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Development of a Novel Inclinometer by Inverse Finite Element Method for Soil Deformation Monitoring
Abstract
The inverse finite element method (iFEM) is a mechanics-based algorithm for dynamic tracking of full-field structural displacements. Accurate deformed structural-shape can be attained without any material properties and loading information, even when some discrete locations lack in-situ strain measurements. The structure is discretized by inverse finite elements, enabling superior predictive capability for complex structural deformations. These exclusive advantages make iFEM suitable for geotechnical deformation monitoring, but very few researches and applications are present in the existing literature. With this in mind, a novel inclinometer that employs iFEM methodology as its underlying design theory is developed in this paper. The inclinometer is fabricated by lowering the fiber optic shape sensor into the conventional inclinometer casing leveraging on sliding fixtures. Distributed strain measures are captured using self-developed fiber Bragg grating sensors and then input into the iFEM model to regenerate the internal displacement of soil structures. The present inclinometer serves to both increase the survival rate of the sensors while at the same time achieving the sensing element reuse. A series of indoor and field validation tests have been performed and demonstrated the high accuracy, robustness, and practical usefulness of the iFEM-based inclinometer.
... Recent studies [30][31] have shown that iFEM methodology provides very robust and reliable deformation reconstruction results for both static and dynamic conditions. The iFEM has the following characteristics for shape sensing process: (1) Shape sensing without material and loading information, (2) modelling capability of complex plate/shell structural geometries, (3) applicability to any geometrical and/or natural boundary conditions, (4) suitability to deformation reconstruction in real time, and (5) insensitivity to measurement errors such as noisy strains. ...
... In Eq. respectively. These can be clearly defined between any two interacting material points as follows: (31) where the N f , S f , and f peridynamic force vectors are aligned with the direction of the relative position of the interacting material points in the deformed configuration, i.e., and shear tests [97]. Accordingly, the material parameters corresponding to each type of interaction can be expressed for three-dimensional PD laminate model as: ...
... In Eqs. (30)(31), the N and S damage parameters define interlaminar failures for mode I and mode II cracks in delamination for an orthotropic laminate (i.e., related with transverse normal and transverse shear deformations), in the given order. In addition to these out-of-plane damage modes, the damage parameter can express the amount of in-plane damage that can be classified as matrix ( M ) and fiber ( F ) failures based on angle. ...
A novel structural health monitoring approach is developed by coupling the inverse finite element method (iFEM) and peridynamic theory (PD) for real-time shape sensing analysis and crack propagation monitoring of plate structures. This hybrid method, called iFEM-PD, can account for deformation, stress, and damage states of any sensor-equipped structure in real time without the need for loading knowledge and regardless of the complexity of structural topology or boundary conditions. The integrated iFEM-PD approach first reconstructs continuous (full-field) deformations from discrete strain measurements and then utilizes them to obtain full-field strains within the structure. Subsequently, iFEM-reconstructed strains are employed with a suitable damage diagnosis index to quantify the critical (possibly damaged/cracked) zone of the structural domain. Next, this critical zone is modelled by populating PD material points and establishing non-local interactions between the material points. Enforcing the real-time deformations predicted by iFEM to the boundary material points of the PD domain as displacement boundary conditions, the deformations of the material points located internal to the damaged zone is recalculated through PD analysis. During this simulation, the damage prognosis is achieved by precisely modelling structural discontinuities (crack etc.) and analyzing crack propagation based on non-local particle interactions. The shape sensing and damage monitoring capabilities of the iFEM-PD method are numerically verified for crack monitoring problems of composite structures subjected to various static/dynamic loads. Also, the high accuracy of the iFEM-PD formulation is experimentally validated by comparing the numerical results with those of digital image correlation. Overall, the merits of the new approach are revealed for precise crack growth monitoring in composite structures.
Field tests were carried out to study the lateral cyclic loading characteristics of rock-socketed monopile in coastal area, and the variation law of pile body accumulative displacement, unloading stiffness, and pile body bending moment with the cyclic number and cyclic loading amplitude were obtained by actual measurement. Test results show that the cyclic loading history with sa small amplitude has a certain effect on the cyclic accumulative displacement and the degradation of unloading stiffness, and both the peak bending moment and locking moment of the pile body increase with the increase of cyclic numbers. By using the normalization method and formula fitting method, it is found that, the accumulative rate of cyclic displacement, the degradation rate of unloading stiffness, and the variation rate of peak bending moment of single piles in weathered rock layers are significantly smaller than those of sand foundation or clay foundation. The cyclic p-y curves at different depths are derived, and the different hysteretic characteristics between shallow cyclic p-y curve and deep cyclic p-y curve are explored. Finally, the static load test after cyclic tests was carried out, and it was found that the lateral bearing performance of the rock-socketed monopile was greatly degraded after a total of 3000 times of lateral cyclic loading.
Shape sensing plays a key role in Structural Health Monitoring (SHM) and has become an excellent methodology for large-scale engineering structures to achieve significant improvement in their safety, reliability, and affordability. The inverse finite element method (iFEM) is an accurate and efficient method for shape sensing to reconstruct the three-dimensional displacements using in situ surface strain data. This study proposes a novel shape sensing method for large deformation monitoring based on strain gradient theory and iFEM method. Initially, the nonlinear displacement fields are described, and Green–Lagrange strain theory is employed to deduce the theoretical section strains, and then the strain–displacement relation is established by using a least-squares variational principle. Next, isogeometric displacement functions and the measured section strain formulations are deduced, and a decoupling method for high order section strains is proposed. Finally, a cantilever beam is used to demonstrate the accuracy and effectiveness of the refined isogeometric iFEM method for large deformations. The numerical results show the superior reconstruction capability and potential applicability of the proposed model for accurate shape prediction of the non-linear deformation of beam structure.
A novel structural health monitoring approach is developed by coupling the inverse finite element method (iFEM) and peridynamic theory (PD) for real-time shape sensing analysis and crack propagation monitoring of plate structures. This hybrid method, called iFEM-PD, can account for deformation, stress, and damage states of any sensor-equipped structure in real time without the need for loading knowledge and regardless of the complexity of structural topology or boundary conditions. The integrated iFEM-PD approach first reconstructs continuous (full-field) deformations from discrete strain measurements and then utilizes them to obtain full-field strains within the structure. Subsequently, iFEM-reconstructed strains are employed with a suitable damage diagnosis index to quantify the critical (possibly damaged/cracked) zone of the structural domain. Next, this critical zone is modeled by populating PD material points and establishing non-local interactions between the material points. Enforcing the real-time deformations predicted by iFEM to the boundary material points of the PD domain as displacement boundary conditions, the deformations of the material points located internal to the damaged zone is recalculated through PD analysis. During this simulation, the damage prognosis is achieved by precisely modeling structural discontinuities (crack etc.) and analyzing crack propagation based on non-local particle interactions. The shape sensing and damage monitoring capabilities of the iFEM-PD method are numerically verified for crack monitoring problems of composite structures subjected to various static/dynamic loads. Also, the high accuracy of the iFEM-PD formulation is experimentally validated by comparing the numerical results with those of digital image correlation. Overall, the merits of the new approach are revealed for precise crack growth monitoring in composite structures.
