Inclinometer Casing Research Articles

A fiber Bragg grating-based inclinometer probe with enhanced sensitivity for a higher slope profiling resolution

A fiber Bragg grating (FBG)-based inclinometer probe with enhanced sensitivity has been developed for slope or ground movement monitoring. The inclinometer probe utilized six FBGs for the tilt measurement and a strain-free FBG that provided the temperature compensation factor. The inclinometer probe was fabricated entirely using a 3D printer and can fit into the standard inclinometer casing, which can be placed into the boreholes. The dimension of the probe is similar to the conventional inclinometer probe, with a total length of 70 cm. Additionally, this design was equipped with three highly compact tilt sensors within the same probe length, providing a better resolution of the inclination profile. Each tilt sensor possesses a flexible middle shaft fabricated using thermoplastic polyurethane (TPU) and was equipped with two FBGs for bi-directional tilt angle measurement (+x and -x). Initially, the tilt sensor was calibrated in the laboratory, which yielded a sensitivity value of 0.0215 nm/°. This value is higher than most previous designs by a factor of two because of the middle shaft's elasticity, which can induce a more significant strain on the FBG. The horizontal displacement of a conventional inclinometer casing could be observed during the field test, which proves the device's functionality. The results have indicated that the inclinometer can be applied in several geotechnical applications, particularly ground movement monitoring.

Slope Monitoring of a Road Embankment by Using Distributed Optical Fibre Sensing Inclinometer

Distributed Optical Fibre Sensor (DOFS) is a versatile sensor considering that it can be used to measure various parameters, such as detection of small strains over large areas and long distances. The application of DOFS is currently being explored as an early warning system, particularly for slope movement detection. In this paper, an embankment slope at Sungai Rual, Jeli, Kelantan, Malaysia was selected to determine the best monitoring layout for landslide detection. Two methods of installation for vertical Fibre Optic (FO) inclinometers (using conventional casing with a special grove) were installed to monitor subsurface horizontal ground movements. The first method of FO inclinometer installation uses concrete (grade 35) grouting, and the second method uses a bentonite mixture. It was found that the first method was unsuccessful due to the fibre breakage during the hardening process of concrete and hence not recommended. However, the measurement was successfully taken for the bentonite mixture method. The displacement reading was taken using both the FO and conventional inclinometer probe. The reading for the 9.5 m depth of the inclinometer casing were recorded for the comparison between FO and conventional inclinometer probes. A comparison between FO and conventional systems showed almost similar results to the conventional method. The FO inclinometer developed in this project shall provide vital information to assist designers and operators, as well as inform stakeholders on the integrity of their structures.

Distributed fibre optic inclinometer with cloud-based monitoring system

Geotechnical instrumentation and monitoring are important aspect of controlling subsurface ground movement, particularly in deep excavations and forewarning of geological disasters such as landslides, land subsidence, ground fissure, and surface collapse. A borehole inclinometer is an example of the established method to monitor such ground movements. For real-time data, a series of in-placed inclinometer probes are installed inside the borehole casing to provide the lateral displacement response over time. The limitation of the conventional system is the high cost and the practicality of inserting a high number of sensing probes into the casing for better measurement accuracy. Recent advancement in Distributed Optical Fibre Strain Sensing (DOFS) enables the incorporation of DOFS as a fibre optic (FO) inclinometer with lateral displacements to be plotted at centimetres spacing and is unaffected by the gauge lengths between the probes. This study enhances the FO inclinometer by developing a web-based application (SmartGeopipe©) to automate data acquisition which also includes complex data processing for the final pipe deflection profile visualisation. The FO inclinometer comprised of optical cables attached externally onto a remodified inclinometer casing and connected to a Brillouin Optical Time Domain Analysis (BOTDA) strain analyser. A standard installation procedure was established to safely secure the sensors during the casing installation process inside the borehole. SmartGeopipe© software is written using a predominantly monolithic Python / Cython backend and a multi-page frontend built with HTML, CSS, and JavaScript ES5 languages. All the computation work and data storing are done in a cloud server and SQL database, respectively. A case study of a station box excavation as part of the Mass Rapid Transit (MRT) tunnelling project was chosen to validate the performance of the FO inclinometer and the web application. Data from the FO inclinometer corroborated well with the manual measurements of the inclinometer probe. This ability supports the realization of an early warning system as an engineering control for hazard management in civil engineering infrastructure projects.

