Deep Underground Instrumentation And Monitoring

The structural integrity of deep, large underground facilities such as tunnels, mines, pumped storage facilities, and physics laboratories requires the ability to predict rock mass stability under loading to ensure the safety of human occupants and the longevity of the underground space. Deformation occurs over time scales that range from milliseconds to decades and spatial scales that range from millimeters to facility scale. Beginning with design, prediction is typically based on finite element models using available or estimated properties. As with most geotechnical problems, much of the difficulty of prediction lies in the inability to sufficiently characterize the rock properties, especially discontinuities. As a consequence, semi-quantitative measures, such as Rock Mass Rating (RMR) or the Hoek-Brown Geological Structure Index (GSI) [1], are used to characterize the rock mass together with empirical charts for design criteria such as rock bolt spacing for ground control. During and following construction, validating model predictions is necessary to assess their performance. Parameter adjustment, or even the physics incorporated within the model, can be made using back analysis. This monitoring should be a continuous or periodic process over the life of the facility. For civil structures, the post-construction era will be measured in decades. With the inherent uncertainties and high stresses associated with the deep underground environment, the potential for rock failure must always be borne in mind. Mitigating the risk is prudent, but formal cost-benefit analysis may be precluded by the uncertainties. Keeping abreast of the condition of the facility through Structural Health Monitoring (SHM) is gaining acceptance for underground construction [2]. One reason for the growth in research in monitoring is that maturing technologies, like fiber-optic sensors and associated instrumentation, can collect data that were not previously achievable. They are robust and geometrically flexible, possess long-term stability, are cost effective, and extend coverage in spatial extent with improved resolution or provide data at a higher sampling rate. In addition to fiber-optic technology, a host of new technologies with potential for underground geotechnical applications exist, including LIDAR, wireless “smart dust”, piezoelectric sensors, and high resolution electrical and seismic imaging [3; 4; 5; 6; 7]. The subject of this paper is mainly to describe preliminary experiments, future needs, and instrumentation and monitoring plans of the authors' research activities in the 2400-meter Deep Underground Science and Engineering Laboratory (DUSEL) in the Black Hills of South Dakota, USA, where fiber-optic sensors and water-level tiltmeter arrays have been installed. Raw project data is available by contacting [e-mail]隧道、矿井、抽水蓄能设施及物理实验室等大型深部地下设施的结构完整性，需具备预测荷载作用下岩体稳定性的能力，以保障人员安全与地下空间的服役寿命。岩体变形的时间尺度跨度从毫秒至数十年，空间尺度则从毫米延伸至设施整体尺度。从设计阶段起，稳定性预测通常基于采用现有或估算参数的有限元模型。与多数岩土工程问题类似，这类预测的核心难点在于无法充分表征岩体特性，尤其是岩体结构面。为此，工程界常采用岩体分级指数（Rock Mass Rating, RMR）、霍克-布朗地质结构指数（Hoek-Brown Geological Structure Index, GSI）[1]等半定量方法，结合围岩控制锚杆间距等设计准则的经验图表，共同完成岩体特性表征。在施工期间及施工结束后，需对模型预测结果进行验证以评估其性能表现。可通过反分析手段开展参数调整，甚至修正模型所纳入的物理机制。此类监测工作应在设施全生命周期内持续或周期性开展。对于民用地下设施，施工后服役周期可达数十年。鉴于深部地下环境固有的不确定性与高应力特性，必须始终警惕岩体失效的潜在风险。降低此类风险是审慎之举，但受不确定性因素制约，正式的成本效益分析往往难以开展。通过结构健康监测（Structural Health Monitoring, SHM）掌握设施运行状态，在地下工程领域正逐步获得广泛认可[2]。监测相关研究日益增多的原因之一在于，光纤传感器及配套仪器等成熟技术可采集此前无法获取的监测数据。这类传感器具备鲁棒性强、几何适配性佳、长期稳定性优异、成本效益高等优势，能够通过提升分辨率扩大空间覆盖范围，或以更高采样率获取数据。除光纤技术外，还有诸多适用于地下岩土工程应用的新兴技术，包括激光雷达（LIDAR）、无线“智能尘埃”、压电传感器，以及高分辨率电学与地震成像技术[3;4;5;6;7]。本文主要旨在介绍作者团队在美国南达科他州布莱克山地区2400米深地下科学与工程实验室（Deep Underground Science and Engineering Laboratory, DUSEL）开展的研究活动中的初步实验、未来需求以及仪器与监测方案，该实验室已部署光纤传感器与水位倾斜仪阵列。原始项目数据可通过联系[e-mail]获取。