Curved beam, plate, and shell finite elements are commonly used in the finite element
modeling of a wide range of civil and mechanical engineering structures. In civil engineering, curved elements are used to model tunnels, arch bridges, pipelines, and domes. Such structures provide a more effcient load transfer than their straight/flat counterparts due to the additional strength provided by their curved geometry. The load transfer is characterized by the bending, shear, and membrane actions. In this paper, a higher-order curved inverse beam element is developed for the inverse Finite Element Method (iFEM), which is aimed at reconstructing the deformed structural shapes based on real-time, in situ strain measurements. The proposed two-node inverse beam
element is based on the quintic-degree polynomial shape functions that interpolate the kinematic variables. The element is C2 continuous and has rapid convergence characteristics. To assess the element predictive capabilities, several circular arch structures subjected to static loading are analyzed, under the assumption of linear elasticity and isotropic material behavior. Comparisons between direct FEM and iFEM results are presented. It is demonstrated that the present inverse beam finite
element is both efficient and accurate, requiring only a few element subdivisions to reconstruct an accurate displacement field of shallow and deep curved beams.
This article investigates the interest of a promising methodology, dealing with the use of an in-situ piezoelectric (PZT) disk to perform real-time Structural Health Monitoring (SHM) of glass fiber-reinforced polymer (GFRP) composites submitted to various four-points bending loadings: monotonic, load-unload and incremental load until failure. The real-time in-situ SHM is conducted acquiring the electrical signature (capacitance) variation of the embedded PZT transducer. To establish the link between the PZT capacitance response and the occurring physical phenomena, especially damage, a multi-instrumentation composed of external Non-Destructive Testing techniques (Acoustic Emission (AE) and Digital Image Correlation (DIC)) was implemented on the tested specimens, so that it was possible to make multi-physical couplings between the various obtained measurements and the PZT capacitance curves. It was shown that the PZT capacitance is very sensitive to damage initiation and progression inside the polymer-matrix composite material until its failure, mainly due to the specimens Neutral Fiber (NF) vertical offset which changes the piezoelectric coupling between the host composite and the transducer. Consequently, the SHM potential of such a PZT disk is highlighted, opening the way for its use in real technical structures submitted to bending loadings, such as aircraft wings.
Structural health monitoring (SHM) is one of the most emerging approaches for early damage detection, which leads to improved safety and efficient maintenance of large-scale civil structures. Data-driven vibration-based SHM techniques rely on sophisticated signal processing methods to analyze and interpret the complex measured data collected from the instrumented structures. Empirical mode decomposition (EMD) is one of the robust time-frequency decomposition techniques that has been widely used in SHM. Numerous studies have used EMD and its variants in different applications specific to structural modal identification and damage detection, which have been presented in various academic journals, conference papers, and technical reports. This paper presents a comprehensive and systematic review and summary of applications of EMD and its variants that have been extensively implemented in SHM. A brief background and illustration of EMD and its variants are presented first to show their performance under various cases, followed by a detailed literature review of their recent applications specific to SHM.
Shape sensing is one of most crucial components of typical structural health monitoring systems and has become a promising technology for future large-scale engineering structures to achieve significant improvement in their safety, reliability, and affordability. The inverse finite element method (iFEM) is an innovative shape-sensing technique that was introduced to perform three-dimensional displacement reconstruction of structures using in situ surface strain measurements. Moreover, isogeometric analysis (IGA) presents smooth function spaces such as non-uniform rational basis splines (NURBS), to numerically solve a number of engineering problems, and recently received a great deal of attention from both academy and industry. In this study, we propose a novel "isogeometric iFEM approach" for the shape sensing of thin and curved shell structures, through coupling the NURBS-based IGA together with the iFEM methodology. The main aim is to represent exact computational geometry, simplify mesh refinement, use smooth basis/shape functions, and allocate a lower number of strain sensors for shape sensing. For numerical implementation, a rotation-free isogeometric inverse-shell element (isogeometric Kirchhoff-Love inverse-shell element (iKLS)) is developed by utilizing the kinematics of the Kirchhoff-Love shell theory in convected curvilinear coordinates. Therefore, the isogeometric iFEM methodology presented herein minimizes a weighted-least-squares functional that uses membrane and bending section strains, consistent with the classical shell theory. Various validation and demonstration cases are presented, including Scordelis-Lo roof, pinched hemisphere, and partly clamped hyperbolic paraboloid. Finally, the effect of sensor locations, number of sensors, and the discretization of the geometry on solution accuracy is examined and the high accuracy and practical aspects of isogeometric iFEM analysis for linear/nonlinear shape sensing of curved shells are clearly demonstrated.
Structural health monitoring (SHM) of civil engineering structures has been widely developed to increase safety and to provide cost-effective maintenance programmes. Although the current approaches of SHM systems using traditional single-point sensors – such as electric strain sensors, accelerometers and global positioning system-based sensors – have appropriate measurement precision for SHM purposes, they present challenges when deployed in real-scale applications, given the limited number of possible points to assess structural behaviour and the harsh environmental conditions during operation. When it comes to reinforced-concrete structures, the development of health monitoring and damage identification presents further challenges, since this type of structure is affected by a variety of chemical, physical and mechanical degradation processes, has a heterogeneous composition and shows non-linear behaviour. On the other hand, fibre optic (FO) technology can provide integrated sensing along with extensive measurement lengths of high sensitivity, durability and stability, which makes it ideal for the SHM of concrete structures. In this paper, FO sensing principles and the different types of FO sensors for civil structure applications are briefly described and a state-of-the-art review of SHM applied to concrete structures using FO sensors in recent decades is presented.
The inverse Finite Element Method (iFEM) is applied to reconstruct the displacement field of a shell structure which undergoes large deformations using discreet strain measurements as the prescribed data. The iFEM computations are carried out using an incremental procedure where at each load step, the incremental strains are used to evaluate the incremental displacements which in turn update the geometry of the deformed structure. The efficacy of the proposed approach to predict large displacements is examined using two case studies involving a cantilevered wing-shaped plate and a clamped plate. The incremental iFEM procedure is demonstrated to be sufficiently accurate in terms of reproducing the correct nonlinear character of the load-displacement curve even when a reduced number of strain sensors is used. Therefore, this approach may have important implications for real-time monitoring of aerospace structures that undergo large displacements.