Guidelines from direct shear modeling in centrifuge for TDR landslide monitoring

The suitability of using time-domain reflectometry (TDR) for landslide monitoring and assessment has been validated by field studies. This method can be used for detecting shear surfaces and monitoring shear displacements at depth, which can provide warning of potential slope failures or landslide reactivations. Although laboratory tests have suggested using the TDR reflection coefficient to estimate shear displacement, quantifying the relationship between these parameters is challenging because it is affected by various cable–grout–ground interactions. Investigating these interactions is challenging because of the limited artificial overburden pressure that can be applied in a laboratory. Therefore, this study improved the TDR applicability through high-gravity centrifuge modeling to simulate sliding at depth. Furthermore, a flexible coaxial cable was modified to improve its sensitivity, which was evaluated using the simple direct shear test. The modified coaxial cable with stiff grout had sufficient sensitivity to detect a small-scale shear displacement of 0.5 mm. Reversed-fault centrifuge modeling was conducted for sandy soil to simulate the shear plane under a gravity level of 55 g, and the shear plane was realistically generated and detected by a cable–grout–ground composite and cables located adjacent to an inclinometer casing. Notably, only the model-scale fault movements directly correlated with TDR reflection peaks and integration areas. Finally, guidelines for conducting TDR-based slope monitoring and directions for further research regarding TDR-based landslide centrifuge modeling are suggested.

Principles of using precise inclinometry for landslides

The main goal of this article is to analyze the possibility of using precise inclinometry for landslides. Inclinometric measurements are widely used to monitor landslides, retaining walls, piles and in places where it is necessary to measure deep ground deformations. The value of deep-seated ground deformations is calculated indirectly by using the difference of the inclination of an inclinometer casing installed on a borehole. Inclinometers are used to measure horizontal movements at various levels, usually within earth fills. Inclinometers are used to monitor slopes, indicating movement on a slope before it is visible on the surface of the slope. Inclinometers have often proved very successful in recognizing movement zones and the size, speed and direction of movement not only on slopes, but also on embankments, etc. There are a number of different types of inclinometers, and within each type there are variations produced by different manufacturers. However, the basic principle of precise inclinometry is the same. A guide tube is installed in the borehole and the inclination of the guide tube from the vertical is measured at predetermined intervals. It is measured using a pendulum which is enclosed in a watertight probe. The probe is lowered through the tube. The inclination of the pendulum is measured using electronic devices that are very accurate. The measurement results are determined by pulling out the measuring instrument (inclinometric probe). After assessing the measured values, the course of the profile is determined, if we compare the profiles from different measurements, we can evaluate the direction and size of the shift for the monitored period.

Development of a novel inclinometer by inverse finite element method for soil deformation monitoring

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.

Deformation calculation method based on FBG technology and conjugate beam theory and its application in landslide monitoring

Prediction of displacement is an important means for evaluating the safety of civil infrastructures. In recent years, fiber Bragg grating (FBG) sensors have been widely used to measure structural strain and temperature in civil engineering for their obvious advantages of light weight, high precision, strong durability, wide measurement range and long-distance transmission. However, the strain signal directly measured by the FBG sensors should be transformed into the displacement response by mathematical algorithms, thus indirectly predicting the deformation behavior of the instrumented structure. Therefore, based on the FBG sensing technology and conjugate beam theory, this paper presents a displacement algorithm for estimating structural deformation of simply supported beam and cantilever beam. The finite element (FE) numerical simulation was conducted to compare the displacement errors of the proposed algorithm and previous methods in solving for the deflections of the deformed beams with and without an inflection point, which shown that the proposed calculation method has higher accuracy and wider application scope. Additionally, the authors pasted the FBG sensor arrays on the opposite sides of the neutral axis of the inclinometer casing to fabricate an in-place FBG-based inclinometer. The laboratory calibration tests and field applications were also conducted, and the monitoring results demonstrate that the FBG measurement system using this proposed calculation method can effectively determine the deformation movement of a slope.

Review of Inclinometer Errors and Provide Correction Methods for Bias shift Error and Depth Position Error of the Probe

The inclinometer is now widely used to measure ground movements and has been used successful in projects associated with proper planning, quality tools, ideal installation of inclinometer casings, committed reading, correct processing of data and intelligent interpretation. When using an inclinometer, a wide range of random or systematic errors can occur. According to previous experiences, in these situations, interpreting data becomes difficult and erroneous conclusions may be drawn about the extent, location, and direction of motion. Errors may be due to equipment defects, improper usage, as well as known system limitations or errors. In this study, the author has reviewed the errors in inclinometer systems. Furthermore, by performing tests on the inclinometer casings of Darian Dam, numbers of common errors were presented such as data bias error, rotation error of sensor axis, and probe positioning error in depth, and in addition, the author tried to find out how to correct the incorrect data. The author also describes the correction method of bias error introduced by Mickelson and provides a method for correcting the depth error of the probe positioning. Based on the proposed method, instead of calculating the amount of correction values due to the depth position error of the probe for each measured dataset, the base data (initial reading) are modified with the relationships provided by the author, and then, the displacements were calculated again. The ability of the proposed method was tested by creating an artificial depth error in one of the inclinometers of Darian Dam, as a result of which the mentioned error was corrected the best.

