coordination and helped to draft the manuscript. HF, KA, KM and HO carried out analysis and interpretation of data of MRI. HF, HK and HS performed experiment and created figures. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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