Landslide monitoring is critical for predicting the stability of slopes to ensure the safety of life and property. Considering the potential advantages of fiber Bragg gratings (FBGs), such as immunity to electromagnetic interference, resistance to hostile environments, light weight, and high measurement precision and real time response, a self-designed, FBG-based in situ inclinometer combining a traditional inclinometer and FBG technology was designed to monitor the inner deformation of a slope. In practical landslide monitoring, the inclinometer can be regarded as a cantilever beam with one end fixed. Based on the deflection curve equation of a normal beam and the composite Simpson integral equation, a theoretical deflection equation of the FBG-based inclinometer versus longitudinal strain was established. A FBG-based inclinometer was fabricated and calibrated in the laboratory and a calibration strain sensitivity coefficient was obtained. The results of calibration tests show that the displacements measured by dial indicators are in good agreement with the theoretical displacements calculated using the proposed equation. A series of FBG-based inclinometers were installed into three vertical boreholes located at different points on the profile of an actual reinforced slope. The in situ monitoring results show that the FBG-based inclinometer can effectively capture the real-time internal displacements and potential sliding surface of the slope, proving the validity of the proposed theoretical equation as well the reliability and practicality of the proposed FBG-based inclinometer in engineering applications.
In this study, Brillouin scattering-based distributed fiber optic sensor is implemented to measure temperature distributions and detect cracks in concrete structures subjected to fire for the first time. A telecommunication-grade optical fiber is characterized as a high temperature sensor with pulse pre-pump Brillouin optical time domain analysis (PPP-BODTA), and implemented to measure spatially-distributed temperatures in reinforced concrete beams in fire. Four beams were tested to failure in a natural gas fueled compartment fire, each instrumented with one fused silica, single-mode optical fiber as a distributed sensor and four thermocouples. Prior to concrete cracking, the distributed temperature was validated at locations of the thermocouples by a relative difference of less than 9%. The cracks in concrete can be identified as sharp peaks in the temperature distribution since the cracks are locally filled with hot air. Concrete cracking did not affect the sensitivity of the distributed sensor but concrete spalling broke the optical fiber loop required for PPP-BOTDA measurements.
It is significant to study the variations in the stability coefficients of hydrodynamic pressure landslides with different permeability coefficients affected by reservoir water level fluctuations and rainstorms. The Sifangbei landslide in Three Gorges Reservoir area is used as case study. Its stability coefficients are simulated based on saturated-unsaturated seepage theory and finite element analysis. The operating conditions of stability coefficients calculation are reservoir water level variations between 175 m and 145 m, different rates of reservoir water level fluctuations, and a three-day continuous rainstorm. Results show that the stability coefficient of the hydrodynamic pressure landslide decreases with the drawdown of the reservoir water level, and a rapid drawdown rate leads to a small stability coefficient when the permeability coefficient ranges from 1.16 × 10−6 m/s to 4.64 × 10−5 m/s. Additionally, the landslide stability coefficient increases as the reservoir water level increases, and a rapid increase in the water level leads to a high stability coefficient when the permeability coefficient ranges from 1.16 × 10−6 m/s to 4.64 × 10−5 m/s. The landslide stability coefficient initially decreases and then increases as the reservoir water level declines when the permeability coefficient is greater than 4.64 × 10−5 m/s. Moreover, for structures with the same landslide, the landslide stability coefficient is most sensitive to the change in the rate of reservoir water level drawdown when the permeability coefficient increases from 1.16 × 10−6 m/s to 1.16 × 10−4 m/s. Additionally, the rate of decrease in the stability coefficient increases as the permeability coefficient increases. Finally, the three-day rainstorm leads to a significant reduction in landslide stability, and the rate of decrease in the stability coefficient initially increases and then decreases as the permeability coefficient increases.
To overcome the shortcomings of conventional slope monitoring methods, this paper presented an in-place inclinometer based on BOTDR (Brillouin Optical Time Domain Reflectometer) which was used to obtain the long-term internal deformation in the slope. The installation process of optical fiber sensors and its measuring principle were introduced. The result of analysis indicated that the error in the measured displacement was proportional to the square of the inclinometer length and the precision of the BOTDR instrument, while it was inversely proportional to the diameter of the inclinometer tube. An actual field slope deformation monitoring case was also introduced. The results show that the BOTDR based inclinometer has a good consistency with the traditional inclinometer. It can effectively access the internal deformation of the slope and help to find the position of potential sliding surface accurately. This technology shows a high reliability and practicality in engineering application that will promote deeper research of slope in the future.
The inverse Finite Element Method (iFEM) is a state-of-the-art methodology originally introduced by Tessler and Spangler for real-time reconstruction of full-field structural displacements in plate and shell structures that are instrumented by strain sensors. This inverse problem is commonly known as shape sensing. In this effort, a new four-node quadrilateral inverse-shell element, iQS4, is developed that expands the library of existing iFEM-based elements. This new element includes hierarchical drilling rotation degrees-of-freedom (DOF) and further extends the practical usefulness of iFEM for shape sensing analysis of large-scale structures. The iFEM/iQS4 formulation is derived from a weighted-least-squares functional that has Mindlin theory as its kinematic framework. Two validation problems, (1) a cantilevered plate under static transverse force near the free tip, and (2) a short cantilever beam under shear loading, are solved and discussed in detail. Following the validation cases, the applicability of the iQS4 element to more complex structures is demonstrated by the analysis of a thin-walled cylinder. For this problem, the effects of noisy strain measurements on the accuracy of the iFEM solution are examined using strain measurements that involve five and ten percent random noise, respectively. Finally, the effect of sensor locations, number of sensors, the discretization of the geometry, and the influence of noise on the strain measurements are assessed with respect to the solution accuracy.
This paper presents a self-designed in-place inclinometer based on fiber Bragg grating (FBG) technology and introduces its application to a landslide monitoring project in Wenzhou, China. The working principle of the FBG-based inclinometer is briefly introduced. The FBG-based inclinometers were installed into nine boreholes that were distributed over different areas of the landslide. After 4 months of monitoring, the deformation of the soil mass at different depths above the bedrock within the landslide was captured by the inclinometers. The results indicated that the soil deformation above the bedrock was relatively large whereas that below the bedrock was minor. Based on the field monitoring data, the potential sliding zones were predicted to be the areas near the bedrock. In addition, a limit equilibrium analysis was carried out to evaluate the stability of the landslide under the circumstance of rainfall. The analyses showed that the calculated failure surface was consistent with the field observations, indicating that the monitoring data recorded by FBGs were accurate. These findings demonstrate that a combination of FBG technique and limit equilibrium analysis can be used to evaluate the stability of landslides effectively.
Shape sensing, i.e., reconstruction of the displacement field of a structure from surface-measured strains, has relevant implications for the monitoring, control and actuation of smart structures. The inverse finite element method (iFEM) is a shape-sensing methodology shown to be fast, accurate and robust. This paper aims to demonstrate that the recently presented iFEM for beam and frame structures is reliable when experimentally measured strains are used as input data.
