Two plastic optical fibers, both having diameter of 1mm, are used to read local disturbance or deformation at a point in granular material, such as sand, soil, or broken rocks. As light is sent into the 1st fiber and reaches its end, it leaves the fiber and hits surfaces of nearby particles. Part of the reflected light then gets into the 2nd fiber, which can be digitally recorded by using photo diode. Two fibers can also be aligned such that a gap is created between the fibers for sensing. The paper describes results of several laboratory and field experiments using these new fiber sensors. The examples include cases in which fiber sensors were used to interpret turbidity of muddy water, liquefaction, soil mass movement in a slope and so on. These results show that the proposed new sensors will be able to support data acquisition procedures in many different engineering projects.


A new method for monitoring complex behaviors of geo-materials using plastic optic fiber (POF) was proposed by Akutagawa et al [1]. Unlike in conventional methods of using optic fibers for engineering monitoring ([2], [3]), the new method uses tip of POF as monitoring points. A series of table-top experiments [4] have shown its capability, limits and potentials to obtain information on deformation of sand particles, passage of water in granular material, passage of air bubble, wetting process of dry sand and so on. Based on these experiences, a new sensor was developed where brightness and color nature of light returning from observation points can be processed by a CMOS-based sensor box. The paper describes the fundamental system of the new device in contrast to the original version in which only brightness of returning light was recorded. Several examples are then introduced to show new potential of the proposed method.

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