This paper introduces a new method for detecting void formation caused primarily by water-induced soil erosion by using a magnet, a compass and a plastic optic fiber sensor. The proposed method starts by putting a magnet at an arbitrary point in a soil mass behind a concrete wall whose thickness would be in the range of 20cm to 35cm. A compass is placed across the concrete wall such that its needle initially points in an arbitrary direction which does not coincide with the Earth's north-south direction. Then, water is supplied onto the soil mass such that portion of the soil mass is eroded and eventually the magnet is forced to move, changing the magnetic field around it. The compass then detects this change in the magnetic field and its needle changes its direction. This needle movement, caused by the soil erosion occurring behind the concrete wall, can be seen by naked eyes or be detected by the plastic optic fiber sensor with ease and accuracy. The original concept in this method is that the important information to be captured in advance, can be remotely obtained across thick concrete walls with the help of magnetic field.
Void formation is a troublesome phenomenon in various geotechnical problems. This void formation can be caused by a number of factors, such as water-induced soil erosion. Void spaces formed beneath a pavement layer, behind a retaining wall, or in a concrete tunnel occasionally lead to serious accidents due to the lack of knowledge of their existence, as they cannot be seen directly because of sight-blocking or shielding layers made of arbitrary materials, such as asphalt or concrete. Advanced geophysical survey methods utilizing ground penetrating radar (e.g., Argentieri et al. 2015, Pazzi et al. 2018) could identify the existence of such voids; however, the survey can only be performed at limited times, which in most cases would be much later than when the void spaces were actually formed. Installation of arbitrary sensors capable of detecting void formation would also be possible. However, these sensors must be equipped with a stable power supply over many years in wet ground, and cables must be laid out, in some form, from the sensor position to the location of a data logger, through a hole made in the shielding layer. When water tightness is required for a given type of infrastructure, constructing this hole for the cables is not preferred and, if possible, should be avoided. Considering this information, a new method for detecting the formation of void spaces using a magnet, a compass, and a plastic optical fiber is proposed in this paper. The main characteristics of the proposed method are as follows; 1) the method enables real-time detection and visualization of the void formation using simple tools that can be operated without electricity, and 2) the construction of a hole in the shielding layer is not required. In this paper, a series of laboratory experiments are described that illustrate the principles of the proposed method and its suitability for detecting soil erosion and void formation in practical projects.