【研究背景】
近几十年来，随着全球气候变暖趋势的加剧，永久冻土融化问题受到越来越多的关注。永久冻土的退化和季节性冻融循环对生态系统和人类活动构成重大威胁，包括由于冻胀和融沉导致的泥石流、地震、滑坡和塌陷等灾害。因此，准确、实时监测冻土变形对预防寒区地质灾害、确保工程建设安全至关重要。冻土变形监测技术主要分为非接触式和接触式两类。非接触式监测方法主要包括全球导航卫星系统（GNSS）、合成孔径雷达（InSAR）、机载激光扫描技术和数字摄影测量学，广泛用于监测寒区地面位移。接触式监测工具，如钻孔水平仪、倾斜仪和位移计，则用于收集地下变形数据。分布式光纤传感（DFOS）技术作为一种先进的地质监测方法，具有诸多优点，在解决诸如斜坡稳定性、基础承载能力、隧道变形和堤坝沉降等地质工程挑战中发挥了关键作用。尤其是在冻土监测方面，DFOS技术主要围绕冰水相变和水分迁移两大科学问题。然而，如何准确监测冻土变形，仍是一个亟待解决的问题。
【研究内容】
南京大学朱鸿鹄教授团队利用自主开发的光缆-冻土界面力学特性测试仪，从土体是否冻结、不同冻结时间和含水量差异分布的角度分析了光缆-冻土界面在拉拔过程中的渐进破坏特征。光纤应变监测结果能够准确反映出光缆-冻土界面的渐进失效特性，应变软化模型能够准确描述界面力学性质。在冻结过程中，土体中的液态水变成冰，导致冻结锋面的移动和水分的迁移，从而导致界面力学性质出现显著差异。光缆-冻土界面在不同深度处的剪切应力演化过程反映了在光缆拔出过程中土体的变形协调状态，表明光缆的测量范围和界面耦合度、土体干密度和初始含水量密切相关。
【研究意义】
本研究为在冻土地区地基变形监测中推广应用光纤传感技术提供了参考。通过研究光缆-冻土界面的失效机制及其力学性质，揭示了界面在冻结过程中的演化规律，为工程建设提供了重要的技术支撑。这一研究成果的创新点在于，利用光纤应变监测技术实现了对冻土变形的精确分布实时监测，为冻土地区的工程建设提供了新的技术手段。
Experimental study on progressive interfacial mechanical behaviors using fiber optic sensing cable in frozen soil
Tian-Xiang Liu1, Hong-Hu Zhu 1*, Qi Li2, Bing Wu1, Hao-Jie Li1, Le-Le Hu1, Du-Min Yan1, 3
- School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China.
- State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China
- China Railway Construction Corporation Limited, Beijing, 100855, China
DOI: 10.16285/j.rsm.2023.0171
Abstract: Frost heave and thaw settlement in cold regions pose a significant threat to engineering construction. Optical frequency domain reflectometry (OFDR) based on Rayleigh scattering can be applied to monitor ground deformation in frozen soil areas, where the interface behavior of soil-embedded fiber optic sensors governs the monitoring accuracy. In this paper, a series of pullout tests were conducted on fiber optic (FO) cables embedded in the frozen soil to investigate the cable‒soil interface behavior. An experimental study was performed on interaction effects, particularly focused on the water content of unfrozen soil, freezing duration, and differential distribution of water content in frozen soil. The high-resolution axial strains of FO cables were obtained using a sensing interrogator, and were used to calculate the interface shear stress. The interfacial mechanical response was analytically modeled using the ideal elasto‒plastic and softening constitutive models. Three freezing periods, correlating with the phase change process between ice and water, were analyzed. The results shows that the freezing effect can amplify the peak shear stress at the cable-soil interface by eight times. A criterion for the interface coupling states was proposed by normalizing the pullout force‒displacement information. Additionally, the applicability of existing theoretical models was discussed by comparing the results of theoretical back‒calculations with experimental measurements. This study provides new insights into the progressive interfacial failure behavior between strain sensing cable and frozen soil, which can be used to assist the interpretation of FO monitoring results of frozen soil deformation.
Keywords: Geotechnical monitoring; Pullout test; Cable‒soil interface; Progressive failure; Frozen soil
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