Fiber Bragg grating (FBG) sensor has been considered as a reliable sensor for health monitoring of structural and geotechnical projects. Various types of FBG based sensors have been proposed in past ...few decades and employed for health monitoring of many geotechnical structures. This paper presents an overview of the recent development and application of FBG based sensors for health monitoring of several key geotechnical structures, including soil nail systems, slopes, and piles. Different sensor design, implementation and packaging methods, advantages and limitations of using FBG based sensors in different projects are reviewed. Comparative analysis of using two mathematical methods for the prediction of ground movement using FBG sensor data are also carried out. The two typical mathematical methods include Finite Difference Method (FDM) and Numerical Integration method (NIM). Possible technical challenges of applying FBG sensors for geotechnical monitoring are discussed.
•Cover conditions have a strong impact on the AH-FBG soil moisture measurements.•Artificial neural network is used to correct water content monitoring errors.•Four AH-FBG-ANN models can effectively ...improve monitoring accuracy.
Actively heated fiber Bragg grating (AH-FBG) can perform quasi-distributed monitoring of soil water content. However, the analysis method needs to be improved to minimize measuring errors. In this study, the artificial intelligence method is proposed and a model test was used to verify its feasibility and to explore the influence of cover conditions. Three cover layers were considered, including bare soil (S), grass (G), and biochar mixed soil (B). The water content measurements based on maximum temperature increase have higher accuracy for G, followed by S, and the worst is B. Fluctuation in the heat power and the longitudinal heat transfer are the main sources of errors. Artificial neural network (ANN) models can effectively improve monitoring accuracy. Cover conditions have a significant influence on the measurements by affecting initial ground temperatures and water content gradients. For field monitoring, the cover layer should be considered when analyzing AH-FBG measurements.
Geotechnical engineering is characterized by many uncertainties, including soil material properties, environmental effects, and engineering design and construction, which bring a significant ...challenge to geotechnical monitoring. However, conventional sensors with several inherent limitations, such as electromagnetic interference, signal loss in long-distance transmission, and low durability in harsh environments cannot fully meet current monitoring needs. Recently, fiber optic sensing technologies have been successfully applied in geotechnical monitoring due to the significant advantages of anti-electromagnetic interference, stable signal long-distance transmission, high durability, high sensitivity, and lightweight, which can be considered an ideal replacement for conventional sensors. In this paper, the working principle of different fiber optic sensing technologies, the development of fiber optic-based sensors, and the recent application status of these sensing technologies for geotechnical monitoring were comprehensively reviewed and discussed in detail. Finally, the challenges and countermeasures of the sensing technologies in geotechnical monitoring were also presented and discussed.
•This paper offers a comprehensive review of using fiber optic sensing technologies for geotechnical monitoring.•This review aims at improving the acknowledgement of fiber optic sensing technologies for geotechnical monitoring.•Possible technical challenges are discussed, which can provide a reference for subsequent research.
Special aspects of the two-year (2018–2019) geotechnical monitoring for the parking buildings in use and their deformational behavior are presented in the article. In 2005, open-type car parks No. 1 ...and No. 2, fell into the inflence zone of a two construction projects, a railway line construction, which connects Vnukovo-1 airport and Kievsky railway station in Moscow, and construction of the new Vnukovo-1 airport building.The article discusses tasks to ensure serviceability of existing buildings. Geotechnical monitoring during building operation allows for a timely documentationof negative processes occurring in building structures and to make prompt decisions to prevent emergencies. Geotechnical monitoring is carried out according to a specially developed program that includes a set of works using various methods. This allows us to assess the state of the supporting structures of the monitored buildings to ensure their strength, reliability, and safe operation.
Thermo-poro-mechanical responses along sliding zone/surface have been extensively studied. However, it has not been recognized that the potential contribution of other crucial engineering geological ...interfaces beyond the slip surface to progressive failure. Here, we aim to investigate the subsurface multi-physics of reservoir landslides under two extreme hydrologic conditions (i.e. wet and dry), particularly within sliding masses. Based on ultra-weak fiber Bragg grating (UWFBG) technology, we employ special-purpose fiber optic sensing cables that can be implanted into boreholes as “nerves of the Earth” to collect data on soil temperature, water content, pore water pressure, and strain. The Xinpu landslide in the middle reach of the Three Gorges Reservoir Area in China was selected as a case study to establish a paradigm for in situ thermo-hydro-poro-mechanical monitoring. These UWFBG-based sensing cables were vertically buried in a 31 m-deep borehole at the foot of the landslide, with a resolution of 1 m except for the pressure sensor. We reported field measurements covering the period 2021 and 2022 and produced the spatiotemporal profiles throughout the borehole. Results show that wet years are more likely to motivate landslide motions than dry years. The annual thermally active layer of the landslide has a critical depth of roughly 9 m and might move downward in warmer years. The dynamic groundwater table is located at depths of 9–15 m, where the peaked strain undergoes a periodical response of leap and withdrawal to annual hydrometeorological cycles. These interface behaviors may support the interpretation of the contribution of reservoir regulation to slope stability, allowing us to correlate them to local damage events and potential global destabilization. This paper also offers a natural framework for interpreting thermo-hydro-poro-mechanical signatures from creeping reservoir bank slopes, which may form the basis for a landslide monitoring and early warning system.
