Abstract In order to study the deformation features of deep thick unconsolidated layer caused by de-watering,the calculation model of fiber Bragg grating(FBG) sensor–Adhesive–Packaging material-borehole sealing material- unconsolidated layer strain transfer system was established. Based on the strain transferring analysis of FBG sensors and the unconsolidated layer,it was found that the formulae deduced in previous researches were not suitable for the strain calculation of the unconsolidated layer monitoring. A new mathematical model of strain transferring of the unconsolidated layer and FBG sensors was proposed,and the factors affecting the strain transferring of the unconsolidated layer were analyzed. The higher strain transfer ratio can be obtained when sealing material elastic modulus was in the range of 8–15 GPa. In a coal mine,24 FBG sensors were installed in the 12 unconsolidated layers,the settlement and deformation monitoring results of the unconsolidated layer with the depth of 92.40–148.69 m were calculated using the new mathematical model proposed in this paper. Field practice indicated that high stress concentration region of the mine was located at the unconsolidated layer with depth of 97.56–104.33 m and 141.94–144.88 m,which was in well accordance with the actual case and the results obtained from other monitoring methods. Study of Fiber Bragg Grating strain transferring theory has important practical significance to promote the development of optical fiber sensing technology and its application.
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