An-Bin Huang

Development of a fibre Bragg grating sensored ground movement monitoring system

Monitoring ground movement manually by lowering an inclinometer probe (IP) in a grouted-in-place casing has been in practice for decades. It is possible to install multiple IP units at various depths in an inclinometer casing to allow automated data logging. The high cost and other problems associated with transmitting signals electrically make an automated IP rather impractical for long term and/or massive deployment. In addition to its small size, available technologies allow optical signals to be transmitted over many kilometres and not be affected by electromagnetic interference. The optical fibre Bragg grating (FBG) is one of the many available fibre optic sensor technologies. The authors used FBG as a sensing medium and developed a ground movement monitoring device referred to as the FBG segmented deflectometer (FBG-SD). The FBG-SD, designed to be inserted into the conventional inclinometer casing, measures the relative deflection between the segments. The amount of lateral movement is computed on the basis of the distribution of the segmented deflections. To verify the effectiveness of the new system, the authors performed indoor as well as field experiments where results from FBG-SDs were compared with conventional IP readings. This paper introduces the design principles of the FBG-SD and describes the performance of the system in their indoor and field applications.

Development of an axisymmetric field simulator for cone penetration tests in sand

The calibration chamber has been an important research tool in establishing interpretation procedures for cone penetration tests (CPTs) in sand. A new simulator system capable of simulating axisymmetric field conditions has been developed. This system is an improvement over conventional calibration chambers and enables CPTs to be calibrated under minimal boundary effects. The new simulator consists of a stack of 20 rings to house the sand specimen. These rings are lined with an inflatable silicone rubber membrane on the inside. The boundary expansion and stress are measured and individually controlled, respectively, at each ring level during the CPT. The soil from physical boundary to infinity is simulated using a nonlinear cavity expansion curve derived from a lateral compression test on the specimen. Results from a series of CPTs show that, at relative densities of approximately 80%, the cone tip resistance values agree within 4% as the diameter ratio of the physical specimen over cone varies from 18 to 22. This paper describes the unique features of this field simulator, presents available CPT data performed in the simulator, and discusses its implications on future calibration tests.

A CALIBRATION CHAMBER FOR COHESIVE SOILS

The paper describes the design and performance of a calibration chamber system for testing in-situ test devices in cohesive soils. Techniques for preparing “undisturbed” clay specimens from a slurry and consolidating them under zero lateral strain K0 conditions are presented. The chamber was used for strain controlled model pressurementer tests during which pore pressure was monitored. Duplicate model pressuremeter tests performed in normally consolidated (NC) kaolinite (σ′vc = 276 kPa) resulted in essentially identical expansion curves. The specimens had an average water content of 44.9% and a coefficient of variation of 1.5%. Also, the predicted lateral stresses essentially coincide with those applied to the chamber specimen.

Development of a chirped/differential optical fiber Bragg grating pressure sensor

With its unique capabilities, the optical fiber Bragg grating has been used as a key component in the development of many sensors. Incorporating the theory of thin plates, the authors have developed an FBG-based pressure sensor by strategically attaching FBGs on the surface of a thin circular plate. The flexural strain in the circular plate induced by pressure applied to the circular plate is sensed by either a single FBG placed radially crossing a neutral point, or two FBGs placed respectively in zones where the strains are of opposite signs. When one FBG is used (i.e., the chirped FBG design), the applied pressure relates to the change in the chirped bandwidth of the FBG reflected waveform. When two FBGs are used (i.e., the differential FBG design), the pressure experienced by the circular plate is correlated to the difference in central wavelength from the two FBGs. In either case the sensing mechanism is immune to temperature fluctuation. The same configuration can potentially be applied for other purposes such as a load cell or displacement transducer. This paper describes the design principles of the FBG pressure sensor and demonstrates its capabilities through laboratory calibrations over a wide range of temperatures.

A Fiber Optic Sensored Triaxial Testing Device

The physical quantities involved in a triaxial testing device have mostly been monitored with electric sensors. These sensors are currently subject to short circuit when submerged under water and electromagnetic interference (EMI). Waterproofing and EMI noise filtration have often been a challenge to the triaxial test set-up. These drawbacks can be substantially minimized when using optic fiber sensors. The optic fiber Bragg grating (FBG) sensors have the additional advantage of being partially distributive where multiple sensors can share the same signal transmission line. Taking advantage of these unique capabilities, the authors explored the possibility of converting all pressure/force and linear displacement transducers in a triaxial testing device into FBG based sensors. A series of shearing tests on unsaturated and saturated soil specimens were carried out using the new FBG sensored triaxial testing device. In most cases, the measurement of physical quantities was paired with electric sensors so that the results can be compared. This paper describes the principles of the individual FBG sensor designs and demonstrates their applications in triaxial testing.

