分布式光纤探测地裂缝的理论基础探讨

A THEORETICAL FRAMEWORK FOR DETECTING AND MONITORING GROUND FISSURES USING DISTRIBUTED FIBER OPTIC SENSING

地裂缝监测是科学认识其成灾机制并进行灾害防治必不可少的工作。高空间分辨率分布式光纤感测(DFOS)技术的发展及其在地质和岩土工程监测中的应用,为地裂缝的精细化探测提供了技术支撑。本文探讨了DFOS技术探测地裂缝的理论基础。以地裂缝形成过程中土体的张拉破坏模式为例,结合传感光缆应变的特点,提出了基于线应变的地裂缝形成判定准则:Rs=ε3/εt(其中,εt为土体的极限拉应变,ε3为最小主应变)。当Rs小于1时,最小主应变小于极限拉应变,土体未发生张拉破坏;当Rs大于1时,土体发生张拉破坏,地裂缝形成。在此基础上,对DFOS探测地裂缝过程中存在的空间分辨率效应、传感光缆-土体界面效应、应变局部化效应、温度效应和应变传递效应及其对策进行了分析和讨论。在实际应用中需针对不同的地质条件和监测需要,对这些效应进行分析和处理,以提高监测结果的准确性。

The detection and monitoring of ground fissures is vital to understand their mechanism and the prevention and control of related hazards. Recently,high-and ultra-high-resolution distributed fiber optic sensing(DFOS) techniques have been developed and applied in the field of geoengineering. This provides a technological basis for improved detection and monitoring of ground fissures. This paper discusses a theoretical framework for ground fissure detection and sensing using the DFOS technique. Based on the tensile failure mode of soil during ground fissure formation and by considering that DFOS captures linear strains,a strain-based failure criterion is proposed as follows: Rs= ε3/εt,where εtis the ultimate tensile strain of soil and ε3 is the minimum principal strain.A value of Rslarger than 1 means that the absolute value of the minimum principal stress has exceeded the ultimate tensile strain of soil. Hence,tensile failure may occur. The ground fissure monitoring results are influenced by a combined effect of spatial resolution,fiber optic cable-soil interaction,strain localization,temperature variation,and strain transfer efficiency. These effects and their countermeasures are also analyzed and discussed. To improve the quality of DFOS-based monitoring,these effects should be carefully addressed according to different geologic conditions and monitoring schemes.