The theoretical framework of the methodology is first reviewed. Timoshenko beam theory is adopted, including stretching, bending, transverse shear and torsion deformation modes. The variational statement and its discretization with C⁰-continuous inverse elements are briefly recalled. The three-dimensional displacement field of the beam structure is reconstructed under the condition that least-squares compatibility is guaranteed between the measured strains and those interpolated within the inverse elements.
The experimental setup is then described. A thin-walled cantilevered beam is subjected to different static and dynamic loads. Measured surface strains are used as input data for shape sensing at first with a single inverse element. For the same test cases, convergence is also investigated using an increasing number of inverse elements. The iFEM-recovered deflections and twist rotations are then compared with those measured experimentally. The accuracy, convergence and robustness of the iFEM with respect to unavoidable measurement errors, due to strain sensor locations, measurement systems and geometry imperfections, are demonstrated for both static and dynamic loadings.
Two kinds of innovative sensors based on optical fiber sensing technologies have been proposed and developed for measuring tilts and displacements in geotechnical structures. The newly developed tilt sensors are based on classical beam theory and were successfully used to measure the inclinations in a physical model test. The conventional inclinometers including in-place and portable types, as a key instrument, are very commonly used in geotechnical engineering. In this paper, fiber Bragg grating sensing technology is used to measure strains along a standard inclinometer casing and these strains are used to calculate the lateral and/or horizontal deflections of the casing using the beam theory and a finite difference method. Finally, the monitoring results are verified by laboratory tests.
The inverse finite element method (iFEM) is a mechanics-based algorithm for deformed-shape estimation of structures. In this paper, an enhanced inverse beam element named iEBT2 is developed based on classical beam theory. The element formulation is derived by minimizing a weighted-errors functional that consists of experimental and numerical section strains. The improved coefficient matrix KR is always non-singular, assuring that the solution of iFEM formulation exists. Location-independent feature of matrix KR simplifies the inverse finite element modeling, especially for complicated structures. Weighting constants are utilized to define error functional, aiming to penalize the contributions from “measure-less” stations. Numerical and experimental cases have been performed and demonstrated excellent predictive capability when iEBT2 model has complete strain measures. In the case of missing strain components, iEBT2 enables deformation estimation and its accuracy is acceptable in general. The enhanced inverse element extends the practical usefulness of iFEM in shape-sensing analysis of civil infrastructures.
The real-time estimation of structural deformations using discrete strain data, known as shape sensing, is critical to the health monitoring of structures such as bridges. An innovative methodology called the inverse finite-element method (iFEM) is proposed to solve this issue. In this paper, a novel two-node inverse beam element, iBeam3, is developed for two-dimensional deformation monitoring of beam type structures. The present iFEM formulation is derived based on the least-squares variational principle involving section strains of Euler-Bernoulli beam theory for stretching and bending. The iBeam3 element is able to reconstruct deformed shapes without any prior material and/or loading information because only the strain-displacement relationship is used in the formulation. Static and dynamic validation cases regarding steel beams with different boundary conditions subjected to transverse force are discussed in detail. In the tests, different discretization strategies are used to perform the iFEM analysis, and the effects of sensor positions, number of sensors, and measurement errors are evaluated with respect to iFEM-predicted accuracy. The experimental results demonstrate that the iBeam3 element is accurate, robust, and highly efficient. The present methodology provides promising potential in the real-time shape sensing of civil infrastructures.
A distributed optical fibre sensing system named Brillouin Optical Time Domain Analysis (BOTDA) is used to monitor the strain development of a laboratory soil slope model. The technology is yet to be fully implemented due to uncertainties of attachment method or the best way to set up optical fibre sensors for geo-structure health monitoring. The aim of study is to evaluate the deformation behaviour subjected to the development of horizontal strains from Brillouin-based optical fibre sensor of a residual soil slope under loading impact using BOTDA technology. In this study, a soil-embedded strain sensor placement approach was proposed to be installed in the 1g model of soil slope which was achieved via the horizontal planting of a three-layered optical fibre cable in S-curve forming slope. In this paper, however, only pilot tests result is demonstrated for preliminary data interpretation purposes. From the preliminary laboratory tests, the results show the soil-embedded sensing fibre arrangement has efficiently detected and measured the horizontal strain deformation due to loading. Therefore, it can be concluded that the sensing fibre was well-responded with the soil movement under loading impact.
Many collapses or severe damages in minarets occur due to large structural deformations during earthquakes. Minarets, which are part of the cultural heritage, require particular attention and appropriate methodologies for their strengthening. The present study proposes a cost-effective seismic strengthening technique, which includes minimal intervention, better workability and reduced cost, for brick-masonry, stone-masonry and Reinforced Concrete (RC) minarets. Fabric Reinforced Cementitious Matrix (FRCM) composites are used in the proposed strengthening technique. 3D finite element models of the minarets with and without strengthening are created considering the minaret-foundation-soil interaction with infinite elements. The nonlinear behaviors of masonry units, concrete and soil materials are modeled using Concrete Damage Plasticity (CDP) and Mohr-Coulomb failure models. The idealized stress-strain diagram for FRCM composite is considered in the analyses. Tie contact is considered between the soil-foundation and FRCM composite layer-wall interfaces. Three earthquake acceleration records selected for the nonlinear seismic analyses are matched to the target response spectrum, and the deconvolved acceleration records are obtained from the matched records. The nonlinear seismic analyses of brick-masonry, stone-masonry and concrete minarets with and without strengthening are implemented under the combined horizontal and vertical ground motions.
Single-measuring point deformation monitoring model is the most popular method in dam health monitoring. Considering that single-point monitoring model cannot comprehensively reflect the overall deformation properties of dams, a spatiotemporal hybrid model of multi-point deformation monitoring for concrete arch dams is proposed. Meanwhile, considering the chaotic effect of residual series, the support vector machine optimized by particle swarm optimization algorithm (PSO-SVM) is adopted to analyze and forecast the residual series. Hence, a spatiotemporal hybrid model for concrete arch dam deformation monitoring considering the chaotic effect of residual series is proposed in the study. Based on the theory of single-measuring point deformation monitoring, a spatiotemporal hybrid model is established by introducing space coordinate and calculating hydraulic component with finite element method. Then, with the good nonlinear processing ability of PSO-SVM, the chaotic effect of residual series is analyzed and predicted by PSO-SVM. Subsequently, a spatiotemporal hybrid model for concrete arch dam deformation monitoring considering chaotic effect of residual series is established by superimposing the residual prediction term with the predicted value of the spatiotemporal hybrid model. Engineering example show that the proposed model has better fitting and predicting precisions compared with the conventional single-point monitoring models, and it can analyze and predict the deformations of multi-point simultaneously. In addition, the proposed model reduces the workload of modelling point by point in single-point monitoring model, which considerably improves the practicality and computational efficiency of deformation-based health monitoring of concrete arch dams.