A micromechanical study of penetration tests in granular material

Abstract Penetration tests such as the cone (Campanella and Robertson (1988), Proc. of the 1st Int. Symp. on Penetration Testing , Vol. 1, p. 93) or flat dilatometer (Marchetti (1980), J. Geotech. Eng. Div. 106, 299) tests are important field testing methods in characterizing soil properties. For granular materials, field testing is essentially the only alternative because of the difficulty in obtaining undisturbed samples. However, lacking an understanding of the penetration mechanisms, the interpretation of these field tests are empirical. A study was carried out to improve our understanding of penetration mechanisms taking advantage of the recent developments in micromechanical behavior of granular soils. A numerical technique which couples the distinct element method and boundary element method was developed to simulate two-dimensional penetration tests in a granular material. A series of simulated penetration tests of a 60° apex angle penetrometer were performed in a normally consolidated granular deposit. Theresults showed that as the penetrometer passed the soil element it experiences a stress loading and unloading. The distribution of contact forces concentrates in the vertical direction below the penetrometer. The direction of particle contacts and contact forces rotates towards the penetrometer tip. The average coordination number does not vary significantly around the penetrometer. It appears that the finer particles experience stresses resulted from contact forces up to three times those of larger particles.

Ground movement monitoring using an optic Fiber Bragg Grating sensored system

The authors have developed a fiber optic ground movement monitoring system using the optic Fiber Bragg Grating (FBG). A series of FBGs are glued to the outside of flexible plastic elements. These flexible elements are connected together to form a single probe. When the flexible element is bent, the FBGs sense the flexural strain as a result of the bending. Twice integration of the strains along the longitudinal axis of the probe yields the distribution of the displacement of the monitoring probe associated with the bending. The sensitivity and range of allowable bending of the monitoring probe can be adjusted according to the need in the field. The FBG based monitoring system has been experimented to measure the displacement distribution of a laterally loaded pile in Yuin-Lin, Taiwan. This paper describes the principles of the FBG sensor monitoring probe system and presents a case of field application of the sensor system.

Development of a Multiple-Purpose Borehole Testing Device for Soft Rock

The authors developed a versatile in situ testing device specifically for soft rock. The main purpose of this device was to provide design parameters for shallow or deep foundations in soft rock. The new device, referred to as the Borehole Testing Device (BTD) consisted of four radial curved platens at the top and a circular steel plate at the bottom. The BTD was designed to be used in a 200-mm-diameter borehole. Driven by a high stress pressuremeter and a hydraulic piston, the BTD can be used to perform a borehole jacking test, plate-loading test, and borehole shear test in the same borehole. A multistage testing procedure was proposed to perform the BTD test. This paper describes the design and operation of the BTD and presents a set of test data to demonstrate the capabilities of the BTD.

Quadruple LVDT local rotational displacement measurement in torsional shear tests

Experience has shown that to obtain pre-failure deformability of geomaterials in laboratory element tests, it is imperative to make local strain measurements. For torsional shear tests, the local measurement is complicated by coupling of the axial, radial, and rotational movement experienced by the soil specimen during shear. The rotational displacement must be isolated from other modes of movement for the interpretation of test results. Existing techniques designed for torsional shear tests under general stress conditions can be complicated and difficult to use. A simplified scheme using two pairs of linear variable displacement transducers (LVDTs) is proposed for local rotational deformation measurement, specifically for cyclic torsional shear tests under constant normal stress conditions. The subtraction of readings within an LVDT pair nullifies displacements at the measurement point caused by axial and radial movement or bulging of the specimen. The same subtraction accumulates and thus isolates LVDT readings caused by rotational movement. The error associated with the simplified set-up is expected to be <0.125%. A series of hollow cylinder torsional shear tests have been performed to verify the effectiveness of the new measurement scheme. The results are repeatable and consistent with well-documented test data.

Development of IoT Sensing Modulus for Surficial Slope Failures

To improve the limitations of rainfall-based slope warning system, a new system that integrates the hydro-mechanical slope analysis and wireless sensing module for surficial ground response monitoring is under development. The proposed system aims to establish a customized, time-dependent warning system for shallow slope failures triggered by rainfalls. A coupled hydro-mechanical analysis considering both the hydraulic infiltrations and mechanical responses of unsaturated soils in slope stability analysis is adopted. A real-time, wireless sensing module adopting the internet of things (IoT) technology is developed. The sensing module integrates the micro-electro-mechanical system (MEMS) sensing components with wireless communication modules. The module measures in-situ surface inclination and water content profile and uploads the data to cloud storage. The wireless sensing modules have been deployed in a potential landslide site for more than one year and current progress shows that feasibility of a customized, time-dependent warning system is promising.