Chang Ye Gue

Fiber-Optic Underground Sensor Networks

Abstract The main motives driving the trend toward increased implementation of structural monitoring systems are the need for structural health monitoring of existing and ageing structures and the desire for a better understanding of increasingly complex designs through performance monitoring of new structures. This drive is sustained by rapid progress in research and technology development on sensors and communications. This chapter presents three case studies focusing on the monitoring of tunnels using distributed fiber-optic sensing. The first case study presents the results of strain development in the sprayed concrete lining (SCL) of a new Crossrail station tunnel during excavation of a cross-passage. The second case study focuses on the short term monitoring of a century old cast iron tunnel during the proximity construction of a large platform tunnel for the Crossrail project. The final case study describes the early stage implementation of a long-term structural health monitoring program to assess the integrity of tunnels at CERN. For all these projects Brillouin-based time domain techniques were used. A brief description of the basic principles of the method and the fiber-optic systems used is presented before the case studies. The variety of optical fibers available today combined with the number of fibre devices, with improved characteristics, recently developed provides a countless potential sensor configurations for environmental monitoring. In this chapter, a background of the use of fiber optic sensor networks in environmental applications are discussed focus in the optical detection for gas leakage sensing, water contamination sensing, soil contamination sensing and mapping with distributed fiber-optic sensors for environment.

Tunnelling close beneath an existing tunnel in clay – perpendicular undercrossing

A series of centrifuge model tests in clay was carried out to investigate the response of an existing tunnel at different clear distances to new tunnelling. A three-dimensional (3D) staged tunnelling model was adopted to simulate a wide range of tail void volume losses for the new tunnel construction while monitoring detailed 3D soil surface settlements and tunnelling-induced strains in the existing tunnel lining. This paper also presents a detailed case study of a similar scenario in the London Underground redevelopment of Bond Street station; various state-of-the-art instrumentation methods, including fibre optic Brillouin optical time domain reflectometry, instrumented tunnel bolts and photogrammetry, were deployed to monitor the response of the existing Royal Mail tunnel due to the new tunnelling works close beneath. The combination of field and centrifuge modelling data provides important new insights into the deformation mechanisms encountered in such complex tunnelling scenarios.

Monitoring the effects of tunnelling under an existing tunnel-fibre optics

Underground tunnel networks are at the heart of United Kingdoms infrastructure, carrying more than 1,100 million passengers each year along its 249 miles long network via 11 underground lines, serving 270 stations.With a complex existing underground rail network already in place; it is inevitable that new tunnels will be constructed within close proximity to existing tunnels. At London Liverpool Street Station, the new eastbound Crossrail platformtunnel was constructed underneath the existing Royal MailTunnel at a parallel alignment over a length of over 100m with a clear distance of less than 2m separating the two. Fibre optic strain sensing system based on Brillouin Optical Time Domain Reflectometry (BOTDR) was installed to measure continuous strain profiles of the cast iron linings of Royal Mail Tunnel. This has provided valuable insights to the deformation mechanisms both during the pilot and final tunnel enlargement.

Distributed fiber optic sensing of axially loaded bored piles

AbstractInstrumented pile tests are vital to establish the performance of a pile and validate the assumptions made during initial design. Conventional instrumentation includes vibrating wire strain...

A tale of two tunnels – understanding the performance of existing and new tunnels during construction works

In many cases, new tunnels in highly populated urban cities like London will need to be constructed underneath a dense network of existing tunnels to avoid them. The construction of new tunnels, however, inevitably results in ground deformations, which are transmitted to adjacent existing structures. The response of existing cast iron tunnels to tunnelling-induced deformation is not well understood and practicing engineers are faced with significant uncertainties about their response. The first part of this paper presents a case study in which distributed fibre optic strain sensing (DFOSS) have been deployed in a section at an existing cast iron tunnel in London underneath which a new, much larger tunnel was being constructed. The DFOSS deployment in this case study provided a detailed response for the cast iron tunnel in a holistic manner that is not encountered in practice. In the second part of the paper, one of the latest case studies being conducted at the Cambridge Centre for Smart Infrastructure and Construction (CSIC), associated with the instrumentation and monitoring of cross-passages in sprayed concrete tunnel linings (SCL) is presented and discussed. The DOFSS provided new data and hence new insights into SCL behaviour that was not available before. doi: 10.12783/SHM2015/197

BOTDR Distributed Fibre Optic Strain Sensing for the Monitoring of an Existing Cast Iron Tunnel

Constructing tunnels in highly congested urban cities is a challenge, as the construction will inevitably take place in close proximity to existing structures both above and below ground. As more tunnels are being constructed, it is inevitable that some tunnels will be constructed in close proximity to existing tunnels and monitoring of the existing tunnels in this case is paramount for the design and construction works. The discrete nature of conventional monitoring instrumentation requires significant interpolation and judgement in order to understand the overall behaviour of the tunnel itself. Subsequently, conservative design approaches will have to be adopted to cater for the gaps in knowledge, which could lead to unnecessary delays and high costs. Distributed fibre optic strain sensing systems based on Brillouin Optical Time Domain Reflectometry (BOTDR) could offer an alternative; in this paper a case study is presented where fibre optic cables were deployed to monitor the response of the cast iron Royal Mail tunnel in the vicinity of London Liverpool Street Station during the construction of Crossrail’s new platform tunnel directly below it. Single mode single core tight-buffered cables were attached directly to the intrados of the cast iron tunnel lining of the Royal Mail tunnel to understand its response during the construction works. This paper focuses on the challenges and considerations in deploying the fibre optic system in the tunnel and presents some of the data, which demonstrates the potential benefit of using such a system in real, complex tunnelling scenarios.