A smoothed inverse finite element method (iFEM(s)) is developed by coupling the inverse finite element method (iFEM) and the smoothing element analysis (SEA) for real-time reconstruction of displacement field utilizing a network of discrete strain-sensor measurements. This reconstruction is commonly referred to as ''shape sensing". The shape-sensing capabilities of iFEM(s) in multilayered composite and sandwich structures are validated using both numerical and experimental strain data. The iFEM(s) approach first recovers continuous (smoothed, full field) strains from discrete strain measurements and subsequently employs these strains in the least-squares variational principle to obtain the deformed structural shape. To model through-the-thickness displacement distributions accurately, the kinematic relations of the refined zigzag theory (RZT) are incorporated into the mathematical formulation of iFEM(s). The least-squares functional accommodates the membrane, bending, zigzag, and full transverse-shear section strains. Moreover, simplified forms of this functional are derived for both woven composite and sandwich structures. Subsequently, a four-node quadrilateral inverse-plate element, iRZT4, is implemented for discretization of the geometry and approximation of kinematic variables. The high accuracy of present computational framework is successfully demonstrated by performing shape-and stress-sensing analyses using numerical strain data. Then, the predictive capabilities of iFEM(s) are also explored on a twill-woven wing-shaped sandwich laminate using experimental strain measurements from surface mounted strain gauges and embedded fiber Bragg grating (FBG) sensors. Finally, the improved shape-sensing predictions of iFEM(s) for both numerical and experimental cases are compared to the conventional iFEM application.
This paper introduces the inverse finite element method using simple brick elements that can be used for shell analysis. The proposed element is the inverse counterpart of an existing Lagrangean-based “direct” trilinear hexahedral finite element that uses the approaches of reduced integration, assumed natural strains and enhanced assumed strain to prevent locking defects in shell modeling. Like the standard trilinear hexahedral element, this locking-free element has eight vertex nodes and three displacement degrees-of-freedom per node. It also has one scalar enhanced-strain degree-of-freedom, which is eliminated at the element level. Both inverse and direct finite element formulations are identical up to the definition of the Lagrangean-based equilibrium equations. For the inverse approach, these equations have as unknowns the positions of the nodes in the undeformed configuration. The current approach is particularly well suited for a category of inverse problems where a given shape must be attained after large elastic deformations. This is the case in the design of turbine blades, to be developed here.
Majority of the existing dam deformation monitoring models focus on the prediction of individual displacement, and ignore the spatial correlation of data. In this study, we propose a method dealing with multi-target prediction called the Maximum Correlated Stacking of Single-Target. The proposed method can provide reliable predictions of multi-target simultaneously, while fully exploiting the internal relationships between target variables via the strategy of targets stacking. Moreover, it can be coupled with different existing baseline models for the prediction and anomaly detection of arch dam deformation.
Jinping–I arch dam is taken as a case study, where the monitoring displacement of 23 different points are analyzed and modeled simultaneously. Three kernel-based machine learning algorithms (i.e., support vector machine, relevance vector machine, and kernel extreme learning machine) and the partial least squares regression are adopted as baseline models for multi-target regression methods. Compared with the single-target regression and two state-of-the-art multi-target regression methods, the simulated results reveal the higher accuracy of the proposed method. Furthermore, model performance is validated in terms of anomaly detection capability, where two progressive anomalous scenarios (i.e., anomalies of single or multiple points) are investigated. The proposed method can be adapted for the health monitoring of other infrastructures in which multiple responses (e.g., displacement, temperature, or stress) need to be predicted simultaneously.
The inverse finite element method (iFEM) is advisable as accurate and efficient method for structure health monitoring (SHM). The previous standard iFEM uses 0th-order and 1st-order shape functions to reconstruct displacement fields under concentrated load and distributed load, respectively. However, in engineering environmental loads, it is difficult to determine which external load is mentioned above. To solve this problem, this paper proposes an improved iFEM based on isogeometric analysis (IGA), which can achieve a unified description of the displacement fields under the effect of concentrated load, distributed load, or concentrated-distributed-hybrid load. Initially, B-spline functions of order two are employed to reconstruct the strain fields. Then, the shape function order of translations and rotations are deduced based on the section strain theory. Next, these deduced displacement shape functions are further used to reconstruct the deformation field using strain measurement. Finally, a wing structure is subjected to concentrated and distributed loads in simulation analysis and experimental tests. The result shows that improved iFEM is more accurate and effective than standard iFEM in displacement reconstruction.
The distributed fiber optic sensing technology has emerged as a promising tool for monitoring soil shear deformation. However, the question remains whether the strain measurements obtained by the soil-embedded optical fiber cables are reliable due to cable-soil slippage. In this paper, a direct shear model test was conducted in laboratory to characterize the cable-soil deformation compatibility considering different anchorage conditions. The optical frequency domain reflectometer was employed to measure cable strains. The measurements were compared with those from the particle image velocity technique. When the strain sensing cable couples well with soil, the cable elongation exhibits a linear relationship with the soil shear displacement. A strain integration method is then proposed to convert strain measurements into shear displacements. Furthermore, the effect of block and tube anchors on the cable-soil coupling behavior is investigated. The conclusions drawn in this study provide a reference for establishing new geotechnical deformation monitoring systems.
With the increasing popularity of wind energy, offshore wind turbines (OWTs) are currently experiencing rapid development. The tower is one of the most significant components of the OWT. However, the tower will not only stand its own weight and weight of the top structure, but also be surrounded by harsh wave and wind loading conditions. Therefore, it is necessary to apply a structural health monitoring (SHM) system to monitor the health condition of the OWT towers in real-time. In this study, inverse Finite Element Method (iFEM) is applied to monitor the tower of an OWT under both static and dynamic loading conditions. The total displacements and von Mises stresses obtained from iFEM analysis are compared against reference results and optimum sensor locations are determined.
The inverse finite element method (IFEM),which is used to reconstruct the displacement field from the discrete surface strain measurements, is of great significance to the management, control and driving of smart structures. However, the iFEM method based on constant cross-section beam elements proposed in previous works were no longer suitable for variable cross-section beam elements. To solve this problem, this paper proposes a new iFEM method for reconstructing the displacement field of variable cross-section beam based on isogeometric analysis. Firstly, the mechanical parameters of beam section are linearized, including section area, axial rigidity, shear rigidity, torsional rigidity and bending rigidity, and a new constitutive relations are established. Then, adhering to the constitutive equations and the small-strain hypothesis, the displacement equations of the theoretical deformation field are deduced. Nevertheless, considering that the deduced displacement equations can not be applied to the iFEM, this paper proposes a method for using isogeometric analysis instead of the original function, and the least-square method is used to establish the strain-displacement relation. Finally, to verify the validity and accuracy of the methodology, a concentrated load and a distributed load were applied to one airfoil in the experiment tests. The predicted displacements with previous iFEM and presented iFEM are compared with those experimentally measured values, respectively. The results show that the presented iFEM exhibited higher accuracy than the previous iFEM in the variable cross-section beam problem.