NSHT: New Smart Hybrid Transducer for Structural and Geotechnical Applications

This work describes the application of a new transducer prototype for continuous monitoring in both the structural and geotechnical fields. The transducer is synthetically constituted by a wire of optical fiber embedded between two fiber tapes (fiberglass or carbon fiber) and glued by a matrix of polyester resin. The fiber optical wire ends have been connected to a control unit whose detection system is based on Brillouin optical time-domain frequency analysis. Three laboratory tests were carried out to evaluate the sensor’s reliability and accuracy. In each experiment, the transducer was applied to a sample of inclinometer casing sets in different configurations and with different constraint conditions. The experimental collected data were compared with theoretical models and with data obtained from the use of different measuring instruments to perform validation and calibration of the transducer at the same time. Several diagrams can compare the transducer and highlight its suitability for the monitoring and maintenance of structures. The characteristic of the transducer suggests its use as a mixed system for reinforcing and monitoring, especially in the lifetime maintenance of critical infrastructures such as transportation and service networks, and historical heritage.

Numerical analysis of field geosynthetic-reinforced retaining walls with secondary reinforcement

Geosynthetic-reinforced retaining (GRR) walls typically have vertical reinforcement spacing of 0·6 m, and this relatively large spacing has been known to cause comparatively high connection forces. To reduce this connection force, short geosynthetic reinforcement layers (referred to as secondary reinforcement layers) are installed between blocks where there are no primary reinforcement layers. This paper presents two-dimensional numerical simulations that were developed to analyse an instrumented GRR wall with secondary reinforcement layers in the field. A finite differential software was employed to develop the numerical model. In addition to the Mohr–Coulomb model, the cap yield model based on the theory of hardening plasticity was used to represent the behaviour of backfill. Inclinometer casings, earth pressure cells and strain gauges were installed in the instrumented GRR walls to measure the facing deflections, lateral earth pressures, vertical earth pressures and geogrid strains. The measured results and numerical predictions were compared and discussed, and reasonable agreement between these results was found. Compared to the measured results, the numerical predictions slightly underestimated the maximum wall facing deflections and vertical earth pressures, and slightly overestimated lateral earth pressures and strains in primary and secondary reinforcement layers. For comparison, a numerical model without secondary reinforcement was developed as well. This comparison revealed that the GRR wall with secondary reinforcement resulted in smaller facing deflections and maximum strains in primary reinforcement layers. Overall, the numerical analysis indicated that secondary reinforcement could provide clear benefits in improving the performance of GRR walls.

Effect of longwall-induced subsurface deformations on shale gas well casing stability under deep covers

This paper presents the results of a 2017 study conducted by the National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Mining Research Division (PMRD), to evaluate the effects of longwall-induced subsurface deformations within a longwall abutment pillar under deep cover. The 2017 study was conducted in a southwestern Pennsylvania coal mine, which extracts 457 m-wide longwall panels under 361 m of cover. One 198 m-deep, in-place inclinometer monitoring well was drilled and installed over a 45 m by 84 m center abutment pillar. In addition to the monitoring well, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 457 m-wide longwall panel on the south side of the abutment pillar was being mined. Prior to the first longwall excavation, a number of simulations using FLAC3D™ were conducted to estimate surface subsidence, increases in underground coal pillar pressure, and subsurface horizontal displacements in the monitoring well. Comparisons of the pre-mining FLAC3D simulation results and the surface, subsurface, and underground instrumentation results show that the measured in-place inclinometer casing deformations are in reasonable agreement with those predicted by the 3D finite difference models. The measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models. Results from this 2017 research clearly indicate that, under deep cover, the measured horizontal displacements within the abutment pillar are approximately one order of magnitude smaller than those measured in a 2014 study under medium cover.

