Pj Bennett

Performance monitoring of a secant piled wall using distributed fiber optic strain sensing

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.

Monitoring Twin Tunnel Interaction Using Distributed Optical Fiber Strain Measurements

In this field trial, a new monitoring technique using distributed strain sensing known as Brillouin optical time-domain reflectometry (BOTDR) was introduced to monitor the behavior of bolted-concrete linings of a recently completed tunnel when a second bored tunnel was constructed side by side at a distance less than one tunnel diameter apart. This was done by measuring circumferential strains in 12 rings using optical fiber that was installed using the point-fixing method. The strain distributions around the circumference of the rings show a generally similar profile. Maximum compressive strains measured below the tunnel springline nearest to the excavated tunnel were larger than the maximum tensile strains measured at the tunnel crown, distorting the circular tunnel into an oval/ellipsoid that was about symmetrical to the horizontal axis. Several methods were introduced to compare strain measurements made by BOTDR and diameter changes recorded by tape extensometer. This involves the use of a symmetrical tunnel distortion model and the basic differential equation for a circular arch. The calculated results showed some degree of similarity between the two methods. The analysis highlighted the importance of measuring the axial strain in the tunnel ring and subtracting the axial strain component to calculate the actual deflection of the lining as a result of bending.

Distributed optical fiber strain sensing in a secant piled wall

An optical fiber strain sensing technique, based on Brillouin Optical Time Domain Reflectometry (BOTDR), was used to obtain the full deformation profile of a secant pile wall during construction of an adjacent basement in London. Details of the installation of sensors as well as data processing are described. By installing optical fiber down opposite sides of the pile, the distributed strain profiles obtained can be used to give both the axial and lateral movements along the pile . Measurements obtained from the BOTDR were found in good agreement with inclinometer data from the adjacent piles. The rela tive merits of the two different techniques are discussed.

The use of fibre optic sensors to monitor pipeline response to tunnelling

This paper describes the use of fibre optic sensing with Brillouin Optical Time-Domain Reflectometry (BOTDR) for near-continuous (distributed) strain monitoring of a large diameter pipeline, buried in predominantly granular material, subjected to a pipe jack tunnelling operation in London Clay. The pipeline, buried at shallow depth, comprises 4.6 m long sections connected with standard bell and spigot type joints, which connect to a continuous steel pipeline. In this paper the suitability of fibre optic sensing with BOTDR for monitoring pipeline behaviour is illustrated. The ability of the fibre optic sensor to detect local strain changes at joints and the subsequent impact on the overall strain profile is shown. The BOTDR strain profile was also used to infer pipe settlement through a process of double-integration and was compared to pipe settlement measurements. The close approximation of the measured pipe settlement provides further confidence in fibre optic strain sensing with BOTDR to investigate the intricacies of pipeline behaviour, pipe-soil interaction and interaction between pipe sections when subjected to ground movement.

Fibre optic installation techniques for pile instrumentation

An innovative technique based on optical fibre sensing that allows continuous strain measurement has recently been introduced in structural health monitoring. Known as Brillouin Optical Time-Domain Reflectometry (BOTDR), this distributed optical fibre sensing technique allows measurement of strain along the full length (up to 10km) of a suitably installed optical fibre. Examples of recent implementations of BOTDR fibre optic sensing in piles are described in this paper. Two examples of distributed optical fibre sensing in piles are demonstrated using different installation techniques. In a load bearing pile, optical cables were attached along the reinforcing bars by equally spaced spot gluing to measure the axial response of pile to ground excavation induced heave and construction loading. Measurement of flexural behaviour of piles is demonstrated in the instrumentation of a secant piled wall where optical fibres were embedded in the concrete by simple endpoint clamping. Both methods have been verified via laboratory works.

Novel polarization selector based on an active waveguide photonic bandgap structure

We describe a photonic bandgap polarization selector based on a photonic crystal placed at junction of two 90° intersecting waveguides to form an ultra-compact device. The photonic crystal consists of 7 layers of a triangular lattice with a radius to pitch ratio (r/a) of 0.24 and a lattice constant of 0.386μm. The PBG is orientated so that the light is incident and collected at 45° to the Γ-K crystallographic direction. Modeling of the PBG shows that TM polarized light is strongly reflected while TE light passes largely into the crystal. Measurements of the fibre-to-fibre transmitted power of the device for each polarization show that the TM collected power is ~6dB higher than the TE light for equal input polarization powers. Further evidence of the strong reflection of TM light comes from an equivalent sample without a 2-D lattice at the waveguide junction. In these samples, no TM light is detected at the output. Furthermore, by taking into account the TE and TM gains within the active waveguides, the TM to TE polarization selection of the PBG is estimated to be up to 22dB.

Analysis of fiber-optic strain-monitoring data from a prestressed concrete bridge

© 2017, American Society of Civil Engineers (ASCE). All rights reserved. This paper presents data from fiber-optic strain monitoring of the Nine Wells Bridge, which is a three-span, pretensioned, prestressed concrete beam-and-slab bridge located in Cambridgeshire in the United Kingdom. The original deployment at the site and the challenges associated with collecting distributed strain data using the Brillouin optical time domain reflectometry (BOTDR) technique are described. In particular, construction and deployment issues of fiber robustness and temperature effects are highlighted. The challenges of interpreting the collected data as well as the potential value of information that may be obtained are discussed. Challenges involved with relating measurements to the expected levels of prestress, including the effects due to debonding, creep, and shrinkage, are discussed and analyzed. This paper provides an opportunity to study whether two commonly used models for creep and shrinkage, adequately model data collected in field conditions.

Semiconductor laser incorporating a short two-dimensional grating

A device has been demonstrated which for the first time incorporates a short two-dimensional grating in the ridge of a ridge-waveguide semiconductor laser. The device used for this work is a standard Fabry-Perot ridge-waveguide laser diode operating at 1.3 /spl mu/m. The active layer contains seven InGaAsP quantum wells. By positioning the grating in the ridge of the laser there is a strong interaction with the lasing mode and only a short grating length is required to dramatically alter the performance of the laser. The output spectrum of the device shows it is lasing on a single longitudinal mode at a considerably shorter wavelength than before the grating was incorporated. Additional structure is observed from the grating and the temperature dependence of the lasing mode is reduced to that expected from a distributed-feedback semiconductor laser. This shows the dominance of the grating in determining the laser characteristics. Incorporating structures into the cavity in this way may allow enhanced functionality in future devices.

Research Data supporting "Analysis of Fiber-Optic Strain-Monitoring Data from a PrestressedConcrete Bridge"

This dataset contains fibre-optic strain and temperature readings taken from two pre-stressed concrete beams on the Nine Wells road-over-rail bridge, located to the south of Cambridge. Each of these beams was instrumented with three fibre-optic cables prior to pouring of the concrete. One of the cables is used to measure strains caused by temperature, while the other two are sensitive to both mechanical strain and temperature strain. Readings were taken on various dates in July and August 2008.

Novel single mode laser operation through transverse mode coupling in a 2-D photonic crystal

An ultrashort 1.5 /spl mu/m photonic crystal is integrated with a laser diode to give single-mode hop-free operation. Theoretical calculations confirm for the first time the role of the spatially dependent spectral response for frequency selection.