Prediction of displacement or strain is an important means and factor for evaluating the safety of geotechnical structures, such as slopes, dams, tunnels and excavation engineering. In recent years, fiber optic displacement sensors have been extensively used in civil engineering due to their obvious advantages of light weight, high precision, strong durability, wide measurement range and long-distance transmission. This paper reviews the development of two common types of fiber optic sensors (fiber Bragg grating sensors and bend loss based fiber optic sensors) for geotechnical health monitoring, and the characteristics and the state of the art research were analyzed and discussed in details. Based on the measured strains, three algorithms for transforming monitored data to required displacement were investigated. Comparison analysis regarding typical advantages and disadvantages of these fiber optic sensing technologies for geotechnical health monitoring was also presented and discussed in this paper.
Excessive subgrade permanent deformation resulting from heavy vehicle traffic and freezing–thawing cycles is one of the primary causes of road damage in seasonally frozen regions. In this paper, an implanted cantilever sensing beam based on fiber Bragg gratings (FBGs) is proposed for subgrade deformation monitoring. Two optical fibers with inscribed FBGs were adhered to the opposite external sides of a PVC pipe which was selected as substrate. After laboratory calibration tests, the cantilever sensing beam was installed horizontally through the subgrade of a testing section. The beam will be bended driven by the deformation of the soils under the action of vehicles, and the deformation is recorded by the FBGs and calculated through principles of continuum mechanics. The subgrade deformation is considered to be the deflection of the sensing beam. The feasibility of the developed method was validated based on the results of field testing conducted for a three‐year period. It has been found that the combination of freezing–thawing cycles with heavy vehicle loading magnifies the extent of permanent subgrade deformation. The developed FBG‐based implanted cantilever sensing beams are thereby demonstrated to provide a feasible and effective approach for the in situ monitoring of subgrade deformation.
In geotechnical engineering, it is critical to measure and control ground settlement. A new method is proposed in this work for quasi-distributed ground settlement monitoring. This method is based on the pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA). The optical fiber is arranged on the monitoring poles through special supports, forming a zigzag pattern. Based on geometric principles, the formulas converting strains in fibers to displacements of supports are derived. Afterwards, the feasibility of the method is verified through laboratory tests. The results show that the proposed method can achieve accuracy within 1 mm. Compared with traditional discrete-point measurement methods, the proposed method can monitor settlement up to 100 points simultaneously with satisfactory precision, facilitating remote and large scale ground settlement monitoring in geotechnical engineering applications.
The methodology known as "shape sensing" allows the reconstruction of the displacement field of a structure starting from strain measurements, with considerable implications for structural monitoring, as well as for the control and implementation of smart structures. An approach to shape sensing is based on the inverse Finite Element Method (iFEM) that uses a variational principle enforcing a least-squares compatibility between measured and analytical strain measures. The structural response is reconstructed without the knowledge of the mechanical properties and load conditions but based only on the relationship between displacements and strains. In order to efficiently apply iFEM to the most common structural typologies of civil engineering, its formulation according to the kinematical assumptions of the Bernoulli-Euler theory is presented. Two beam inverse finite elements are formulated for different loading conditions. Depending on the type of element, the relationship between the minimum number of required measurement stations and the interpolation order is defined. Several examples representing common applications of civil engineering and involving beams and frames are presented. To simulate the experimental strain data at the station points and to verify the accuracy of the displacements obtained with the iFEM shape sensing procedure, a direct FEM analysis of the considered structures is performed using the LUSAS software.
Known as "shape sensing", real-time reconstruction of a structure's three-dimensional displacements using a network of in situ strain sensors and measured strains is a vital technology for structural health monitoring (SHM). The inverse finite element method (iFEM) is a mechanics-based shape-sensing algorithm shown to be fast, accurate, and robust for usage as a part of SHM systems. In this study, a new eight-node curved inverse-shell element, named as iCS8, is developed based on iFEM methodology. The kinematic relations of iCS8 element are established through combining kinematics of solid shell together with kinematic assumptions of first-order shear deformation plate theory. The new weighted-least-squares functional of iFEM uses the complete set of section strains consistent with the iCS8 element, i.e., (1) coupled membrane-bending and (2) transverse-shear section strains. The iCS8 element accommodates a curvilinear isoparametric coordinate system, thus it can be effectively utilized to model cylindrical/curved geometries with a coarse discretization. This practical modelling capability can allow a relatively sparse placement of sensors, therefore providing an advantage for real-time shape sensing of curvilinear geometries. The high accuracy and practical utility of the iCS8 element is demonstrated for different cylindrical marine structures through examining coarse iCS8 discretizations with dense and sparse sensor deployments.
Soil, as an indispensable composition for building foundation materials, plays a significant role in safety and service life of buildings. However, in different regions, soil properties are distinct and complex. With the technology development, studies on soil properties based on soil strain has become important. In this paper, a method of soil strain measurement was proposed based on fiber Bragg grating (FBG) sensing technology. The new FBG soil strain sensor is structurally simple and incorporates clamping devices that are designed to better transfer soil strain. Calibration tests were conducted to verify the performance of the sensor. Then, to satisfy the long-term soil strain monitoring, the sensor's fatigue properties were studied by using an MTS testing machine. Finally, field experiments were carried out, and the experimental results demonstrate that the sensor is sensitive to soil strain and the proposed novel method is effective and reliable for soil strain measurement. In summary, the new FBG soil strain sensor can be embedded in soil for real time measurement of soil strain with good performance and long fatigue life. Benefitting from the FBG technology, many FBG soil strain sensors with different central wave lengths can share one optical fiber, which greatly simplifies the installation and operation when multiple FBG soil strain sensors are deployed.
Slope instabilities are a common occurrence in hilly areas, mine, and dam region, which bring a grave threat to the safety of transportation business and human life. The effective slope monitoring methods and instruments can provide information on the current deformation and historical trends, which is important scientific information for researchers to analyze slope stability and provide a timely warning to the public. Optical fiber sensors (OFS) have attracted many researchers to explore due to their have unique advantages in comparison with conventional electronic based sensing technologies, such as immunity to electromagnetic interference, remote transmission, easy integration and high sensitivity. This paper presents an overview of the latest development and application of OFS based slope deformation monitoring from two aspects: 1) Fiber Bragg grating (FBG) based sensing and monitoring technology; 2) Distributed optical fiber sensors (DOFS) including Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Analysis (BOTDA) for slope monitoring. Meanwhile, current challenges in implementing the OFS-based slope monitoring technology are discussed.