FIELD TRIALS ON THE SOIL CEMENT MIXING TECHNOLOGY TO REINFORCE EARTH LEVEES IN THE MEKONG DELTA, VIETNAM

The effectiveness of seepage cutoff and sliding stability of an earth levee reinforced by soil cement deep mixing technology (SCDM) was evaluated by field monitored data. Groundwater observation was conducted via nine ground water wells installed in the research site. Lateral displacement of the whole earth levee was measured by the 15-m inclinometer casing and a 0.5-m inclinometer probe. The research assessed effects of soilcrete reinforcement by comparing with simulations before applying widely in the Mekong Delta, Vietnam. The results indicate that the soilcrete walls can effectively prevent the seepage through the earth levee’s body and improve the factor of safety about 1.2 times.

Comparison of performance of inclinometer casing and TDR technique

TDR (Time Domain Reflectometry) and GPR (Ground Penetrating Radar) are two of the electromagnetic methods in applied geophysics, which using them for various applications are developing. The Time Domain Reflectometry is a remote sensing method that has been used for years to determine the nature of the materials and spatial location. The use of TDR system has led to innovative applications of it and comparing it with previous measuring techniques, since it has developed. In this study, not only a summary of the basics of TDR application for monitoring of ground deformation is offered, but also a comparison of this technology with other measurement techniques (inclinometer casing) is provided. Actually, this paper presents a case study in which the opportunity arose to compare these two technologies in detecting subsurface deformation in slopes. A TDR system includes a radar wave receiver & generator, a transmission line and a waveguide. The generated electro-magnetic pulse moves toward the waveguide within the conductor cable and enters the test environment. For this study, slopes overlooking the Darian dam bottom outlet, power house and spillway were instrumented with RG59/U coaxial cables for TDR monitoring and slope inclinometer. Coaxial cables - as a TDR sensor - and inclinometer casings were installed in a same bore hole where coaxial cable was attached to the inclinometer casing. Shear and tensile deformations of the cable, which is caused by ground movements, significantly impacts on cable reflection coefficient. In Darian dam boreholes, the cable points subject to the shear and stretch were correlated with deformation points of the inclinometer casings in incremental displacement graphs. This study shows that TDR technique is more sensitive than inclinometer casing for small movement in the slide planes. Because manual processing of TDR data is hard and need experienced personnel, the authors have designed an algorithm to compare the shape of the new TDR waveforms with the base reading waveform in order to monitor the subsurface deformations.

Testing of laterally loaded piles in natural sandy gravels

In this paper, the authors present the results of field tests on a single laterally loaded pile and two piles in line. Bored reinforced concrete piles with a diameter of 0·25 m and a depth of 5 m were constructed in natural sandy gravels. The lateral loading on a head of tested piles is applied using a hydraulic press with a maximum force of 500 kN. A single pile and two piles in line were incrementally loaded with a controlled head displacement of 2 mm until the ultimate capacity of the pile was attained. The deflections of the piles were measured using an inclinometer probe through inclinometer casings installed in the axes of the piles. The results are analysed and approximated using the Fourier series and the Cesáro sum technique. The measurements are compared using a three-dimensional finite-element method and Rocscience RS3 software, whereas the modelling soil and pile material are compared using simple constitutive laws. The comparison results of the measured and computed deflections of the piles correlate, which indicates that granular materials, such as sandy gravels, provide acceptable approximation and can be modelled using simple constitutive laws.

Parameters affecting the lateral movements of compound deep cement mixing walls by numerical simulations and parametric analyses

A compound deep cement mixing retaining wall system is a combination of deep cement mixing (DCM) columns and precast reinforced concrete walls. This type of retaining wall was used in a deep excavation for a reservoir construction project in a soft clay area in Thailand. During construction, an inclinometer casing was installed to monitor the lateral movement profiles of this retaining wall system until construction had been completed. Studies on the parameters that affect the lateral movements of retaining walls of this type are limited because of the complex geometry involved. In this paper, a three-dimensional numerical model is first calibrated using an instrumented case history. Then, an analysis of the results for this case history is presented to characterize the wall behavior in terms of the ground settlement induced by wall deflection during excavation and in terms of lateral wall movement. Finally, a parametric study is performed. The results provide information on the influence exerted on the lateral wall movement by the following factors: the elastic modulus of the DCM columns, the embedded length of the DCM columns, the size of the DCM columns, the thickness of the precast wall, the thickness of the soft clay layer and the berm size. The influences of these factors are compared and rated in terms of their degree of importance.

