If we bury it in the ground, can it measure how many tons of vehicle weight are pressing down on it?
From the perspective of optical sensing principles and civil engineering mechanics, directly embedding ordinary optical fibers into soil cannot accurately measure the specific tonnage of vehicles passing overhead; however, by structuring and packaging optical fibers into weighing sensors, earth pressure cells, or installing them in the load-bearing layers of the road surface, vehicle loads can be completely calculated.
I. Why can’t directly burying optical fibers in the soil measure vehicle weight directly?
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Mismatch in Sensing Mechanism and Sensitivity Direction
Optical fiber sensing (including Fiber Bragg Grating FBG and Distributed Optical Fiber DAS/DTS) is inherently highly sensitive to axial strain (\\varepsilon) and temperature (\\Delta T):\frac{\Delta \lambda_B}{\lambda_B} = (1 - P_e)\varepsilon + (\\alpha + \\xi)\\Delta TThe lateral/radial pressure directly applied to the cross-section of an optical fiber has a very low photoelastic coefficient, resulting in minimal wavelength drift. Furthermore, the non-uniform lateral squeezing of bare fibers by soil particles can easily cause birefringence (leading to broadening of the grating reflection spectrum or even splitting into two peaks), making it impossible to establish a single-valued, linear mechanical correspondence.
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Mechanical Uncertainty of Soil Medium
Soil is a porous, non-uniform, and discrete medium. When the load from vehicles above is transmitted downwards, it follows the semi-infinite elastic body theory (such as Boussinesq stress distribution formulas), where stress significantly attenuates and diffuses non-linearly with burial depth and radial distance. The moisture content, compaction, particle size distribution, and creep relaxation of the soil layer will all cause dynamic drift in the stress transfer coefficient. Directly embedded optical fibers can only qualitatively detect