The roof of Dalian gymnasium was designed in the form of suspen-dome structure. A structural health–monitoring system has been developed for the roof structure to guarantee the safety condition during construction process as well as in future service. In this article, a monitoring scheme was proposed in detail according to the mechanical characteristics of the roof structure. Fiber Bragg grating sensors, inclinometers, and accelerometers were applied to measure necessary structural information. In order to interrogate different types of sensors, a novel data acquisition device of the structural health–monitoring system was also introduced and has achieved multitudinous physical variable synchronization acquisition. By analyzing the data obtained during the construction and normal operation of the gymnasium, the structural health condition was evaluated and the structural damage could subsequently be located.
This paper describes an interesting landslide located in the mountainous area of Zhejiang Province, China, which was caused by highway construction. Due to the complex geological conditions, the failure mechanism of the landslide is particularly difficult to analyze. The soil deformations were initially judged to be shallow and partial sliding. However, engineering measures used to stabilize the sliding masses failed. By establishing a comprehensive monitoring system composed of GPS, inclinometers and groundwater level monitors, the deformation of landslide was monitored. It is found that the previous deformation mechanism assumed without the monitoring data is incorrect. On the other hand, there is a large rock ancient landslide in the site, and deformation of the soil mass was caused by the reactivation of this ancient landslide. This case study illustrates the importance to deeply analyze the geological data and the field monitoring data and to correctly understand the geological conditions.
Pipeline is an important structure to transport oil and gas through long distances. However, pipeline also suffers from many threats especially corrosion and leakage. Therefore, it is necessary to conduct pipeline safety monitoring. With the advantage of high precision in distributed strain measurement, the optical frequency domain reflectometry (OFDR) technique is more suitable for pipeline monitoring. In this paper, a new application of the OFDR technique is introduced to monitor both corrosion and leakage. In order to verify this method, simulation tests of corrosion and leakage were conducted. In the corrosion test, several optical fiber sensors were bonded to the pipe surface with the same interval, forming a sensor array. Based on the sensor array, a hoop strain nephogram was created to show the corrosion level and corrosion location. In the leakage test, the results indicated that pipeline leakage can be detected by the distributed optical fiber sensor (DOFS). All the test results demonstrate that it is possible to monitor pipeline corrosion and leakage based on the hoop strain theory and the DOFS.
A novel distributed fiber optic strain sensing technology, named Brillouin optical time-domain analysis (BOTDA), has been used to study the performance of large-diameter bored piles subjected to a slope excavation in Hong Kong. A new installation method for the distributed fiber optic sensors (FOS) in the bored piles was proposed in this study. Distributed strains along the instrumented bored piles were obtained by BOTDA sensors during multi-stage excavations. Details of sensor design, field installations, sensor protections, and data analysis are presented in this paper. Axial and bending strains along the instrumented pile were obtained by two sets of BOTDA sensors installed on diametrical opposite sides along the pile. The calculation method for deriving the lateral deflections from distributed strains is described. Thus, the calculated lateral deflections from BOTDA sensors were compared with the traditional inclinometers which were installed at the center of the same instrumented bored pile. Measurements obtained from the BOTDA sensors were found to be in good agreement with the inclinometer data. The maximum lateral wall deflection over the excavation depth was between 0.05% and 0.1%. The lessons learned from this field implementation are discussed and suggestions provided for further similar applications. Field application in this study reveals that the BOTDA measurement has great potential to be used for performance monitoring of large diameter piles.
The inverse finite element method (iFEM) is an innovative framework for dynamic tracking of full-field structural displacements and stresses in structures that are instrumented with a network of strain sensors. In this study, an improved iFEM formulation is proposed for displacement and stress monitoring of laminated composite and sandwich plates and shells. The formulation includes the kinematics of Refined Zigzag Theory (RZT) as its baseline. The present iFEM methodology minimizes a weighted-least-squares functional that uses the complete set of strain measures of RZT. The main advantage of the current formulation is that highly accurate through-the-thickness distributions of displacements, strains, and stresses are attainable using an element based on simple C0-continuous displacement interpolation functions. Moreover, a relatively small number of strain gauges is required. A three-node inverse-shell element, named i3-RZT, is developed. Two example problems are examined in detail: (1) a simply supported rectangular laminated composite plate and (2) a wedge structure with a hole near one of the clamped ends. The numerical results demonstrate the superior capability and potential applicability of the i3-RZT/iFEM methodology for performing accurate shape and stress sensing of complex composite structures.
Recently developed piezoceramic-based transducers, known as smart aggregates (SAs), have shown their applicability and versatility in various applications of structural health monitoring (SHM). The lead zirconate titanate (PZT) patches embedded inside SAs have different modes that are more suitable for generating or receiving different types of stress waves (e.g. P and S waves, each of which has a unique role in SHM). However, due to the geometry of the 2D PZT patch, the embedded SA can only generate or receive the stress wave in a single direction and thus greatly limits its applications. This paper is the first of a series of two companion papers that introduces the authors' latest work in developing a novel, embeddable spherical smart aggregate (SSA) for the health monitoring of concrete structures. In addition to the 1D guided wave produced by SA, the SSA embedded in concrete structures can generate or receive omni-directional stress waves that can significantly improve the detection aperture and provide additional functionalities in SHM. In the first paper (Part I), the detailed fabrication procedures with the help of 3D printing technology and electrical characterization of the proposed SSA is presented. The natural frequencies of the SSA were experimentally obtained and further compared with the numerical results. In addition, the influence of the components' thickness (spherical piezoceramic shell and epoxy) and outer radius (spherical piezoceramic shell and protection concrete) on the natural frequencies of the SSA were analytically studied. The results will help elucidate the key parameters that determine the natural frequencies of the SSA. The natural frequencies of the SSA can thus be designed for suitability in the damage detection of concrete structures. In the second paper (Part II), further numerical and experimental verifications on the performance of the proposed SSA in concrete structures will be discussed.
Fiber optic sensing technology has been widely used in civil infrastructure health monitoring due to its various advantages, e.g., anti-electromagnetic interference, corrosion resistance, etc. This paper investigates a new method for stiffness monitoring and damage identification of bridges under moving vehicle loads using spatially-distributed optical fiber sensors. The relationship between the element stiffness of the bridge and the long-gauge strain history is firstly studied, and a formula which is expressed by the long-gauge strain history is derived for the calculation of the bridge stiffness. Meanwhile, the stiffness coefficient from the formula can be used to identify the damage extent of the bridge. In order to verify the proposed method, a model test of a 1:10 scale bridge-vehicle system is conducted and the long-gauge strain history is obtained through fiber Bragg grating sensors. The test results indicate that the proposed method is suitable for stiffness monitoring and damage assessment of bridges under moving vehicular loads.