Development and operation of a fiber Bragg grating based online monitoring strategy for slope deformation

Purpose – The purpose of this paper is to investigate an online monitoring strategy that incorporates fiber Bragg gratings (FBGs) for deformation displacement detection, with the background that slope deformation monitoring is crucial to engineering safety supervision and disaster prevention. Design/methodology/approach – A “beam element” method has been proposed, introduced and experimentally verified in detail. The deformation displacement along a flexible bar can be obtained based on this method, using the distributed strain detected by the FBGs embedded in the bar. A novel sensor structure containing inclinometer casings and a series of connected flexible pipes with FBGs embedded has been proposed. Based on the features of this structure, two FBG deformation sensors have been manufactured and installed into a slope. A matched monitoring station which permits real-time supervision, warning and remote access across the Internet was established and operated. Findings – Displacement data from September 2013 to August 2014 are obtained, which is basically consistent with the practical situation. Originality/value – The FBG deformation sensors demonstrated a robust and reliable measurement performance, which is promising for real-time disaster warning in slope engineering.

Effect of Tidal Fluctuation on the Stability of Estuarine Structures in Sarawak

Several incidents of lateral movement and failures of estuarine structures have been reported in Sarawak. These structures located in very soft and deep sedimentary soils are usually supported on pile foundations. There is a 4 to 7 m daily tidal fluctuation in these locations, the effect of which on the ground and the piles is usually neglected in design. A study has been undertaken to formulate improved approaches for the design of riverine and estuarine structures. The validation of a theoretical model requires data on ground movement and pore water pressure changes due to tidal fluctuation. Accordingly, piezometers and inclinometer casings were installed at the sites of two structures where a bridge and jetty are proposed to be constructed. The inclinometers measure the lateral movement of the river banks and a pile installed in the riverbed. The piezometer and inclinometer readings are being recorded periodically. The paper explains the background of the study, case histories of failures, soil conditions at the two sites, details of instrumentation, results of measurement, and the interpretations.

Quantifying Errors in Manual Inclinometer Field Measurements

Inclinometer systems are commonly used to measure horizontal ground displacements at landslides, pile wall construction, tunneling, and excavations. These systems have been in widespread use since the late 1960s (Dunnicliff, 1988). My primary goal with this research was to determine the accuracy and precision achievable with an inclinometer. I also sought to determine the variables that contribute to or reduce accuracy. Inclinometer systems consist of an inclinometer casing (with four vertical grooves spaced 90 degrees apart), a portable traversing probe, a control cable, a readout/data recorder, and monitoring accessories (a pulley and wheel). One groove is selected as the A0 direction, which is typically oriented toward the expected direction of movement. The inclinometer probe used in this research is an English-style probe, with a wheelbase of 24 in. (61 cm) between the upper and lower spring-loaded wheel assemblies. The inclinometer probe is typically run down the casing with the wheels oriented in the direction of anticipated movement. Two accelerometers are mounted within the body of the probe to measure tilt (deviation from vertical). The two accelerometers are perpendicular to each other, with one in-line with the wheel assemblies. The direction 90 degrees clockwise from the A0 direction is designated as B0 (Figure 1). The probe measures its vertical tilt in the A0 and B0 axes. Two sets of readings are taken by lowering the probe to the bottom of the casing and taking readings every 2 ft (0.61 m) as the probe is pulled back up. The probe is then turned 180 degrees, and the process is repeated. These readings are then compared to generate an average orientation value. When the casing is set up, an initial set of readings is taken. Baseline measurements are then documented for the casing, and subsequent readings are compared to these readings to identify changes in conditions. …

Development of novel optical fiber sensors for measuring tilts and displacements of geotechnical structures

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.

Plastic and viscous shear displacements of a deep and very slow landslide in stiff clay formation

Deep and superficial displacements have been measured since 2005 in a slow active landslide that has occurred in a stiff clay formation of the Italian Southern Apennines. Recently, new inclinometer casings have been installed to achieve further information on the displacement field. In this paper, standard inclinometer profiles and fixed-in-place probe data are analysed over time. New data confirm previous hypotheses on geometry and kinematics and add more information on the viscous component of displacements. The mechanism of movement, in most of the track, essentially corresponds to sliding localized on a shear surface, consistently with stress and strength distribution. However, locally, internal viscous deformations also occur, especially in the weathered and softened zones of the landslide, and contribute to the soil discharge continuity. Viscous displacements can be interpreted by a simplified rheological model based on the Bingham equation. The residual shear strength has been considered as the creep threshold value and the dynamic viscosity has been considered dependent on the stress level, on the basis of long term direct shear tests under controlled shear stresses which were carried out in laboratory.