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.
Despite the extensive use of distributed fiber optic sensing (DFOS) in monitoring underground structures, its potential in detecting structural anomalies, such as cracks and cavities, is still not ...fully understood. To contribute to the identification of defects in underground structures, this study conducted a four-point bending test of a reinforced concrete (RC) beam and uniaxial loading tests of an RC specimen with local cavities. The experimental results revealed the disparity in DFOS strain spike profiles between these two structural anomalies. The effectiveness of DFOS in the quantification of crack opening displacement (COD) was also demonstrated, even in cases where perfect bonding was not achievable between the cable and structures. In addition, DFOS strain spikes observed in two diaphragm wall panels of a twin circular shaft were also reported. The most probable cause of those spikes was identified as the mechanical behavior associated with local concrete contamination. With the utilization of the strain profiles obtained from laboratory tests and field monitoring, three types of multi-classifiers, based on support vector machine (SVM), random forest (RF), and backpropagation neural network (BP), were employed to classify strain profiles, including crack-induced spikes, non-crack-induced spikes, and non-spike strain profiles. Among these classifiers, the SVM-based classifier exhibited superior performance in terms of accuracy and model robustness. This finding suggests that the SVM-based classifier holds promise as a potential solution for the automatic detection and classification of defects in underground structures during long-term monitoring.
•The relationship between strain profiles and soil failure mechanisms was explored.•The layout of sensing cables for ground collapse monitoring was optimized.•A corrected strain measurement-ground ...settlement model was derived.
Ground collapse events frequently lead to significant economic losses, catastrophic injuries and fatalities. There is a growing need for sensitive monitoring technologies to locate and predict potential ground collapses. This study aims to evaluate the performance of fiber optic (FO) cables for ground collapse monitoring. A model test was conducted in laboratory, and the optical frequency domain reflectometry (OFDR) and particle image velocimetry (PIV) techniques were combined to monitor the whole collapse process. The strain profiles measured by FO cables under different soil failure mechanisms were analyzed in detail. In the case of shallow sinkholes, the soil failure surfaces emanate from the edges of sinkholes and eventually propagate to the ground surface, and the shape of failure surfaces gradually changes from curvy to vertical. Micro-anchors significantly enhance the interface bonding between cables and surrounding soil. Moreover, the settlements calculated based on strain measurements match well with actual settlements.
•A monitoring scheme for stability of underground oil storage caverns in the construction phase.•The results of geotechnical monitoring on a pilot storage caverns in China were presented.•The spatial ...and time dependent effects of the excavation face were analyzed.•The effect of the geological conditions on rock mass deformation and rockbolt stress was analyzed.•This study compared the data from the pilot caverns with those from underground powerhouse caverns of hydropower stations.
Geotechnical monitoring is one of the important means for the control of safety of underground structure in both construction and operation phase. In this study, geotechnical monitoring on the stability was performed on a pilot underground crude-oil storage facility during the construction phase in China. Considering the engineering characteristics and geological conditions of the caverns, this study proposed a monitoring scheme for the stability of the underground oil storage caverns during the construction phase. This study systematically summarizes the results of geotechnical monitoring on a pilot underground water-sealed crude-oil storage caverns during construction phase in China. The monitoring results of the deformation of the surrounding rock showed that the convergence of the surrounding rocks, the crown settlement and the internal displacement of the caverns were within the ranges of 4–8mm, 3–6mm and 4–8mm, respectively. The monitoring results of the stress on rockbolts show that the stresses were mostly less than 50MPa. The spatial and time dependent effects of the excavation face and the effect of the geological conditions on the rock mass deformation and on the rockbolt stress were analyzed using the geotechnical monitoring data. This study compared the data from the pilot underground crude-oil storage caverns with those from underground powerhouse caverns of hydropower stations. The comparison showed that the stability of the underground pilot crude-oil storage caverns during the construction phase was relatively superior to that of the underground powerhouse caverns of the hydropower stations due to the relatively good quality of the rock masses.
This study surveyed the response of diaphragm walls with a length of 51 m during a large basement excavation (approximately 9310 m2, depth of 30.2 m) in soft soil in Hangzhou, China. The field data ...of wall deflections, maximum wall deflection rate, stress development of diaphragm walls, axial force of struts, and measured horizontal earth pressures were extensively investigated. The following conclusions were drawn: deep excavation can produce greater stress unloading compared with shallow excavation, which results in the diaphragm walls producing more significant deflection; the load transfer to the struts caused by the excavation is completed within two to three weeks after strut casting; the deeper basement excavation only has a minor effect on the distant axial forces of the struts; the horizontal earth pressure behind the diaphragm wall is linearly distributed along the depth. This paper proposes a general earth pressure AEP envelope based on the investigated case and similar cases.
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