This paper introduces a new measurement technology characterized by the use of distributed optical fiber sensor (OFSs) for monitoring the strain and temperature distribution of glass fiber reinforced polymer (GFRP) bar soil nails. Laboratory tension tests were used to verify the performance of the OFSs for strain and elongation monitoring of GFRP bars. The measured strain data from the OFSs agree fairly well with the data from strain gauges in calibration tests. In field monitoring tests, two GFRP bar soil nails were installed with OFSs and pure strain data were used to evaluate the performance of GFRP bar soil nails after installation in a practical slope. Both the strain and temperature distributions measured by the OFSs show symmetric features. A Brillouin optical time domain analysis (BOTDA) measurement unit was used to collect temperature and strain data from the OFSs. The monitoring data show that the accumulative elongations of the soil nails present a continuous but limited increase with time in the field. The achieved maximum elongations of soil nails were less than 0.4 mm. The measured axial elongations of the soil nails were also validated using corresponding data predicted by a theoretical model. The test results from the present study prove that BOTDA based sensors are useful for the investigation of the average strain distributions (or elongation) of long soil nails and these data are useful for the estimation of the potential sliding surface of the entire soil nailing system.
An optical fiber strain-sensing technique, on the basis of Brillouin optical time domain reflectometry (BOTDR), was used to monitor the performance of a secant pile wall subjected to multiple props during construction of an adjacent basement in London. Details of the installation of sensors and data processing are described. Distributed strain profiles were obtained by deriving strain measurements from optical fibers installed on opposite sides of the pile to allow monitoring of both axial and lateral movements along the pile. Methods for analyzing the thermal strain and temperature compensation are also presented. Measurements obtained from the BOTDR were found to be in good agreement with inclinometer data from the adjacent piles. The relative merits of the two different techniques are discussed.
In recent decades, conventional electric instruments have already been widely used to monitor the performance of geotechnical structures. However, there are several inherent limitations of electric instruments for engineering including: electromagnetic interference, a large number of cables for multipoint measurement, signal loss in long distance transmission, and poor durability. Since the first Fiber Bragg Grating (FBG) sensor was fabricated in 1978, a significant progress has been made on the commercialization of optical fiber sensing technologies. In 1980s, a fully distributed sensing technology named Brillouin Optical Time Domain Analysis (BOTDA) has been proposed and developed for measuring strain and temperature. In this paper, the authors review previous studies on the development and application of fiber optic sensors. Based on the measured strains, various analysis methods were transferred to required parameters such as displacement, force and pressure which can more directly reflect the safety of geotechnical structures under complex engineering stress condition.
In the past few years, fiber optic sensing technologies have played an increasingly important role in the health monitoring of civil infrastructures. These innovative sensing technologies have recently been successfully applied to the performance monitoring of a series of geotechnical structures. Fiber optic sensors have shown many unique advantages in comparison with conventional sensors, including immunity to electrical noise, higher precision and improved durability and embedding capabilities; fiber optic sensors are also smaller in size and lighter in weight. In order to explore the mechanism of seepage-induced slope instability, a small-scale 1 g model test of the soil slope has been performed in the laboratory. During the model's construction, specially fabricated sensing fibers containing nine fiber Bragg grating (FBG) strain sensors connected in a series were horizontally and vertically embedded into the soil mass. The surcharge load was applied on the slope crest, and the groundwater level inside of the slope was subsequently varied using two water chambers installed besides the slope model. The fiber optic sensing data of the vertical and horizontal strains within the slope model were automatically recorded by an FBG interrogator and a computer during the test. The test results are presented and interpreted in detail. It is found that the gradually accumulated deformation of the slope model subjected to seepage can be accurately captured by the quasi-distributed FBG strain sensors. The test results also demonstrate that the slope stability is significantly affected by ground water seepage, which fits well with the results that were calculated using finite element and limit equilibrium methods. The relationship between the strain measurements and the safety factors is further analyzed, together with a discussion on the residual strains. The performance evaluation of a soil slope using fiber optic strain sensors is proved to be a potentially effective approach.
Recently fiber-optic sensing technologies have been applied for condition monitoring of geo-structures such as slopes, foundations, and earth-retaining walls. However, the validity of measured data from soil-embedded optical fibers is strongly influenced by the properties of the interface between the sensing fiber and the soil mass. This paper presents a study of the interfacial properties of an optical fiber embedded in soil with an emphasis on the effect of overburden pressure. Laboratory pullout tests were conducted to investigate the load-deformation characteristics of a 0.9 mm tight-buffered optical fiber embedded in soil. Based on a tri-linear interfacial shear stress-displacement relationship, an analytical model was derived to describe the progressive pullout behavior of an optical fiber from soil matrix. A comparison between the experimental and predicted results verified the effectiveness of the proposed model. The test results are further interpreted and discussed. It is found that the interfacial bond between an optical fiber and soil is enhanced under high overburden pressures. The apparent coefficients of friction of the optical fiber/soil interface decrease as the overburden pressure increases, due to the restrained soil dilation around the optical fiber. Furthermore, to facilitate the analysis of strain measurement, three working states of a soil-embedded sensing fiber were defined in terms of two characteristic displacements.
Extensive studies of Norwegian rock-slope failure areas support a subdivision into three principle types: (1) Rockfall areas, (2) rockslide areas, and (3) complex fields. The classification is based on structural geometry and style of deformation, slope gradient, and the volumes involved. Rockfall areas, or toppling sites, are found in sub-vertical mountain sides. One or more unstable blocks are bound by a steep crevasse near the slope edge that is nearly cliff-parallel, whereas cliff-oblique extension fractures limit the blocks laterally. Rockslide areas are found on moderately dipping slopes, where slope-parallel, basal sliding planes, or detachments (along foliation, exfoliation surfaces), bound unstable blocks. The block size is controlled by steep fractures. Complex fields reveal complicated structural geometries and a rough morphology. Typical structures include a back-bounding graben, trenches and local depressions, fault scarps, crevasses, rotated fault blocks and deep-seated, low-angle detachments. They also involve significantly larger rock volumes (> 10 mill m3) than the other types. Complex fields can be subdivided into either listric or planar styles based on their internal fault geometry. Several structural features are diagnostic for areas undergoing rock-slope deformation above a basal detachment. On a large scale, the areas consist of detached blocks resting on a low-angle fault (rock on rock) or fault-rock layer (membrane) above non-deformed bedrock. Fault rocks appear to be common. This is consistent with a two-layer model, with an upper layer of more or less fractured bedrock, and a lower detachment layer of non-cohesive fault rocks that have mechanical properties more like soil (soft sediments). Development of non-cohesive fault breccia and gouge along a basal detachment, and especially if altered to clay, may drastically reduce the stability of a rock-failure area. Driving forces and deformation mechanisms in rock-slope failure areas can be evaluated from short-term factors, such as seismic activity, water pressure and/or frost-related processes. In addition, an important long-term factor is gradual change in mechanical properties of slide planes. High water pressure and/or frost wedging are presumably most important for present day rockfall areas. Gradual reduction in the shear resistance of a detachment layer in combination with water pressure and freeze-thaw processes are probably critical aspects of rockslide areas and complex fields. Seismic activity above a critical surface acceleration could cause the final triggering leading to an avalanche for all types of rock-slope failures.